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Dissolved Organic Nitrogen Leaching from Rice-Wheat Rotated Agroecosystem in Southern China
Pedosphere 25(1): 93–102, 2015 ISSN 1002-0160/CN 32-1315/P c 2015 Soil Science Society of China ° Published by Elsevier B.V. and Science Press
Dissolved Organic Nitrogen Leaching from Rice-Wheat Rotated Agroecosystem in Southern China SONG Ge1,2,3 , ZHAO Xu1,2,∗ , WANG Shen-Qiang1,2 , XING Guang-Xi1 and ZHU Zhao-Liang1 1 State
Key Laboratory of Soil and Sustainable Agriculture, Institute e of Soil Science, Chinese Academy of Sciences, Nanjing 210008 (China) 2 Changshu National Agro-Ecosystem Observation and Research Station, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008 (China) 3 University of Chinese Academy of Sciences, Beijing 100049 (China) (Received May 27, 2014; revised November 13, 2014)
ABSTRACT The rice-wheat rotation in southern China is characterized by frequent flooding-draining water regime and heavy nitrogen (N) fertilization. There is a substantial lack of studies into the behavior of dissolved organic nitrogen (DON) in the intensively managed agroecosystem. A 3-year in situ field experiment was conducted to determine DON leaching and its seasonal and yearly variations as affected by fertilization, irrigation and precipitation over 6 consecutive rice/wheat seasons. Under the conventional N practice (300 kg N ha−1 for rice and 200 kg N ha−1 for wheat), the seasonal average DON concentrations in leachate (100 cm soil depth) for the three rice and wheat seasons were 0.6–1.1 and 0.1–2.3 mg N L−1 , respectively. The cumulative DON leaching was estimated to be 1.1–2.3 kg N ha−1 for the rice seasons and 0.01–1.3 kg N ha−1 for the wheat seasons, with an annual total of 1.1–3.6 kg N ha−1 . In the rice seasons, N fertilizer had little effect (P > 0.05) on DON leaching; precipitation and irrigation imported 3.6–9.1 kg N ha−1 of DON, which may thus conceal the fertilization effect on DON. In the wheat seasons, N fertilization had a positive effect (P < 0.01) on DON. Nevertheless, this promotive effect was strongly influenced by variable precipitation, which also carried 1.8–2.9 kg N ha−1 of DON into fields. Despite a very small proportion to chemical N applied and large variations driven by water regime, DON leaching is necessarily involved in the integrated field N budget in the rice-wheat rotation due to its relatively greater amount compared to other natural ecosystems. Key Words:
irrigation, N fertilization, paddy soil, precipitation, seasonal variation
Citation: Song, G., Zhao, X., Wang, S. Q., Xing, G. X. and Zhu, Z. L. 2015. Dissolved organic nitrogen leaching from rice-wheat rotated agroecosystem in southern China. Pedosphere. 25(1): 93–102.
Dissolved organic nitrogen (DON) is increasingly being recognized as an indispensable cog of N cycling in terrestrial ecosystem (Siemens and Kaupenjohann, 2002; Cookson and Murphy, 2004). Neff et al. (2003) suggested that DON leaching was a significant leak of N due to some recalcitrant forms of DON flushed from ecosystems during rapid rates of leaching, even in times of high N demand. Moreover, DON leached from terrestrial ecosystem detrimentally affects water quality due to its odor and color and the potential in carrying with toxic metals and pesticides (Zsolnay, 2003). Multiple studies in the forest ecosystems have addressed that DON was a dominant source of N leached into steams or groundwater (Stevens and Wannop, 1987; Perakis and Hedin, 2002; Cairns et al., 2009). Campbell et al. (2000) reported that the concentrations of DON even exceeded the concentrations of mineral N and DON accounted for up to 90% of total dissolved N (TDN) in forest streamwater. Unlike in undisturbed ∗ Corresponding
author. E-mail: [email protected].
natural forest ecosystems where the TDN leached was dominated by DON and DON flux was commonly less fluctuative (Cairns et al., 2009), leaching of DON as − well as NH+ 4 or NO3 in agroecosystem varied widely upon management practice (i.e., tillage, rotation, fertilization, and irrigation) (Chantigny, 2003). For example, input of mineral N can promote not only leaching of inorganic N (IN) but also the release and leaching of DON (Lu et al., 2011); moreover, application of manure has been shown to increase DON leaching compared to fertilization with mineral N (Embacher et al., 2008). A number of studies have shown that DON was a considerable N carrier in leachate for agroecosystem (Siemens and Kaupenjohann, 2002; Neff et al., 2003; Cookson and Murphy, 2004). However, these studies have previously tended to focus on upland soils rather than paddy soil (Siemens and Kaupenjohann, 2002; Christou et al., 2005; Cookson et al., 2008). The rice-wheat rotation is the primary cropping sy-
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stem adopted in paddy soil in the Taihu Lake region (Dawe et al., 2004). This rotated paddy field typically exhibits heavy chemical N fertilization (500–600 kg N ha−1 ) and alternate flooding-drying cycling. The paddy soil is submerged for the most period of rice growth except for mid-season aerations, but typically dry and aerated during the wheat season (Zhao et al., 2012). This unique water regime in fact generates a varying pattern of N leaching during two crop growing seasons (Tian et al., 2007). In the rice season, leaching of NO− 3 is generally low because flooding conditions create an anaerobic soil environment for denitrification, but in contract, NH+ 4 can flow downward with infiltrated water after quick hydrolysis of applied NH+ 4 -based fertilizer with oxygen-limited nitrification (Qiao et al., 2013). In the wheat season, leaching of NO− 3 becomes significant and increases with increasing N application rates due to strong nitrification under aerobic soil conditions (Zhang et al., 2013). Besides + NO− 3 and NH4 , leaching of DON was always detected in the rice-wheat rotated paddy soil from most previous studies but unfortunately has not been paid enough attention (Zhu et al., 2000; Tian et al., 2007; Tan et al., 2013). Information on DON leaching as affected by the distinct water regime adopted in the rice and wheat seasons is sparse. Currently, there are still considerable gaps in our understanding about the behavior of DON in the rice ecosystem. To quantify the functional role of DON in the rice-wheat rotated paddy soil, it is needed to know first how much DON would contribute to N leaching. What are the differences in DON leaching on different farming practices (e.g., fertilization and irrigation) and its variation patterns and influence factors? To address these questions, we established in situ field observations of DON leaching over the course of three consecutive rice-wheat rotations. We hypothesized that high N fertilization would increase DON leaching in the rice-wheat agroecosystem like as in upland agroecosystem (Siemens and Kaupenjohann, 2002; M¨oller et al., 2005; Mattsson et al., 2009) and intensive flooding-draining water regime would cause great variations of DON leaching between the rice and wheat seasons. MATERIALS AND METHODS Study area A 3-year field experiment was conducted at the Yixing Base for Agri-Environment Research, Changshu National Agro-Ecosystem Observation and Research Station, Chinese Academy of Sciences. The base
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is located in the northwest part of the Taihu Lake basin of the Yangtze Delta (31◦ 160 N, 119◦ 540 E). The region has a subtropical monsoon climate with an average temperature of 15.7 ◦ C and an annual rainfall of 1 177 mm. For the most part, this region has adopted the rice-wheat rotation with approximately 240 to 300 kg N ha−1 of chemical N fertilizer applied for rice and 200 to 250 kg N ha−1 for wheat. The soil is a Gleyic Hydragric Anthrosol (Eutric, Siltic) (IUSS Working Group WRB, 2006), which originated from the lacustrine deposit. The soil is silty and consists of 8.3% (v/v) sand, 81.5% (v/v) silt and 10.2% (v/v) clay. The soil contains 15.4 g kg−1 of organic carbon and 1.79 g kg−1 of total nitrogen. The cation exchange capacity of the soil is 11.8 cmol kg−1 and the pH (KCl) of the topsoil (0–20 cm) is 4.8. Experiment plots The experiment utilized three adjoining paddy fields each with area of 0.07 ha, belonging to local farmers. These paddy fields have been planted for many years with rice (from June to October)-wheat (from November to May of next year) rotations. The experiment was conducted over three consecutive years, including three rice and three wheat growing seasons from June 2007 to May 2010. In each rotation during the experimental period, the three fields were treated with urea at the rates of 0 kg N ha−1 for both crops as a control, 100 kg N ha−1 for both crops, and 300 kg N ha−1 for rice and 200 kg N ha−1 for wheat (representing conventional N application rate in this region), respectively. For N application, 30% was basally applied, 40% was top-dressed at tillering stage, and the remaining 30% was top-dressed at the ear differentiation stage for each crop. Phosphate and potassium fertilizers were applied basally in the form of calcium superphosphate at a rate of 60 kg P2 O5 ha−1 , and in the form of potassium chloride at a rate of 45 kg K2 O ha−1 . A local crop grower was employed to control normal cultivation and field management practices during the experimental period. Direct sowing and surface fertilizer application were consistently adopted in the three fields. During the rice seasons, the flooded water was mostly maintained at a depth of 3–5 cm in the field except when drainage was caused after steeping field, mid-season aerations, and the final drainage before harvest. During the wheat seasons, there was no irrigation, and drainage ditches were opened to prevent water logging injuries to the wheat plants caused by excessive rainwater during the winter under the subtropical monsoon climate.
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Sampling and chemical analysis Porous pipes made of polyvinyl chloride were installed at 80–100 cm soil depth and evenly distributed in each field in triplicate to collect percolation water. Leachate water was collected with a hand-operated vacuum pump at approximately 10-d intervals during the rice season, and more frequently immediately after fertilization. Likewise, leachate was collected after every rainfall during the wheat season. Water preexisting in the porous pipe was drained and discarded before sampling to avoid contamination. Approximately 250 mL of leachate was sampled for N concentration analysis. So far, there is a lack of appropriate methods for directly quantifying water via leaching in rice-wheat rotated paddy soil. Water balance method is usually conducted for large-scale estimates using multifarious parameters from soil hydroconductibility coefficients, soil evaporation, crop transpiration, irrigation, precipitation, etc. (Allen et al., 1998; Tian et al., 2007), which may not reflect the true conditions of the microenvironment. Here we used rapid-response percolation meters and field undisturbed tension-free monolith lysimeters described by Zhao et al. (2012) to directly measure the soil water leaching amount during the rice and wheat growing seasons, respectively. Briefly, the percolation meters were used to measure the rate of surface water vertical percolation in flooded rice seasons. Considering an approximately constant rate of surface water moving through the paddy soil (mm d−1 ) when the field is flooded, the total amount of leaching water in the rice seasons was, therefore, calculated as the rate of surface water vertical percolation (mm d−1 ) × flooded period (d) × 10. In the current study region, the rate of water vertical percolation was averaged at 2 mm d−1 . Unlike in the rice seasons, water leaching varies greatly upon rainfall amount, intensity and frequency in the wheat seasons. The direct observation of the amount of leaching water is even more difficult in situ. The lysimeters equipped with auto water-supply system were cultivated as planting rice or wheat in the plots under the same water and fertilizer management practices. As a result, the amount of leaching water from the lysimeters was expected to be similar to that in the field during the wheat seasons. Irrigation water during the rice seasons was pumped from a river near the field. Electromagnetic flow meters (LDBP-150L-M2X2-200, Yixing Xiangming Instrument Co., China), with a detection range of 1 to 200 m3 h−1 and detection error < 1%, were installed at the irrigation inlet to precisely measure the water volume of individual irrigation events. When irrigation
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events occurred, a 250-mL water sample was collected for analysis of various N concentrations. A precipitation collector (APS-III, Wuhan Tianhong Inc., China) was installed near the experimental field to collect rainwater. An impulse from the precipitation sensor activates an electronic control system, which causes the lid open to collect rain water once rain events occur. The lid swaps back to cover the container when there is no precipitation signal anymore. The rainwater volume was first recorded after each rain event during the experimental period, and then 250 mL rainwater was sampled for analysis. In order to prevent conversion of N, all the leachate, irrigation and precipitation water samples were deepfrozen at −20 ◦ C in a freezer immediately for next analysis (Fellman et al., 2008). The concentrations of − NH+ 4 , NO3 and TDN in these water samples were analyzed with a continuous flow analyzer (Skalar, Netherlands) after the water samples are filtered through a 0.45-µm membrane. The concentration of IN was cal− culated as the sum of NH+ 4 and NO3 . The concentration of DON was calculated by subtracting IN from TDN. Cumulative DON leaching (kg N ha−1 ) during each crop season was obtained by multiplying the timeinterval weighted DON concentration in leachate from 100 cm soil depth (mg N L−1 ) by total amount of leaching water (m3 ha−1 ) and 0.001, where the timeinterval weighted DON concentration is defined as the sum of [individual DON concentration in leachate (mg N L−1 ) × interval between two adjacent samplings (d)]/total growth time (d). Cumulative DON input by precipitation during each crop season was calculated as the sum of amount of individual precipitation (mm) × DON concentration in the corresponding rainwater samples (mg N L−1 ) × 0.01. Cumulative DON input by irrigation during each rice season was calculated as the sum of amount of individual irrigation (m3 ha−1 ) × DON concentration in the corresponding water samples (mg N L−1 ) × 0.001. Statistical analysis The data were statistically analyzed by SPSS. Bivariate correlation analysis was used to examine the correlation between DON input from precipitation and irrigation, the amount of precipitation and irrigation, and the amount of DON leaching. One-way analysis of variance (ANOVA) was employed to analyze the influence of N application rate on DON leaching. Two-way ANOVA was used to investigate the effects of N application rate and DON input by precipitation and irrigation on the amount of DON leaching and the ratio of
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DON/TDN. Stepwise linear regression was used to explain the variations of DON leaching and the ratio of DON/TDN resulting from N application, the DON input by precipitation and irrigation or soil leachate amount. For all statistical analyses, a significance level of P < 0.05 was used. RESULTS DON leaching in rice and wheat seasons DON leaching showed yearly and inter-seasonal variations during three rice and wheat crop growing
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seasons (Fig. 1). In the rice seasons, the DON concentrations in the leaching water had the consistent trend of higher values during the time frame from June to July than during that from August to October. The peak values in the 2008 and 2009 rice seasons were greater than the value in the 2007 season. For the wheat seasons, the DON concentrations were very low during the first 2007–2008 season, but soared during the following 2008–2009 and 2009–2010 seasons. Because of less rainfall in the 2007–2008 season, percolation water was not collected until 61 d of wheat sowing, and there were only 5 samples collected during the
Fig. 1 Temporal variations of dissolved organic nitrogen (DON) concentrations in soil leachates and DON input by irrigation and precipitation during the three rice and wheat seasons under the conventional N application rates of 300 kg N ha−1 for rice and 200 kg N ha−1 for wheat from 2007 to 2010. Vertical bars indicate the standard deviations of the means (n = 3). Vertical arrows show the N fertilization times of basal fertilization (BF), the first top-dressing (TD1) and the second top-dressing (TD2).
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whole season. The DON concentrations in the 2008– 2009 and 2009–2010 seasons rose consistently during the period from February to March, which differed from the 2007–2008 season. Under the conventional N application rate, the seasonal time-interval-weighted average of DON concentration ranged from 0.55 to 1.05 mg N L−1 during the three rice seasons and from 0.08 to 2.29 mg N L−1 during the three wheat seasons (Table I). The fluctuation of DON concentration was much smaller in the rice seasons than in the wheat seasons. The amounts of leaching water were 1 920–2 180 m3 ha−1 during the rice seasons and 138–1 180 m3 ha−1 during the wheat seasons. The more regular and higher level of water leaching during the rice seasons caused more DON leaching compared to the wheat seasons. Using measured DON concentrations and amounts of leaching water, DON leaching from a 100-cm soil depth in the rice-
wheat rotated paddy soil was calculated. The data were 1.06–2.29 kg N ha−1 (corresponding to 18.5%–47.6% of TDN) for the three rice seasons and 0.01–1.29 kg N ha−1 (corresponding to 2.77%–14.6% of TDN) for the three wheat seasons. The percentages of DON to TDN were lower for the wheat seasons than those for the rice seasons. DON input by precipitation and irrigation The rain water introduced 1.42–5.97 kg N ha−1 of DON by 533–1 056 mm precipitation during the three rice seasons and 1.76–2.87 kg N ha−1 of DON by 434–563 mm precipitation during the three wheat seasons. The irrigation water (5 079–5 726 m3 ha−1 ) carried 2.22–3.78 kg N ha−1 of DON to the soil during the three rice seasons (Table II). The total DON input was positively correlated to the precipitation and irrigation rates (R2 = 0.8060, P = 0.015). Although the
TABLE I Seasonal time-interval weighted average of dissolved organic nitrogen (DON) concentration in soil leachates and the amounts of DON leaching during the rice and wheat seasons under the conventional N application rates of 300 kg N ha−1 for rice and 200 kg N ha−1 for wheat from 2007 to 2010 Crop
Season
DON concentration
Growth period
Leaching watera)
DON leaching
DON/TDNb)
Rice
2007 2008 2009 Average 2007–2008 2008–2009 2009–2010 Average
mg N L−1 0.55±0.30c) 0.59±0.54 1.05±0.16 0.73±0.28 0.08±0.05 2.29±0.59 1.09±0.09 1.15±1.10
d 144 129 141
m3 ha−1 1 920 1 940 2 180 2 013±145 138±4.46 230±17.9 1 180±163 516±577
kg N ha−1 1.06±0.57 1.15±1.04 2.29±0.36 1.50±0.68 0.01±0.007 0.53±0.14 1.29±0.10 0.61±0.64
% 33.5±7.8 18.5±7.3 47.6±4.3 33.2±14.6 2.8±2.4 14.6±4.9 12.4±1.7 9.9±6.3
Wheat
192 186 210
a) Amounts
of leaching water for the rice seasons were estimated based on the vertical leaching rate of 2 mm d−1 during the flooded period; these for the wheat seasons were obtained from the field undisturbed tension-free monolith lysimeter under the same conditions as the field experiment. b) TDN = total dissolved nitrogen. c) Means±standard deviations (n = 3). TABLE II Dissolved organic nitrogen (DON) input from precipitation and irrigation in the rice and wheat seasons from 2007 to 2010
Crop
Season
Rice
2007 2008 2009 Average
Wheat
2007–2008 2008–2009 2009–2010 Average
a) Means±standard b) No
Precipitation
Irrigation
Amount
DON concentration
DON input
Amount
DON concentration
DON input
mm 661 533 1 056 750±272a)
mg N L−1 0.21 0.31 0.57 0.36±0.18
kg N ha−1 1.42 1.66 5.97 3.02±2.56
m3 ha−1 5 168 5 726 5 079 5 324±351
mg N L−1 0.43 0.66 0.61 0.57±0.12
kg N 2.22 3.78 3.09 3.03±0.78
0.41 0.65 0.32 0.46±0.17
1.76 2.87 1.82 2.15±0.63
-b) -
-
-
434 442 563 480±73
deviations (n = 3). irrigation was practiced in the wheat seasons.
Total DON input ha−1 3.64 5.44 9.06 6.05±2.76 1.76 2.87 1.82 2.15±0.63
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average DON concentrations in soil leachate for both seasons (0.73–1.15 mg N L−1 ) (Table I) were higher than those in precipitation and irrigation water (0.36– 0.57 mg N L−1 ), DON leaching were lower overall than DON input from precipitation and irrigation (Tables I and II). A positive correlation was found between DON leaching and DON input from precipitation and irrigation (R2 = 0.6761, P = 0.045). Effects of N application and DON input on DON leaching Looking at only Fig. 1, it seems difficult to obtain the clear relationship between DON concentrations and N fertilization events due to the large yearly and inter-seasonal variations of DON concentration and the interfering effects from irregular precipitation and irrigation. We also failed to examine the effects of different N fertilization rates on the DON leaching (Fig. 2). Although the DON leaching appeared to increase with the N application rate during the wheat seasons and decreased with it during the rice season, no significant difference (P > 0.05) was found among DON leaching under the three N application rates. However, the two-way ANOVA analysis revealed that DON leaching was only significantly affected (P < 0.01) by DON input by precipitation and irrigation in the rice seasons, whereas it was significantly affected (P < 0.01) by both DON input by precipitation and N application in the wheat seasons (Table III). DON inputs by precipitation and irrigation and N application also had extremely significant effects (P < 0.01) on the ratios of DON/TDN during both rice and wheat seasons (Table III). The stepwise linear regression analysis showed that only DON input by precipitation and irrigation could explain 16.2% (P = 0.038) of the variation of DON leaching in the rice seasons, but N application, DON input by precipitation and soil leachate amount could explain 62.2% of the variation of DON leaching in the wheat seasons (32.4% from DON input by pre-
Fig. 2 Effect of N fertilization rate (0 kg N ha−1 for both crops as a control, 100 kg N ha−1 for both crops, and 300 kg N ha−1 for rice and 200 kg N ha−1 for wheat) on dissolved organic nitrogen (DON) leaching in the rice and wheat seasons from 2007 to 2010. The bar is the seasonal average of DON leaching from the corresponding three rice or wheat seasons. Vertical bars indicate the standard deviations of the means (n = 3).
cipitation, P < 0.001; 20.1% from soil leachate amount, P < 0.001; 9.7% from N application rate, P = 0.023) (Table IV). For the ratio of DON/TDN, the contribution percentage increased to 47.4% (P < 0.001) from DON input by precipitation and irrigation in the rice seasons and to 77.2% from DON input by precipitation, soil leachate amount and N application in the wheat seasons (50.1% from DON input by precipitation, P < 0.001; 13.7% from soil leachate amount, P = 0.001; 13.4% from N application, P = 0.001) (Table IV). DISCUSSION Under the local conventional N application rates (300 kg N ha−1 for rice and 200 kg N ha−1 for wheat) and water management practices in the rice-wheat rotated agroecosystem, the DON concentration in leachate from 100 cm soil depth averaged 0.73±0.28 mg
TABLE III Results of analysis of variance examining the effects of N application and dissolved organic nitrogen (DON) input by precipitation and irrigation on DON leaching and the ratio of DON:total dissolved nitrogen (DON/TDN) during the rice and wheat seasons Crop
Rice
Wheat
Factor
N application rate DON input by precipitation and irrigation N application rate × DON input by precipitation and irrigation N application rate DON input by precipitation N application rate × DON input by precipitation
DON leaching
DON/TDN
F value
P value
F value
P value
2.467 6.550 3.798 14.934 77.299 22.962
0.113 0.007 0.021 < 0.001 < 0.001 < 0.001
10.066 34.734 8.380 15.224 67.741 7.142
0.001 < 0.001 0.001 < 0.001 < 0.001 0.001
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TABLE IV Results of stepwise linear regression analysis explaining the variations of dissolved organic nitrogen (DON) leaching and the ratio of DON:total dissolved nitrogen (DON/TDN) resulted from N application (Napp ), DON input (DONinput ) by precipitation and irrigation and soil leachate amount (Vleach ) Crop
Rice
Dependent variable (Y )
Entered variable
DON leaching
DONinput by precipitation and irrigation DONinput by precipitation and irrigation DONinput by precipitation Vleach Napp DONinput by precipitation Vleach Napp
DON/TDN Wheat
DON leaching
DON/TDN
Variable
Excluded variable Regression equation R2
Variable
P
0.038
0.162
< 0.001
0.474
< 0.001 < 0.001 0.023 < 0.001 0.001 0.001
0.324 0.201 0.097 0.501 0.137 0.134
Napp Vleach Napp Vleach -
0.090 0.094 0.263 0.736 -
P
N L−1 during the rice season and 1.15±1.10 mg N L−1 during the wheat season. The corresponding DON leaching was calculated to be 1.50±0.68 and 0.61±0.64 kg N ha−1 (Table I). On an annual basis, the DON concentration in soil leachate was 0.79±0.27 mg N L−1 and the DON leaching was 2.11±1.31 kg N ha−1 for the entire rice-wheat rotation. These values were higher than those in natural ecosystems, e.g., forest soil (1.3±0.5 kg N ha−1 , Campbell et al., 2000; 0.008–0.135 mg N L−1 , Perakis and Hedin, 2002; 0.34–0.40 kg N ha−1 , Hood et al., 2003; 0.5–0.8 kg N ha−1 , M¨oller et al., 2005; 0.35±0.07 kg N ha−1 , Cairns et al., 2009; 0.26– 0.89 mg N L−1 , Mattsson et al., 2009) and peat soil (0.18±0.13 mg N L−1 , Chapman et al., 2001; 0.005– 0.065 mg N L−1 , Bragazza and Limpens, 2004; 0.455 mg N L−1 in average, Cundill et al., 2007). However, the DON/TDN ratio of the rice-wheat rotated farmland was only 23.0%±6.3%, much lower than the ratios of the natural ecosystems (59%±20%, Campbell et al., 2000; 40%, Chapman et al., 2001; 61%–97%, Perakis and Hedin, 2002; 36.8%–94.1%, Streeter et al., 2003; 61%–82%, Bragazza and Limpens, 2004; 71%– 80%, M¨oller et al., 2005; 82%, Cundill et al., 2007; 64%, Cairns et al., 2009). The reason for this may be the high inorganic N distribution in this intensively fertilized paddy soil, which is largely different from the undisturbed natural ecosystems where organic matter dominantly regulates N cycling. Hagedorn et al. (2000, 2001) demonstrated that the DON concentrations in the subsoil were higher under reducing conditions than oxidizing conditions and were significantly related to the dissolved Fe concentration. The water flooding during the rice seasons usually results in the reduction and dissolution of Fe oxyhydroxides, on which DON is mainly immobilized through the ligand exchange process (Qualls, 2000). This might be
Y = 0.563 + 0.250DONinput Y = 9.222 + 4.110DONinput Y = −1.145 + 0.001Vleach + 0.521DONinput + 0.002Napp Y = −26.019 + 18.738DONinput + 0.009Vleach − 0.050Napp
the explanation for the moderately high DON/TDN ratio found during the rice seasons (33.2%±14.6%), compared to those found during the wheat seasons (9.93%±6.29%) and other studies conducted on upland agroecosystem (7.9%–14.9%, Murphy et al., 2000; 6%– 21%, Siemens and Kaupenjohann, 2002; 17.6%, M¨oller et al., 2005). Nevertheless, the annual DON leaching from the rice-wheat rotated paddy soil was comparable with those from aerobic uplands (1.1–2.5 kg N ha−1 , Murphy et al., 2000; 0.4–2.3 mg N L−1 , Siemens and Kaupenjohann, 2002; 0.9 kg N ha−1 , M¨oller et al., 2005). In the current study, the chemical N application did not significantly affect (P > 0.05) DON leaching during the three rice seasons (Fig. 2). This was in contrast to our initial hypothesis and inconsistent with the general knowledge that the input of mineral N promotes the release and leaching of DON (Currie et al., 1996; McDowell et al., 1998; Neff et al., 2000; Shand et al., 2000; Kalbitz and Geyer, 2002). This phenomenon may be explained by the concealing effects of a large amount of DON input by precipitation and irrigation. The current DON input from precipitation and irrigation averaged 6.05 kg N ha−1 (Table II), which was 3 times larger than the average accumulated leaching DON (1.50 kg N ha−1 in average). In addition, the DON concentration peaks for leachate coincided frequently with the DON input from precipitation and irrigation (Fig. 1). Moreover, DON input by precipitation and irrigation had an extremely significant effect on the DON leaching amount (Table III). These results can support the notion that DON inputs from precipitation and irrigation are the most important factors for DON leaching in the rice seasons, which could largely conceal the influence of N fertilization. However, it should be admitted that application of organic mat-
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ter instead of chemical N fertilizer may still increase DON leaching from rice paddy (Wang et al., 2013). In a temperate rice system, Bird et al. (2002, 2003) found 30% of straw 15 N was recovered in the mobile humic acid and about 19% was in the microbial biomass after straw incorporation. Both the mobile humic acid and microbial biomass belong to the active and labile N pool in soil and substantially contribute to soil DON leaching. Unlike the rice season, the wheat season was completely rain-fed in the rice-wheat rotation (Zhao et al., 2012). Therefore, the sample time and amount of soil leachate were governed by precipitation events during the wheat seasons (Fig. 1). DON leaching in the three wheat seasons showed higher variability than those values in the three rice seasons due to the large variations in precipitation (Table I, Fig. 1). Many studies conducted on upland soils (Siemens and Kaupenjohann, 2002; M¨oller et al., 2005; Embacher et al., 2008; Mattsson et al., 2009) have shown that N fertilization in spite of organic or inorganic fertilizers increased DON concentration and/or leaching due to the stimulation to net N mineralization and microbial activity and the increments of crop plant biomass (Embacher et al., 2008; Lu et al., 2011). In the current study, not only chemical N application but also DON input by precipitation had significant effects on the DON leaching during the wheat seasons (Tables III and IV). N application, DON input by precipitation and soil leachate amount explained 62.2% of the variation of DON leaching. Moreover, precipitation seemed to influence DON leaching responses to N fertilization during the wheat seasons. Compared to the amount of DON leaching, the ratio of DON/TDN was a more feasible parameter to convey the effects of extrinsic factors on DON leaching in the rice-wheat rotated agroecosystem. The two-way ANOVA analysis showed that N application and DON input by precipitation and irrigation significantly affected the ratio of DON/TDN in the rice and wheat seasons, even though N application did not seem to impact the amount of DON leaching during the rice seasons (Table III). The stepwise linear regression analysis clearly indicated that DON input by precipitation and irrigation explained 47.4% variation (P < 0.001) of the ratio of DON/TDN, compared to 16.2% variation (P = 0.038) of DON leaching amount during the rice seasons (Table IV). Moreover, the variation of the ratio of DON/TDN in the wheat seasons was well explained by N application and DON input by precipitation together with soil leachate amount (77.2%, P < 0.001). It should be noted that some measurement errors in DON leaching possibly remained due to the limitations
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in techniques for direct determination of DON concentration in leachate and uniform metering methods for the leaching water amount. Nevertheless, the significant DON input from irrigation and precipitation accurately measured by electromagnetic flowmeters and rain collectors supports the notion that DON leaching was greatly influenced by irrigation and precipitation rather than by chemical N fertilization. The great variations in DON leaching also validate the necessity of involving DON in the integrated field N budget because the leached DON was higher in the rice-wheat rotation than in natural ecosystems. CONCLUSIONS The 3-year field observation showed 2.1±1.3 kg N ha of DON leached from the whole rice-wheat rotation and, correspondingly, the DON input by irrigation and precipitation was 8.3±3.4 kg N ha−1 . Although DON leaching was less than inorganic N leaching, it is still necessary to include DON in the overall N budget because of a greater amount of DON leached from the rice-wheat rotation compared to natural ecosystems. Anthropogenic N addition (N deposition, fertilization and irrigation) and water management (frequent flooding-draining water regime) have even more evident influences on DON leaching. For the wheat season, DON leaching was prompted by N fertilization and strongly influenced by precipitation. For the rice season, high precipitation and irrigation rates carried more DON to paddy soil, thus concealing the direct influence of N fertilization on DON leaching. The mechanisms of DON production, transformation and migration in paddy soil were ultimately complex, and different from those of other ecosystems. Further laboratory and field-scale studies are needed in this highly fertilized and intensively flooded/drained ricewheat rotated agroecosystem. −1
ACKNOWLEDGEMENTS This study was supported by the Jiangsu Provincial Natural Science Foundation of China (No. BK2010612), the Foundation of State Key Laboratory of Soil and Sustainable Agriculture, China (No. Y052010034), and the National Natural Science Foundation of China (No. 41001147). We especially thank the anonymous reviewers for their constructive comments that have greatly improved the manuscript. REFERENCES Allen, R. G., Pereira, L. S., Raes, D. and Smith, M. 1998. Crop Evapotranspiration—Guidelines for Computing Crop Water
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Paddy Water Environ. 11: 381–395. Tian, Y. H., Yin, B., Yang, L. Z., Yin, S. X. and Zhu, Z. L. 2007. Nitrogen runoff and leaching losses during rice-wheat rotations in Taihu Lake region, China. Pedosphere. 17: 445– 456. Wang, Y. F., Liu, X. M., Butterly, C., Tang, C. X. and Xu, J. M. 2013. pH change, carbon and nitrogen mineralization in paddy soils as affected by Chinese milk vetch addition and soil water regime. J. Soils Sediments. 13: 654–663. Zhang, J. H., Liu, J. L., Zhang, J. B., Cheng, Y. N. and Wang, W. P. 2013. Nitrate-nitrogen dynamics and nitrogen budgets
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Dissolved Organic Nitrogen Leaching from Rice-Wheat Rotated Agroecosystem in Southern China SONG Ge1,2,3 , ZHAO Xu1,2,∗ , WANG Shen-Qiang1,2 , XING Guang-Xi1 and ZHU Zhao-Liang1 1 State
Key Laboratory of Soil and Sustainable Agriculture, Institute e of Soil Science, Chinese Academy of Sciences, Nanjing 210008 (China) 2 Changshu National Agro-Ecosystem Observation and Research Station, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008 (China) 3 University of Chinese Academy of Sciences, Beijing 100049 (China) (Received May 27, 2014; revised November 13, 2014)
ABSTRACT The rice-wheat rotation in southern China is characterized by frequent flooding-draining water regime and heavy nitrogen (N) fertilization. There is a substantial lack of studies into the behavior of dissolved organic nitrogen (DON) in the intensively managed agroecosystem. A 3-year in situ field experiment was conducted to determine DON leaching and its seasonal and yearly variations as affected by fertilization, irrigation and precipitation over 6 consecutive rice/wheat seasons. Under the conventional N practice (300 kg N ha−1 for rice and 200 kg N ha−1 for wheat), the seasonal average DON concentrations in leachate (100 cm soil depth) for the three rice and wheat seasons were 0.6–1.1 and 0.1–2.3 mg N L−1 , respectively. The cumulative DON leaching was estimated to be 1.1–2.3 kg N ha−1 for the rice seasons and 0.01–1.3 kg N ha−1 for the wheat seasons, with an annual total of 1.1–3.6 kg N ha−1 . In the rice seasons, N fertilizer had little effect (P > 0.05) on DON leaching; precipitation and irrigation imported 3.6–9.1 kg N ha−1 of DON, which may thus conceal the fertilization effect on DON. In the wheat seasons, N fertilization had a positive effect (P < 0.01) on DON. Nevertheless, this promotive effect was strongly influenced by variable precipitation, which also carried 1.8–2.9 kg N ha−1 of DON into fields. Despite a very small proportion to chemical N applied and large variations driven by water regime, DON leaching is necessarily involved in the integrated field N budget in the rice-wheat rotation due to its relatively greater amount compared to other natural ecosystems. Key Words:
irrigation, N fertilization, paddy soil, precipitation, seasonal variation
Citation: Song, G., Zhao, X., Wang, S. Q., Xing, G. X. and Zhu, Z. L. 2015. Dissolved organic nitrogen leaching from rice-wheat rotated agroecosystem in southern China. Pedosphere. 25(1): 93–102.
Dissolved organic nitrogen (DON) is increasingly being recognized as an indispensable cog of N cycling in terrestrial ecosystem (Siemens and Kaupenjohann, 2002; Cookson and Murphy, 2004). Neff et al. (2003) suggested that DON leaching was a significant leak of N due to some recalcitrant forms of DON flushed from ecosystems during rapid rates of leaching, even in times of high N demand. Moreover, DON leached from terrestrial ecosystem detrimentally affects water quality due to its odor and color and the potential in carrying with toxic metals and pesticides (Zsolnay, 2003). Multiple studies in the forest ecosystems have addressed that DON was a dominant source of N leached into steams or groundwater (Stevens and Wannop, 1987; Perakis and Hedin, 2002; Cairns et al., 2009). Campbell et al. (2000) reported that the concentrations of DON even exceeded the concentrations of mineral N and DON accounted for up to 90% of total dissolved N (TDN) in forest streamwater. Unlike in undisturbed ∗ Corresponding
author. E-mail: [email protected].
natural forest ecosystems where the TDN leached was dominated by DON and DON flux was commonly less fluctuative (Cairns et al., 2009), leaching of DON as − well as NH+ 4 or NO3 in agroecosystem varied widely upon management practice (i.e., tillage, rotation, fertilization, and irrigation) (Chantigny, 2003). For example, input of mineral N can promote not only leaching of inorganic N (IN) but also the release and leaching of DON (Lu et al., 2011); moreover, application of manure has been shown to increase DON leaching compared to fertilization with mineral N (Embacher et al., 2008). A number of studies have shown that DON was a considerable N carrier in leachate for agroecosystem (Siemens and Kaupenjohann, 2002; Neff et al., 2003; Cookson and Murphy, 2004). However, these studies have previously tended to focus on upland soils rather than paddy soil (Siemens and Kaupenjohann, 2002; Christou et al., 2005; Cookson et al., 2008). The rice-wheat rotation is the primary cropping sy-
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stem adopted in paddy soil in the Taihu Lake region (Dawe et al., 2004). This rotated paddy field typically exhibits heavy chemical N fertilization (500–600 kg N ha−1 ) and alternate flooding-drying cycling. The paddy soil is submerged for the most period of rice growth except for mid-season aerations, but typically dry and aerated during the wheat season (Zhao et al., 2012). This unique water regime in fact generates a varying pattern of N leaching during two crop growing seasons (Tian et al., 2007). In the rice season, leaching of NO− 3 is generally low because flooding conditions create an anaerobic soil environment for denitrification, but in contract, NH+ 4 can flow downward with infiltrated water after quick hydrolysis of applied NH+ 4 -based fertilizer with oxygen-limited nitrification (Qiao et al., 2013). In the wheat season, leaching of NO− 3 becomes significant and increases with increasing N application rates due to strong nitrification under aerobic soil conditions (Zhang et al., 2013). Besides + NO− 3 and NH4 , leaching of DON was always detected in the rice-wheat rotated paddy soil from most previous studies but unfortunately has not been paid enough attention (Zhu et al., 2000; Tian et al., 2007; Tan et al., 2013). Information on DON leaching as affected by the distinct water regime adopted in the rice and wheat seasons is sparse. Currently, there are still considerable gaps in our understanding about the behavior of DON in the rice ecosystem. To quantify the functional role of DON in the rice-wheat rotated paddy soil, it is needed to know first how much DON would contribute to N leaching. What are the differences in DON leaching on different farming practices (e.g., fertilization and irrigation) and its variation patterns and influence factors? To address these questions, we established in situ field observations of DON leaching over the course of three consecutive rice-wheat rotations. We hypothesized that high N fertilization would increase DON leaching in the rice-wheat agroecosystem like as in upland agroecosystem (Siemens and Kaupenjohann, 2002; M¨oller et al., 2005; Mattsson et al., 2009) and intensive flooding-draining water regime would cause great variations of DON leaching between the rice and wheat seasons. MATERIALS AND METHODS Study area A 3-year field experiment was conducted at the Yixing Base for Agri-Environment Research, Changshu National Agro-Ecosystem Observation and Research Station, Chinese Academy of Sciences. The base
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is located in the northwest part of the Taihu Lake basin of the Yangtze Delta (31◦ 160 N, 119◦ 540 E). The region has a subtropical monsoon climate with an average temperature of 15.7 ◦ C and an annual rainfall of 1 177 mm. For the most part, this region has adopted the rice-wheat rotation with approximately 240 to 300 kg N ha−1 of chemical N fertilizer applied for rice and 200 to 250 kg N ha−1 for wheat. The soil is a Gleyic Hydragric Anthrosol (Eutric, Siltic) (IUSS Working Group WRB, 2006), which originated from the lacustrine deposit. The soil is silty and consists of 8.3% (v/v) sand, 81.5% (v/v) silt and 10.2% (v/v) clay. The soil contains 15.4 g kg−1 of organic carbon and 1.79 g kg−1 of total nitrogen. The cation exchange capacity of the soil is 11.8 cmol kg−1 and the pH (KCl) of the topsoil (0–20 cm) is 4.8. Experiment plots The experiment utilized three adjoining paddy fields each with area of 0.07 ha, belonging to local farmers. These paddy fields have been planted for many years with rice (from June to October)-wheat (from November to May of next year) rotations. The experiment was conducted over three consecutive years, including three rice and three wheat growing seasons from June 2007 to May 2010. In each rotation during the experimental period, the three fields were treated with urea at the rates of 0 kg N ha−1 for both crops as a control, 100 kg N ha−1 for both crops, and 300 kg N ha−1 for rice and 200 kg N ha−1 for wheat (representing conventional N application rate in this region), respectively. For N application, 30% was basally applied, 40% was top-dressed at tillering stage, and the remaining 30% was top-dressed at the ear differentiation stage for each crop. Phosphate and potassium fertilizers were applied basally in the form of calcium superphosphate at a rate of 60 kg P2 O5 ha−1 , and in the form of potassium chloride at a rate of 45 kg K2 O ha−1 . A local crop grower was employed to control normal cultivation and field management practices during the experimental period. Direct sowing and surface fertilizer application were consistently adopted in the three fields. During the rice seasons, the flooded water was mostly maintained at a depth of 3–5 cm in the field except when drainage was caused after steeping field, mid-season aerations, and the final drainage before harvest. During the wheat seasons, there was no irrigation, and drainage ditches were opened to prevent water logging injuries to the wheat plants caused by excessive rainwater during the winter under the subtropical monsoon climate.
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Sampling and chemical analysis Porous pipes made of polyvinyl chloride were installed at 80–100 cm soil depth and evenly distributed in each field in triplicate to collect percolation water. Leachate water was collected with a hand-operated vacuum pump at approximately 10-d intervals during the rice season, and more frequently immediately after fertilization. Likewise, leachate was collected after every rainfall during the wheat season. Water preexisting in the porous pipe was drained and discarded before sampling to avoid contamination. Approximately 250 mL of leachate was sampled for N concentration analysis. So far, there is a lack of appropriate methods for directly quantifying water via leaching in rice-wheat rotated paddy soil. Water balance method is usually conducted for large-scale estimates using multifarious parameters from soil hydroconductibility coefficients, soil evaporation, crop transpiration, irrigation, precipitation, etc. (Allen et al., 1998; Tian et al., 2007), which may not reflect the true conditions of the microenvironment. Here we used rapid-response percolation meters and field undisturbed tension-free monolith lysimeters described by Zhao et al. (2012) to directly measure the soil water leaching amount during the rice and wheat growing seasons, respectively. Briefly, the percolation meters were used to measure the rate of surface water vertical percolation in flooded rice seasons. Considering an approximately constant rate of surface water moving through the paddy soil (mm d−1 ) when the field is flooded, the total amount of leaching water in the rice seasons was, therefore, calculated as the rate of surface water vertical percolation (mm d−1 ) × flooded period (d) × 10. In the current study region, the rate of water vertical percolation was averaged at 2 mm d−1 . Unlike in the rice seasons, water leaching varies greatly upon rainfall amount, intensity and frequency in the wheat seasons. The direct observation of the amount of leaching water is even more difficult in situ. The lysimeters equipped with auto water-supply system were cultivated as planting rice or wheat in the plots under the same water and fertilizer management practices. As a result, the amount of leaching water from the lysimeters was expected to be similar to that in the field during the wheat seasons. Irrigation water during the rice seasons was pumped from a river near the field. Electromagnetic flow meters (LDBP-150L-M2X2-200, Yixing Xiangming Instrument Co., China), with a detection range of 1 to 200 m3 h−1 and detection error < 1%, were installed at the irrigation inlet to precisely measure the water volume of individual irrigation events. When irrigation
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events occurred, a 250-mL water sample was collected for analysis of various N concentrations. A precipitation collector (APS-III, Wuhan Tianhong Inc., China) was installed near the experimental field to collect rainwater. An impulse from the precipitation sensor activates an electronic control system, which causes the lid open to collect rain water once rain events occur. The lid swaps back to cover the container when there is no precipitation signal anymore. The rainwater volume was first recorded after each rain event during the experimental period, and then 250 mL rainwater was sampled for analysis. In order to prevent conversion of N, all the leachate, irrigation and precipitation water samples were deepfrozen at −20 ◦ C in a freezer immediately for next analysis (Fellman et al., 2008). The concentrations of − NH+ 4 , NO3 and TDN in these water samples were analyzed with a continuous flow analyzer (Skalar, Netherlands) after the water samples are filtered through a 0.45-µm membrane. The concentration of IN was cal− culated as the sum of NH+ 4 and NO3 . The concentration of DON was calculated by subtracting IN from TDN. Cumulative DON leaching (kg N ha−1 ) during each crop season was obtained by multiplying the timeinterval weighted DON concentration in leachate from 100 cm soil depth (mg N L−1 ) by total amount of leaching water (m3 ha−1 ) and 0.001, where the timeinterval weighted DON concentration is defined as the sum of [individual DON concentration in leachate (mg N L−1 ) × interval between two adjacent samplings (d)]/total growth time (d). Cumulative DON input by precipitation during each crop season was calculated as the sum of amount of individual precipitation (mm) × DON concentration in the corresponding rainwater samples (mg N L−1 ) × 0.01. Cumulative DON input by irrigation during each rice season was calculated as the sum of amount of individual irrigation (m3 ha−1 ) × DON concentration in the corresponding water samples (mg N L−1 ) × 0.001. Statistical analysis The data were statistically analyzed by SPSS. Bivariate correlation analysis was used to examine the correlation between DON input from precipitation and irrigation, the amount of precipitation and irrigation, and the amount of DON leaching. One-way analysis of variance (ANOVA) was employed to analyze the influence of N application rate on DON leaching. Two-way ANOVA was used to investigate the effects of N application rate and DON input by precipitation and irrigation on the amount of DON leaching and the ratio of
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DON/TDN. Stepwise linear regression was used to explain the variations of DON leaching and the ratio of DON/TDN resulting from N application, the DON input by precipitation and irrigation or soil leachate amount. For all statistical analyses, a significance level of P < 0.05 was used. RESULTS DON leaching in rice and wheat seasons DON leaching showed yearly and inter-seasonal variations during three rice and wheat crop growing
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seasons (Fig. 1). In the rice seasons, the DON concentrations in the leaching water had the consistent trend of higher values during the time frame from June to July than during that from August to October. The peak values in the 2008 and 2009 rice seasons were greater than the value in the 2007 season. For the wheat seasons, the DON concentrations were very low during the first 2007–2008 season, but soared during the following 2008–2009 and 2009–2010 seasons. Because of less rainfall in the 2007–2008 season, percolation water was not collected until 61 d of wheat sowing, and there were only 5 samples collected during the
Fig. 1 Temporal variations of dissolved organic nitrogen (DON) concentrations in soil leachates and DON input by irrigation and precipitation during the three rice and wheat seasons under the conventional N application rates of 300 kg N ha−1 for rice and 200 kg N ha−1 for wheat from 2007 to 2010. Vertical bars indicate the standard deviations of the means (n = 3). Vertical arrows show the N fertilization times of basal fertilization (BF), the first top-dressing (TD1) and the second top-dressing (TD2).
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whole season. The DON concentrations in the 2008– 2009 and 2009–2010 seasons rose consistently during the period from February to March, which differed from the 2007–2008 season. Under the conventional N application rate, the seasonal time-interval-weighted average of DON concentration ranged from 0.55 to 1.05 mg N L−1 during the three rice seasons and from 0.08 to 2.29 mg N L−1 during the three wheat seasons (Table I). The fluctuation of DON concentration was much smaller in the rice seasons than in the wheat seasons. The amounts of leaching water were 1 920–2 180 m3 ha−1 during the rice seasons and 138–1 180 m3 ha−1 during the wheat seasons. The more regular and higher level of water leaching during the rice seasons caused more DON leaching compared to the wheat seasons. Using measured DON concentrations and amounts of leaching water, DON leaching from a 100-cm soil depth in the rice-
wheat rotated paddy soil was calculated. The data were 1.06–2.29 kg N ha−1 (corresponding to 18.5%–47.6% of TDN) for the three rice seasons and 0.01–1.29 kg N ha−1 (corresponding to 2.77%–14.6% of TDN) for the three wheat seasons. The percentages of DON to TDN were lower for the wheat seasons than those for the rice seasons. DON input by precipitation and irrigation The rain water introduced 1.42–5.97 kg N ha−1 of DON by 533–1 056 mm precipitation during the three rice seasons and 1.76–2.87 kg N ha−1 of DON by 434–563 mm precipitation during the three wheat seasons. The irrigation water (5 079–5 726 m3 ha−1 ) carried 2.22–3.78 kg N ha−1 of DON to the soil during the three rice seasons (Table II). The total DON input was positively correlated to the precipitation and irrigation rates (R2 = 0.8060, P = 0.015). Although the
TABLE I Seasonal time-interval weighted average of dissolved organic nitrogen (DON) concentration in soil leachates and the amounts of DON leaching during the rice and wheat seasons under the conventional N application rates of 300 kg N ha−1 for rice and 200 kg N ha−1 for wheat from 2007 to 2010 Crop
Season
DON concentration
Growth period
Leaching watera)
DON leaching
DON/TDNb)
Rice
2007 2008 2009 Average 2007–2008 2008–2009 2009–2010 Average
mg N L−1 0.55±0.30c) 0.59±0.54 1.05±0.16 0.73±0.28 0.08±0.05 2.29±0.59 1.09±0.09 1.15±1.10
d 144 129 141
m3 ha−1 1 920 1 940 2 180 2 013±145 138±4.46 230±17.9 1 180±163 516±577
kg N ha−1 1.06±0.57 1.15±1.04 2.29±0.36 1.50±0.68 0.01±0.007 0.53±0.14 1.29±0.10 0.61±0.64
% 33.5±7.8 18.5±7.3 47.6±4.3 33.2±14.6 2.8±2.4 14.6±4.9 12.4±1.7 9.9±6.3
Wheat
192 186 210
a) Amounts
of leaching water for the rice seasons were estimated based on the vertical leaching rate of 2 mm d−1 during the flooded period; these for the wheat seasons were obtained from the field undisturbed tension-free monolith lysimeter under the same conditions as the field experiment. b) TDN = total dissolved nitrogen. c) Means±standard deviations (n = 3). TABLE II Dissolved organic nitrogen (DON) input from precipitation and irrigation in the rice and wheat seasons from 2007 to 2010
Crop
Season
Rice
2007 2008 2009 Average
Wheat
2007–2008 2008–2009 2009–2010 Average
a) Means±standard b) No
Precipitation
Irrigation
Amount
DON concentration
DON input
Amount
DON concentration
DON input
mm 661 533 1 056 750±272a)
mg N L−1 0.21 0.31 0.57 0.36±0.18
kg N ha−1 1.42 1.66 5.97 3.02±2.56
m3 ha−1 5 168 5 726 5 079 5 324±351
mg N L−1 0.43 0.66 0.61 0.57±0.12
kg N 2.22 3.78 3.09 3.03±0.78
0.41 0.65 0.32 0.46±0.17
1.76 2.87 1.82 2.15±0.63
-b) -
-
-
434 442 563 480±73
deviations (n = 3). irrigation was practiced in the wheat seasons.
Total DON input ha−1 3.64 5.44 9.06 6.05±2.76 1.76 2.87 1.82 2.15±0.63
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average DON concentrations in soil leachate for both seasons (0.73–1.15 mg N L−1 ) (Table I) were higher than those in precipitation and irrigation water (0.36– 0.57 mg N L−1 ), DON leaching were lower overall than DON input from precipitation and irrigation (Tables I and II). A positive correlation was found between DON leaching and DON input from precipitation and irrigation (R2 = 0.6761, P = 0.045). Effects of N application and DON input on DON leaching Looking at only Fig. 1, it seems difficult to obtain the clear relationship between DON concentrations and N fertilization events due to the large yearly and inter-seasonal variations of DON concentration and the interfering effects from irregular precipitation and irrigation. We also failed to examine the effects of different N fertilization rates on the DON leaching (Fig. 2). Although the DON leaching appeared to increase with the N application rate during the wheat seasons and decreased with it during the rice season, no significant difference (P > 0.05) was found among DON leaching under the three N application rates. However, the two-way ANOVA analysis revealed that DON leaching was only significantly affected (P < 0.01) by DON input by precipitation and irrigation in the rice seasons, whereas it was significantly affected (P < 0.01) by both DON input by precipitation and N application in the wheat seasons (Table III). DON inputs by precipitation and irrigation and N application also had extremely significant effects (P < 0.01) on the ratios of DON/TDN during both rice and wheat seasons (Table III). The stepwise linear regression analysis showed that only DON input by precipitation and irrigation could explain 16.2% (P = 0.038) of the variation of DON leaching in the rice seasons, but N application, DON input by precipitation and soil leachate amount could explain 62.2% of the variation of DON leaching in the wheat seasons (32.4% from DON input by pre-
Fig. 2 Effect of N fertilization rate (0 kg N ha−1 for both crops as a control, 100 kg N ha−1 for both crops, and 300 kg N ha−1 for rice and 200 kg N ha−1 for wheat) on dissolved organic nitrogen (DON) leaching in the rice and wheat seasons from 2007 to 2010. The bar is the seasonal average of DON leaching from the corresponding three rice or wheat seasons. Vertical bars indicate the standard deviations of the means (n = 3).
cipitation, P < 0.001; 20.1% from soil leachate amount, P < 0.001; 9.7% from N application rate, P = 0.023) (Table IV). For the ratio of DON/TDN, the contribution percentage increased to 47.4% (P < 0.001) from DON input by precipitation and irrigation in the rice seasons and to 77.2% from DON input by precipitation, soil leachate amount and N application in the wheat seasons (50.1% from DON input by precipitation, P < 0.001; 13.7% from soil leachate amount, P = 0.001; 13.4% from N application, P = 0.001) (Table IV). DISCUSSION Under the local conventional N application rates (300 kg N ha−1 for rice and 200 kg N ha−1 for wheat) and water management practices in the rice-wheat rotated agroecosystem, the DON concentration in leachate from 100 cm soil depth averaged 0.73±0.28 mg
TABLE III Results of analysis of variance examining the effects of N application and dissolved organic nitrogen (DON) input by precipitation and irrigation on DON leaching and the ratio of DON:total dissolved nitrogen (DON/TDN) during the rice and wheat seasons Crop
Rice
Wheat
Factor
N application rate DON input by precipitation and irrigation N application rate × DON input by precipitation and irrigation N application rate DON input by precipitation N application rate × DON input by precipitation
DON leaching
DON/TDN
F value
P value
F value
P value
2.467 6.550 3.798 14.934 77.299 22.962
0.113 0.007 0.021 < 0.001 < 0.001 < 0.001
10.066 34.734 8.380 15.224 67.741 7.142
0.001 < 0.001 0.001 < 0.001 < 0.001 0.001
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TABLE IV Results of stepwise linear regression analysis explaining the variations of dissolved organic nitrogen (DON) leaching and the ratio of DON:total dissolved nitrogen (DON/TDN) resulted from N application (Napp ), DON input (DONinput ) by precipitation and irrigation and soil leachate amount (Vleach ) Crop
Rice
Dependent variable (Y )
Entered variable
DON leaching
DONinput by precipitation and irrigation DONinput by precipitation and irrigation DONinput by precipitation Vleach Napp DONinput by precipitation Vleach Napp
DON/TDN Wheat
DON leaching
DON/TDN
Variable
Excluded variable Regression equation R2
Variable
P
0.038
0.162
< 0.001
0.474
< 0.001 < 0.001 0.023 < 0.001 0.001 0.001
0.324 0.201 0.097 0.501 0.137 0.134
Napp Vleach Napp Vleach -
0.090 0.094 0.263 0.736 -
P
N L−1 during the rice season and 1.15±1.10 mg N L−1 during the wheat season. The corresponding DON leaching was calculated to be 1.50±0.68 and 0.61±0.64 kg N ha−1 (Table I). On an annual basis, the DON concentration in soil leachate was 0.79±0.27 mg N L−1 and the DON leaching was 2.11±1.31 kg N ha−1 for the entire rice-wheat rotation. These values were higher than those in natural ecosystems, e.g., forest soil (1.3±0.5 kg N ha−1 , Campbell et al., 2000; 0.008–0.135 mg N L−1 , Perakis and Hedin, 2002; 0.34–0.40 kg N ha−1 , Hood et al., 2003; 0.5–0.8 kg N ha−1 , M¨oller et al., 2005; 0.35±0.07 kg N ha−1 , Cairns et al., 2009; 0.26– 0.89 mg N L−1 , Mattsson et al., 2009) and peat soil (0.18±0.13 mg N L−1 , Chapman et al., 2001; 0.005– 0.065 mg N L−1 , Bragazza and Limpens, 2004; 0.455 mg N L−1 in average, Cundill et al., 2007). However, the DON/TDN ratio of the rice-wheat rotated farmland was only 23.0%±6.3%, much lower than the ratios of the natural ecosystems (59%±20%, Campbell et al., 2000; 40%, Chapman et al., 2001; 61%–97%, Perakis and Hedin, 2002; 36.8%–94.1%, Streeter et al., 2003; 61%–82%, Bragazza and Limpens, 2004; 71%– 80%, M¨oller et al., 2005; 82%, Cundill et al., 2007; 64%, Cairns et al., 2009). The reason for this may be the high inorganic N distribution in this intensively fertilized paddy soil, which is largely different from the undisturbed natural ecosystems where organic matter dominantly regulates N cycling. Hagedorn et al. (2000, 2001) demonstrated that the DON concentrations in the subsoil were higher under reducing conditions than oxidizing conditions and were significantly related to the dissolved Fe concentration. The water flooding during the rice seasons usually results in the reduction and dissolution of Fe oxyhydroxides, on which DON is mainly immobilized through the ligand exchange process (Qualls, 2000). This might be
Y = 0.563 + 0.250DONinput Y = 9.222 + 4.110DONinput Y = −1.145 + 0.001Vleach + 0.521DONinput + 0.002Napp Y = −26.019 + 18.738DONinput + 0.009Vleach − 0.050Napp
the explanation for the moderately high DON/TDN ratio found during the rice seasons (33.2%±14.6%), compared to those found during the wheat seasons (9.93%±6.29%) and other studies conducted on upland agroecosystem (7.9%–14.9%, Murphy et al., 2000; 6%– 21%, Siemens and Kaupenjohann, 2002; 17.6%, M¨oller et al., 2005). Nevertheless, the annual DON leaching from the rice-wheat rotated paddy soil was comparable with those from aerobic uplands (1.1–2.5 kg N ha−1 , Murphy et al., 2000; 0.4–2.3 mg N L−1 , Siemens and Kaupenjohann, 2002; 0.9 kg N ha−1 , M¨oller et al., 2005). In the current study, the chemical N application did not significantly affect (P > 0.05) DON leaching during the three rice seasons (Fig. 2). This was in contrast to our initial hypothesis and inconsistent with the general knowledge that the input of mineral N promotes the release and leaching of DON (Currie et al., 1996; McDowell et al., 1998; Neff et al., 2000; Shand et al., 2000; Kalbitz and Geyer, 2002). This phenomenon may be explained by the concealing effects of a large amount of DON input by precipitation and irrigation. The current DON input from precipitation and irrigation averaged 6.05 kg N ha−1 (Table II), which was 3 times larger than the average accumulated leaching DON (1.50 kg N ha−1 in average). In addition, the DON concentration peaks for leachate coincided frequently with the DON input from precipitation and irrigation (Fig. 1). Moreover, DON input by precipitation and irrigation had an extremely significant effect on the DON leaching amount (Table III). These results can support the notion that DON inputs from precipitation and irrigation are the most important factors for DON leaching in the rice seasons, which could largely conceal the influence of N fertilization. However, it should be admitted that application of organic mat-
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ter instead of chemical N fertilizer may still increase DON leaching from rice paddy (Wang et al., 2013). In a temperate rice system, Bird et al. (2002, 2003) found 30% of straw 15 N was recovered in the mobile humic acid and about 19% was in the microbial biomass after straw incorporation. Both the mobile humic acid and microbial biomass belong to the active and labile N pool in soil and substantially contribute to soil DON leaching. Unlike the rice season, the wheat season was completely rain-fed in the rice-wheat rotation (Zhao et al., 2012). Therefore, the sample time and amount of soil leachate were governed by precipitation events during the wheat seasons (Fig. 1). DON leaching in the three wheat seasons showed higher variability than those values in the three rice seasons due to the large variations in precipitation (Table I, Fig. 1). Many studies conducted on upland soils (Siemens and Kaupenjohann, 2002; M¨oller et al., 2005; Embacher et al., 2008; Mattsson et al., 2009) have shown that N fertilization in spite of organic or inorganic fertilizers increased DON concentration and/or leaching due to the stimulation to net N mineralization and microbial activity and the increments of crop plant biomass (Embacher et al., 2008; Lu et al., 2011). In the current study, not only chemical N application but also DON input by precipitation had significant effects on the DON leaching during the wheat seasons (Tables III and IV). N application, DON input by precipitation and soil leachate amount explained 62.2% of the variation of DON leaching. Moreover, precipitation seemed to influence DON leaching responses to N fertilization during the wheat seasons. Compared to the amount of DON leaching, the ratio of DON/TDN was a more feasible parameter to convey the effects of extrinsic factors on DON leaching in the rice-wheat rotated agroecosystem. The two-way ANOVA analysis showed that N application and DON input by precipitation and irrigation significantly affected the ratio of DON/TDN in the rice and wheat seasons, even though N application did not seem to impact the amount of DON leaching during the rice seasons (Table III). The stepwise linear regression analysis clearly indicated that DON input by precipitation and irrigation explained 47.4% variation (P < 0.001) of the ratio of DON/TDN, compared to 16.2% variation (P = 0.038) of DON leaching amount during the rice seasons (Table IV). Moreover, the variation of the ratio of DON/TDN in the wheat seasons was well explained by N application and DON input by precipitation together with soil leachate amount (77.2%, P < 0.001). It should be noted that some measurement errors in DON leaching possibly remained due to the limitations
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in techniques for direct determination of DON concentration in leachate and uniform metering methods for the leaching water amount. Nevertheless, the significant DON input from irrigation and precipitation accurately measured by electromagnetic flowmeters and rain collectors supports the notion that DON leaching was greatly influenced by irrigation and precipitation rather than by chemical N fertilization. The great variations in DON leaching also validate the necessity of involving DON in the integrated field N budget because the leached DON was higher in the rice-wheat rotation than in natural ecosystems. CONCLUSIONS The 3-year field observation showed 2.1±1.3 kg N ha of DON leached from the whole rice-wheat rotation and, correspondingly, the DON input by irrigation and precipitation was 8.3±3.4 kg N ha−1 . Although DON leaching was less than inorganic N leaching, it is still necessary to include DON in the overall N budget because of a greater amount of DON leached from the rice-wheat rotation compared to natural ecosystems. Anthropogenic N addition (N deposition, fertilization and irrigation) and water management (frequent flooding-draining water regime) have even more evident influences on DON leaching. For the wheat season, DON leaching was prompted by N fertilization and strongly influenced by precipitation. For the rice season, high precipitation and irrigation rates carried more DON to paddy soil, thus concealing the direct influence of N fertilization on DON leaching. The mechanisms of DON production, transformation and migration in paddy soil were ultimately complex, and different from those of other ecosystems. Further laboratory and field-scale studies are needed in this highly fertilized and intensively flooded/drained ricewheat rotated agroecosystem. −1
ACKNOWLEDGEMENTS This study was supported by the Jiangsu Provincial Natural Science Foundation of China (No. BK2010612), the Foundation of State Key Laboratory of Soil and Sustainable Agriculture, China (No. Y052010034), and the National Natural Science Foundation of China (No. 41001147). We especially thank the anonymous reviewers for their constructive comments that have greatly improved the manuscript. REFERENCES Allen, R. G., Pereira, L. S., Raes, D. and Smith, M. 1998. Crop Evapotranspiration—Guidelines for Computing Crop Water
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