Nitrogen mobility, ammonia volatilization, and estimated leaching loss from long-term manure incorporation in red soil

Journal of Integrative Agriculture 2017, 16(9): 2082–2092 Available online at www.sciencedirect.com

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RESEARCH ARTICLE

Nitrogen mobility, ammonia volatilization, and estimated leaching loss from long-term manure incorporation in red soil HUANG Jing1, 2, 3, DUAN Ying-hua2, XU Ming-gang2, ZHAI Li-mei2, ZHANG Xu-bo4, WANG Bo-ren2, 3, ZHANG Yang-zhu1, GAO Su-duan5, SUN Nan2 1

College of Resources and Environment, Hunan Agricultural University, Changsha 410128, P.R.China

2

Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences (CAAS)/National Engineering Laboratory for Improving Quality of Arable Land, Beijing 100081, P.R.China 3 Red Soil Experimental Station of CAAS in Hengyang/National Observation and Research Station of Farmland Ecosystem in Qiyang, Qiyang 426182, P.R.China 4 Key Laboratory of Ecosystem Network Observation and Modeling/Yucheng Comprehensive Experiment Station, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, P.R.China 5 USDA Agricultural Research Service, San Joaquin Valley Agricultural Sciences Center, CA 93648-9757, USA

Abstract Nitrogen (N) loss from fertilization in agricultural fields has an unavoidable negative impact on the environment and a better understanding of the major pathways can assist in developing the best management practices. The aim of this study was to evaluate the fate of N fertilizers applied to acidic red soil (Ferralic Cambisol) after 19 years of mineral (synthetic) and manure fertilizer treatments under a cropping system with wheat-maize rotations. Five field treatments were examined: control (CK), chemical nitrogen and potash fertilizer (NK), chemical nitrogen and phosphorus fertilizer (NP), chemical nitrogen, phosphorus and potash fertilizer (NPK) and the NPK with manure (NPKM, 70% N from manure). Based on the soil total N storage change in 0–100 cm depth, ammonia (NH3) volatilization, nitrous oxide (N2O) emission, N plant uptake, and the potential N leaching loss were estimated using a mass balance approach. In contrast to the NPKM, all mineral fertilizer treatments (NK, NP and NPK) showed increased nitrate (NO3–) concentration with increasing soil depth, indicating higher leaching potential. However, total NH3 volatilization loss was much higher in the NPKM (19.7%) than other mineral fertilizer treatments (≤4.2%). The N2O emissions were generally low (0.2–0.9%, the highest from the NPKM). Total gaseous loss accounted for 1.7, 3.3, 5.1, and 21.9% for NK, NP, NPK, and NPKM treatments, respectively. Estimated N leaching loss from the NPKM was only about 5% of the losses from mineral fertilizer treatments. All data demonstrated that manure incorporation improved soil productivity, increased yield, and reduced potential leaching, but with significantly higher NH3 volatilization, which could be reduced by improving the application method. This study confirms that manure incorporation

Received 12 June, 2016 Accepted 28 October, 2016 HUANG Jing, Tel: +86-746-3841027, E-mail: huangjing@caas. cn; Correspondence ZHANG Yang-zhu, E-mail: [email protected]; SUN Nan, E-mail: [email protected] © 2017 CAAS. Publishing services by Elsevier B.V. All rights reserved. doi: 10.1016/S2095-3119(16)61498-3

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is an essential strategy in N fertilization management in upland red soil cropping system. Keywords: soil NO3–-N, ammonia volatilization, nitrogen leaching, long-term field experiment, mass balance, nitrous oxide emission

1. Introduction Overuse of synthetic nitrogen (N) fertilizers due to intensification of agricultural productions has led to high losses from agricultural soils and caused damage to the environment (Erisman et al. 2007). China has become the largest N fertilizer consumer accounting for about 30% of the world’s consumption since 2002 (FAO 2010). However, for a popular cropping system with winter wheat-summer maize rotation, the N use efficiency (NUE) is less than 30% (Zhao et al. 2006). Nitrogen loss has led to surface water eutrophication, ground water contamination, air quality degradation, and contributed to global warming by forming greenhouse gases. Nitrate leaching losses from soil into water not only reduces soil fertility but also causes threat to environment and human health (Cameron et al. 2013). Groundwater contamination has been reported as a major concern in northern China. A survey of NO3– concentration in 600 groundwater samples showed that about 45% of the samples exceeded the drinking water standard of 50.0 mg L–1 proposed by major developed countries with the highest reported concentration reaching 113 mg L–1 (Zhang et al. 2004). Increased concentration and mobility of NO3– in soil profile indicates high risk of leaching and groundwater contaimination. In wheat-maize fields, NO3–-N in the 0−90 cm soil layer was found to accumulate above 200 kg N ha–1 at N application rate of 553 kg N ha–1 yr–1 (Ju et al. 2006). The N leaching loss in subtropical areas such as the red soil region in southern China was expected to be worse than northern China because of higher precipitation. However, we have insufficient data to validate (Xu et al. 2010). Sun et al. (2008) found about 16.8% of N fertilizer applied at 150 kg N ha–1 yr–1 was leached in a rain-fed peanut rape rotation system in an acidic red soil. Long et al. (2012) found that pig manure applied at 150 kg N ha−1 yr–1 did not result in elevated NO3– concentrations in soil, and addition of lime with high manure incorporation rate (600 kg N ha–1 yr–1) in surface soil layer (0−15 cm) had no significant effect to reduce NO3– concentrations in soil below (15−150 cm). This potentially indicates that N leaching can be significant, if manure application rate is too high. Ammonia (NH3) volatilization is one of the major N losses from soil fertilization. High NH3 loss is caused by a

chemical reaction shifting from ammonium (NH4+) to NH3 at high pH (NH4++OH–→NH3↑+H2O). The worldwide NH3 losses range from 10 to 19% (average 14%) of the used N fertilizers (Ferm 1998). In China, high NH3 volatilization has been reported from calcareous soil (high soil pH) in the Northern China Plain. The NH3 losses were 30−39, 11−48, and 1−20% of total N applied to rice (urea or ammonium bicarbonate), maize (urea) and wheat (urea), respectively (Cai et al. 2002). Measurements in paddy soils (i.e., under flooded conditions) showed that NH3 volatilization accounted for 27.6−59.7% of urea-N applied from different N fertilizers and application methods (Zhang et al. 2011). Ammonia losses from rice paddies were generally lower (13.2−31.1%) under different combined irrigation systems, including non-flooding and wet-dry cycles. The losses were also lower under different nutrient managements, including compound fertilizer treatment with ammonium bicarbonate or urea and/or control released urea (Xu et al. 2012). Limited studies have assessed NH3 volatilization under upland farming (e.g., wheat, maize) in red soil. One of the studies reported NH3 volatilization was in the range of 0.7−4.0% when 70−250 kg ha–1 urea was applied in red soil under upland farming with the crop rotation of smooth crabgrass (Digitaria ischaemum) in spring and winter radish (Raphanus sativus) in autumn (Zhou et al. 2007). These values were much lower than those from the calcareous soils in different regions and paddy soils in the same region. More accurate assessment on the NH3 volatilization loss is needed from upland red soil. In addition to NH3, other volatile forms of N include nitrous oxide (N2O), dinitrogen (N2), and nitrogen oxides (NOx, nitric oxide (NO) and nitrogen dioxide (NO2)). N2O is a potent greenhouse gas with the warming potential ~300 times greater than the equivalent mass of CO2. It also contributes to the destruction of stratospheric ozone (Cicerone 1987). NO is a precursor of NO2. It reacts with O2 and further with water to form nitric acid (HNO3), which is a major component of acid rain. The NOx can also react with volatile organic compounds under sunlight to form ozone, a ground level air pollutant that is a respiratory hazard and a greenhouse gas (Williams et al. 1992). Understanding N loss in gaseous forms has significance in N cycling and developing management practices. More attention has been paid to NH3 and N2O because of their role in mass and global warming. The other forms of gaseous N are minor

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except N2, which could become significant under anaerobic conditions such as flooded paddy soil, but insignificant in dry land production (Mosier et al. 1989). The N2O and NO are intermediate products of nitrification and denitrification in soil (Sahrawat and Keeney 1986). N2O is primarily produced from nitrification process at low and moderate soil moistures and denitrification processes at soil moisture containing over 70% water-filled pore space due to a decreased O2 supply (Ruser et al. 2006). Thus, in non-irrigated upland conditions, nitrification is expected to be the dominant process in N2O production, which results in much lower N2O emissions than those from high soil moisture conditions in the laboratory (Cai et al. 2016). Field data also indicated that the gas loss from denitrification (mainly as N2 and N2O) can be as high as 33% of applied N in paddy field (Zhu et al. 1989), but substantially lower in upland production of maize and wheat (0.8−2% of applied N) (Cai et al. 2002). Annual losses of N2O and NO from wheat-maize rotation system under different fertilization (urea or organic fertilizers) treatments were reported in the range of 0−1.7% (Akiyama et al. 2004; Cui et al. 2012). The red soils (equivalent to Ferralic Cambisol in the World Soil Classification by FAO) cover about 2 million ha in tropical and subtropical regions in southern China (Xu et al. 2003). The productivity of the red soils is low especially in upland crop production (e.g., wheat and maize) due to low soil pH. To increase grain yield, high amount of chemical fertilizers have been used in the last few decades, which resulted in serious environmental problems, including intensified acidification and reduction in soil productivity. To address these issues and develop nutrient management strategies in this region, a long-term field experiment was established in 1990 in southern China to determine the effects of various fertilization (including synthetic fertilizer and manure) regimes on crop response and soil nutrients in a wheat-maize rotation system in red soil. Previous studies have reported crop yield, N uptake, N2O emissions and soil properties (Duan et al. 2011; Zhai et al. 2011). A recent research has used a mass balance approach to estimate the total N loss (including both volatilization and leaching loss) to the environment (Duan et al. 2016). In the present study, we adopted this approach to further quantify N losses via specific pathways. Specifically we aim to: 1) determine NH3 volatilization amount during the maize and wheat growing seasons; 2) examine NO3–-N distribution or mobility in soil profiles; and 3) estimate total gaseous loss and the potential leaching loss by integrating all available data.

2. Materials and methods 2.1. Experimental site This study was conducted on a red soil at the Qiyang Exper-

imental Station (26°45´12´´N, 111°52´32´´E) of the Chinese Academy of Agricultural Sciences, Qiyang, Hunan Province, China. The annual rainfall, sunshine hours, and average temperature were 1 250 mm, 1 620 h and 18.0°C, respectively. The top soil (0−20 cm) of the experimental field at the beginning of the long-term experiment (i.e., in 1990) had a soil organic carbon of 7.89 g kg−1, total N of 1.07 g kg−1, total P of 0.45 g kg−1, total K of 13.7 g kg−1, available N of 79 mg kg−1, available P of 13.9 mg kg−1, available K of 104 mg kg−1, and bulk density (BD) of 1.19 g cm–3.

2.2. Field treatment and experimental design Five fertilizer treatments investigated in this study were: control (CK), inorganic N with K (NK), inorganic N with P (NP), combined inorganic N, K, and P (NPK), and NPK with manure (NPKM, 70% N applied was from manure). All treatments were established in 1990 and repeated annually until 2009 (total 19 years). The soil profiles were sampled for N analysis every year. The treatments were duplicated and plots were laid out in a randomized block design. Each plot size was 196 m2 (20 m×9.8 m). The N fertilizer was supplied as urea and pig manure at an annual rate of 300 kg N ha–1, P as single superphosphate at 53 kg P ha–1, and K as KCl at 100 kg K ha–1. The amount of pig manure to provide 70% of total N (210 kg N ha–1 from pig manure) was calculated from the total N content in the dry manure (average 16.7 g kg–1). The annual application of fresh pig manure was 41.7 t ha–1. For the annual supply, 30% of the total amount of fertilizer was applied for wheat and 70% for maize. All mineral fertilizers and manure were applied as basal fertilizer before crop planting. To simulate local farmers’ practices, both mineral fertilizers and manure were applied by banding at a depth of 10 cm, followed by sowing of crop seeds, and then covering with soil. Wheat (cultivar Xiangmai 4) was seeded in November and was harvested in early May each year. Maize (cultivar hybrid Yedan 13) was seeded between wheat rows in late March and harvested in middle of July each year. The period after maize harvest before the next season’s wheat seeding was referred to fallow period (about 3.5 mon).

2.3. NO3–-N distribution in soil profile and determination of NH3 volatilization To evaluate N leaching potential from various fertilization regimes, soil samples were collected from the top 1 m profile in fall 2008 after harvesting maize, and NO3–-N concentration in the profile was determined. To collect a representative sample in the plot, five cores of soil were randomly collected in each plot using a 5-cm inner diameter (i.d.) auger and combined. The samples were separated

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into 0–20, 20–40, 40–60, 60–80 and 80–100 cm depths. Fresh soil samples were used for soil NO3–-N and NH4+-N analysis. The well mixed 10 g fresh soil samples were extracted with 50 mL distilled water. After shaking for 30 min, the suspension was filtered and analyzed for NO3– -N and NH4+-N by phenoldisulfonic acid and indophenol blue colorimetry, respectively (Bao 2000). The amount of NH4+-N in each extract was found in the range of 1.8–9.6% (average 5.0%) of total extracted mineral N, which means each extract contained >95% NO3–-N. Ammonia volatilization rate was measured from 2009 to 2010 during wheat and maize growing seasons, using a continuous airflow chamber method as described by Tian et al. (1998). Immediately following fertilizer applications, a PVC chamber base (20 cm in diameter, and 5 cm in height) was inserted about 5 cm deep into the soil, between crop rows at the center of each plot and the base top was fitted with a water trough. During measurement, a cylindrical chamber (20 cm in diameter, and 10 cm in height) fitted with two plastic tubes was inserted into the water trough at the top of the chamber base. The chamber air inlet was a plastic tube (25 mm i.d.) at a height of 2 m above the soil surface. The background air NH3 concentration was monitored with an inlet at a height of 2 m above the soil surface. The NH3 volatilized into each chamber was collected through the outlet plastic tubing (10 mm i.d.) immersed into the bottom of a 1 000-mL glass erlenmeyer flask, which contained 250 mL of 2% boric acid solution to trap NH3. The air flow through a group of multiple chambers was controlled by a flow meter (air flow through each chamber was 0.8 L s–1). Sampling time for each measurement was 2 h (between 2:00 and 4:00 p.m.) when it likely represented the daily average flux according to Tong et al. (2009). The NH3 dissolved in the acid solution was determined by titration with sulfuric acid (0.005 mol L–1 H2SO4) in the laboratory (Xu et al. 2013). The NH3 volatilization from all treatments was corrected for the background air NH3 concentration in each measurement. Cumulative NH3 volatilization (kg N ha–1 d–1) during each

(maize or wheat) growing season was calculated as follows: (Fj+Fi) (1) Cumulative NH3−Nemission=∑ ×(tj−ti)×24 2 Where, Fi and Fj are the NH3 flux (kg N ha–1 h–1) in the ith and jth day, ti and tj are the time (day) between any of the two sampling events (i and j) after fertilizer application. The NH3 volatilization for each treatment was measured daily following fertilizer application for about two weeks. After two weeks, measurements were made occasionally for other times during the year because little differences were found between fertilizer treatments and the control. The differences in NH3 emission between fertilizer treatments and the control were considered to be the result of fertilizer application.

2.4. Nitrogen mass balance and estimate of N leaching loss Nitrogen balance (N input-N export by crop) has been used to estimate the risk of N losses from arable land (Constantin et al. 2010). However, the loss estimated using this method only reflects plant use efficiency. To more accurately assess N loss to the environment from fertilization regime, Duan et al. (2016) incorporated soil N status change into the mass balance equation and estimated the total loss as follows: Nloss to the environment=Ninput from fertilizer and manure+Ninput from the environment– Nplant uptake–Nsoil total N storage change (2) Where, Nloss to the environment is all losses to the environment including gases and leaching, Ninput from fertilizer and manure is N from fertilizer applications, Ninput from the environment is N via soil N mineralization, N fixation, precipitation, etc., which was estimated from the plant uptake in the control (CK) that no N fertilizer was applied, Nplant uptake is the nitrogen output from soil by crops uptake, Nsoil total N storage change refers to soil total N storage. In this study, soil for total N analysis and soil bulk density (SBD) were collected in the fall of 2009, i.e., after 19 years of fertilizer treatment; thus N mass balance was conducted for the period of 1990−2009. The annual soil N change rate was calculated from:

Soil total N storage change rate (kg ha–1 y–1)=(TN2009–TN1990)/(2009–1990)×SBD×Ss×Sh×10–3 Where, TN1990 or TN2009 is the soil total N content (g kg–1) determined at the beginning of the experiment, i.e., in 1990 or the end of the study period (2009), SBD represents soil bulk density (g cm–3), Ss is the area (ha) and Sh is the soil depth (m). In this study, we chose the boundary of 1 m soil depth to define the rooting zone, i.e., soil N in top 1 m soil is available for crops and the N below this layer is subjected to leaching because the majority of maize and wheat roots are found in the surface soil (Shi et al. 2012). The total N loss to the environment estimated in eq. (2) included all possible pathways including runoff/erosion, leaching, NH3 volatilization, and all other gaseous (N2O,

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(3)

NOx and N2) losses. The field was flat and had little runoff and erosion. In this study, we collected new data on NH3 volatilization for each crop growing season. Previous investigations have determined N2O emissions (Zhai et al. 2011), while other gaseous (N2 and NOx) losses were not measured. However, these losses were minor based on the studies below. Under the upland cropping conditions, the N2O/(N2O+N2) ratio averaged 0.5 in upland soil (Schlesinger 2009). The NOx (mainly NO) can be estimated in relation to N2O with a NO/N2O ratio, which is about 0.4 in fertilized cropland (Stehfest and Bouwman 2006). So other gaseous (N2 and NOx) losses were estimated as 1.4 times N2O emis-

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sion. Under these assumptions and with all available data, eq. (4) can be rearranged to estimate the potential leaching: Nleaching loss=Ninput from fertilizer and manure+Ninput from the environment– Nplant uptake–Nsoil total N storage change–NNH3 volatilization– NN2O emission–Nother gaseous loss (NOx+N2) (4) –1 –1 Where, all units in eq. (4) are kg N ha y . All the data were measured in our experiments, except the minor gas loss such as NOx and N2, which were estimated based on available literature (Stehfest and Bouwman 2006; Schlesinger 2009). The NH3 volatilization and N2O emissions were measured at different years from the total soil N determination. However, we assume they represent annual loss and consider that the same treatment was applied to the same plot and there were no extreme weather conditions in terms of temperature during the entire study period, although climate was relatively drier in 2009 and 2010 (Fig. 1). The fate of N fertilizer applied include plant uptake, soil total N storage change, NH3 volatilization, N2O emissions, other gas emissions and N leaching. Their proportions to total N input were calculated as follows, and plant uptake was taken for instance. Plant uptake (%)=Nplant uptake/(Ninput from fertilizer and manure+ Ninput from the environment)×100 (5)

2.5. Data analyses Statistical analyses on the data of NH3 or N2O volatilization and soil NO3–-N were performed using SAS 9.2 (SAS Institute 2008). Because there were only two replicates due to limitations on the field size and preference to using large plots, the analysis power is limited. The analysis relies on the fact that if the error variance is small and the treatment effect size is large, significance is still possible (Steel and Torrie 1980; Milliken and Johnson 1989). Thus the purpose of performing such statistical analysis in this study was to provide some useful information on treatment effects with available data.

For the NH3 or N2O volatilization data, a model based on the randomized completed block design was used to fit the treatment effects and produce residual diagnostics. The means separation between the different treatments was performed using Tukey’s adjustment. For the soil profile NO3–-N data that indicate N mobility, a SAS PROC MIXED Program was used to fit a mixed model with repeated measures. The treatment, soil depth, and their interaction are the fixed effects and the replications and treatments×replications are the random effects. This random effect defines the experimental units for incorporating a first order, autoregressive covariance structure among the repeated measures on the soil depths. Log-transformed data were used in order to improve residual diagnostics. The soil depth×treatment interaction was significant (P<0.001). So, the corresponding least square means and 95% confidence intervals were produced, which were back transformed to the original units of measurement. These are reported in the results.

3. Results 3.1. Nitrate movement in soil profile To examine soil N mobility after long-term fertilization regimes, the NO3– contents in the 0−100 cm soil profile after 18 years of different fertilizer treatments were analyzed and the results are shown in Fig. 2. Soil NO3–-N concentration in the CK was the lowest in the profile, ranging from 1.4 to 5.6 mg kg–1. It also exhibited some downward movement from surface to 40 cm depth or below. The NO3– contents in fertilizer treatments at each depth were all significantly higher than those in the CK (P<0.05). All the mineral fertilizer treatments (NK, NP and NPK) showed a trend of Soil NO3–-N content (mg kg–1)

20 19 18 17 16

Soil depth (cm)

Annual rainfall Annual temperature

Annual temperature (°C)

2 000 1 800 1 600 1 400 1 200 1 000 800 600 400 200 0

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Annual rainfall (mm)

0

0

10

20

30

40

50

60

70

80

90

20 40 60 80 100

CK NP NK NPK NPKM

15

Year

Fig. 1 Trends of annual rainfall and temperature over the entire study period (1990–2010) at the long-term experimental site in Qiyang, Hunan Province, China.

Fig. 2 Soil NO3–-N contents in soil after 18 years of fertilizer treatments after maize harvest in October 2008. CK, control; NP, inorganic nitrogen with phosphorus; NK, inorganic nitrogen with potassium; NPK, inorganic nitrogen, phosphorus and potassium combination; NPKM, NPK with manure. Error bars (SE) denote the lower and upper values at the 95% confidence level generated by a PROC MIXED Program.

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3.2. Dynamics of NH3 volatilization during wheat and maize growing seasons The NH3 volatilization, measured following the fertilizer applications during the maize and wheat growing seasons, is shown in Fig. 3. The highest volatilization rate was observed on the first day, ranging from 2.3 to 10.4 kg N ha–1 d–1 (Fig. 3-A) with the highest from the NPKM treatment. After the initial high values, the volatilization rates dropped substantially (especially for the NPKM treatment) and continued to decrease until no differences were observed from the control in two weeks. Beyond two weeks, until wheat harvest, the average NH3 volatilization rate of CK, NK, NP, NPK and NPKM in wheat growing season was about 1.2, 1.3, 1.1, 1.3 and 1.2 kg N ha–1 d–1, respectively, with no significant differences among these values. The NH3 volatilization rate during the maize growing season (Fig. 3-B) showed a similar pattern as that during the wheat growing season (Fig. 3-A). The highest NH3 volatilization rates (up to 11.4 kg N ha–1 d–1) were observed during the first two days following fertilizer applications and then declined continuously. Among the treatments, the NPKM had the highest NH3 volatilization rates, but declined at a faster rate with time, compared with the NH3 volatilization rates during the wheat growing season. For all the mineral fertilizer treatments (NK, NP, NPK), the NH3 volatilization rates were almost the same as that during the wheat growing season, in terms of both value and decreasing rate. The NH3 volatilization rate for NPKM treatment, 10 days after application, was still substantially higher than the mineral fertilizer treatments, but reduced to 1.8 kg N ha–1 d–1 15 days later, which was not significantly different from other treatments. The NH3 volatilization data from both growing seasons indicated significant NH3 volatilization loss during a short period of time, immediately following fertilizer applications. At or after two weeks until wheat harvest, during the maize growing season, the average NH3 volatilization rate ranged from 1.2 to 1.3 kg N ha–1 d–1 with no significant differences among the five treatments. Cumulative NH3 volatilization losses from the CK, NK, NP, NPK and NPKM treatments during the wheat and maize growing seasons were 1.0, 2.6, 1.1, 6.0 and 18.2 kg N ha–1

Ammonia flux (kg N ha–1 d–1)

A

CK NP NK NPK NPKM

12 10 8 6 4 2 0

0

2

B 12 Ammonia flux (kg N ha–1 d–1)

increasing NO3– concentrations with increasing soil depth, although an exceptionally high NO 3–-N concentration (70.7 mg kg–1) was observed in the NK treatment at 20− 40 cm soil depth. Soil NO3– contents in NPKM treatment were the lowest among all fertilizer treatments and unlike other treatments, the NO3– content below 60 cm depth was significantly lower than those above (P<0.05) indicating less downward N movement.

4 6 8 10 12 Days after fertilization (d)

14

16

CK NP NK NPK NPKM

10 8 6 4 2 0

0

2

4 6 8 10 12 Days after fertilization (d)

14

16

Fig. 3 Ammonia volatilization rate from different treatments following fertilizer application in wheat growing seasons (A) and maize growing seasons (B). CK, control; NP, inorganic nitrogen with phosphorus; NK, inorganic nitrogen with potassium; NPK, inorganic nitrogen, phosphorus and potassium combination; NPKM, NPK with manure. Error bars (SE) denote the lower and upper values at the 95% confidence level generated by a PROC MIXED Program.

and 0.7, 1.0, 6.9, 7.4 and 44.7 kg N ha–1, respectively. The losses during maize growing season were generally higher than those during wheat growing season for all fertilizer treatments. The application of manure with mineral fertilizer resulted in the highest or significantly higher NH3 volatilization than those from all other mineral fertilizer treatments

(Table 1). There were no significant differences in NH3

volatilization loss between the mineral fertilizer treatments (NK, NP and NPK) and the non-fertilized control.

3.3. The fate of N fertilizer applied and estimates of N loss Using the mass balance approach, the fate of N fertilizer applied including plant uptake, soil total N storage change and N loss to the environment after 19 years of different fertilizer treatments is shown in Table 2. With the same amount of total N input, there were significant differences in plant uptake, due to the effects of fertilizer treatments on yield (Duan et al. 2011). Soil total N storage change (19.0–41.0%)

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was also significantly different among the treatments. The total N loss from mineral fertilizer treatments (NK, NP, and NPK) was about twice as that from NPKM, which had the lowest total N loss. The total N loss to the environment was broken down to gaseous loss and leaching loss (Table 2). Among the gaseous losses, the NH3 volatilization measured was the highest in mass, accounting for 65−90% of total volatilization loss. Ammonia loss was generally higher than the N2O emissions with the highest loss from NPKM. The N2O emissions were also the highest from NPKM. Both NH3 volatilization and N2O emissions from NPKM were significantly higher than other treatments (P<0.05). Based on Stehfest and Bouwman (2006) and Schlesinger (2009), the other minor gas emissions (N2, NO, and NOx) were estimated to be about 0.3−1.3% of total N input. Total gaseous N loss accounted for 1.7, 3.3, 5.1, and 21.9% for NK, NP, NPK, and NPKM, respectively. The potential N leaching loss was estimated to be the lowest (2.1%) from NPKM compared to 35% or higher loss from all other mineral fertilizer treatments. These data show the relative differences among the treatments. However, the absolute percentage loss might be underestimated due to potential overestimation of NH3 from the method used. Table 1 Treatment effect comparisons (P-value) for total NH3 volatilization loss during winter wheat and maize growing season Treatment1) CK CK 1.0000 NK NP NPK NPKM

NK 0.7706 1.0000

NP 0.9716 0.5002 1.0000

NPK 0.1518 0.4350 0.0928 1.0000

1)

NPKM 0.0001 0.0002 0.0001 0.0003 1.0000

CK, control; NK, inorganic nitrogen with potassium; NP, inorganic nitrogen with phosphorus; NPK, inorganic nitrogen, phosphorus and potassium combination; NPKM, NPK with manure. P<0.05 means significant difference between treatments.

4. Discussion 4.1. High NO3– mobility and its accumulation in soil from mineral fertilizer applications Nitrate concentration data, determined after 18 years of fertilization, showed that the NPKM had the lowest soil NO3–-N content in the 0−100 cm soil profile among all treatments and significantly lower nitrate concentrations (P<0.05) in the depths below 60 cm than those above (Fig. 2). There were significant positive correlations among the population sizes of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaeon (AOA), soil pH, and potential nitrification rates in NO3– production (He et al. 2007). The presumably more abundant AOB and AOA in NPKM did not appear to result in higher NO3– than the mineral fertilizer treatments. The NO3– distribution in soil profile appeared to be more affected by the N source and plant uptake. Fig. 2 suggests that the combined applications of mineral fertilizers with manure, decreased soil NO3–-N accumulation, which was partially due to higher N uptake and higher accumulation of organic N, especially in the surface soil. Higher yield from the NPKM treatment was attributed to the soil improvement on pH (Duan et al. 2011). The mineral fertilizer treatments, especially NK and NP treatments, showed strong acidification due to nitrification, with pH between 4.2–4.4, compared to 6.3 from the NPKM. This severely retarded plant growth, reduced crop yields and N uptake, increased NO3–-N accumulation in the rooting zone and enhanced mobility to deeper soil layer, which was shown by the higher concentration in lower depths in the profile. The NPKM had the highest total N in surface soil, but most of them were in organic form (Duan et al. 2016), which indicated increased soil N storage or improved soil fertility. The repeated use of mineral fertilizer resulted in much higher NO3–-N in the soil rooting zone. In the red soil subtropical region, seasonal and annual nitrate leaching was found to differ with fertilizer

Table 2 Fertilizer nitrogen (N) balance from 19 years of different fertilization treatments N input (kg N ha–1 yr–1)2) Treatment1) CK NK NP NPK NPKM 1)

N output (%)3)

Fertilizer N

Environment N

Plant uptake

0 300 300 300 300

20 20 20 20 20

100 18 26 33 49

Soil total N storage change (0–1 m) 0 41 36 19 27

NH3 volatilization 0 1.1 b 2.5 b 4.2 b 19.7 a

N2O emissions 0 0.2 c 0.3 b 0.4 a 0.9 a

Other gas emissions

Potential N leaching

0 0.3 0.4 0.6 1.3

0 39.3 34.7 42.9 2.1

CK, control; NK, inorganic nitrogen with potassium; NP, inorganic nitrogen with phosphorus; NPK, inorganic nitrogen, phosphorus and potassium combination; NPKM, NPK with manure. N input from the environment was estimated by the plant N uptake in the control (CK) without N fertilizer application. 3) Plant uptake data were from Duan et al. (2016); the N2O emission data were from Zhai et al. (2011); other gas emissions (N2, NO, NOx) were estimated as 1.4 times N2O emission based on Stehfest and Bouwman (2006) and Schlesinger (2009). Values followed by the same letter in the same column do not differ significantly according to the Tukey’s test at the 0.05 level. 2)

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application and precipitation. The monthly precipitation and nitrate leaching losses were significantly correlated (Long et al. 2012). Other studies have also supported that the combined application of fertilizers with manure increase grain yield and reduce soil NO3–-N accumulation in soil, compared with the application of chemical fertilizers alone (Dauden and Quilez 2004; Yang et al. 2004).

4.2. High volatilization loss from manure application Gaseous N loss was dominated by NH 3 volatilization, accounting for 65.3−89.7% of the total gas loss. The highest NH3 volatilization (near 20% of total N input) was determined from the NPKM (Fig. 3 and Table 2) among all fertilizer treatments. During each wheat or maize growing season, the NH3 volatilization rate peaked on the first or second day (Fig. 3) and then decreased gradually with no differences among the various treatments after about two weeks. Similar observations were reported by Zhang et al. (2011). The NH3 volatilization from a maize field increased quickly after fertilizations, with peak emissions during the first 1−4 d and most losses were measured in less than 7 d. In our study, the NH3 volatilization rates after the peak declined at a slower rate during the maize growing season, compared with the NH3 volatilization rates during the wheat growing season. This might be due to the fact that 70% of annual N was applied during the maize growing season and the average temperature and accumulated rainfall during the measurement period in maize (14.4°C and 142.4 mm) were higher than the measurement period in wheat (8.2°C and 22.2 mm). The highest NH3 loss from manure was most likely due to its higher NH4+ content, which is common in most fresh manure with high pH (~8.8) that favors NH3 formation and subsequent loss to the atmosphere. The NH3 volatilization from mineral fertilizer treatments (NK, NP, and NPK) was much lower and accounted for only 1.3−3.5% of total N input, which could be explained largely by lower soil pH (4.2−4.5). This result is similar to the measurement in an earlier study in upland red soil under smooth crabgrass-winter radish rotation, where NH3 volatilization accounted for 2.1−6.9% of the total N applied (160−480 kg N ha–1 yr–1) (Zhou et al. 2007). Soil pH has significant effect on the abiotic NH3 volatilization (Dewes 1996). Even within the narrow pH range (4.2−6.3) among the different treatments in this study, there is a significant positive correlation between NH3 volatilization loss and soil pH (y=21.0x–86.8, where, y is NH3 volatilization cumulative loss (kg N ha–1) and x is soil pH, R2=0.55, P<0.05). Application method may be another reason for high NH3 volatilization from NPKM. In this study, to simulate farmers’ practices, the mineral fertilizers with manure were mixed and buried at about 10 cm depth from the surface. Huijsmans

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et al. (2003) has shown that deep placement could reduce NH3 volatilization by 20% compared to surface incorporation and by 75% compared to surface spreading in 3 soil types (sand, sandy loam and clay). Thus, deeper placement of manure should be promoted to reduce NH3 volatilization losses in the red soil. Cumulative N2O emissions were generally lower than NH3 (Table 2). Although its loss is insignificant in mass (0.2−0.9% of total N input), minimizing N2O emissions is necessary because of its much stronger global warming potential than other greenhouse gases. The NPKM treatment also resulted in twice or higher N2O emissions than other treatments, which is consistent with Meng et al. (2005). In a sandy loam soil under wheat-maize rotation, the relative N2O emissions were 0.24, 0.21, 0.15 and 0.15% of applied N in M, NPK, NP and NK treatments, respectively (Meng et al. 2005). The high N2O loss from manure could be due to enhanced microbial activity as N2O is produced from both nitrification and denitrification processes. On the other hand, N2O may originate from the hot spots in soil induced by the organic C and N of the manure when denitrification and nitrification processes are simulated (Dambreville et al. 2008).

4.3. Estimated N leaching loss - high from mineral fertilization and low from manure incorporation Significant differences were found in both total N loss to the environment and the estimated N leaching loss based on the field data collected from the 19 years of mineral fertilizer treatments and manure incorporation (Table 2). The total N loss (including both gaseous loss and leaching) to the environment were 24.0−48.0% of total N input for various fertilizer treatments. These values were obtained after taking soil total N storage changes into the mass balance equation, thus more accurately reflecting the N loss and impact on the environment (Duan et al. 2016). The total loss was determined by both soil total N storage changes and plant uptake. The manure incorporation accumulated up to 27.0% soil N and so did the NK and NP treatments (36.0−41.0%), but by totally different means. Manure incorporation leads to organic build-up with nutrient storage in surface soil (Duan et al. 2016) and the N accumulation in NK and NP treatment was most likely due to low NUE because of lower yield. The plant N uptake from NPKM (49%) was 1.5−2.7 times higher (higher yield) than other mineral fertilizer treatments (NK, NP, and NPK) (Table 2). The NPK application had higher yield or N uptake, but similar total N loss as compared to the NK and NP treatments because of lower accumulation in the rooting zone. The data indicated that multiple factors were affecting plant uptake that resulted in the different N losses. The accumulation of N in red soil

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was significantly high, indicating lower rate or split applications of N fertilizer during the growing seasons is better than all of the fertilizer N as a basal dressing once. Organic N in manure can be incorporated into the soil organic matter pool, and can be converted to NH4+ through mineralization or absorbed by soil microorganisms for their growth. The manure N is likely more efficient during crop growing season because it only becomes available to plants through slow mineralization to NH4+ and NO3– . Although there is a competition by microbes, a combination of manure with mineral fertilizer would prevent this potential problem. In China, soil organic matter has been found to correlate with cereal crop productivity and yield stability across several provinces (Pan et al. 2009) and combined application of organic fertilizer can promote immobilization of N fertilizer to reduce N loss (Liu et al. 2009). Our study was able to quantify the total N loss from the wheat-maize production system into gaseous volatilization (mainly as NH3) based on field measurements and leaching loss. However, it should be noted that the NH3 volatilization losses measured from the field may only provide relative differences among the treatments, i.e., do not necessarily represent absolute losses because of the method utilized. We used the continuous airflow chamber method to measure the NH3 volatilization, but the air flow rate in the chamber might be exceeding the ambient flow rate that likely caused overestimation of NH3 volatilization. These may lead to overestimation in leaching loss. Nonetheless, the results showed a much lower leaching loss from the NPKM treatment (2.1%) than those from the mineral fertilization (34.7−42.9% of total N input) (Table 2). The NO3–-N data (Fig. 2) in soil profile supports this assessment by showing a much more downward movement of NO3– in mineral N fertilizer treatments and much lower concentration and mobility in NPKM treatment. The long-term experiment in this study has clearly shown a positive impact of manure incorporation on crop yield (plant uptake) and soil total N storage as well as reduction in leaching loss. The significantly lower leaching loss from NPKM treatment than all other mineral fertilizer treatments is a proof of the multiple benefits of manure on soil productivity and plant growth. Many studies have reported that manure resulted in good establishment of crop with efficient nutrient uptake and improvement on soil physical properties (Stenberg et al. 2012). Our study has demonstrated that the integrated benefit of manure incorporation resulted in significantly reduced N leaching loss.

5. Conclusion The aim of agricultural N management is to provide sufficient N to plants to maximize crop growth and yield, as well as

minimize environmental impact. Our results have shown that long-term use of mineral N fertilizer (300 kg N ha–1 y–1) will not only lead to reduced crop yield or low plant N uptake, but also high leaching risk (estimated about 34.7−42.9% of total N inputs). When combining mineral N fertilizers with manure to provide 70% total N (NPKM), crop yield and soil total N storage were significantly improved and had the lowest leaching loss (2.1%). Although the NH3 volatilization losses (19.7%) and N2O emissions (0.9%) were substantially higher from the manure incorporation than mineral fertilizer treatments (NP, NK, and NPK), improving the application method such as deeper placement in soil, could reduce the gaseous loss. Overall, this research has demonstrated that incorporation of manure with mineral fertilizer (NPKM) is one of the most effective fertilization management strategies in upland red soil for sustainable crop production.

Acknowledgements We appreciate Mr. Bruce Mackey (USDA Agricultural Research Service, USA) for his advice and help in data analyses. We also appreciate Mr. Tom Pflaum from USDA Agricultural Research Service, San Joaquin Valley Agricultural Sciences Center for his editorial help. This work was supported by the National Key Research and Development Program of China (2016YFD0200301), the open fund of Key Laboratory of Non-point Source Pollution Control, Ministry of Agriculture, China (20130104) and the Key Technologies R&D Program of China during the 12th Five-year Plan period (2012BAD14B04).

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