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Nitrogen deposition differentially affects soil gross nitrogen transformations in organic and mineral horizons
Journal Pre-proof Nitrogen deposition differentially affects soil gross nitrogen transformations in organic and mineral horizons
Yi Cheng, Jing Wang, Jinyang Wang, Shenqiang Wang, Scott X. Chang, Zucong Cai, Jinbo Zhang, Shuli Niu, Shuijin Hu PII:
S0012-8252(19)30411-8
DOI:
https://doi.org/10.1016/j.earscirev.2019.103033
Reference:
EARTH 103033
To appear in:
Earth-Science Reviews
Received date:
24 June 2019
Revised date:
11 November 2019
Accepted date:
14 November 2019
Please cite this article as: Y. Cheng, J. Wang, J. Wang, et al., Nitrogen deposition differentially affects soil gross nitrogen transformations in organic and mineral horizons, Earth-Science Reviews(2019), https://doi.org/10.1016/j.earscirev.2019.103033
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© 2019 Published by Elsevier.
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Type of paper: Review Articles Date of preparation: April 2019 Text pages: 25; Tables: 0; Figures: 4 (plus 1 Table and 10 Figures in Supplementary Information)
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Nitrogen deposition differentially affects soil gross nitrogen transformations in
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organic and mineral horizons
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Yi Cheng1†, Jing Wang2†, Jinyang Wang3, Shenqiang Wang4, Scott X. Chang5,
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Zucong Cai1,6, Jinbo Zhang1,7,8,9*, Shuli Niu10*, Shuijin Hu11,12* School of Geography, Nanjing Normal University, Nanjing 210023, China
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College of Forestry, Nanjing Forestry University, Nanjing 210037, China
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School of the Environment, Natural Resources and Geography, Bangor University,
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Gwynedd LL57 2UW, UK
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State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science,
Chinese Academy of Sciences, Nanjing 210008, China 5
Department of Renewable Resources, 442 Earth Sciences Building, University of
Alberta, Edmonton T6G 2E3, Canada 6
Key Laboratory of Virtual Geographic Environment (Nanjing Normal University),
Ministry of Education, Nanjing, 210023, China 7
Jiangsu Center for Collaborative Innovation in Geographical Information Resource
Development and Application, Nanjing, 210023, China 1
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State Key Laboratory Cultivation Base of Geographical Environment Evolution
(Jiangsu Province), Nanjing, 210023, China 9
Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Control,
Nanjing, 210023, China 10
Key Laboratory of Ecosystem Network Observation and Modeling, Institute of
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Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, 100101, China
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Department of Entomology & Plant Pathology, North Carolina State University,
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Raleigh, NC 27695, USA
College of Resources and Environmental Sciences, Nanjing Agricultural
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University, Nanjing, 210095, China
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*: Corresponding authors
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JBZ: [email protected] Tel.: +86 25 8589 1203; Fax: +86 25 8589 1347 SLN: [email protected] Tel.: +86 10 64888062; Fax: +86 10 64889399 SJH: [email protected] Tel.: +86 919 515 2097; Fax: +86 919 515 7716 †
These authors contributed equally to this work.
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Journal Pre-proof Abstract Reactive nitrogen (N) input can profoundly alter soil N transformations and long-term productivity of forest ecosystems. However, critical knowledge gaps exist in our understanding of N deposition effects on internal soil N cycling in forest ecosystems. It is well established that N addition enhances soil N availability based on traditional
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net mineralization rate assays. Yet, experimental additions of inorganic N to soils broadly show a suppression of microbial activity and protein depolymerization. Here
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we show, from a global meta-analysis of 15N-labelled studies that gross N
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transformation rates in forest soil organic and mineral horizons differentially respond
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to N addition. In carbon (C)-rich organic horizons, N addition significantly enhanced
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soil gross rates of N mineralization, nitrification and microbial NO3¯ immobilization
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rates, but decreased gross microbial NH4+ immobilization rates. In C-poor mineral soils, in contrast, N addition did not change gross N transformation rates except for
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increasing gross nitrification rates. An initial soil C/N threshold of approx. 14.6, above which N addition enhanced gross N mineralization rates, could explain why gross N mineralization was increased by N deposition in organic horizons alone. Enhancement of gross N mineralization by N deposition was also largely attributed to enhanced N mineralization activity per unit microbial biomass. Our results indicate that the net effect of N input on forest soil gross N transformations are highly stratified by soil C distribution along the soil profile, and thus challenge the perception that N availability ubiquitously limits N mineralization. These findings suggest that these differences should be integrated into models to better predict forest ecosystem N 3
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cycle and C sequestration potential under future N deposition scenarios.
KEYWORDS Net N mineralization, gross mineralization, gross nitrification, gross NH4+ and NO3¯
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immobilization, nitrogen deposition, microbial biomass.
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Journal Pre-proof 1 INTRODUCTION
Nitrogen (N) is the primary limiting factor for the growth of organisms in forest ecosystems. In natural forests, available N for plants mainly comes from soil-available N and external N inputs, such as atmospheric N deposition and N2-fixation. Among these sources, atmospheric N deposition is becoming an important source of N input,
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alleviating N limitations in forest ecosystems on a global scale (Levy-Booth et al.,
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2014). Atmospheric N deposition has rapidly increased worldwide due to elevated
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fossil fuel combustion and widespread use of chemical N fertilizer, which increased
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reactive N losses to the environment (Galloway et al., 2008; Templer et al., 2012).
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Thus, a better understanding of the response of soil N cycling to N enrichment in forest ecosystems is critical to predicting future changes in soil N availability and N
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losses, and to understanding the related ecological and environmental issues under
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elevated atmospheric N deposition.
Soil N mineralization, which is an index of soil N availability for plants and microorganisms, can be affected by N addition. On one hand, N addition can enhance foliar and litter N, and enhance litter quality (reduced litter C:N ratio) (Lu et al., 2011; Chen et al., 2018). Increased litter quality is often associated with enhanced residue decomposition and thus soil N mineralization rates (Booth et al., 2005). On the other hand, N addition to forest ecosystems has been shown to decrease soil microbial activity and suppress depolymerization of proteins (a rate-limiting step of soil organic N mineralization), likely reducing N mineralization (Treseder, 2008; Chen et al., 2018; 5
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Ye et al., 2018; Zhang et al., 2018). These contrasting results illustrate the complexity of N deposition on soil N mineralization and highlight the need to better understand the underlying mechanisms. In addition, forest ecosystems commonly contain organic and mineral soil horizons with differences in chemical and physical properties (i.e. organic matter content, soil pore space, and N availability). Interactions of these
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properties with soil microorganisms, particularly in in boreal and temperate forests, make the effects of N addition on gross N transformation rates in forest ecosystems
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more complex.
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Previous studies have attempted to unravel the mechanisms behind the complex
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effects of N addition on soil N availability by calculating soil net N mineralization and
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nitrification rates (Nave et al., 2009; Lu et al., 2011; Chen et al., 2018). However,
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several simultaneously counteracting N cycling processes, e.g., gross N mineralization and immobilization, interact to determine the net N transformation
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rates and subsequent N losses (Niu et al., 2016). Consequently, the exact mechanisms through which N deposition affects soil N availability remain largely uncertain. Many studies have synthesized the effects of N addition on soil pH (Tian & Niu, 2015), soil microbial biomass (Treseder, 2008), soil N acquisition enzymes (Chen et al., 2018), soil amoA gene abundances of ammonia-oxidizing archaea (AOA) and bacteria (AOB) (Carey et al., 2016), and microbial composition (Zhang et al., 2018) in an attempt to establish the linkages between N deposition and soil net N transformation rates. Limited success, however, has so far been achieved because net N transformation rates mask the true rates of multiple nitrogen transformation 6
Journal Pre-proof processes, and thus can’t represent the real status of N cycling in soils. A comprehensive picture that incorporates soil gross N transformation rates would provide new insights into the complex mechanisms and processes that govern soil N availability in response to N deposition. Benefiting from recent rapid progress in 15N dilution techniques and 15N tracing
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models quantifying gross rates of N processes, we are able to conduct this data synthesis to identify the general pattern of the effects of N addition on soil gross N
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transformation rates in forest ecosystems. We also provide a process-based
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understanding of changes in forest soil N availability and subsequent N losses in
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response to N addition. We specifically address a fundamental question: if
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decomposition of organic matter is N-limited, why does N addition often supress
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microbial activity but enhance N mineralization? In addition, considering that a decrease in microbial N limitation with soil depth from organic to mineral horizons
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(Wild et al., 2015), we hypothesize that N addition would alleviate N limitation and thus promote gross N mineralization rates in organic horizons, while such effects would be weakened with soil depth especially in mineral horizons. All abbreviations used in the text are explained in Box 1.
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Box 1 Abbreviations used in the text Abbreviation Description N Nitrogen C Carbon P Phosphorus AOA Ammonia-oxidizing archaea AOB Ammonia-oxidizing bacteria MBC Microbial biomass C MBN Microbial biomass N MAT Mean annual temperature MAP Mean annual precipitation GMR Gross mineralization rates GNR Gross nitrification rates
Gross NH4+ immobilization rates
GINR GHNR GANR DNRA NMR NNR N/IA Specific GMR
Gross NO3¯ immobilization rates Gross heterotrophic nitrification rates Gross autotrophic nitrification rates Dissimilatory NO3¯ reduction to NH4+ Net mineralization rates Net nitrification rates Ratios of GNR to GIAR Gross mineralization rates per MBN
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GIAR
Gross NH4+ immobilization rates per MBN
Specific GINR
Gross NO3¯ immobilization rates per MBN
NO3¯MRT
Average residence time of NO3¯
NH4+MRT MBNMRT QT QW QM RR RR++ CI POM MAOM Den
Average residence time of NH4+ Average residence time of MBN Total heterogeneity Within-group Among-group The response ratio The weighted response ratio Confidence interval Particulate organic matter Mineral-associated organic matter Denitrification
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Specific GIAR
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2 MATERIALS AND METHODS
2.1 Data collection
We searched peer-reviewed papers that assessed the response of soil gross N
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transformation rates to N addition in forest ecosystems using the Web of Science and Google Scholar. Our own data from one submitted manuscript was also included (see
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Supporting Information). The following criteria were applied in selecting the studies:
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(1) N treatment and control plots were deployed under the same climate, soil and
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vegetation conditions to avoid confounding noise; (2) studies conducted along natural
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N deposition gradients were excluded due to possible effects of factors other than N;
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(3) the N application rate and the duration of the experiment were recorded; and (4) for studies with multiple factors being manipulated (e.g., a full factorial design with
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four treatments: control, phosphorus (P) addition, N addition, and combined N and P addition), we only extracted data from control and N addition treatments. We also extracted soil pH, soil organic C, total N, soil C/N ratio, microbial biomass C (MBC), microbial biomass N (MBN), MBC/MBN ratio, and respiration from control and N treatment plots when these data were reported along with gross N transformation rates. In addition, background information such as altitude, ambient N deposition, mean annual temperature (MAT), mean annual precipitation (MAP), soil horizon, forest type, climate region, initial values of soil pH, organic C, total N and C/N, respiration, N-addition form, N-addition rate, and N-addition duration were also 9
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extracted. All original data were extracted from the text, tables, figures, and appendices of the publications. When data were graphically presented, the g3data software (version 1.5.1) was used to obtain numeric data. Our final dataset included 619-paired observations derived from 27 papers with a total of 19 variables (Supporting Information). Among these sources, we evaluated a total of 68 paired gross mineralization rates, 60 paired gross nitrification rates, 51 paired gross NH4+
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immobilization rates, 50 paired gross NO3¯ immobilization rates, 51 paired ratios of
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gross nitrification to gross NH4+ immobilization rates (N/IA), 23 paired specific gross
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mineralization rates (expressed as gross N mineralization rates per MBN), 16 paired
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specific gross NH4+ immobilization rates (expressed as gross NH4+ immobilization
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rates per MBN), 16 paired specific gross NO3¯ immobilization rates (expressed as
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gross NO3¯ immobilization rates per MBN), 19 paired NO3¯MRT (average residence time of NO3¯, NO3¯/gross nitrification rates), 33 paired NH4+MRT (NH4+/gross N
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mineralization rates), and 16 paired MBNMRT (MBN /gross N immobilization rates) . In addition to examining the overall effects of N addition on soil gross N transformation rates, we determined how N treatments, forest types, and environmental conditions influence the responses of gross N transformation rates to N addition. We grouped the data based on N addition levels (<5, 5–10, and >10 g m−2 yr−1), N chemical species (CO(NH2)2, NH4NO3, NaNO3, NH4(SO4)2, NH4Cl), N-fertilizer forms (NH4+ based, NO3¯ based, NH4++NO3¯ based) and experimental durations (<5, 5–10, and >10 yr) (Lu et al., 2011). For forest type, we categorized the data into coniferous, broad-leaved, and mixed-wood forests. For environmental 10
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conditions, we classified the data according to altitude (high: 1500–3500 m, intermediate: 100–1500 m, and low: <100 m), annual precipitation (humid: >800 mm, semi-humid: 400–800 mm, semi-aid: 200–400 mm and arid: <200 mm) and climate region (tropical/subtropical, temperate and boreal).
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2.2 Meta-analysis
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Data were analyzed using the meta-analysis approach described by Hedges et al.
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(1999). For each study, the response ratio (RR) was calculated using the equation:
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RR = ln(Xt/Xc), where Xt and Xc were means of the chosen variable in the N addition
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and control treatments, respectively. The weighted response ratio (RR++) and 95%
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bootstrap confidence interval (CI) were calculated using MetaWin 2.1 software with a random effects model (Rosenberg et al., 2000). N-induced changes of soil gross N
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transformation rates were calculated using the equation: Change (%) = (e RR++ –1) × 100. The change is considered to be significant (P < 0.05) if the 95% CI does not overlap zero (Hedges et al., 1999). For each categorical variable (soil horizon, altitude, annual precipitation, climate region, forest type, N species, N chemical form, N application rate, and N application duration), total heterogeneity (QT) was partitioned into within-group (QW) and among-group (QM) heterogeneity (Table S1). We considered a particular categorical variable to have a significant impact on the response ratio when QM was larger than the critical value (Gurevitch & Hedges,
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1993), and we examined this significance using Prandom values (produced from randomized tests with 4999 permutations and sample size as the weighting function). To clarify the mechanisms by which N addition caused the change in soil gross N transformation rates, correlation analyses were conducted to test the relationships between the RRs of various gross N transformation rates and RRs of
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soil pH, organic C, total N, C/N ratio, MBC, MBN, MBC/MBN ratio, and respiration, as well as the relationships between the RRs of various gross N transformation rates
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and altitude, ambient N deposition, mean annual temperature, mean annual
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precipitation, N-addition rate, N-addition duration, total N load (determined by
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multiplying the N application rate by total number of applications), and background
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soil pH, organic C, total N, C/N ratio, MBC, MBN, MBC/MBN ratio and respiration
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(values in controls). In addition, the relationships among RRs of various gross N transformation rates were examined. The correlation analysis was conducted with
3 RESULTS
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SPSS software (SPSS 16.0 for windows, SPSS Inc., Chicago, IL, USA).
Averaged across all studies, N addition significantly increased gross N mineralization rates by 15.4% and gross nitrification rates by 42.5%, but had no significant effects on gross NH4+ immobilization rates and gross NO3¯ immobilization rates (Fig. 1). The response of gross N transformation rates to N addition varied with soil horizons (Fig. 2 and 3). In the organic horizon, N addition significantly enhanced soil gross N 12
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the relative importance of the two microbial consumptive fates of NH4+ and the risk of NO3¯ leaching losses, were significantly enhanced by 100.4% and to a lesser degree
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by 53.1% (-0.93 to 124%) in response to N addition in the organic and mineral soil
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layers, respectively (Fig. 2).
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(a)
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GMR GNR
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GIAR
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GINR
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N/IA
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Specific GMR
(b)
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Specific GIAR
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Specific GINR
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NO NO3-N 3 MRT
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+ NH4+M NH 4 MRT
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MBNM
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MBNMRT -50
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100 150 200 250 300 350
N-induced change (%)
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Figure 1 Effects of N deposition on eleven variables related to N transformation rates. Data points are the summary statistics from the meta-analysis. Error bars represent 95% bootstrap confidence intervals (CIs) and values next to the dots denote the number of observations. The effect of N addition is significant if the CI of effect size does not cover zero. GMR, gross mineralization rates; GNR, gross nitrification rates; GIAR, gross NH4+ immobilization rates; GINR, gross NO3¯ immobilization rates; N/IA, the ratio of soil gross nitrification to gross NH4+
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immobilization rates; Specific GMR, GIAR and GINR represent N mineralization, NH4+
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immobilization and NO3¯ immobilization activity per microbial biomass, respectively; NO3¯MRT,
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NH4+MRT and MBNMRT represent the average residence time of NO3¯, NH4+ and MBN,
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respectively. See Box 1 for explanation of abbreviations.
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Specific GMR, GIAR and GINR (expressed as gross N mineralization, NH4+ immobilization and NO3¯ immobilization rates per microbial biomass N, respectively),
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as an index of the influence of substrate quantity and quality, showed no significant change in response to N addition (Fig. 1). However, specific GMR tended to increase by 19.3% (-1.13 to 42.4%) in response to N addition. The NH4+MRT and MBNMRT (the average residence time of NH4+ and MBN, respectively) also showed no significant change, whereas NO3¯MRT significantly increased by 138% in response to N addition (Fig. 1).
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GINR (d)
GMR (a)
All
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Mineral
Mineral
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Organic
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Organic
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50 100 150 200 250 300
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GNR (b)
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GIAR (c)
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N-induced change (%)
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N-induced change (%)
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Figure 2 Effects of soil horizon (organic vs. mineral) on (a) N-induced change of soil gross mineralization rates (GMR), (b) gross nitrification rates (GNR), (c) gross NH4+ immobilization rates (GIAR), (d) gross NO3¯ immobilization rates (GINR) and (e) ratio of soil gross nitrification to gross NH4+ immobilization rates (N/IA). Data points are the summary statistics from the meta-analysis. Error bars represent 95% bootstrap confidence intervals (CIs) and values next to the dots denote the number of observations. The effect of N addition is significant if the CI of effect size does not cover zero. See Box 1 for explanation of abbreviations.
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Figure 3 Synthesized response of forest ecosystem N pools and biogeochemical processes
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to N deposition. GMR, gross mineralization rates; GNR, gross nitrification rates; GIAR, gross NH4+ immobilization rates; GINR, gross NO3¯ immobilization rates; N/IA, the ratio of soil
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gross nitrification to gross NH4+ immobilization rates; NMR, net mineralization rates; NNR, net nitrification rates; GHNR, gross heterotrophic nitrification rates; GANR, gross autotrophic nitrification rates; DNRA, dissimilatory NO3¯ reduction to NH4+; Den, denitrification; NMR, GMR-(GIAR+GINR); NNR, GNR-GINR; Red and blue numbers refer to the data based on the meta-analysis by this study and another previous meta-analysis by Lu et al., (2011), respectively; +, increase in response to N addition; -, decrease in response to N addition; ?, there are currently not enough studies, No, no significant response. See Box 1 for explanation of abbreviations.
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Altitude, climate region and duration of N application were found to significantly affect N-addition induced changes in gross mineralization rates, while annual precipitation, forest type, N species, N chemical form and N application rate did not exhibit significant effects (Fig. S1). In addition, the response ratio of gross mineralization rates in response to N addition was negatively correlated with initial
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soil pH, and the response ratio of soil C/N and MBC/MBN, but was positively correlated with N load, duration of N fertilizer, initial soil C/N, and the response ratio of
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soil organic C and total N (Fig. 4). Nitrogen-induced changes of gross nitrification
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rates varied with annual precipitation, N application rate, and duration of N application
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(Fig. S2). Additionally, the response ratio of gross nitrification rates in response to N
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addition was significantly and positively correlated with mean annual temperature,
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mean annual precipitation, N application rate, N load and duration of N fertilizer (Fig. S3). In contrast, none of variables significantly affected N-induced changes of gross
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NH4+ immobilization rates (Fig. S4). There was a significant relationship between the response ratio of gross NH4+ immobilization rates and gross mineralization rates in response to N addition (Fig. S5).The response of gross NO3¯ immobilization rates to N addition depended on climate zone and duration of N application (Fig. S6). The response ratio of gross NO3¯ immobilization rates and gross nitrification rates was significantly and positively correlated (Fig. S5). Additionally, the response ratio of gross NO3¯ immobilization rates in response to N addition was significantly and negatively correlated with mean annual temperature, but was significantly and positively correlated with N load and duration of N fertilizer (Fig. S7). The response of 17
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N/IA value to N addition was dependent on annual precipitation and duration of N application (Fig. S8). There were significant and positive relationships between the response ratio of N/IA value in response to N addition and N application rate, N load
1 0 -1 0
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1 0 -1 -2
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-1
(d)
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R =0.212, P<0.01
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R =0.138, P<0.05
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GMR response ratio
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(g)
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Soil total C response ratio
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Soil C/N response ratio
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R =0.108, P<0.05
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Initial soil pH 3 2
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R =0.285, P<0.01
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Soil total N response ratio
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(h) R =0.236, P<0.05
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GMR response ratio
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and duration of N fertilizer (Fig. S9).
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MBC/MBN response ratio
Figure 4 Relationships between gross mineralization rates (GMR) response ratio and (a) N load, (b) duration, (c) initial soil pH, (d) initial soil C/N, and between GMR and response ratio of (e) soil total C, (f) total N, (g) C/N, and (h) MBC/MBN. N load and duration represent the total amount of N added over the course of the experiment and duration of fertilization, respectively. The closed and open squares indicate that the data were derived from mineral and organic layers, respectively. 18
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4 DISCUSSION
To our knowledge, this study presents the first comprehensive synthesis of process-based understanding of how N enrichment affects soil gross N
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transformation rates and N availability (Fig. 3). One phenomenon has been puzzling to the ecology community is: while decomposition of organic matter is N-limited, N
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addition often supresses microbial activity but enhances N mineralization. Our results
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showed that gross N mineralization rates increased significantly in response to N
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addition in the organic horizon, but were unaffected in the mineral horizon (Fig. 2 and
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3). Such results were in contrast to the prevailing view that N availability limits N
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mineralization in terrestrial ecosystems (Nave et al., 2009; Lu et al., 2011; Chen et al., 2018), and also were inconsistent with previous findings that organic horizons and
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mineral soils did not exhibit significantly different net N mineralization responses to N inputs in forest ecosystems (Nave et al., 2009). We propose a new explanation for how N addition affects gross N mineralization rates in different soil horizons (Fig. 3). We found that there was an initial soil C/N threshold of approx. 14.6, above which N addition had a positive effect on gross N mineralization rates (Fig. 4). The soil C/N in the organic horizon ranged from 16.4 to 40.2 with an average of 31.9, while it averaged 15.0 (6.9-17.5) in the mineral horizon (Fig. 4). Under higher C/N ratios, soil microbes were generally N-limited, and N addition might have alleviated such limitation, thereby increasing gross C and N mineralization rates (Averill & Waring, 19
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2017). The possible explanation was that under N limitation, large decreases in particulate organic matter (POM, microbial-accessible) following N fertilization overwhelmed gains in mineral-associated organic matter (MAOM, microbial-inaccessible), leading to a net decrease in total soil C and N (Averill & Waring, 2017). Several studies have documented increases in MAOM under N
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fertilization, accompanied by decreases in POM in forests (Cusack et al., 2010) and agroecosystems (Dou et al., 2016). When microbes N limitation are removed by N
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addition, the microbial population size and decomposition rates increase, driving
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down the size of the POM pool, at the same time the increased microbial population
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size, microbial C use efficiency and turnover of the microbial biomass accelerate
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sorption of microbial necromass onto mineral binding sites, causing the MAOM pool
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to increase (Averill & Waring, 2017). In contrast, microbial biomass in the mineral horizon was considered to be C-limited, and hence N addition has no effect on gross
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C and N mineralization. Therefore, the difference in C/N may account for the different sensitivities of organic and mineral horizons to N addition in gross N mineralization. Alternatively, the different sensitivities of organic and mineral horizons to N addition was assumed to be associated with the fact that forest floor is the initial entry point of N addition. Microbial activity in the forest floor has been found to be likely more responsive to NO3¯ deposition compared to mineral soil (Deforest et al., 2004a, b). However, we found that N addition significantly increased gross nitrification rates in both forest organic and mineral layers. Such results indicated that different sensitivities of organic and mineral horizons to N addition appeared to not be 20
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associated with the initial entry point of N addition. Consistent with previous findings that gross mineralization rates were positively related to soil N concentration (substrate quantity) and negatively related to soil C/N (substrate quality) (Booth et al., 2005; Högberg et al., 2007), our study also showed that the response ratio of gross mineralization rates was negatively correlated with the response ratio of soil C/N and
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MBC/MBN, but was positively correlated with the response ratio of total N (Fig. 4). Thus, the increased soil N concentration and decreased soil C/N in response to N
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addition (Lu et al., 2011; Chen et al., 2018) was likely responsible for the enhanced
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soil gross mineralization rates in the organic horizon.
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In addition, microbial activity is also an important factor controlling mineralization
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rates. Generally, increased N mineralization is expected with elevated microbial
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activities (Aber et al., 1998). However, numerous studies have confirmed that N addition had a global negative effect on microbial index, such as soil microbial
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composition (Zhang et al., 2018), depolymerization of protein (a rate-limiting step of soil organic N mineralization) (Chen et al., 2018) and microbial biomass (Treseder, 2008; Lu et al., 2011; Ye et al., 2018). An increased gross N mineralization rate accompanied by reduced microbial activity in response to N addition is particularly perplexing. Chen et al. (2018) accordingly concluded that changes in microbial activity are not a dominant mechanism for increased N mineralization. In fact, changes in N availability are frequently linked to shifts in soil pH or osmotic stress, which may have masked N stimulation of microbial activity (Averill & Waring, 2017). We thus propose that another microbial activity related index should be introduced to 21
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address such problem. We used specific gross N mineralization rates (gross N mineralization rate ÷ microbial N) as a proxy of activity per unit microbial biomass (Corre et al., 2010), and found that specific gross N mineralization rates were enhanced by 19.3% under N addition. This well explained why N addition often supress microbial activity but enhances N mineralization. The findings highlight that
a crucial role in regulating soil N mineralization.
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substrate quantity and quality, reflected by specific gross N mineralization rates, play
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Our synthesis also showed that gross nitrification rates significantly increased by
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N addition in forest ecosystems (Fig. 2 and 3). NH4+ addition increased gross
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nitrification rates probably by directly providing NH4+ substrate for nitrification (Corre
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et al., 2014; Baldos et al., 2015). A positive relationship between the response ratio of
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gross nitrification rates and gross mineralization rates indicated that increasing NH4+ availability from mineralization was at least partially responsible for the enhanced
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gross nitrification rates under N addition. Alternatively, heterotrophic microorganisms may compete against nitrifiers for available NH4+ under high C/N (Murphy et al., 2003; Booth et al., 2005). Soil C/N has been demonstrated to decrease significantly by N addition (Lu et al., 2011; Chen et al., 2018), which might have caused lower microbial demand for NH4+ and thus more opportunity for nitrifiers for available NH4+, thereby causing increased nitrification. On the contrary, N addition-induced soil acidification might have inhibited gross nitrification rates (Cheng et al., 2013; Tian & Niu, 2015). There was a possibility that the stimulatory effect of N addition might offset the
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inhibitory effect of N addition-induced soil acidification, resulting in increasing gross nitrification rates by N addition in forest ecosystem. Microbial immobilization of N is considered an underlying mechanism for soil N retention. Considering that N addition generally causes a decrease in various microbial indices and soil pH (Treseder, 2008; Tian & Niu, 2015; Chen et al., 2018;
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Zhang et al., 2018), reduced microbial immobilization of N in response to N addition would be expected. However, we found that N addition increased gross NO3¯
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immobilization rates by 51.4% and decreased gross NH4+ immobilization rates by
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25.6% in the organic horizon. Generally, soil microbes prefer NH4+ over NO3¯ for their
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growth (Lindell & Post, 2001), and greater gross NH4+ immobilization rates could be
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expected. We indeed found that gross NH4+ immobilization rates were on average 25
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times higher than gross NO3¯ immobilization rates in our data set (Fig. S10). Our results may suggest that under N addition decreased immobilization of NH4+ into the
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MBN pool outweighed increased immobilization of NO3¯ into MBN pool, resulting in a reduced microbial biomass N (Lu et al., 2011). However, no significant response of MBNMRT to N addition suggests that MBN turnover rate was not affected by N addition. Such results further demonstrate that there was no certain relationship between MBN and gross N transformation rates in response to N addition. A previous review suggests that much of the enhanced N leaching into streams under enhanced N deposition is derived from soil N cycling through organic and inorganic pools, rather than from its passing directly from the atmosphere to streams, which underlined the significance of soil microbial N cycles that govern N availability 23
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and related N losses in soils (Niu et al., 2016). In general, a tightly coupled microbial N cycle, where NH4+ and NO3¯ immobilization rates were comparable to gross mineralization and nitrification rates, respectively, signifies a conservative N cycle and low N losses (Corre et al., 2003). In this study, we found forest ecosystems appeared to move towards an uncoupled microbial N cycle under N addition, where
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gross mineralization rates increased but gross NH4+ immobilization rates decreased, and gross nitrification rates increased faster than gross NO3¯ immobilization rates,
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resulting in potential risks of high N losses. Likewise, the N/IA value, considered as
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the relative rates of NH4+ being nitrified or immobilized by soil microorganisms, was
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significantly enhanced by N addition, indicating increasing risk of NO3¯ losses
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(Stockdale et al., 2002; Murphy et al., 2003). Longer NO3¯MRT in response to N
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addition also indicated a slower NO3¯ pool turnover and a less dynamic pool, and hence implied a higher tendency for N losses. Indeed, from global syntheses, N
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addition has been shown to promote N2O emission and NO3¯ leaching (Liu & Greaver, 2009; Lu et al., 2011), and reduce total ecosystem 15N recovery (Templer et al., 2012). Thus, the strategies to reduce gross nitrification rates and simultaneously enhance gross N immobilization rates could be recommended to keep soil NO3¯ concentration low in forest ecosystems in response to N deposition, thereby increasing N retention in soils and avoiding N losses to environments. This study for the first time proposes a new mechanistic understanding that incorporates gross N transformations into the effects of N deposition on forest soil N availability and N losses. In contrast to the prevailing view that N availability limits N 24
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mineralization, we propose a new conceptual model that gross N mineralization rates in response to N enrichment depend on forest soil layers. Mineralization was more stimulated by N addition in organic soil horizons with higher C/N compared to mineral soil horizons. Our results suggest that enhanced activity per unit microbial biomass largely explained why N addition enhanced gross N mineralization rates
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accompanied by reduced microbial activity. The greater increase of gross nitrification rates compared to gross NO3¯ immobilization rates under elevated N deposition
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indicated that forest ecosystems tend toward an uncoupled microbial N cycle. The
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increasing N/IA value in response to N addition also suggested an increasing risk of
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NO3¯ losses in forest ecosystems. These findings highlight changes in soil gross N
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transformation rates in specific soil layers should be integrated into models to better
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predict forest ecosystem N cycles and C sequestration potential under future N deposition scenarios. Although a limited number of observations exist for this
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meta-analysis of very complex processes, we believe they provide a useful addition in understanding the responses of N cycling processes to experimental N addition, and should improve our understanding of the underlying mechanisms of N deposition effects on plant and ecosystem function.
ACKNOWLEDGEMENTS
We thank the authors whose work was included in the meta-analyses. This work was financially supported by the National Natural Science Foundation of China [grant 25
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numbers 41977081, 41807093, 41622104, and 41830642], and the National Key Research and Development Program of China [grant numbers 2017YFD0200106 and 2017YFD0800103]. We thank Dr. Robert B. Jacobson and two anonymous reviewers for their constructive comments that substantially improved the quality of an earlier
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version of this manuscript.
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The authors declare no competing interests.
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COMPETING INTERESTS
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Figure 1
Figure 2
Figure 3
Figure 4
Yi Cheng, Jing Wang, Jinyang Wang, Shenqiang Wang, Scott X. Chang, Zucong Cai, Jinbo Zhang, Shuli Niu, Shuijin Hu PII:
S0012-8252(19)30411-8
DOI:
https://doi.org/10.1016/j.earscirev.2019.103033
Reference:
EARTH 103033
To appear in:
Earth-Science Reviews
Received date:
24 June 2019
Revised date:
11 November 2019
Accepted date:
14 November 2019
Please cite this article as: Y. Cheng, J. Wang, J. Wang, et al., Nitrogen deposition differentially affects soil gross nitrogen transformations in organic and mineral horizons, Earth-Science Reviews(2019), https://doi.org/10.1016/j.earscirev.2019.103033
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Type of paper: Review Articles Date of preparation: April 2019 Text pages: 25; Tables: 0; Figures: 4 (plus 1 Table and 10 Figures in Supplementary Information)
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Nitrogen deposition differentially affects soil gross nitrogen transformations in
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organic and mineral horizons
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Yi Cheng1†, Jing Wang2†, Jinyang Wang3, Shenqiang Wang4, Scott X. Chang5,
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Zucong Cai1,6, Jinbo Zhang1,7,8,9*, Shuli Niu10*, Shuijin Hu11,12* School of Geography, Nanjing Normal University, Nanjing 210023, China
2
College of Forestry, Nanjing Forestry University, Nanjing 210037, China
3
School of the Environment, Natural Resources and Geography, Bangor University,
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Gwynedd LL57 2UW, UK
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1
State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science,
Chinese Academy of Sciences, Nanjing 210008, China 5
Department of Renewable Resources, 442 Earth Sciences Building, University of
Alberta, Edmonton T6G 2E3, Canada 6
Key Laboratory of Virtual Geographic Environment (Nanjing Normal University),
Ministry of Education, Nanjing, 210023, China 7
Jiangsu Center for Collaborative Innovation in Geographical Information Resource
Development and Application, Nanjing, 210023, China 1
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State Key Laboratory Cultivation Base of Geographical Environment Evolution
(Jiangsu Province), Nanjing, 210023, China 9
Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Control,
Nanjing, 210023, China 10
Key Laboratory of Ecosystem Network Observation and Modeling, Institute of
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Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, 100101, China
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Department of Entomology & Plant Pathology, North Carolina State University,
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Raleigh, NC 27695, USA
College of Resources and Environmental Sciences, Nanjing Agricultural
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University, Nanjing, 210095, China
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11
*: Corresponding authors
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JBZ: [email protected] Tel.: +86 25 8589 1203; Fax: +86 25 8589 1347 SLN: [email protected] Tel.: +86 10 64888062; Fax: +86 10 64889399 SJH: [email protected] Tel.: +86 919 515 2097; Fax: +86 919 515 7716 †
These authors contributed equally to this work.
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Journal Pre-proof Abstract Reactive nitrogen (N) input can profoundly alter soil N transformations and long-term productivity of forest ecosystems. However, critical knowledge gaps exist in our understanding of N deposition effects on internal soil N cycling in forest ecosystems. It is well established that N addition enhances soil N availability based on traditional
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net mineralization rate assays. Yet, experimental additions of inorganic N to soils broadly show a suppression of microbial activity and protein depolymerization. Here
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we show, from a global meta-analysis of 15N-labelled studies that gross N
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transformation rates in forest soil organic and mineral horizons differentially respond
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to N addition. In carbon (C)-rich organic horizons, N addition significantly enhanced
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soil gross rates of N mineralization, nitrification and microbial NO3¯ immobilization
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rates, but decreased gross microbial NH4+ immobilization rates. In C-poor mineral soils, in contrast, N addition did not change gross N transformation rates except for
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increasing gross nitrification rates. An initial soil C/N threshold of approx. 14.6, above which N addition enhanced gross N mineralization rates, could explain why gross N mineralization was increased by N deposition in organic horizons alone. Enhancement of gross N mineralization by N deposition was also largely attributed to enhanced N mineralization activity per unit microbial biomass. Our results indicate that the net effect of N input on forest soil gross N transformations are highly stratified by soil C distribution along the soil profile, and thus challenge the perception that N availability ubiquitously limits N mineralization. These findings suggest that these differences should be integrated into models to better predict forest ecosystem N 3
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cycle and C sequestration potential under future N deposition scenarios.
KEYWORDS Net N mineralization, gross mineralization, gross nitrification, gross NH4+ and NO3¯
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immobilization, nitrogen deposition, microbial biomass.
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Journal Pre-proof 1 INTRODUCTION
Nitrogen (N) is the primary limiting factor for the growth of organisms in forest ecosystems. In natural forests, available N for plants mainly comes from soil-available N and external N inputs, such as atmospheric N deposition and N2-fixation. Among these sources, atmospheric N deposition is becoming an important source of N input,
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alleviating N limitations in forest ecosystems on a global scale (Levy-Booth et al.,
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2014). Atmospheric N deposition has rapidly increased worldwide due to elevated
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fossil fuel combustion and widespread use of chemical N fertilizer, which increased
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reactive N losses to the environment (Galloway et al., 2008; Templer et al., 2012).
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Thus, a better understanding of the response of soil N cycling to N enrichment in forest ecosystems is critical to predicting future changes in soil N availability and N
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losses, and to understanding the related ecological and environmental issues under
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elevated atmospheric N deposition.
Soil N mineralization, which is an index of soil N availability for plants and microorganisms, can be affected by N addition. On one hand, N addition can enhance foliar and litter N, and enhance litter quality (reduced litter C:N ratio) (Lu et al., 2011; Chen et al., 2018). Increased litter quality is often associated with enhanced residue decomposition and thus soil N mineralization rates (Booth et al., 2005). On the other hand, N addition to forest ecosystems has been shown to decrease soil microbial activity and suppress depolymerization of proteins (a rate-limiting step of soil organic N mineralization), likely reducing N mineralization (Treseder, 2008; Chen et al., 2018; 5
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Ye et al., 2018; Zhang et al., 2018). These contrasting results illustrate the complexity of N deposition on soil N mineralization and highlight the need to better understand the underlying mechanisms. In addition, forest ecosystems commonly contain organic and mineral soil horizons with differences in chemical and physical properties (i.e. organic matter content, soil pore space, and N availability). Interactions of these
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properties with soil microorganisms, particularly in in boreal and temperate forests, make the effects of N addition on gross N transformation rates in forest ecosystems
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more complex.
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Previous studies have attempted to unravel the mechanisms behind the complex
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effects of N addition on soil N availability by calculating soil net N mineralization and
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nitrification rates (Nave et al., 2009; Lu et al., 2011; Chen et al., 2018). However,
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several simultaneously counteracting N cycling processes, e.g., gross N mineralization and immobilization, interact to determine the net N transformation
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rates and subsequent N losses (Niu et al., 2016). Consequently, the exact mechanisms through which N deposition affects soil N availability remain largely uncertain. Many studies have synthesized the effects of N addition on soil pH (Tian & Niu, 2015), soil microbial biomass (Treseder, 2008), soil N acquisition enzymes (Chen et al., 2018), soil amoA gene abundances of ammonia-oxidizing archaea (AOA) and bacteria (AOB) (Carey et al., 2016), and microbial composition (Zhang et al., 2018) in an attempt to establish the linkages between N deposition and soil net N transformation rates. Limited success, however, has so far been achieved because net N transformation rates mask the true rates of multiple nitrogen transformation 6
Journal Pre-proof processes, and thus can’t represent the real status of N cycling in soils. A comprehensive picture that incorporates soil gross N transformation rates would provide new insights into the complex mechanisms and processes that govern soil N availability in response to N deposition. Benefiting from recent rapid progress in 15N dilution techniques and 15N tracing
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models quantifying gross rates of N processes, we are able to conduct this data synthesis to identify the general pattern of the effects of N addition on soil gross N
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transformation rates in forest ecosystems. We also provide a process-based
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understanding of changes in forest soil N availability and subsequent N losses in
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response to N addition. We specifically address a fundamental question: if
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decomposition of organic matter is N-limited, why does N addition often supress
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microbial activity but enhance N mineralization? In addition, considering that a decrease in microbial N limitation with soil depth from organic to mineral horizons
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(Wild et al., 2015), we hypothesize that N addition would alleviate N limitation and thus promote gross N mineralization rates in organic horizons, while such effects would be weakened with soil depth especially in mineral horizons. All abbreviations used in the text are explained in Box 1.
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Box 1 Abbreviations used in the text Abbreviation Description N Nitrogen C Carbon P Phosphorus AOA Ammonia-oxidizing archaea AOB Ammonia-oxidizing bacteria MBC Microbial biomass C MBN Microbial biomass N MAT Mean annual temperature MAP Mean annual precipitation GMR Gross mineralization rates GNR Gross nitrification rates
Gross NH4+ immobilization rates
GINR GHNR GANR DNRA NMR NNR N/IA Specific GMR
Gross NO3¯ immobilization rates Gross heterotrophic nitrification rates Gross autotrophic nitrification rates Dissimilatory NO3¯ reduction to NH4+ Net mineralization rates Net nitrification rates Ratios of GNR to GIAR Gross mineralization rates per MBN
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GIAR
Gross NH4+ immobilization rates per MBN
Specific GINR
Gross NO3¯ immobilization rates per MBN
NO3¯MRT
Average residence time of NO3¯
NH4+MRT MBNMRT QT QW QM RR RR++ CI POM MAOM Den
Average residence time of NH4+ Average residence time of MBN Total heterogeneity Within-group Among-group The response ratio The weighted response ratio Confidence interval Particulate organic matter Mineral-associated organic matter Denitrification
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Specific GIAR
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2 MATERIALS AND METHODS
2.1 Data collection
We searched peer-reviewed papers that assessed the response of soil gross N
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transformation rates to N addition in forest ecosystems using the Web of Science and Google Scholar. Our own data from one submitted manuscript was also included (see
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Supporting Information). The following criteria were applied in selecting the studies:
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(1) N treatment and control plots were deployed under the same climate, soil and
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vegetation conditions to avoid confounding noise; (2) studies conducted along natural
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N deposition gradients were excluded due to possible effects of factors other than N;
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(3) the N application rate and the duration of the experiment were recorded; and (4) for studies with multiple factors being manipulated (e.g., a full factorial design with
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four treatments: control, phosphorus (P) addition, N addition, and combined N and P addition), we only extracted data from control and N addition treatments. We also extracted soil pH, soil organic C, total N, soil C/N ratio, microbial biomass C (MBC), microbial biomass N (MBN), MBC/MBN ratio, and respiration from control and N treatment plots when these data were reported along with gross N transformation rates. In addition, background information such as altitude, ambient N deposition, mean annual temperature (MAT), mean annual precipitation (MAP), soil horizon, forest type, climate region, initial values of soil pH, organic C, total N and C/N, respiration, N-addition form, N-addition rate, and N-addition duration were also 9
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extracted. All original data were extracted from the text, tables, figures, and appendices of the publications. When data were graphically presented, the g3data software (version 1.5.1) was used to obtain numeric data. Our final dataset included 619-paired observations derived from 27 papers with a total of 19 variables (Supporting Information). Among these sources, we evaluated a total of 68 paired gross mineralization rates, 60 paired gross nitrification rates, 51 paired gross NH4+
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immobilization rates, 50 paired gross NO3¯ immobilization rates, 51 paired ratios of
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gross nitrification to gross NH4+ immobilization rates (N/IA), 23 paired specific gross
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mineralization rates (expressed as gross N mineralization rates per MBN), 16 paired
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specific gross NH4+ immobilization rates (expressed as gross NH4+ immobilization
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rates per MBN), 16 paired specific gross NO3¯ immobilization rates (expressed as
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gross NO3¯ immobilization rates per MBN), 19 paired NO3¯MRT (average residence time of NO3¯, NO3¯/gross nitrification rates), 33 paired NH4+MRT (NH4+/gross N
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mineralization rates), and 16 paired MBNMRT (MBN /gross N immobilization rates) . In addition to examining the overall effects of N addition on soil gross N transformation rates, we determined how N treatments, forest types, and environmental conditions influence the responses of gross N transformation rates to N addition. We grouped the data based on N addition levels (<5, 5–10, and >10 g m−2 yr−1), N chemical species (CO(NH2)2, NH4NO3, NaNO3, NH4(SO4)2, NH4Cl), N-fertilizer forms (NH4+ based, NO3¯ based, NH4++NO3¯ based) and experimental durations (<5, 5–10, and >10 yr) (Lu et al., 2011). For forest type, we categorized the data into coniferous, broad-leaved, and mixed-wood forests. For environmental 10
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conditions, we classified the data according to altitude (high: 1500–3500 m, intermediate: 100–1500 m, and low: <100 m), annual precipitation (humid: >800 mm, semi-humid: 400–800 mm, semi-aid: 200–400 mm and arid: <200 mm) and climate region (tropical/subtropical, temperate and boreal).
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2.2 Meta-analysis
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Data were analyzed using the meta-analysis approach described by Hedges et al.
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(1999). For each study, the response ratio (RR) was calculated using the equation:
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RR = ln(Xt/Xc), where Xt and Xc were means of the chosen variable in the N addition
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and control treatments, respectively. The weighted response ratio (RR++) and 95%
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bootstrap confidence interval (CI) were calculated using MetaWin 2.1 software with a random effects model (Rosenberg et al., 2000). N-induced changes of soil gross N
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transformation rates were calculated using the equation: Change (%) = (e RR++ –1) × 100. The change is considered to be significant (P < 0.05) if the 95% CI does not overlap zero (Hedges et al., 1999). For each categorical variable (soil horizon, altitude, annual precipitation, climate region, forest type, N species, N chemical form, N application rate, and N application duration), total heterogeneity (QT) was partitioned into within-group (QW) and among-group (QM) heterogeneity (Table S1). We considered a particular categorical variable to have a significant impact on the response ratio when QM was larger than the critical value (Gurevitch & Hedges,
11
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1993), and we examined this significance using Prandom values (produced from randomized tests with 4999 permutations and sample size as the weighting function). To clarify the mechanisms by which N addition caused the change in soil gross N transformation rates, correlation analyses were conducted to test the relationships between the RRs of various gross N transformation rates and RRs of
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soil pH, organic C, total N, C/N ratio, MBC, MBN, MBC/MBN ratio, and respiration, as well as the relationships between the RRs of various gross N transformation rates
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and altitude, ambient N deposition, mean annual temperature, mean annual
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precipitation, N-addition rate, N-addition duration, total N load (determined by
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multiplying the N application rate by total number of applications), and background
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soil pH, organic C, total N, C/N ratio, MBC, MBN, MBC/MBN ratio and respiration
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(values in controls). In addition, the relationships among RRs of various gross N transformation rates were examined. The correlation analysis was conducted with
3 RESULTS
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SPSS software (SPSS 16.0 for windows, SPSS Inc., Chicago, IL, USA).
Averaged across all studies, N addition significantly increased gross N mineralization rates by 15.4% and gross nitrification rates by 42.5%, but had no significant effects on gross NH4+ immobilization rates and gross NO3¯ immobilization rates (Fig. 1). The response of gross N transformation rates to N addition varied with soil horizons (Fig. 2 and 3). In the organic horizon, N addition significantly enhanced soil gross N 12
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the relative importance of the two microbial consumptive fates of NH4+ and the risk of NO3¯ leaching losses, were significantly enhanced by 100.4% and to a lesser degree
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by 53.1% (-0.93 to 124%) in response to N addition in the organic and mineral soil
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layers, respectively (Fig. 2).
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(a)
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GMR GNR
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GIAR
51
GINR
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50
N/IA
51
Specific GMR
(b)
23
Specific GIAR
16
Specific GINR
16
NO NO3-N 3 MRT
19
+ NH4+M NH 4 MRT
33
MBNM
16
MBNMRT -50
0
50
100 150 200 250 300 350
N-induced change (%)
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Figure 1 Effects of N deposition on eleven variables related to N transformation rates. Data points are the summary statistics from the meta-analysis. Error bars represent 95% bootstrap confidence intervals (CIs) and values next to the dots denote the number of observations. The effect of N addition is significant if the CI of effect size does not cover zero. GMR, gross mineralization rates; GNR, gross nitrification rates; GIAR, gross NH4+ immobilization rates; GINR, gross NO3¯ immobilization rates; N/IA, the ratio of soil gross nitrification to gross NH4+
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immobilization rates; Specific GMR, GIAR and GINR represent N mineralization, NH4+
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immobilization and NO3¯ immobilization activity per microbial biomass, respectively; NO3¯MRT,
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NH4+MRT and MBNMRT represent the average residence time of NO3¯, NH4+ and MBN,
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respectively. See Box 1 for explanation of abbreviations.
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Specific GMR, GIAR and GINR (expressed as gross N mineralization, NH4+ immobilization and NO3¯ immobilization rates per microbial biomass N, respectively),
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as an index of the influence of substrate quantity and quality, showed no significant change in response to N addition (Fig. 1). However, specific GMR tended to increase by 19.3% (-1.13 to 42.4%) in response to N addition. The NH4+MRT and MBNMRT (the average residence time of NH4+ and MBN, respectively) also showed no significant change, whereas NO3¯MRT significantly increased by 138% in response to N addition (Fig. 1).
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GINR (d)
GMR (a)
All
All
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Mineral
Mineral
40
Organic
0
34
Organic
28
-50
50
16
-50
50 100 150 200 250 300
0
50 100 150 200 250 300 N/IA (e)
GNR (b)
All
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Organic
0
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-50
-50
50 100 150 200 250 300
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GIAR (c)
All
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N-induced change (%)
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-50
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50 100 150 200 250 300
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N-induced change (%)
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Figure 2 Effects of soil horizon (organic vs. mineral) on (a) N-induced change of soil gross mineralization rates (GMR), (b) gross nitrification rates (GNR), (c) gross NH4+ immobilization rates (GIAR), (d) gross NO3¯ immobilization rates (GINR) and (e) ratio of soil gross nitrification to gross NH4+ immobilization rates (N/IA). Data points are the summary statistics from the meta-analysis. Error bars represent 95% bootstrap confidence intervals (CIs) and values next to the dots denote the number of observations. The effect of N addition is significant if the CI of effect size does not cover zero. See Box 1 for explanation of abbreviations.
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Figure 3 Synthesized response of forest ecosystem N pools and biogeochemical processes
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to N deposition. GMR, gross mineralization rates; GNR, gross nitrification rates; GIAR, gross NH4+ immobilization rates; GINR, gross NO3¯ immobilization rates; N/IA, the ratio of soil
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gross nitrification to gross NH4+ immobilization rates; NMR, net mineralization rates; NNR, net nitrification rates; GHNR, gross heterotrophic nitrification rates; GANR, gross autotrophic nitrification rates; DNRA, dissimilatory NO3¯ reduction to NH4+; Den, denitrification; NMR, GMR-(GIAR+GINR); NNR, GNR-GINR; Red and blue numbers refer to the data based on the meta-analysis by this study and another previous meta-analysis by Lu et al., (2011), respectively; +, increase in response to N addition; -, decrease in response to N addition; ?, there are currently not enough studies, No, no significant response. See Box 1 for explanation of abbreviations.
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Altitude, climate region and duration of N application were found to significantly affect N-addition induced changes in gross mineralization rates, while annual precipitation, forest type, N species, N chemical form and N application rate did not exhibit significant effects (Fig. S1). In addition, the response ratio of gross mineralization rates in response to N addition was negatively correlated with initial
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soil pH, and the response ratio of soil C/N and MBC/MBN, but was positively correlated with N load, duration of N fertilizer, initial soil C/N, and the response ratio of
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soil organic C and total N (Fig. 4). Nitrogen-induced changes of gross nitrification
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rates varied with annual precipitation, N application rate, and duration of N application
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(Fig. S2). Additionally, the response ratio of gross nitrification rates in response to N
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addition was significantly and positively correlated with mean annual temperature,
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mean annual precipitation, N application rate, N load and duration of N fertilizer (Fig. S3). In contrast, none of variables significantly affected N-induced changes of gross
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NH4+ immobilization rates (Fig. S4). There was a significant relationship between the response ratio of gross NH4+ immobilization rates and gross mineralization rates in response to N addition (Fig. S5).The response of gross NO3¯ immobilization rates to N addition depended on climate zone and duration of N application (Fig. S6). The response ratio of gross NO3¯ immobilization rates and gross nitrification rates was significantly and positively correlated (Fig. S5). Additionally, the response ratio of gross NO3¯ immobilization rates in response to N addition was significantly and negatively correlated with mean annual temperature, but was significantly and positively correlated with N load and duration of N fertilizer (Fig. S7). The response of 17
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N/IA value to N addition was dependent on annual precipitation and duration of N application (Fig. S8). There were significant and positive relationships between the response ratio of N/IA value in response to N addition and N application rate, N load
1 0 -1 0
1000
2000
3000
1 0 -1 -2
0
10
-1
(d)
2
R =0.212, P<0.01
1 0 -1 -2
0
10 20 30 40 50 60
0 -1
3 2
(e)
1 0
50
2
R =0.138, P<0.05
-1
-0.3 0.0
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1
2
R =0.350, P<0.01
GMR response ratio
2
(g)
40
0.3
0.6
0.9
1.2
Soil total C response ratio
3 2
(c)
2
-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2
Soil C/N response ratio
2
R =0.108, P<0.05
1 0
-1 -2
3
4
5
6
7
8
Initial soil pH 3 2
(f)
2
R =0.285, P<0.01
1 0 -1 -2
-0.3 0.0 0.3 0.6 0.9 1.2 1.5
Soil total N response ratio
2
(h) R =0.236, P<0.05
1
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GMR response ratio
Initial soil C/N 3
30
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2
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3
3
Duration (yr)
GMR response ratio
GMR response ratio
N load (kg N ha )
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GMR response ratio
-2
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R =0.312, P<0.01
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GMR response ratio
R =0.089, P<0.01
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(a)
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2
3
GMR response ratio
GMR response ratio
and duration of N fertilizer (Fig. S9).
0
-1 -0.4
-0.2
0.0
0.2
0.4
MBC/MBN response ratio
Figure 4 Relationships between gross mineralization rates (GMR) response ratio and (a) N load, (b) duration, (c) initial soil pH, (d) initial soil C/N, and between GMR and response ratio of (e) soil total C, (f) total N, (g) C/N, and (h) MBC/MBN. N load and duration represent the total amount of N added over the course of the experiment and duration of fertilization, respectively. The closed and open squares indicate that the data were derived from mineral and organic layers, respectively. 18
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4 DISCUSSION
To our knowledge, this study presents the first comprehensive synthesis of process-based understanding of how N enrichment affects soil gross N
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transformation rates and N availability (Fig. 3). One phenomenon has been puzzling to the ecology community is: while decomposition of organic matter is N-limited, N
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addition often supresses microbial activity but enhances N mineralization. Our results
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showed that gross N mineralization rates increased significantly in response to N
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addition in the organic horizon, but were unaffected in the mineral horizon (Fig. 2 and
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3). Such results were in contrast to the prevailing view that N availability limits N
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mineralization in terrestrial ecosystems (Nave et al., 2009; Lu et al., 2011; Chen et al., 2018), and also were inconsistent with previous findings that organic horizons and
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mineral soils did not exhibit significantly different net N mineralization responses to N inputs in forest ecosystems (Nave et al., 2009). We propose a new explanation for how N addition affects gross N mineralization rates in different soil horizons (Fig. 3). We found that there was an initial soil C/N threshold of approx. 14.6, above which N addition had a positive effect on gross N mineralization rates (Fig. 4). The soil C/N in the organic horizon ranged from 16.4 to 40.2 with an average of 31.9, while it averaged 15.0 (6.9-17.5) in the mineral horizon (Fig. 4). Under higher C/N ratios, soil microbes were generally N-limited, and N addition might have alleviated such limitation, thereby increasing gross C and N mineralization rates (Averill & Waring, 19
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2017). The possible explanation was that under N limitation, large decreases in particulate organic matter (POM, microbial-accessible) following N fertilization overwhelmed gains in mineral-associated organic matter (MAOM, microbial-inaccessible), leading to a net decrease in total soil C and N (Averill & Waring, 2017). Several studies have documented increases in MAOM under N
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fertilization, accompanied by decreases in POM in forests (Cusack et al., 2010) and agroecosystems (Dou et al., 2016). When microbes N limitation are removed by N
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addition, the microbial population size and decomposition rates increase, driving
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down the size of the POM pool, at the same time the increased microbial population
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size, microbial C use efficiency and turnover of the microbial biomass accelerate
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sorption of microbial necromass onto mineral binding sites, causing the MAOM pool
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to increase (Averill & Waring, 2017). In contrast, microbial biomass in the mineral horizon was considered to be C-limited, and hence N addition has no effect on gross
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C and N mineralization. Therefore, the difference in C/N may account for the different sensitivities of organic and mineral horizons to N addition in gross N mineralization. Alternatively, the different sensitivities of organic and mineral horizons to N addition was assumed to be associated with the fact that forest floor is the initial entry point of N addition. Microbial activity in the forest floor has been found to be likely more responsive to NO3¯ deposition compared to mineral soil (Deforest et al., 2004a, b). However, we found that N addition significantly increased gross nitrification rates in both forest organic and mineral layers. Such results indicated that different sensitivities of organic and mineral horizons to N addition appeared to not be 20
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associated with the initial entry point of N addition. Consistent with previous findings that gross mineralization rates were positively related to soil N concentration (substrate quantity) and negatively related to soil C/N (substrate quality) (Booth et al., 2005; Högberg et al., 2007), our study also showed that the response ratio of gross mineralization rates was negatively correlated with the response ratio of soil C/N and
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MBC/MBN, but was positively correlated with the response ratio of total N (Fig. 4). Thus, the increased soil N concentration and decreased soil C/N in response to N
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addition (Lu et al., 2011; Chen et al., 2018) was likely responsible for the enhanced
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soil gross mineralization rates in the organic horizon.
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In addition, microbial activity is also an important factor controlling mineralization
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rates. Generally, increased N mineralization is expected with elevated microbial
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activities (Aber et al., 1998). However, numerous studies have confirmed that N addition had a global negative effect on microbial index, such as soil microbial
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composition (Zhang et al., 2018), depolymerization of protein (a rate-limiting step of soil organic N mineralization) (Chen et al., 2018) and microbial biomass (Treseder, 2008; Lu et al., 2011; Ye et al., 2018). An increased gross N mineralization rate accompanied by reduced microbial activity in response to N addition is particularly perplexing. Chen et al. (2018) accordingly concluded that changes in microbial activity are not a dominant mechanism for increased N mineralization. In fact, changes in N availability are frequently linked to shifts in soil pH or osmotic stress, which may have masked N stimulation of microbial activity (Averill & Waring, 2017). We thus propose that another microbial activity related index should be introduced to 21
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address such problem. We used specific gross N mineralization rates (gross N mineralization rate ÷ microbial N) as a proxy of activity per unit microbial biomass (Corre et al., 2010), and found that specific gross N mineralization rates were enhanced by 19.3% under N addition. This well explained why N addition often supress microbial activity but enhances N mineralization. The findings highlight that
a crucial role in regulating soil N mineralization.
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substrate quantity and quality, reflected by specific gross N mineralization rates, play
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Our synthesis also showed that gross nitrification rates significantly increased by
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N addition in forest ecosystems (Fig. 2 and 3). NH4+ addition increased gross
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nitrification rates probably by directly providing NH4+ substrate for nitrification (Corre
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et al., 2014; Baldos et al., 2015). A positive relationship between the response ratio of
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gross nitrification rates and gross mineralization rates indicated that increasing NH4+ availability from mineralization was at least partially responsible for the enhanced
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gross nitrification rates under N addition. Alternatively, heterotrophic microorganisms may compete against nitrifiers for available NH4+ under high C/N (Murphy et al., 2003; Booth et al., 2005). Soil C/N has been demonstrated to decrease significantly by N addition (Lu et al., 2011; Chen et al., 2018), which might have caused lower microbial demand for NH4+ and thus more opportunity for nitrifiers for available NH4+, thereby causing increased nitrification. On the contrary, N addition-induced soil acidification might have inhibited gross nitrification rates (Cheng et al., 2013; Tian & Niu, 2015). There was a possibility that the stimulatory effect of N addition might offset the
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inhibitory effect of N addition-induced soil acidification, resulting in increasing gross nitrification rates by N addition in forest ecosystem. Microbial immobilization of N is considered an underlying mechanism for soil N retention. Considering that N addition generally causes a decrease in various microbial indices and soil pH (Treseder, 2008; Tian & Niu, 2015; Chen et al., 2018;
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Zhang et al., 2018), reduced microbial immobilization of N in response to N addition would be expected. However, we found that N addition increased gross NO3¯
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immobilization rates by 51.4% and decreased gross NH4+ immobilization rates by
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25.6% in the organic horizon. Generally, soil microbes prefer NH4+ over NO3¯ for their
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growth (Lindell & Post, 2001), and greater gross NH4+ immobilization rates could be
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expected. We indeed found that gross NH4+ immobilization rates were on average 25
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times higher than gross NO3¯ immobilization rates in our data set (Fig. S10). Our results may suggest that under N addition decreased immobilization of NH4+ into the
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MBN pool outweighed increased immobilization of NO3¯ into MBN pool, resulting in a reduced microbial biomass N (Lu et al., 2011). However, no significant response of MBNMRT to N addition suggests that MBN turnover rate was not affected by N addition. Such results further demonstrate that there was no certain relationship between MBN and gross N transformation rates in response to N addition. A previous review suggests that much of the enhanced N leaching into streams under enhanced N deposition is derived from soil N cycling through organic and inorganic pools, rather than from its passing directly from the atmosphere to streams, which underlined the significance of soil microbial N cycles that govern N availability 23
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and related N losses in soils (Niu et al., 2016). In general, a tightly coupled microbial N cycle, where NH4+ and NO3¯ immobilization rates were comparable to gross mineralization and nitrification rates, respectively, signifies a conservative N cycle and low N losses (Corre et al., 2003). In this study, we found forest ecosystems appeared to move towards an uncoupled microbial N cycle under N addition, where
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gross mineralization rates increased but gross NH4+ immobilization rates decreased, and gross nitrification rates increased faster than gross NO3¯ immobilization rates,
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resulting in potential risks of high N losses. Likewise, the N/IA value, considered as
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the relative rates of NH4+ being nitrified or immobilized by soil microorganisms, was
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significantly enhanced by N addition, indicating increasing risk of NO3¯ losses
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(Stockdale et al., 2002; Murphy et al., 2003). Longer NO3¯MRT in response to N
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addition also indicated a slower NO3¯ pool turnover and a less dynamic pool, and hence implied a higher tendency for N losses. Indeed, from global syntheses, N
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addition has been shown to promote N2O emission and NO3¯ leaching (Liu & Greaver, 2009; Lu et al., 2011), and reduce total ecosystem 15N recovery (Templer et al., 2012). Thus, the strategies to reduce gross nitrification rates and simultaneously enhance gross N immobilization rates could be recommended to keep soil NO3¯ concentration low in forest ecosystems in response to N deposition, thereby increasing N retention in soils and avoiding N losses to environments. This study for the first time proposes a new mechanistic understanding that incorporates gross N transformations into the effects of N deposition on forest soil N availability and N losses. In contrast to the prevailing view that N availability limits N 24
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mineralization, we propose a new conceptual model that gross N mineralization rates in response to N enrichment depend on forest soil layers. Mineralization was more stimulated by N addition in organic soil horizons with higher C/N compared to mineral soil horizons. Our results suggest that enhanced activity per unit microbial biomass largely explained why N addition enhanced gross N mineralization rates
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accompanied by reduced microbial activity. The greater increase of gross nitrification rates compared to gross NO3¯ immobilization rates under elevated N deposition
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indicated that forest ecosystems tend toward an uncoupled microbial N cycle. The
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increasing N/IA value in response to N addition also suggested an increasing risk of
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NO3¯ losses in forest ecosystems. These findings highlight changes in soil gross N
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transformation rates in specific soil layers should be integrated into models to better
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predict forest ecosystem N cycles and C sequestration potential under future N deposition scenarios. Although a limited number of observations exist for this
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meta-analysis of very complex processes, we believe they provide a useful addition in understanding the responses of N cycling processes to experimental N addition, and should improve our understanding of the underlying mechanisms of N deposition effects on plant and ecosystem function.
ACKNOWLEDGEMENTS
We thank the authors whose work was included in the meta-analyses. This work was financially supported by the National Natural Science Foundation of China [grant 25
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numbers 41977081, 41807093, 41622104, and 41830642], and the National Key Research and Development Program of China [grant numbers 2017YFD0200106 and 2017YFD0800103]. We thank Dr. Robert B. Jacobson and two anonymous reviewers for their constructive comments that substantially improved the quality of an earlier
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version of this manuscript.
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The authors declare no competing interests.
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COMPETING INTERESTS
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