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Gross Nitrification Rates and Nitrous Oxide Emissions in an Apple Orchard Soil in Northeast China
Pedosphere 25(4): 622–630, 2015 ISSN 1002-0160/CN 32-1315/P c 2015 Soil Science Society of China Published by Elsevier B.V. and Science Press
Gross Nitrification Rates and Nitrous Oxide Emissions in an Apple Orchard Soil in Northeast China GE Shunfeng, JIANG Yuanmao∗ and WEI Shaochong State Key Laboratory of Crop Biology, College of Horticulture Science and Engineering, Shandong Agricultural University, Tai’an 271018 (China) (Received July 10, 2014; revised March 17, 2015)
ABSTRACT A better understanding of nitrogen (N) transformation in agricultural soils is crucial for the development of sustainable and environmental-friendly N fertilizer management and the proposal of effective N2 O mitigation strategies. This study aimed: i) to elucidate the seasonal dynamic of gross nitrification rate and N2 O emission, ii) to determine the influence of soil conditions on the gross nitrification, and iii) to confirm the relationship between gross nitrification and N2 O emissions in the soil of an apple orchard in Yantai, Northeast China. The gross nitrification rates and N2 O fluxes were examined from March to October in 2009, 2010, and 2011 using the barometric process separation (BaPS) technique and the static chamber method. During the wet seasons gross nitrification rates were 1.64 times higher than those under dry season conditions. Multiple regression analysis revealed that gross nitrification rates were significantly correlated with soil temperature and soil water-filled pore space (WFPS). The relationship between gross nitrification rates and soil WFPS followed an optimum curve peaking at 60% WFPS. Nitrous oxide fluxes varied widely from March to October and were stimulated by N fertilizer application. Statistically significant positive correlations were found between gross nitrification rates and soil N2 O emissions. Further evaluation indicated that gross nitrification contributed significantly to N2 O formation during the dry season (about 86%) but to a lesser degree during the wet season (about 51%). Therefore, gross nitrification is a key process for the formation of N2 O in soils of apple orchard ecosystems of the geographical region. Key Words:
barometric process separation system, mineral N, N transformation, soil water-filled pore space, static chamber
Citation: Ge S F, Jiang Y M, Wei S C. 2015. Gross nitrification rates and nitrous oxide emissions in an apple orchard soil in Northeast China. Pedosphere. 25(4): 622–630.
INTRODUCTION The biogeochemical cycle of nitrogen (N) has long been considered an important issue because of its great significance in crop production and its impact on atmospheric environment by producing nitrogen oxides. Nitrification is one of the microbial key processes regulating N transformation and N availability in soils. Denitrification is another process that produces nitrogen oxides such as NO and N2 O (Cao et al., 2006). Hereby, the significance of nitrification is not only referred to the production of NO− 3 , but also to the direct production of nitrogen oxides. Among all the nitrogen oxides, N2 O is important as a persistent greenhouse gas that generates stratospheric NO. N2 O has an essential role in ozone-layer chemistry. Its atmospheric concentration has increased at a rate of 0.2% to 0.3% per year since 1990 (H´enault et al., 2012). Over 65% of atmospheric N2 O comes from soil as a result of nitrification and denitrification (Bouwman, 1998). ∗ Corresponding
author: E-mail: [email protected].
Hart et al. (1994) and Ingwersen et al. (1999) reported that gross and net rates of nitrification were not well correlated. Various studies have demonstrated that net nitrification rates greatly underestimate the total rates of N turnover in soils (Stark and Hart, 1997; Neill et al., 1999; Verchot et al., 2001; Ross et al., 2004). Compared with the measurement of net N turnover rates, the determination of gross N turnover rates was more difficult. However, gross nitrification may be the more suitable indicator to comprehensively understand soil N transformations. Common methods for measuring gross nitrification rates are the 15 N soil pool dilution (Davidson et al., 1991) and acetylene inhibition method (Davidson et al., 1986). These methods have the disadvantage of introducing labeled material or changing the composition of soil atmosphere. These disadvantages can be overcome by the use of the barometric process separation (BaPS) method, which was first introduced by Ingwersen et al. (1999). In the BaPS method P, CO2 , and O2 are measured to com-
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pute the gross nitrification rate. To prove its capacity to determine the gross nitrification rate, the BaPS method was successfully validated against the 15 N pool dilution method for temperate forests (Ingwersen et al., 1999; Rosenkranz et al., 2006), tropical rainforests (Breuer et al., 2002), grasslands (M¨ uller et al., 2004), and agricultural soils (Stange and Neue, 2009). To date, China has the largest apple cultivation area and production in the world. Its fruit production has gradually become an important pillar industry for increasing the income of farmers. During the last decade an increasing amount of N fertilizers is used to improve yield with the corresponding intensive production of fruits. At present, N utilization has reached 400 to 600 kg ha−1 in apple orchards in China with an annually increasing trend (Gao et al., 2002; Peng and Jiang, 2006). The excessive use of N fertilizer decreases the utilization rate and the agricultural production benefits, with negative consequences such as aggravated fruit physiological disease and environmental pollution (Yi et al., 2008). Gross nitrification and N2 O emissions are particularly controlled by soil water-filled pore space (WFPS), soil temperature, organic carbon, soil pH, and soil inorganic N concentrations (Dobbie and Smith, 2003; Martin et al., 2003). These factors are strongly controlled by fertilizer management, weed control practices, and phenology characteristics of fruit trees in orchards. In contrast to the tropical forest ecosystems (Breuer et al., 2002; Kiese et al., 2008), information on gross nitrification and N2 O emissions in apple orchard system is extremely scarce (Pang et al., 2009), and seasonal detailed information in apple orchards is unavailable. Therefore, we carried out a 3year study in an apple orchard in Northeast China. The main objectives were: i) to understand the seasonal variations of gross nitrification and N2 O emissions, ii) to determine the influence of soil conditions on the gross nitrification, and iii) to confirm the relationship between gross nitrification and N2 O emissions.
2006), and the main characteristics are presented in Table I. The apple orchard was dominated by ‘Gala’ apple trees (Malus domestica/Malus hupehensis) and the distance between rows of apple trees was 5 m. The size of the crown of each apple tree was about 3.5 m. The apple orchard was fertilized twice at a soil depth of 10 cm by digging a fertilization band (10 cm depth, 30 cm width) 1.5 m away from tree row in late March in 2009 and mid-March in 2010 and 2011 (250 kg N ha−1 as urea), and in late June in 2009 and midJune in 2010 and 2011 (200 kg N ha−1 as compound fertilizer, N:P2 O5 :K2 O = 15:15:15).
MATERIALS AND METHODS Sampling sites Field experiments were performed from 2009 to 2011 in an apple orchard at Laishan, Yantai City, Shandong Province, Northeast China (121◦ 21 00 E, 37◦ 22 47 N). The climate is classified as semi-humid, with annual average precipitation of 672.5 mm, of which nearly 70% occurs from June to September. The annual mean temperature (1984–2009) is 12.5 ◦ C, and there are about 210 frost-free days each year. The soil was classified as a Lixisol (IUSS Working Group WRB,
TABLE I Some basic physicochemical characteristics of the apple orchard soil studied Soil layer
Organic matter
cm 0–5 5–10
g kg−1 g cm−3 a) 10.36 ± 0.19 1.21 ± 0.04 5.61 ± 0.02 1.13 ± 0.05 10.04 ± 0.27 1.24 ± 0.02 5.59 ± 0.01 1.28 ± 0.06
a) Means
Total nitrogen
pH (H2 O)
Bulk density
± standard deviations (n = 15).
Measurement of gross nitrification rate The gross nitrification rates of intact soil cores were analyzed with the BaPS method (UMS GrmbH, Germany). The theory of the determination of gross nitrification by BaPS technique is based on the determination of the total pressure change, as well as the changes of O2 and CO2 partial pressure in an isothermal gas tight system. Nitrification leads to a pressure decrease by net O2 consumption, denitrification leads to a pressure increase, and soil respiration is neutral for pressure for a respiration quotient (RQ) = 1. The central equation of the BaPS method is: ΔNx Oy = Δnet − ΔO2 − ΔCO2
(1)
where ΔNx Oy stands for the rate of N gases produced by denitrification (mol h−1 ). The Δnet denotes the net rate of the total gas production (Δnet > 0) or consumption (Δnet < 0) (mol h−1 ), and ΔCO2 and ΔO2 are the net rates of CO2 formation and O2 depletion (mol h−1 ), respectively, in the closed chamber’s atmosphere. Since the total gas production rate (Δnet) and the net change rates of O2 (ΔO2 ) and CO2 (ΔCO2 ) are measured, the production rate of N-trace gases (ΔNx Oy ) via denitrification can be calculated. The total O2 consumption can be divided into three parts: 1) the O2 consumption by respiration (ΔO2,res ), 2) the O2 consumption by nitrification (ΔO2,nit ), and 3) the change in the dissolved O2 in soil water (ΔO2,dis ):
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ΔO2 = ΔO2,res + ΔO2,nit + ΔO2,dis
(2)
Also, the net CO2 production rate can be expressed as: ΔCO2 = ΔCO2,res + ΔCO2,nit + ΔCO2,den + ΔCO2,dis
(3)
where ΔCO2,res , ΔCO2,nit , ΔCO2,den , and ΔCO2,dis refer to the CO2 production rates by respiration, nitrification, and denitrification, and the change rate in the dissolved CO2 in soil water, respectively. The terms ΔO2,dis and ΔCO2,dis take into account that due to concentration changes during incubation, O2 is released from soil solution to the chamber’s atmosphere and CO2 is transferred from the chamber’s atmosphere to soil solution. In the BaPS system these terms are calculated using Henry’s law. By combining the total net gas balance equation (Eq. 1) with the O2 and CO2 balance equations (Eqs. 2 and 3) and taking into the account that −ΔO2,res = ΔCO2,res by a RQ of 1, gross nitrification rate can be obtained by calculating the O2 consumption by nitrification as follows: ΔO2,nit = [δ/(δ + 1)](Δnet − ΔCO2,dis − ΔNx Oy − ΔCO2,dis − ΔO2,dis )
(4)
where δ stands for the ratio of ΔO2,nit to ΔCO2,nit , which is a fixed value of 7.3. Further details including discussion of the uncertainties are given by Ingwersen et al. (1999, 2008), Breuer et al. (2002), M¨ uller et al. (2004) and Stange and Neue (2009). The respiration quotient (RQ) is one of the most sensitive parameters in the calculation of gross nitrification rates by the BaPS method. Thus, an independent identification of the RQ is advisible (Ingwersen et al., 1999; M¨ uller et al., 2004). An earlier study in the same region (Liu et al., 2005) showed a very good agreement of soil respiration rates by gas chromatography and the BaPS method while using RQ = 1. Therefore, RQ = 1 was used to calculate the gross nitrification rate in this study. According to the original BaPS software (UMS, M¨ unchen, Germany), the ratio of autotrophic nitrification to heterotrophic nitrification was set to 3:1, and the ratio between N2 O and N2 production was set to 1:2, as there was no additional information for this study site. Ingwersen et al. (2008) reported some problems in the BaPS method due to the calculation of the amount of CO2 dissolving into soil solution during incubation at soil pH > 6.5. Soil pH at the study site was about 5.6. Thus, this methodological limitation does not hold for this site.
To investigate the seasonal dynamic of gross nitrification rate, circular stainless cores (diameter of 7 cm) were used to take soil samples from March to October in 2009, 2010, and 2011. Soil samples were obtained monthly to include all tree growth periods. Each datum of gross nitrification rate is based on the incubation of a set of 7 intact soil cores sampled from the top soil layer (0 to 5 cm). The set of 7 intact soil cores were taken at fertilization band (three cores), 0.5 m (two cores) and 1.0 m (two cores) away from the fertilization band. The provided nitrification rates in this study are based on three replicated measurements involving 21 soil cores. Fresh soil cores from the field were put into the BaPS system for 6–18 h to acclimatize under field conditions (soil moisture and soil temperature) and subsequently gross nitrification rates were determined during a 24-h BaPS run. Measurement of N2 O emission For the period (March 2009–October 2011) when gross nitrification rates were investigated, N2 O emission was also continuously monitored, and the arrangement of N2 O sampling positions in the apple orchard was consistent with soil cores. N2 O measurement was performed using static chambers in 8:00 a.m.–10:00 a.m. and 4:00 p.m.–6:00 p.m. The chambers (50 cm × 50 cm × 50 cm) were of transparent PVC fixed to aluminum alloy frames. A buffer pipe (1.5 m, 0.6 mm outer diameter, 0.4 mm inner diameter) was inserted through the lid to keep the air pressure balanced between the inside and outside of the chamber. A fan (12 V, 0.5 A, 8 cm in diameter) was installed on the top wall of each chamber to mix the air when chamber was closed. Permanently positioned chamber bases (50 cm length × 50 cm width × 5 cm height) with water-filled grooves in the upper end were used to ensure gas tightness. During the flux measurement, gas within the chamber was sampled into 0.5-L polyethylene-coated aluminum bags using a membrane pump at 0, 20, 40, 60, 80, 100 and 120 min after the set of the chamber. N2 O concentrations of samples were analyzed by gas chromatography (Model SP3410, Beijing Analytical Instrument Factory, China) with an electron capture detector (ECD). The gas samples were loaded into a 2-mL loop connected to a 10-port valve. Both a precolumn (2 m, 4 mm outer diameter) and an analytical column (2 m, 4 mm outer diameter) were packed with Porapak Q (80–100 mesh) and hold at 72 ◦ C. Pure N2 was used as carrier gas with a flow rate of 30 mL min−1 . The temperatures of the ECD and injection port were 390 and 72 ◦ C, respectively. A small amount of CO2 (6 mL min−1 ) was added into makeup gas of
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ECD so as to improve the sensitivity and repeatability of ECD. Analytical variability was less than 1%. The N2 O emission rate was calculated using the change in the N2 O concentration of the chamber, which was estimated as the slope of the linear regression between the N2 O concentration and time. The N2 O emission rates could be disregarded if the regression coefficient (r2 ) was less than 0.85.
(version 13.0, SPSS Inc., USA). The relationships among gross nitrification rates and soil variables, name− ly soil temperature, WFPS, and NH+ 4 and NO3 concentrations, were analyzed by multiple-linear regression. The relationship between soil WFPS and gross nitrification rate was described by O’Neill functions (Stange and Neue, 2009). RESULTS
Determinations of rainfall, soil temperature, soil WFPS, and soil mineral N Rainfall data were obtained from the Laishan Agroecological Station of Yantai Academy of Agricultural Sciences, China, located 400 m away from the experimental site. Soil temperature was measured with an SN2202 digital thermodetector (Sinan Instrument Plant of Beijing Normal University, China). After the BaPS incubation, soil samples from the 7 cores were removed and mixed to measure soil moisture content using the thermo-gravimetrical method, and then soils were dried at 105 ◦ C for 24 h. Soil mineral N (NH+ 4 −1 and NO− KCl and 3 ) was extracted with 0.01 mol L measured by a colorimetric continuous flow analyzer (SAN++, Skalar Company, the Netherlands; Schloter et al., 2003). Considering the influence of soil texture, soil WFPS was used in the data analysis. Soil bulk density was determined at each sampling time, thereby allowing soil WFPS to be calculated from the soil weight moisture content values (Merino et al., 2004). WFPS is calculated as follows: WFPS (%) = (gravimetric soil weight moisture content × bulk density)/total porosity. The total porosity in percentage was computed as (1 − bulk density/2.65) × 100. Statistical analysis All statistical analyses were performed using SPSS
Fig. 1
Environmental conditions From March 2009 to October 2011 the precipitation at the experiment site was in total 2 452.6 mm (Fig. 1). The seasonal pattern of precipitation was evident at this site. About 70% of total rainfall was observed during wet season conditions (June to September), whereas only 30% of total rainfall occurred during the dry season (October to May; Fig. 1). The annual mean soil temperatures in 2009, 2010, and 2011 were 12.8, 12.7, and 12.4 ◦ C, and the variation ranges were −1.98–25.32, −1.78–25.68, and −3.67–25.34 ◦ C, respectively. The soil temperature gradually increased from January to August, and then decreased from September to January. Seasonal dynamic of gross nitrification rate, N2 O fluxes, and soil mineral N Gross nitrification rates from March to October of 2009 to 2011 gradually increased, peaked from June to August, and then declined. Exceptions from this general trend were July 2009 and August 2011 (Fig. 2). Gross nitrification rates were significantly higher during the wet season (June to September) than during the dry season (October to May). Mean gross nitrification rates were 13.9, 11.6, and 12.3 mg N kg−1 d−1 for the wet seasons of 2009, 2010, and 2011, respectively,
Mean monthly rainfall and soil temperature at 5 cm depth during the study period of 2009–2011.
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− Fig. 2 Soil water-filled pore space (WFPS), gross nitrification rates, N2 O fluxes, and mineral N (NH+ 4 and NO3 ) concentrations during the study period of 2009–2011. Arrows indicate the timing of fertilizer application. Vertical bars indicate standard deviations
of the means (n = 3).
whereas the values for the dry seasons were 8.8, 7.3, and 8.4 mg N kg−1 d−1 , respectively. The minimum nitrification rate of 6.5 mg N kg−1 d−1 was observed in April 2011 during the dry season when soil WFPS was 26.5%. The maximum gross nitrification rate of 18.06 mg N kg−1 d−1 was observed in August 2009 after a rainfall, which increased soil WFPS from 38.6% to 56.9%. However, the gross nitrification decreased in July 2009, August and September 2011 because of soil WFPS above 60% (Fig. 2). Nitrous oxide fluxes varied widely from March to October and were stimulated by N fertilizer application (Fig. 2). The changing trend of the N2 O fluxes was consistent in 2009, 2010 and 2011. For example,
the highest N2 O flux (240.9 μg N m−2 h−1 ) was measured 9 d after fertilizer application in mid-June 2009. In contrast, only a small increase in the N2 O flux (63.6 μg N m−2 h−1 ) occurred after applying the fertilizer in mid-March 2009. The variation of soil NH+ 4 concentrations in 0– 5 cm soil layer was highly influenced by fertilization events. The apple orchard was fertilized twice at a soil depth of 10 cm in mid-March and mid-June. Thus, soil NH+ 4 concentrations were higher at that time every year. The maximum NO− 3 concentrations occurred in September 2009, September 2010, and July 2011, whereas the lowest NO− 3 concentrations appeared in October 2009, 2010, and 2011 (Fig. 2).
NITRIFICATION AND N2 O EMISSION IN ORCHARD SOIL
Effects of soil variables on gross nitrification The gross nitrification rates were significantly correlated with soil temperature (P < 0.01, r = 0.68), WFPS (P < 0.01, r = 0.75), and NH+ 4 concentration (P < 0.05, r = 0.51). There was no significant correlation between gross nitrification rate and soil NO− 3 concentration. The multiple regression of gross nitrification rate with soil variables (WFPS, soil tempera− ture, NH+ 4 and NO3 concentrations) showed that soil WFPS and soil temperature could be identified as two key factors determining the seasonal variability of gross nitrification rate, accounting for 38% (P < 0.006) and 20% (P < 0.011) of the seasonal variation, respectively. The relationship between soil WFPS and gross nitrification rate could be best described by O’Neill functions with an optimum nitrification rate at a soil WFPS of 60%. Relationship between gross nitrification and N2 O emissions Regression analysis revealed a significant relationship between the gross nitrification rate and the N2 O fluxes measured at the same day. The relationship could be described best by a linear regression line during the whole seasons (r = 0.67, P < 0.01; Fig. 3a). During the dry season, the relationship between gross nitrification rate and the N2 O emissions could be described best by a linear regression line (r = 0.81, P < 0.01; Fig. 3b). DISCUSSION This work presents information on the seasonal dynamics of gross nitrification over a 3-year period, based on monthly measurements using the BaPS method, in an apple orchard ecosystem under semi-humid con-
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ditions. These measurements were accompanied by the N2 O flux measurements, thereby enabling to establish a relationship between gross nitrification and N2 O emission. During the 3 years, gross nitrification rates showed a strong seasonal pattern with significantly higher rates during the wet season as compared with the dry season (Fig. 2). Rates under wet season conditions were 1.6 times higher than those under dry season conditions. These findings are consistent with the studies that were performed in tropical rainforests (Kiese et al., 2008) and grassland (Gao et al., 2008). Gross nitrification rates of the present study (6.8–18.1 mg N kg−1 d−1 ) are in the same range as those reported by other studies, such as in tropical rainforest (0.5–20 mg N kg−1 d−1 ; Breuer et al., 2002; Booth et al., 2005; Kiese et al., 2008), grassland (0.7–8.2 mg N kg−1 d−1 ; Gao et al., 2008; Stange and Neue, 2009), and winter wheat-summer maize double cropping systems (4–24 mg N kg−1 d−1 ; Wan et al., 2009). N2 O is a relatively stable greenhouse gas that has a significantly contribute to the destruction of the ozone layer in the stratosphere (Crutzen, 1981; Bouwman, 1998). Agricultural soils are known to be responsible for a large proportion of the increased N2 O concentration in the atmosphere, mainly due to the use of N fertilizer (Dobbie and Smith, 2003; Stehfest and Bouwman, 2006; Barton et al., 2008). In this study, N2 O fluxes varied widely from March to October and were stimulated by N fertilizer application (Fig. 2). However, N2 O fluxes varied widely in response to the timing of N fertilization and were usually greater after the fertilization in June than after the fertilization in March, because the soil conditions were more conducive for N2 O production in late June compared to late March. In late June, soil temperature was generally higher than 20.0 ◦ C and the top soil was subje-
Fig. 3 Relationships between gross nitrification rates and N2 O fluxes throughout the study period (a) and in the dry season (from March to June, and in October) (b).
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cted to regular wetting and drying cycles (Dobbie et al., 1999; Du et al., 2006; Pang et al., 2009). Several factors such as soil moisture, soil O2 concentration, soil organic matter content, NH+ 4 availability, and pH may act as important physiological constraints, and consequently affect the nitrification process in terrestrial ecosystems (Booth et al., 2005; Silva et al., 2005; Cookson et al., 2006). Despite the numerous factors that may potentially affect the nitrification process, only a few were key parameters. Cookson et al. (2006) found that soil moisture, temperature, and inorganic N availability were the major factors among the 15 investigated soil factors, which control the survival and activity of microorganisms and consequently strongly influence soil N transformations. In our study, gross nitrification rate was obviously influenced by soil temperature. The rates were higher in summer (June, July, and August) or autumn (September and October) as compared with the rates measured in spring (March, April, and May). Similar results were shown by Davidson et al. (1992) and Stark and Hart (1997) for various grassland and forest ecosystems of North America. Breuer et al. (2002) found that nitrification increased at an average rate of 1.17 mg NH4 -N m−2 h−1 per 1 ◦ C increase in soil temperature. A Q10 (temperature coefficient, a measure of the change rate of a biological or chemical system as a consequence of increasing the temperature by 10 ◦ C) value of 3.60 was calculated for the temperature range between 14 and 24 ◦ C. Ingwersen et al. (1999) found a slightly higher Q10 value (Q10 = 4.13), which can be derived from the data presented on the temperature dependency of the gross nitrification in the soil taken from a temperate spruce forest ecosystem (temperature range of 15 to 25 ◦ C). Aside from soil temperature, a statistically significant positive correlation was found between soil WFPS and gross nitrification. The relationship between gross nitrification and soil WFPS seemed to follow an optimum curve, peaking at 60% WFPS. This finding is in line with the significantly decreased gross nitrification rates measured in July 2009, August 2011, and September 2011 when soil WFPS was 82%, 83%, and 80%, respectively. Linn and Doran (1984) noted that soil aeration, which is directly affected by soil moisture, might be a limiting factor for the aerobic processes like gross nitrification. Our study is consistent with the previous studies (Breuer et al., 2002; Corre et al., 2002; Khalil and Baggs, 2005; Kiese et al., 2008), which found that under the more saturated soil moisture conditions gross nitrification was reduced. Soil N2 O emission is a complex process that is simultaneously influenced by several factors. Nitrifica-
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tion and denitrification by soil microbes are the main sources of N2 O emission in soil. However, gross nitrification and denitrification may simultaneously occur in soil. Therefore, the real contribution of each process to the observed N2 O fluxes is difficult to ascertain (Arah, 1997). However, denitrification is regarded as the most important N2 O forming process in agricultural soils (Azam et al., 2002; Dobbie and Smith, 2003), whereas nitrification is reported to make a substantial contribution to N2 O emission under aerobic conditions (Klemedtsson et al., 1988; Williams et al., 1998). Recently, several studies have indicated that nitrification is the dominant source of N2 O under aerobic conditions as long as the WFPS is in the range between 30% and 70% (Williams et al., 1998; Chen and Huang, 2006; Ciarlo et al., 2008; Zhang and Han, 2008; Wan et al., 2009). As demonstrated by Breuer et al. (2002) and Kiese et al. (2008) gross nitrification rates were positively correlated with N2 O fluxes. The relationship could be described by a linear regression line (r = 0.67, P < 0.01; Fig. 3a) through the analysis of all data. Since both nitrification and denitrification contribute to the formation of N2 O emission, the relationship between gross nitrification rates and N2 O emissions was further analyzed by selecting data only for the dry seasons. We assumed that N2 O emissions during the dry season should be dominated by nitrification. This hypothesis was confirmed by a linear regression analysis with a significantly better fit between N2 O emissions and gross nitrification (Fig. 3b). Applying the regression equation in Fig. 3b to all the measurements revealed that gross nitrification may contribute to only 51% N2 O emission in the wet season, 86% in the dry season, and 64% in the study period. These findings are in good agreement with the reports of Chen and Huang (2006) and Wan et al. (2009), who observed that nitrification was the major source of N2 O emission from agricultural soils of China. Therefore, the gross nitrification was concluded to be the main source of N2 O production in this region. But, this conclusion was reached based on a hypothesis that N2 O emissions in the dry season predominantly originated from gross nitrification, so the outcome of this approach was subject to uncertainty and need further studies. In this study, the average N2 O emitted from the soil in apple orchard accounted for 1.16% of the fertilized N, including the contributions of background emissions during 8 months (from March to October) in 3 years. This value is slightly higher than the results (0.9%) presented by Lu et al. (2006) for croplands and grasslands in China where fertilizer N was applied, and 1.8 times higher than the value determined by Pang
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et al. (2009) for apple orchards in the semiarid Loess Plateau of China. Although the present study and the study of Pang et al. (2009) were performed in the apple orchard soils, climate and soil conditions were different among the two sites. For example, at our study site annual rainfall was about 844 mm and the annual mean soil temperature was 12.1 ◦ C. In contrast, the site studied by Pang et al. (2009) had an annual precipitation of 584 mm and an annual mean soil temperature of 9.1 ◦ C. Therefore, to obtain a precise and reliable N2 O emission factor for semi-humid agricultural soils, multi-year and multi-site measurements are required.
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CONCLUSIONS The gross nitrification rates in the apple orchard showed an obvious seasonal pattern, with the highest rates during the wet season and significantly lower rates during the dry season. Fertilizer application had a positive effect on the gross nitrification and N2 O emissions. The relationship between the gross nitrification rates and soil WFPS seemed to follow an optimum curve, peaking at 60% soil WFPS. Statistically significant positive correlations existed between the gross nitrification rate and N2 O emission. Further investigations indicated that gross nitrification contributed significantly to N2 O formation during the dry season (86%) but less during the wet season (51%). Therefore, the gross nitrification was the primary source of N2 O emissions from the apple orchard soil in this region. However, the combined measurements of N2 O emission and the underlying N turnover processes would be necessary to further our understanding of processes and mechanisms governing biosphere-atmosphere N gas exchange. ACKNOWLEDGEMENT This study was supported by the Special Fund for Agro-scientific Research in the Public Interest of China (No. 201103003) and the Earmarked Fund for China Agriculture Research System (No. CARS-28). REFERENCES Arah J R M. 1997. Apportioning nitrous oxide fluxes between nitrification and denitrification using gas-phase mass spectrometry. Soil Biol Biochem. 29: 1295–1299. Azam F, M¨ uller C, Weiske A, Benckiser G, Ottow J C G. 2002. Nitrification and denitrification as sources of atmospheric nitrous oxide—role of oxidizable carbon and applied nitrogen. Biol Fert Soils. 35: 54–61. Barton L, Kiese R, Gatter D, Butterbach-Bahl K, Buck R, Hinz C, Murphy D V. 2008. Nitrous oxide emissions from a cropped soil in a semi-arid climate. Glob Change Biol. 14:
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Pang J Z, Wang X K, Mu Y J, Ouyang Z Y, Liu W Z. 2009. Nitrous oxide emissions from an apple orchard soil in the semiarid Loess Plateau of China. Biol Fert Soils. 46: 37–44. Peng F T, Jiang Y M. 2006. Characteristics of N, P, and K nutrition in different yield level apple orchards. Sci Agr Sin (in Chinese). 39: 361–367. Rosenkranz P, Br¨ uggemann N, Papen H, Xu Z, Horv´ ath L, Butterbach-Bahl K. 2006. Soil N and C trace gas fluxes and microbial soil N turnover in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary. Plant Soil. 286: 301–322. Ross D S, Lawrence G B, Fredriksen G. 2004. Mineralization and nitrification patterns at eight northeastern USA forested research sites. Forest Ecol Manag. 188: 317–335. Schloter M, Bach H J, Metz S, Sehy U, Munch J C. 2003. Influence of precision farming on the microbial community structure and functions in nitrogen turnover. Agr Ecosyst Environ. 98: 295–304. Silva R G, Jorgensen E E, Holub S M, Gonsoulin M E. 2005. Relationships between culturable soil microbial populations and gross nitrogen transformation processes in a clay loam soil across ecosystems. Nutr Cycl Agroecosys. 71: 259–270. Stange C F, Neue H U. 2009. Measuring and modelling seasonal variation of gross nitrification rates in response to long-term fertilisation. Biogeosciences. 6: 2181–2192. Stark J M, Hart S C. 1997. High rates of nitrification and nitrate turnover in undisturbed coniferous forests. Nature. 385: 61– 64. Stehfest E, Bouwman L. 2006. N2 O and NO emission from agricultural fields and soils under natural vegetation: Summarizing available measurement data and modeling of global annual emissions. Nutr Cycl Agroecosys. 74: 207–228. Verchot L V, Holmes Z, Mulon L, Groffman P M, Lovett G M. 2001. Gross vs net rates of N mineralization and nitrification as indicators of functional differences between forest types. Soil Biol Biochem. 33: 1889–1901. Wan Y J, Ju X T, Ingwersen J, Stange C F, Zhang F S, Streck T. 2009. Gross nitrogen transformations and related nitrous oxide emissions in an intensively used calcareous soil. Soil Sci Soc Am J. 73: 102–112. Williams P H, Jarvis S C, Dixon E. 1998. Emission of nitric oxide and nitrous oxide from soil under field and laboratory conditions. Soil Biol Biochem. 30: 1885–1893. Yi Z X, Wang P, Chen P P, Tu N M. 2008. Effect of different types of nitrogen fertilizer on nitrogen absorption and utilization of summer maize (Zea mays L.). Plant Nutr Fert Sci (in Chinese). 14: 472–478. Zhang J F, Han X G. 2008. N2 O emission from the semi-arid ecosystem under mineral fertilizer (urea and superphosphate) and increased precipitation in northern China. Atmos Environ. 42: 291–302.
Gross Nitrification Rates and Nitrous Oxide Emissions in an Apple Orchard Soil in Northeast China GE Shunfeng, JIANG Yuanmao∗ and WEI Shaochong State Key Laboratory of Crop Biology, College of Horticulture Science and Engineering, Shandong Agricultural University, Tai’an 271018 (China) (Received July 10, 2014; revised March 17, 2015)
ABSTRACT A better understanding of nitrogen (N) transformation in agricultural soils is crucial for the development of sustainable and environmental-friendly N fertilizer management and the proposal of effective N2 O mitigation strategies. This study aimed: i) to elucidate the seasonal dynamic of gross nitrification rate and N2 O emission, ii) to determine the influence of soil conditions on the gross nitrification, and iii) to confirm the relationship between gross nitrification and N2 O emissions in the soil of an apple orchard in Yantai, Northeast China. The gross nitrification rates and N2 O fluxes were examined from March to October in 2009, 2010, and 2011 using the barometric process separation (BaPS) technique and the static chamber method. During the wet seasons gross nitrification rates were 1.64 times higher than those under dry season conditions. Multiple regression analysis revealed that gross nitrification rates were significantly correlated with soil temperature and soil water-filled pore space (WFPS). The relationship between gross nitrification rates and soil WFPS followed an optimum curve peaking at 60% WFPS. Nitrous oxide fluxes varied widely from March to October and were stimulated by N fertilizer application. Statistically significant positive correlations were found between gross nitrification rates and soil N2 O emissions. Further evaluation indicated that gross nitrification contributed significantly to N2 O formation during the dry season (about 86%) but to a lesser degree during the wet season (about 51%). Therefore, gross nitrification is a key process for the formation of N2 O in soils of apple orchard ecosystems of the geographical region. Key Words:
barometric process separation system, mineral N, N transformation, soil water-filled pore space, static chamber
Citation: Ge S F, Jiang Y M, Wei S C. 2015. Gross nitrification rates and nitrous oxide emissions in an apple orchard soil in Northeast China. Pedosphere. 25(4): 622–630.
INTRODUCTION The biogeochemical cycle of nitrogen (N) has long been considered an important issue because of its great significance in crop production and its impact on atmospheric environment by producing nitrogen oxides. Nitrification is one of the microbial key processes regulating N transformation and N availability in soils. Denitrification is another process that produces nitrogen oxides such as NO and N2 O (Cao et al., 2006). Hereby, the significance of nitrification is not only referred to the production of NO− 3 , but also to the direct production of nitrogen oxides. Among all the nitrogen oxides, N2 O is important as a persistent greenhouse gas that generates stratospheric NO. N2 O has an essential role in ozone-layer chemistry. Its atmospheric concentration has increased at a rate of 0.2% to 0.3% per year since 1990 (H´enault et al., 2012). Over 65% of atmospheric N2 O comes from soil as a result of nitrification and denitrification (Bouwman, 1998). ∗ Corresponding
author: E-mail: [email protected].
Hart et al. (1994) and Ingwersen et al. (1999) reported that gross and net rates of nitrification were not well correlated. Various studies have demonstrated that net nitrification rates greatly underestimate the total rates of N turnover in soils (Stark and Hart, 1997; Neill et al., 1999; Verchot et al., 2001; Ross et al., 2004). Compared with the measurement of net N turnover rates, the determination of gross N turnover rates was more difficult. However, gross nitrification may be the more suitable indicator to comprehensively understand soil N transformations. Common methods for measuring gross nitrification rates are the 15 N soil pool dilution (Davidson et al., 1991) and acetylene inhibition method (Davidson et al., 1986). These methods have the disadvantage of introducing labeled material or changing the composition of soil atmosphere. These disadvantages can be overcome by the use of the barometric process separation (BaPS) method, which was first introduced by Ingwersen et al. (1999). In the BaPS method P, CO2 , and O2 are measured to com-
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pute the gross nitrification rate. To prove its capacity to determine the gross nitrification rate, the BaPS method was successfully validated against the 15 N pool dilution method for temperate forests (Ingwersen et al., 1999; Rosenkranz et al., 2006), tropical rainforests (Breuer et al., 2002), grasslands (M¨ uller et al., 2004), and agricultural soils (Stange and Neue, 2009). To date, China has the largest apple cultivation area and production in the world. Its fruit production has gradually become an important pillar industry for increasing the income of farmers. During the last decade an increasing amount of N fertilizers is used to improve yield with the corresponding intensive production of fruits. At present, N utilization has reached 400 to 600 kg ha−1 in apple orchards in China with an annually increasing trend (Gao et al., 2002; Peng and Jiang, 2006). The excessive use of N fertilizer decreases the utilization rate and the agricultural production benefits, with negative consequences such as aggravated fruit physiological disease and environmental pollution (Yi et al., 2008). Gross nitrification and N2 O emissions are particularly controlled by soil water-filled pore space (WFPS), soil temperature, organic carbon, soil pH, and soil inorganic N concentrations (Dobbie and Smith, 2003; Martin et al., 2003). These factors are strongly controlled by fertilizer management, weed control practices, and phenology characteristics of fruit trees in orchards. In contrast to the tropical forest ecosystems (Breuer et al., 2002; Kiese et al., 2008), information on gross nitrification and N2 O emissions in apple orchard system is extremely scarce (Pang et al., 2009), and seasonal detailed information in apple orchards is unavailable. Therefore, we carried out a 3year study in an apple orchard in Northeast China. The main objectives were: i) to understand the seasonal variations of gross nitrification and N2 O emissions, ii) to determine the influence of soil conditions on the gross nitrification, and iii) to confirm the relationship between gross nitrification and N2 O emissions.
2006), and the main characteristics are presented in Table I. The apple orchard was dominated by ‘Gala’ apple trees (Malus domestica/Malus hupehensis) and the distance between rows of apple trees was 5 m. The size of the crown of each apple tree was about 3.5 m. The apple orchard was fertilized twice at a soil depth of 10 cm by digging a fertilization band (10 cm depth, 30 cm width) 1.5 m away from tree row in late March in 2009 and mid-March in 2010 and 2011 (250 kg N ha−1 as urea), and in late June in 2009 and midJune in 2010 and 2011 (200 kg N ha−1 as compound fertilizer, N:P2 O5 :K2 O = 15:15:15).
MATERIALS AND METHODS Sampling sites Field experiments were performed from 2009 to 2011 in an apple orchard at Laishan, Yantai City, Shandong Province, Northeast China (121◦ 21 00 E, 37◦ 22 47 N). The climate is classified as semi-humid, with annual average precipitation of 672.5 mm, of which nearly 70% occurs from June to September. The annual mean temperature (1984–2009) is 12.5 ◦ C, and there are about 210 frost-free days each year. The soil was classified as a Lixisol (IUSS Working Group WRB,
TABLE I Some basic physicochemical characteristics of the apple orchard soil studied Soil layer
Organic matter
cm 0–5 5–10
g kg−1 g cm−3 a) 10.36 ± 0.19 1.21 ± 0.04 5.61 ± 0.02 1.13 ± 0.05 10.04 ± 0.27 1.24 ± 0.02 5.59 ± 0.01 1.28 ± 0.06
a) Means
Total nitrogen
pH (H2 O)
Bulk density
± standard deviations (n = 15).
Measurement of gross nitrification rate The gross nitrification rates of intact soil cores were analyzed with the BaPS method (UMS GrmbH, Germany). The theory of the determination of gross nitrification by BaPS technique is based on the determination of the total pressure change, as well as the changes of O2 and CO2 partial pressure in an isothermal gas tight system. Nitrification leads to a pressure decrease by net O2 consumption, denitrification leads to a pressure increase, and soil respiration is neutral for pressure for a respiration quotient (RQ) = 1. The central equation of the BaPS method is: ΔNx Oy = Δnet − ΔO2 − ΔCO2
(1)
where ΔNx Oy stands for the rate of N gases produced by denitrification (mol h−1 ). The Δnet denotes the net rate of the total gas production (Δnet > 0) or consumption (Δnet < 0) (mol h−1 ), and ΔCO2 and ΔO2 are the net rates of CO2 formation and O2 depletion (mol h−1 ), respectively, in the closed chamber’s atmosphere. Since the total gas production rate (Δnet) and the net change rates of O2 (ΔO2 ) and CO2 (ΔCO2 ) are measured, the production rate of N-trace gases (ΔNx Oy ) via denitrification can be calculated. The total O2 consumption can be divided into three parts: 1) the O2 consumption by respiration (ΔO2,res ), 2) the O2 consumption by nitrification (ΔO2,nit ), and 3) the change in the dissolved O2 in soil water (ΔO2,dis ):
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ΔO2 = ΔO2,res + ΔO2,nit + ΔO2,dis
(2)
Also, the net CO2 production rate can be expressed as: ΔCO2 = ΔCO2,res + ΔCO2,nit + ΔCO2,den + ΔCO2,dis
(3)
where ΔCO2,res , ΔCO2,nit , ΔCO2,den , and ΔCO2,dis refer to the CO2 production rates by respiration, nitrification, and denitrification, and the change rate in the dissolved CO2 in soil water, respectively. The terms ΔO2,dis and ΔCO2,dis take into account that due to concentration changes during incubation, O2 is released from soil solution to the chamber’s atmosphere and CO2 is transferred from the chamber’s atmosphere to soil solution. In the BaPS system these terms are calculated using Henry’s law. By combining the total net gas balance equation (Eq. 1) with the O2 and CO2 balance equations (Eqs. 2 and 3) and taking into the account that −ΔO2,res = ΔCO2,res by a RQ of 1, gross nitrification rate can be obtained by calculating the O2 consumption by nitrification as follows: ΔO2,nit = [δ/(δ + 1)](Δnet − ΔCO2,dis − ΔNx Oy − ΔCO2,dis − ΔO2,dis )
(4)
where δ stands for the ratio of ΔO2,nit to ΔCO2,nit , which is a fixed value of 7.3. Further details including discussion of the uncertainties are given by Ingwersen et al. (1999, 2008), Breuer et al. (2002), M¨ uller et al. (2004) and Stange and Neue (2009). The respiration quotient (RQ) is one of the most sensitive parameters in the calculation of gross nitrification rates by the BaPS method. Thus, an independent identification of the RQ is advisible (Ingwersen et al., 1999; M¨ uller et al., 2004). An earlier study in the same region (Liu et al., 2005) showed a very good agreement of soil respiration rates by gas chromatography and the BaPS method while using RQ = 1. Therefore, RQ = 1 was used to calculate the gross nitrification rate in this study. According to the original BaPS software (UMS, M¨ unchen, Germany), the ratio of autotrophic nitrification to heterotrophic nitrification was set to 3:1, and the ratio between N2 O and N2 production was set to 1:2, as there was no additional information for this study site. Ingwersen et al. (2008) reported some problems in the BaPS method due to the calculation of the amount of CO2 dissolving into soil solution during incubation at soil pH > 6.5. Soil pH at the study site was about 5.6. Thus, this methodological limitation does not hold for this site.
To investigate the seasonal dynamic of gross nitrification rate, circular stainless cores (diameter of 7 cm) were used to take soil samples from March to October in 2009, 2010, and 2011. Soil samples were obtained monthly to include all tree growth periods. Each datum of gross nitrification rate is based on the incubation of a set of 7 intact soil cores sampled from the top soil layer (0 to 5 cm). The set of 7 intact soil cores were taken at fertilization band (three cores), 0.5 m (two cores) and 1.0 m (two cores) away from the fertilization band. The provided nitrification rates in this study are based on three replicated measurements involving 21 soil cores. Fresh soil cores from the field were put into the BaPS system for 6–18 h to acclimatize under field conditions (soil moisture and soil temperature) and subsequently gross nitrification rates were determined during a 24-h BaPS run. Measurement of N2 O emission For the period (March 2009–October 2011) when gross nitrification rates were investigated, N2 O emission was also continuously monitored, and the arrangement of N2 O sampling positions in the apple orchard was consistent with soil cores. N2 O measurement was performed using static chambers in 8:00 a.m.–10:00 a.m. and 4:00 p.m.–6:00 p.m. The chambers (50 cm × 50 cm × 50 cm) were of transparent PVC fixed to aluminum alloy frames. A buffer pipe (1.5 m, 0.6 mm outer diameter, 0.4 mm inner diameter) was inserted through the lid to keep the air pressure balanced between the inside and outside of the chamber. A fan (12 V, 0.5 A, 8 cm in diameter) was installed on the top wall of each chamber to mix the air when chamber was closed. Permanently positioned chamber bases (50 cm length × 50 cm width × 5 cm height) with water-filled grooves in the upper end were used to ensure gas tightness. During the flux measurement, gas within the chamber was sampled into 0.5-L polyethylene-coated aluminum bags using a membrane pump at 0, 20, 40, 60, 80, 100 and 120 min after the set of the chamber. N2 O concentrations of samples were analyzed by gas chromatography (Model SP3410, Beijing Analytical Instrument Factory, China) with an electron capture detector (ECD). The gas samples were loaded into a 2-mL loop connected to a 10-port valve. Both a precolumn (2 m, 4 mm outer diameter) and an analytical column (2 m, 4 mm outer diameter) were packed with Porapak Q (80–100 mesh) and hold at 72 ◦ C. Pure N2 was used as carrier gas with a flow rate of 30 mL min−1 . The temperatures of the ECD and injection port were 390 and 72 ◦ C, respectively. A small amount of CO2 (6 mL min−1 ) was added into makeup gas of
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ECD so as to improve the sensitivity and repeatability of ECD. Analytical variability was less than 1%. The N2 O emission rate was calculated using the change in the N2 O concentration of the chamber, which was estimated as the slope of the linear regression between the N2 O concentration and time. The N2 O emission rates could be disregarded if the regression coefficient (r2 ) was less than 0.85.
(version 13.0, SPSS Inc., USA). The relationships among gross nitrification rates and soil variables, name− ly soil temperature, WFPS, and NH+ 4 and NO3 concentrations, were analyzed by multiple-linear regression. The relationship between soil WFPS and gross nitrification rate was described by O’Neill functions (Stange and Neue, 2009). RESULTS
Determinations of rainfall, soil temperature, soil WFPS, and soil mineral N Rainfall data were obtained from the Laishan Agroecological Station of Yantai Academy of Agricultural Sciences, China, located 400 m away from the experimental site. Soil temperature was measured with an SN2202 digital thermodetector (Sinan Instrument Plant of Beijing Normal University, China). After the BaPS incubation, soil samples from the 7 cores were removed and mixed to measure soil moisture content using the thermo-gravimetrical method, and then soils were dried at 105 ◦ C for 24 h. Soil mineral N (NH+ 4 −1 and NO− KCl and 3 ) was extracted with 0.01 mol L measured by a colorimetric continuous flow analyzer (SAN++, Skalar Company, the Netherlands; Schloter et al., 2003). Considering the influence of soil texture, soil WFPS was used in the data analysis. Soil bulk density was determined at each sampling time, thereby allowing soil WFPS to be calculated from the soil weight moisture content values (Merino et al., 2004). WFPS is calculated as follows: WFPS (%) = (gravimetric soil weight moisture content × bulk density)/total porosity. The total porosity in percentage was computed as (1 − bulk density/2.65) × 100. Statistical analysis All statistical analyses were performed using SPSS
Fig. 1
Environmental conditions From March 2009 to October 2011 the precipitation at the experiment site was in total 2 452.6 mm (Fig. 1). The seasonal pattern of precipitation was evident at this site. About 70% of total rainfall was observed during wet season conditions (June to September), whereas only 30% of total rainfall occurred during the dry season (October to May; Fig. 1). The annual mean soil temperatures in 2009, 2010, and 2011 were 12.8, 12.7, and 12.4 ◦ C, and the variation ranges were −1.98–25.32, −1.78–25.68, and −3.67–25.34 ◦ C, respectively. The soil temperature gradually increased from January to August, and then decreased from September to January. Seasonal dynamic of gross nitrification rate, N2 O fluxes, and soil mineral N Gross nitrification rates from March to October of 2009 to 2011 gradually increased, peaked from June to August, and then declined. Exceptions from this general trend were July 2009 and August 2011 (Fig. 2). Gross nitrification rates were significantly higher during the wet season (June to September) than during the dry season (October to May). Mean gross nitrification rates were 13.9, 11.6, and 12.3 mg N kg−1 d−1 for the wet seasons of 2009, 2010, and 2011, respectively,
Mean monthly rainfall and soil temperature at 5 cm depth during the study period of 2009–2011.
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− Fig. 2 Soil water-filled pore space (WFPS), gross nitrification rates, N2 O fluxes, and mineral N (NH+ 4 and NO3 ) concentrations during the study period of 2009–2011. Arrows indicate the timing of fertilizer application. Vertical bars indicate standard deviations
of the means (n = 3).
whereas the values for the dry seasons were 8.8, 7.3, and 8.4 mg N kg−1 d−1 , respectively. The minimum nitrification rate of 6.5 mg N kg−1 d−1 was observed in April 2011 during the dry season when soil WFPS was 26.5%. The maximum gross nitrification rate of 18.06 mg N kg−1 d−1 was observed in August 2009 after a rainfall, which increased soil WFPS from 38.6% to 56.9%. However, the gross nitrification decreased in July 2009, August and September 2011 because of soil WFPS above 60% (Fig. 2). Nitrous oxide fluxes varied widely from March to October and were stimulated by N fertilizer application (Fig. 2). The changing trend of the N2 O fluxes was consistent in 2009, 2010 and 2011. For example,
the highest N2 O flux (240.9 μg N m−2 h−1 ) was measured 9 d after fertilizer application in mid-June 2009. In contrast, only a small increase in the N2 O flux (63.6 μg N m−2 h−1 ) occurred after applying the fertilizer in mid-March 2009. The variation of soil NH+ 4 concentrations in 0– 5 cm soil layer was highly influenced by fertilization events. The apple orchard was fertilized twice at a soil depth of 10 cm in mid-March and mid-June. Thus, soil NH+ 4 concentrations were higher at that time every year. The maximum NO− 3 concentrations occurred in September 2009, September 2010, and July 2011, whereas the lowest NO− 3 concentrations appeared in October 2009, 2010, and 2011 (Fig. 2).
NITRIFICATION AND N2 O EMISSION IN ORCHARD SOIL
Effects of soil variables on gross nitrification The gross nitrification rates were significantly correlated with soil temperature (P < 0.01, r = 0.68), WFPS (P < 0.01, r = 0.75), and NH+ 4 concentration (P < 0.05, r = 0.51). There was no significant correlation between gross nitrification rate and soil NO− 3 concentration. The multiple regression of gross nitrification rate with soil variables (WFPS, soil tempera− ture, NH+ 4 and NO3 concentrations) showed that soil WFPS and soil temperature could be identified as two key factors determining the seasonal variability of gross nitrification rate, accounting for 38% (P < 0.006) and 20% (P < 0.011) of the seasonal variation, respectively. The relationship between soil WFPS and gross nitrification rate could be best described by O’Neill functions with an optimum nitrification rate at a soil WFPS of 60%. Relationship between gross nitrification and N2 O emissions Regression analysis revealed a significant relationship between the gross nitrification rate and the N2 O fluxes measured at the same day. The relationship could be described best by a linear regression line during the whole seasons (r = 0.67, P < 0.01; Fig. 3a). During the dry season, the relationship between gross nitrification rate and the N2 O emissions could be described best by a linear regression line (r = 0.81, P < 0.01; Fig. 3b). DISCUSSION This work presents information on the seasonal dynamics of gross nitrification over a 3-year period, based on monthly measurements using the BaPS method, in an apple orchard ecosystem under semi-humid con-
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ditions. These measurements were accompanied by the N2 O flux measurements, thereby enabling to establish a relationship between gross nitrification and N2 O emission. During the 3 years, gross nitrification rates showed a strong seasonal pattern with significantly higher rates during the wet season as compared with the dry season (Fig. 2). Rates under wet season conditions were 1.6 times higher than those under dry season conditions. These findings are consistent with the studies that were performed in tropical rainforests (Kiese et al., 2008) and grassland (Gao et al., 2008). Gross nitrification rates of the present study (6.8–18.1 mg N kg−1 d−1 ) are in the same range as those reported by other studies, such as in tropical rainforest (0.5–20 mg N kg−1 d−1 ; Breuer et al., 2002; Booth et al., 2005; Kiese et al., 2008), grassland (0.7–8.2 mg N kg−1 d−1 ; Gao et al., 2008; Stange and Neue, 2009), and winter wheat-summer maize double cropping systems (4–24 mg N kg−1 d−1 ; Wan et al., 2009). N2 O is a relatively stable greenhouse gas that has a significantly contribute to the destruction of the ozone layer in the stratosphere (Crutzen, 1981; Bouwman, 1998). Agricultural soils are known to be responsible for a large proportion of the increased N2 O concentration in the atmosphere, mainly due to the use of N fertilizer (Dobbie and Smith, 2003; Stehfest and Bouwman, 2006; Barton et al., 2008). In this study, N2 O fluxes varied widely from March to October and were stimulated by N fertilizer application (Fig. 2). However, N2 O fluxes varied widely in response to the timing of N fertilization and were usually greater after the fertilization in June than after the fertilization in March, because the soil conditions were more conducive for N2 O production in late June compared to late March. In late June, soil temperature was generally higher than 20.0 ◦ C and the top soil was subje-
Fig. 3 Relationships between gross nitrification rates and N2 O fluxes throughout the study period (a) and in the dry season (from March to June, and in October) (b).
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cted to regular wetting and drying cycles (Dobbie et al., 1999; Du et al., 2006; Pang et al., 2009). Several factors such as soil moisture, soil O2 concentration, soil organic matter content, NH+ 4 availability, and pH may act as important physiological constraints, and consequently affect the nitrification process in terrestrial ecosystems (Booth et al., 2005; Silva et al., 2005; Cookson et al., 2006). Despite the numerous factors that may potentially affect the nitrification process, only a few were key parameters. Cookson et al. (2006) found that soil moisture, temperature, and inorganic N availability were the major factors among the 15 investigated soil factors, which control the survival and activity of microorganisms and consequently strongly influence soil N transformations. In our study, gross nitrification rate was obviously influenced by soil temperature. The rates were higher in summer (June, July, and August) or autumn (September and October) as compared with the rates measured in spring (March, April, and May). Similar results were shown by Davidson et al. (1992) and Stark and Hart (1997) for various grassland and forest ecosystems of North America. Breuer et al. (2002) found that nitrification increased at an average rate of 1.17 mg NH4 -N m−2 h−1 per 1 ◦ C increase in soil temperature. A Q10 (temperature coefficient, a measure of the change rate of a biological or chemical system as a consequence of increasing the temperature by 10 ◦ C) value of 3.60 was calculated for the temperature range between 14 and 24 ◦ C. Ingwersen et al. (1999) found a slightly higher Q10 value (Q10 = 4.13), which can be derived from the data presented on the temperature dependency of the gross nitrification in the soil taken from a temperate spruce forest ecosystem (temperature range of 15 to 25 ◦ C). Aside from soil temperature, a statistically significant positive correlation was found between soil WFPS and gross nitrification. The relationship between gross nitrification and soil WFPS seemed to follow an optimum curve, peaking at 60% WFPS. This finding is in line with the significantly decreased gross nitrification rates measured in July 2009, August 2011, and September 2011 when soil WFPS was 82%, 83%, and 80%, respectively. Linn and Doran (1984) noted that soil aeration, which is directly affected by soil moisture, might be a limiting factor for the aerobic processes like gross nitrification. Our study is consistent with the previous studies (Breuer et al., 2002; Corre et al., 2002; Khalil and Baggs, 2005; Kiese et al., 2008), which found that under the more saturated soil moisture conditions gross nitrification was reduced. Soil N2 O emission is a complex process that is simultaneously influenced by several factors. Nitrifica-
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tion and denitrification by soil microbes are the main sources of N2 O emission in soil. However, gross nitrification and denitrification may simultaneously occur in soil. Therefore, the real contribution of each process to the observed N2 O fluxes is difficult to ascertain (Arah, 1997). However, denitrification is regarded as the most important N2 O forming process in agricultural soils (Azam et al., 2002; Dobbie and Smith, 2003), whereas nitrification is reported to make a substantial contribution to N2 O emission under aerobic conditions (Klemedtsson et al., 1988; Williams et al., 1998). Recently, several studies have indicated that nitrification is the dominant source of N2 O under aerobic conditions as long as the WFPS is in the range between 30% and 70% (Williams et al., 1998; Chen and Huang, 2006; Ciarlo et al., 2008; Zhang and Han, 2008; Wan et al., 2009). As demonstrated by Breuer et al. (2002) and Kiese et al. (2008) gross nitrification rates were positively correlated with N2 O fluxes. The relationship could be described by a linear regression line (r = 0.67, P < 0.01; Fig. 3a) through the analysis of all data. Since both nitrification and denitrification contribute to the formation of N2 O emission, the relationship between gross nitrification rates and N2 O emissions was further analyzed by selecting data only for the dry seasons. We assumed that N2 O emissions during the dry season should be dominated by nitrification. This hypothesis was confirmed by a linear regression analysis with a significantly better fit between N2 O emissions and gross nitrification (Fig. 3b). Applying the regression equation in Fig. 3b to all the measurements revealed that gross nitrification may contribute to only 51% N2 O emission in the wet season, 86% in the dry season, and 64% in the study period. These findings are in good agreement with the reports of Chen and Huang (2006) and Wan et al. (2009), who observed that nitrification was the major source of N2 O emission from agricultural soils of China. Therefore, the gross nitrification was concluded to be the main source of N2 O production in this region. But, this conclusion was reached based on a hypothesis that N2 O emissions in the dry season predominantly originated from gross nitrification, so the outcome of this approach was subject to uncertainty and need further studies. In this study, the average N2 O emitted from the soil in apple orchard accounted for 1.16% of the fertilized N, including the contributions of background emissions during 8 months (from March to October) in 3 years. This value is slightly higher than the results (0.9%) presented by Lu et al. (2006) for croplands and grasslands in China where fertilizer N was applied, and 1.8 times higher than the value determined by Pang
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et al. (2009) for apple orchards in the semiarid Loess Plateau of China. Although the present study and the study of Pang et al. (2009) were performed in the apple orchard soils, climate and soil conditions were different among the two sites. For example, at our study site annual rainfall was about 844 mm and the annual mean soil temperature was 12.1 ◦ C. In contrast, the site studied by Pang et al. (2009) had an annual precipitation of 584 mm and an annual mean soil temperature of 9.1 ◦ C. Therefore, to obtain a precise and reliable N2 O emission factor for semi-humid agricultural soils, multi-year and multi-site measurements are required.
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CONCLUSIONS The gross nitrification rates in the apple orchard showed an obvious seasonal pattern, with the highest rates during the wet season and significantly lower rates during the dry season. Fertilizer application had a positive effect on the gross nitrification and N2 O emissions. The relationship between the gross nitrification rates and soil WFPS seemed to follow an optimum curve, peaking at 60% soil WFPS. Statistically significant positive correlations existed between the gross nitrification rate and N2 O emission. Further investigations indicated that gross nitrification contributed significantly to N2 O formation during the dry season (86%) but less during the wet season (51%). Therefore, the gross nitrification was the primary source of N2 O emissions from the apple orchard soil in this region. However, the combined measurements of N2 O emission and the underlying N turnover processes would be necessary to further our understanding of processes and mechanisms governing biosphere-atmosphere N gas exchange. ACKNOWLEDGEMENT This study was supported by the Special Fund for Agro-scientific Research in the Public Interest of China (No. 201103003) and the Earmarked Fund for China Agriculture Research System (No. CARS-28). REFERENCES Arah J R M. 1997. Apportioning nitrous oxide fluxes between nitrification and denitrification using gas-phase mass spectrometry. Soil Biol Biochem. 29: 1295–1299. Azam F, M¨ uller C, Weiske A, Benckiser G, Ottow J C G. 2002. Nitrification and denitrification as sources of atmospheric nitrous oxide—role of oxidizable carbon and applied nitrogen. Biol Fert Soils. 35: 54–61. Barton L, Kiese R, Gatter D, Butterbach-Bahl K, Buck R, Hinz C, Murphy D V. 2008. Nitrous oxide emissions from a cropped soil in a semi-arid climate. Glob Change Biol. 14:
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