Nitrate-Nitrogen Dynamics and Nitrogen Budgets in Rice-Wheat Rotations in Taihu Lake Region, China

Nitrate-Nitrogen Dynamics and Nitrogen Budgets in Rice-Wheat Rotations in Taihu Lake Region, China

Pedosphere 23(1): 59–69, 2013 ISSN 1002-0160/CN 32-1315/P c 2013 Soil Science Society of China  Published by Elsevier B.V. and Science Press Nitrate...

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Pedosphere 23(1): 59–69, 2013 ISSN 1002-0160/CN 32-1315/P c 2013 Soil Science Society of China  Published by Elsevier B.V. and Science Press

Nitrate-Nitrogen Dynamics and Nitrogen Budgets in Rice-Wheat Rotations in Taihu Lake Region, China∗1 ZHANG Jun-Hua1,2 , LIU Jian-Li1,∗2 , ZHANG Jia-Bao1 , CHENG Ya-Nan1,3 and WANG Wei-Peng1,3 1 Institute

of Soil Science, Chinese Academy of Sciences, Nanjing 210008 (China) Key Laboratory of Rice Biology, China National Rice Research Institute, Hangzhou 310006 (China) 3 University of Chinese Academy of Sciences, Beijing 100049 (China) 2 State

(Received April 27, 2012; revised November 18, 2012)

ABSTRACT Nitrate-nitrogen (NO− 3 -N) dynamics and nitrogen (N) budgets in rice (Oryza sativa L.)-wheat (Triticum aestivum L.) rotations in the Taihu Lake region of China were studied to compare the effects of N fertilizer management over a two-year period. The experiment included four N rates for rice and wheat, respectively: N1 (125 and 94 kg N ha−1 ), N2 (225 and 169 kg N ha−1 ), N3 (325 and 244 kg N ha−1 ), and N0 (0 kg N ha−1 ). The results showed that an overlying water layer during the rice growing seasons contributed to − moderate concentrations of NO− 3 -N in sampled waters and the concentrations of NO3 -N only showed a rising trend during the field -N concentrations in leachates during the wheat seasons were much higher than those during the rice seasons, drying stage. The NO− 3 particularly in the wheat seedling stage. In the wheat seedling stage, the NO− 3 -N concentrations of leachates were significantly higher −1 ) at a depth in N treatments than in N0 treatment and increased with increasing N rates. As the NO− 3 -N content (below 2 mg N L -N in the groundwater of paddy of 80 cm during the rice-wheat rotations did not respond to the applied N rates, the high levels of NO − 3 − fields might not be directly related to NO− 3 -N leaching. Crop growth trends were closely related to variations of NO 3 -N in leachates. A reduction in N application rate, especially in the earlier stages of crop growth, and synchronization of the peak of N uptake by the crop with N fertilizer application are key measures to reduce N loss. Above-ground biomass for rice and wheat increased significantly with increasing N rate, but there was no significant difference between N2 and N3. Increasing N rates to the levels greater than N2 not only decreased N use efficiency, but also significantly increased N loss. After two cycles of rice-wheat rotations, the apparent N losses of N1, N2 and N3 amounted to 234, 366 and 579 kg N ha−1 , respectively. With an increase of N rate from N0 to N3, the percentage of N uptake in total N inputs decreased from 63.9% to 46.9%. The apparent N losses during the rice seasons were higher than those during the wheat seasons and were related to precipitation; therefore, the application of fertilizer should take into account climate conditions and avoid application before heavy rainfall. Key Words:

above-ground biomass, crop uptake, nitrate-nitrogen leaching, nitrogen mineralization, nitrogen transport

Citation: Zhang, J. H., Liu, J. L., Zhang, J. B., Cheng, Y. N and Wang, W. P. 2013. Nitrate-nitrogen dynamics and nitrogen budgets in rice-wheat rotations in Taihu Lake region, China. Pedosphere. 23(1): 59–69.

The rice-wheat (R-W) cropping system is practiced in South and East Asia from subtropical to warm-temperate climatic zones (Timsina and Connor, 2001). In China, R-W rotation is conducted in an area of 13 million ha in the Yangtze River Basin, and produces 8 and 6 t ha−1 of rice and wheat, respectively (Liu et al., 2003b). Farmlands in the Taihu Lake region have a typical R-W cropping system, and the amount of nitrogen (N) input in this region has significantly increased in the past two decades, ranging from 550 to 650 kg N ha−1 , which is much higher than the national average of about 300 kg N ha−1 (Zhu and Chen, 2002). High inputs of N and soil anaerobic-aerobic cycles in R-W rotations easily result in N leaching (Alam ∗1 Supported

et al., 2006), particularly NO− 3 -N leaching (Gheysari et al., 2009; Thimonier et al., 2010). It was found that NO− 3 -N concentrations in most of the lakes and rivers in the Taihu Lake region exceeded the up limit value of drinking water regulated by the WHO (Zhu et al., 2003; Council of European Communities, 1998). Recent surveys on well water in Suzhou, a city within the Taihu Lake region, showed that more than 40% of the −1 (Tian wells contained more than 11.3 mg NO− 3 -N L et al., 2007). When N application rates exceed crop demand, considerable NO− 3 -N accumulates in the soil (Granstedt, 2000; St´ephane et al., 2011). Accumulated NO− 3 -N is prone to leaching into the subsoil after heavy rainfall (Ju et al., 2006), and poses a high risk of gro-

by the National Basic Research Program (973 Program) of China (No. 2011CB100506) and the National Natural Science Foundation of China (Nos. 41171179 and 40871105). ∗2 Corresponding author. E-mail: [email protected]

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undwater contamination when combined with high N fertilizer and water input (Stites and Kraft, 2000; Zhao et al., 2009). Therefore, pollution from agricultural sources may be an important contributor to the high NO− 3 -N levels contaminating the water bodies, and it is imperative to reduce pollution from agricultural practices. Many studies have detailed the mechanisms and pathways for NO− 3 -N leaching under different farming regimes and fertilization managements, and have been performed in single crop, undisturbed soil columns or double-crop field experiments (Yang and Su, 2009). However, there were few in situ integrated studies on NO− 3 -N leaching, particularly in continuous R-W cropping systems. In addition, little attention has been devoted to the relationship between NO− 3 -N leaching and crop growth in the Taihu Lake region (Song et al., 2011). The amount of N that is supplied in excess of crop needs and cannot be fixed in the soil is prone to enter into the adjacent ecosystems and cause water pollution (Grignani et al., 2007; Yang et al., 2011). For this reason, N loss has widely been considered to be an indicator of potential environmental damage (Fan et al., 2007). Calculation of N budgets is a potentially useful method for predicting the risk of N loss (Sogbedji et al., 2000; Liu et al., 2003a). N budgets or balances are often evaluated by comparing various N inputs and outputs in soil-crop systems (Mishima et al., 2010). Because of their relative simplicity, the use of N budgets to evaluate the effects and interactions of crop systems and fertilization management is of great interest. Schleef and Kleihanss (1994) reported that 100 kg N ha−1 of annual N surplus could be regarded as a baseline for NO− 3 -N leaching into ground- or surface-water on a regional scale. As excessive N input has been common in the Taihu Lake region and water pollution has become an increasingly serious problem since the 1980s, N budgets and the fate of N fertilizer in R-W rotations are becoming important issues. Because there is lack of field studies on N budgets and N loss pathways under high N fertilizer inputs in R-W cropping systems, integrated research is essential to understand N behaviors and balances in specific soil-crop systems. An in situ field study was conducted in R-W rotations in the Taihu Lake region for two consecutive years. The aims of this study were: 1) to ascertain the NO− 3 -N leaching dynamics in the soil profile and determine when leaching is most likely to occur; (2) to investigate the above-ground production response to different N rates and determine the relationship between NO− 3 -N transport and crop growth; and 3) to study the N budgets and pathways for N loss as re-

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lated to N application rates. MATERIALS AND METHODS Site and soil characteristics A field experiment was carried out for two consecutive years (Jun. 2007–Oct. 2009) on a typical plot of agricultural soil in the Taihu Lake region at the Changshu Agroecological Experimental Station (31◦ 32 45 N, 120◦ 41 57 E), Chinese Academy of Sciences. The site, located in the Taihu Lake region, belongs to the northern subtropical humid climate zone, with altitude of 1.3 m and frost-free period of 224 days a year. During the experimental period from 2007 to 2009, the average annual precipitation was 1 131 mm and the average daily temperature was 17.6 ◦ C (Fig. 1). The soil is developed from lacustrine deposits that results in a weakly calcareous paddy soil and is classified as Gleyi-Stagnic Anthrosol according to the FAO soil taxonomy system. Table I shows the general characteristics of the three layers of soil where the trial was carried out.

Fig. 1 Monthly temperatures and precipitations during the experimental period from 2007 to 2009.

Experimental design and treatments Prior to the experiment, an R-W rotation was grown for one year without any NPK fertilizer inputs to make the soil at the field site homogeneously. The experiment included four N fertilizer rates for rice and wheat, respectively: N1 (125 and 95 kg N ha−1 ), N2 (225 and 170 kg N ha−1 ), N3 (325 and 245 kg N ha−1 ), and N0 control (no nitrogen). The N1, N2 and N3 application rates represented the lowest, current recommended and high traditional N rates, respectively. The treatments were randomized blocks with four repli-

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TABLE I General physical and chemical characteristics of the soil in the experimental field Depth

Bulk density

cm 0–20 20–40 40–80

g cm−3 1.2 1.3 1.4

Clay (< 0.002 mm)

Silt (0.002–0.05 mm)

Sand (> 0.05 mm)

Total N

7.8 6.4 3.8

g N kg−1 1.62 1.78 0.33

% 24.2 32.4 24.6

68.0 61.3 71.6

cates. The plots, each 16 m2 (3.2 m × 5 m), were separated by plastic film buried to a depth of 25 cm, and isolation wells were built around the plot to prevent lateral seepage of water and fertilizers. Varieties of rice and wheat used in this study and the dates of fertilization are presented in Table II. Urea (46% N), superphosphate (14% P2 O5 ) and potassium chloride (65% K2 O) were used to supply N, P and K, respectively. Overall, 40% of the urea was basally applied, 30% was top-dressed at the tillering stage, and 30% was applied at the tassel stage for rice and wheat. P2 O5 (75 kg ha−1 ) and K2 O (75 kg ha−1 ) were applied basally in the rice seasons; P and K applied in the wheat seasons were 75% of the levels for the rice seasons, which were also applied basally. Only the basal fertilizer for the rice and wheat seasons were mixed with surface soil, and the other fertilizers were applied to the soil surface. The mean seedling age of rice at transplant was approximately one month, and four seedlings were transplanted per hill. The field was flooded prior to transplanting the rice and an overlying water layer of 3–5 cm was maintained during the rice growing seasons, except for a few days of drying at the end of the tilling stage and for approximately two weeks drying before harvest. With a sowing rate of 270 kg ha−1 , winter wheat was broadcasted into the soil by mixing the seeds with topsoil, which is a practice that is widely adopted by local farmers. There was no irrigation during the winter wheat growing seasons due to the abundant rainfall, which is a practice widely adopted by local farmers. Sample collection and crop management Suction cups made of clay materials were installed

vertically at depths of 20, 40 and 80 cm to collect leachates. The suction cup was connected to a polyvinyl chloride (PVC) tube, and a vacuum pump was used to collect water samples from different soil depths. Soil surface water (only for the rice seasons) and leachates at different depths for the rice seasons were collected every day after fertilization until NO− 3N concentrations in the water samples stabilized, at which point samples were collected every 10 days. In this time, if heavy rain occurred (more than 15 mm d−1 ), samples were also collected. Leachates for the wheat seasons at depths of 20, 40 and 80 cm were collected every 3 days after fertilizer application until NO− 3 -N concentrations stabilized, at which point samples were collected at 15-day intervals (no soil surface water in the wheat seasons). In addition, if heavy rainfall occurred in these time, additional samples were also collected. Soil samples were also collected from all plots at depths of 0–20, 20–40 and 40–80 cm before planting and after harvest to determine the initial and + final soil inorganic N concentration (NO− 3 -N + NH4 -N; − Rhoades, 1996). The concentration of NO3 -N in water samples was analyzed using a continuous flow analyzer (AA3, Bran and Luebbe, Norderstedt, Germany). Soil samples were extracted with 2 mol L−1 KCl to deter+ mine the contents of NO− 3 -N and NH4 -N using the continuous flow analyzer. Population densities for rice and wheat were determined from randomly selected 1-m2 area in each plot. The above-ground biomass for rice and wheat were assessed during the main growing season, and the production was expressed as oven-dried matter. In addition, plant samples in maturity for rice and wheat were collected to analyze the N content by the micro-

TABLE II Varieties and fertilization dates in the rice-wheat cropping system Year

2007–2008 2008–2009

Variety (rice/wheat)

Wuyujing/Yangmai 10 Wuyujing/Yangmai 10

Rice season

Wheat season

Basal fertilizer

Tillering fertilizer

Tassel fertilizer

Basal fertilizer

Tillering fertilizer

Tassel fertilizer

Jun. 18, 2007 Jun. 21, 2008

Jul. 2, 2007 Jul. 5, 2008

Aug. 14, 2007 Aug. 8, 2008

Nov. 5, 2007 Nov. 8, 2008

Jan. 7, 2008 Jan. 5, 2009

Mar. 17, 2008 Mar. 6, 2009

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Kjeldahl method (Bremner, 1996). N uptake for rice and wheat were estimated from the above-ground dry matter production multiplied by their N concentrations. Data analyses A simple N budget was estimated for each plot from Jun. 2007 to Oct. 2009 as the difference between the inputs (N from fertilizer, initial inorganic N and mineralized N) and outputs (crop uptake of N and residual soil N) (Oenema et al., 2003; Schr¨ oder et al., 2003; Pinitpaitoon et al., 2011). This difference, called the apparent N loss, represents N that is lost by leaching, ammonia volatilization, runoff, denitrification and other ways. The 0–80 cm soil profile was used for inorganic N in the N budget calculations because most of the crop roots were distributed in the 0– 80 cm depth under the experimental conditions (Tian, 2007). As N from atmospheric deposition and irrigation water are the same in all treatments, they were not considered in the N budgets. N mineralization was estimated by the balance of inputs (initial soil inorganic N) and outputs (N uptake and residual soil N) in the control plots (N0) according to Ju et al. (2002) and Jing et al. (2007). Statistical analysis of the data was accomplished by the standard analysis of variance (ANOVA) and pairs of mean values compared by least significant difference (LSD) at the 5% level using the SAS software package (SAS Institute, 1996). RESULTS NO− 3 -N leaching in rice-wheat cropping systems Variations of NO− NO− 3 -N for rice seasons 3 -N was formed from the hydrolysis and nitrification process after urea was applied to the paddy fields. For soil surface water, NO− 3 -N content tended to rise first and then fall after fertilization, and generally peaked in the third day. The maximum did not exceed 5.0 mg N L−1 (Fig. 2). There were significant differences across all treatments during basal and tillering fertilization. No significant differences were observed in the treatments during tassel fertilization, and the NO− 3 -N contents were less than 2.5 mg N L−1 . The NO− 3N concentrations in the soil surface water among all treatments in the tassel fertilization period remained at a low level. At depths of 20, 40 and 80 cm, no significant differences were observed for NO− 3 -N concentrations in all treatments during the entire rotation, and the contents were below 2.0 mg N L−1 (Fig. 2). However, ob-

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vious differences were observed in the late stage of rice growths across all treatments in 2008. The maximum NO− 3 -N concentrations for 20, 40 and 80 cm depths in this period were 5.7, 1.5, and 1.2 mg N L−1 , respectively, which is much higher than those in 2007. These results are associated with incomplete drying at the same time in 2007, which was caused by abundant rainfall (shown in Fig. 1). Complete drying in the later rice growths in 2008 resulted in a rising trend of − NO− 3 -N concentrations in leaching samples. The NO3 N concentrations in leachates at a depth of 80 cm in the 2007 and 2008 rice seasons were all below 2 mg N L−1 , and were independent of the fertilization rates (Fig. 2), which illustrates that the high levels of NO− 3 -N in the groundwater in this region may not be directly related to NO− 3 -N leaching from paddy fields. The compact structure of plough pan in the paddy soil contributes to a weak permeability, which inhibits the downward flow of NO− 3 -N, and the denitrification in the saturated layer may result in low NO− 3 -N concentrations in leachates. NO− Variations of NO− 3 -N for wheat seasons 3N concentrations in leachates for wheat seasons were much higher than those for rice seasons, and the maximum concentrations at depths of 20, 40 and 80 cm were 11.0, 8.0 and 1.0 mg N L−1 , respectively (Fig. 3). After basal fertilizer application, NO− 3 -N contents showed little change in each soil layer within a few days and no significant differences were shown in all treatments. A peak was not observed until approximately two weeks after basal fertilization, which was presumably due to the weak soil nitrification for heavy soil and high water content in this period. After the tillering fertilizer was applied to the soil, NO− 3 -N contents at a depth of 20 cm rapidly peaked and remained at a high level, and the 40 and 80 cm depths shared this trend. The NO− 3 -N contents in tassel fertilization showed little change because wheat reached the peak of N uptake. The NO− 3 -N contents in leachates for N1, N2 and N3 were significantly higher than N0 and increased with increasing N rates, which suggests that part of the NO− 3 -N in leachates is from seasonally applied N fertilizer. Consistent with the rice seasons, NO− 3 -N concentrations at a depth of 80 cm for the wheat seasons were also less than the drinking water quality criteria regulated by the WHO (Fig. 3). Above-ground biomass in rice-wheat cropping systems Growths of above-ground biomass for rice and wheat at different growth stages were studied over two consecutive years (Fig. 4). Prior to the rice jointing stage, the above-ground biomass did not respond to

NO− 3 -N DYNAMICS AND N BUDGETS

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Fig. 2 Variations of NO− 3 -N contents in leachates at different soil depths during 2007–2008 rice seasons under four application rates of N fertilizer: N1 (125 kg N ha−1 ), N2 (225 kg N ha−1 ), N3 (325 kg N ha−1 ), and N0 (no N). The arrows represent fertilization dates. Bars with the same letter(s) for each growth stage are not significantly different at P < 0.05 according to the least significant difference test. Vertical bars indicate standard errors of the means (n = 4).

the applied N rates; however, the above-ground biomass significantly increased with increasing N rates after shooting. Contrary to less than 0.5% of mature biomass in the rice seedling stage, the biomass after the shooting stage accounted for more than 60% of the mature biomass. The NO− 3 -N concentrations in the water samples for rice at the late growth stage were relatively low (Fig. 2), which illustrates that the N uptake capacity for rice at this stage is high. Therefore, fertilization at the peak of N uptake of crop is an effective measure to improve N use efficiency. Generally, the above-ground biomass for rice significantly increased with increasing N rates, with the exception of N2 and N3. N3 did not markedly increase biomass compared to N2 in 2007 or 2008. On average, the biomass of N3 in maturity for rice was 86.0% higher than that for N0, 42.1% higher than that for N1, and only 2.9% higher than that for N2, which suggests that N rates higher than N2 can not significantly increase the rice

biomass. In addition, the above-ground biomass across the growth season in 2008 was higher than that in 2007, presumably due to more available N in the 2008 rice season. Before wheat turns green, the average aboveground biomass accounted for about 4.5%–11.9% of maturity and no significant differences were observed in all treatments. After the reviving stage, the biomass rapidly increased and remarkable differences were observed in each treatment. However, after peaking in the dough stage, the biomass in wheat seasons rapidly decreased. Different to the S-type distribution in rice seasons, the biomass in wheat seasons showed a parabolic distribution. Whether for the rice seasons or for the wheat seasons, the variations of biomass were closely related to the variations of NO− 3 -N in the leachates (Figs. 2 and 3). Consistent with rice, the above-ground biomass for wheat increased with increasing N rates; however, N3 showed no significant difference compared

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Fig. 3 Variations of NO− 3 -N contents in leachates at different soil depths during 2007–2009 wheat seasons under four application rates of N fertilizer: N1 (95 kg N ha−1 ), N2 (170 kg N ha−1 ), N3 (245 kg N ha−1 ), and N0 (no N). The arrows represent fertilization dates. Bars with the same letter(s) for each growth stage are not significantly different at P < 0.05 according to the least significant difference test. Vertical bars indicate standard errors of the means (n = 4).

to N2. It is suggested that no real fertilizer efficiency is produced, and the additional parts of N is easily enter into surface water or groundwater by leaching, ammonia volatilization or runoff, which would contribute to an increased N loss to the environment. N application rates in this region could meet the requirements of crops at the N2 level, which means the best N rates for rice and wheat in the Taihu Lake region are 225 and 169 kg N ha−1 , respectively. Nitrogen budgets in rice-wheat cropping systems For sustainable land use, N budgets have to be balanced to avoid negative impacts on the environment. We calculated the N budget for each growing season and both crops from the top 80 cm of soil during the experimental period (Table III). The N application rate accounted for the highest proportion in the total N inputs under the R-W rotations. Although the share of indigenous N in the soil compared to the total

N inputs declined as N rates were increased, it still exceeded N uptake of crops in N0 ranging from 158 to 199 kg N ha−1 (Table III). N mineralization ranging from 71 to 92 kg N ha−1 was a significant N input, particularly during the rice seasons. N mineralization often determines the crop response to N fertilizer. It should be pointed out that N mineralization in our study included the N from the environment (N deposition and biological N fixation) as well. N accumulations in the crops increased with increasing N rates in the R-W rotations, and the corresponding apparent N losses also increased. Residual N was the major part of N surplus during the wheat seasons, and the proportion was lower during the rice seasons. In the 2007 rice season, the apparent N loss in N3 accounted for 69.1% of the N surplus and only a small amount of the residual N into the soil pool. This was associated with the high intensity rainfall during the tillering and tassel fertilization periods, which resu-

NO− 3 -N DYNAMICS AND N BUDGETS

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Fig. 4 Above-ground biomass at different growth stages in the rice-wheat rotations from 2007 to 2009 under four application rates of N fertilizer: N1 (125 and 95 kg N ha−1 for rice and wheat, respectively), N2 (225 and 170 kg N ha−1 ), N3 (325 and 245 kg N ha−1 ), and N0 (no N). Bars with the same letter(s) for each growth stage are not significantly different at P < 0.05 according to the least significant difference test. Vertical bars indicate standard errors of the means (n = 4).

lted in large amounts of N fertilizer lost from runoff or other ways (Fig. 1). As precipitation in this region mostly occurred during the summer (Jun.–Aug.), N loss during the rice seasons was much higher than that during the wheat seasons. After two cycles of R-W cropping (Table III), residual N increased with increasing N rates; however, the apparent N losses were much higher than the residual N. Apparent N loss of N1 accounted for 58.1% of the N surplus, N2 accounted for 64.0%, and N3 accounted for 70.6%. The amount of apparent N loss drastically increased with increasing N rates, particularly when the N rate exceeded the N2 level, which indicated that N fertilizer losses occurred mainly under high N inputs. With the increase of N rates, the percentage of N uptake occupied in the total N output decreased from 63.9% to 46.9%. The lack of response of N uptake by crop to higher fertilizer rates indicates that the extra N lost from leaching, runoff and other means increased. The apparent N losses were so high that they contributed to the waste of N fertilizer and the pollution of the environment. The heavy soil structure of paddy soil (Figs. 2 and 3) determined the leaching

losses of NO− 3 -N were low. The amount of apparent N loss was 579 kg N ha−1 for N3, which suggests that lots of applied N loss through other ways (Table III). DISCUSSION A great deal of researches have reported that the increasing NO− 3 -N concentrations in the groundwater is related to large applications of N and unreasonable fertilization management (Goulding, 2000; Di and Cameron, 2002; Zhang et al., 2005; Engstr¨ om et al., − 2011). In our study, the NO3 -N contents of all treatments at a soil depth of 80 cm for the rice and wheat seasons were lower than 2 mg N L−1 , which is far below 11.3 mg N L−1 suggested by the drinking water criteria of the WHO. The NO− 3 -N concentrations at a depth of 80 cm did not show a response to N application rates, which indicates that the high levels of NO− 3 -N in the groundwater of this region may not be directly related to NO− 3 -N leaching from the paddy fields. Recent researches have shown that leaching may not be the main pathway of N loss in paddy fields, and abundant NO− 3N losses may only occur during heavy rainfall or pon-

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TABLE III Nitrogen (N) budget in the rice-wheat rotations from 2007 to 2009 Cropping system Rice season in 2007

Wheat season during 2007–2008

Rice season in 2008

Wheat season during 2008–2009

Two cycles of rice-wheat rotations

a) Mineral

Nitrogen budget

N0

1) N application rate 2) 3) 4) 5) 6) 7) 1) 2) 3) 4) 5) 6) 7) 1) 2) 3) 4) 5) 6) 7) 1) 2) 3) 4) 5) 6) 7) 1) 2) 3) 4) 5) 6) 7)

0

a)

Nmin before sowing Net mineralization Crop uptake (grain + straw) Nmin after harvest Apparent N loss, = 1) + 2) + N surplus, = 5) + 6) N application rate Nmin before sowing Net mineralization Crop uptake (grain + straw) Nmin after harvest Apparent N loss, = 1) + 2) + N surplus, = 5) + 6) N application rate Nmin before sowing Net mineralization Crop uptake (grain + straw) Nmin after harvest Apparent N loss, = 1) + 2) + N surplus, = 5) + 6) N application rate Nmin before sowing Net mineralization Crop uptake (grain + straw) Nmin after harvest Apparent N loss, = 1) + 2) + N surplus, = 5) + 6) N application rate Nmin before sowing Net mineralization Crop uptake (grain + straw) Nmin after harvest Apparent N loss, = 1) + 2) + N surplus, = 5) + 6)

3) − 4) − 5)

3) − 4) − 5)

3 − 4 − 5)

3) − 4) − 5)

3) − 4) − 5)

77 89 86 80 0 80 0 80 78 51 107 0 107 0 107 92 75 124 0 124 0 124 71 48 147 0 147 0 77 330 260 147 0 147

N1

N2

kg N ha−1 125 225 77 89 117 85 89 174 94 85 78 87 131 39 170 125 131 92 134 151 63 214 94 151 71 104 169 43 212 438 77 330 442 169 234 403

77 89 147 97 147 244 169 97 78 129 162 53 215 225 162 92 203 165 111 276 169 165 71 144 206 55 261 788 77 330 623 206 366 572

N3 325 77 89 154 104 233 337 244 104 78 158 172 96 268 325 172 92 238 218 133 351 244 218 71 175 241 117 358 1 138 77 330 725 241 579 820

nitrogen.

ding prior to transplanting (Cookson et al., 2001; Huang et al., 2007). The lack of conditions that oxi− dized NH+ 4 -N to NO3 -N at 80 cm depth in the trial and the denitrification in the saturated layer may contribute to the low NO− 3 -N concentrations in the leachates (Zhou et al., 2010). Denitrification in the saturated layer is a source for green house gas emissions (N2 O), and certain measures, such as reasonable cultivation, should be performed to inhibit pollution to the environment (Yao et al., 2010). Another reason for the low leaching concentration of NO− 3 -N was the absence of irrigation in the paddy fields, which resulted in weak downward flow, and this was different from Northern China and vegetable fields (Zhu et al., 2003; Li et al., 2007; Song et al., 2009; Yang and Su, 2009). However, the impact of NO− 3 -N in the leachates on water quality should be addressed, because leachates could enter

into the surrounding rivers through lateral flow. NO− 3N can also move continuously downwards and be lost even if it is not leached during the season of application. Davies and Sylvester-Bradley (1995) found that the annual amount of NO− 3 -N leached in agricultural land in Britain increased by 36 kg N ha−1 over a 50year period and one-third was derived from residual NO− 3 -N. For soil surface water in the rice seasons, NO− 3 -N concentrations across all treatments showed no significant differences after the tassel fertilizer was applied. Vigorous crop growth and well-developed root system during the tassel fertilization period (Tian, 2007) contributed to a higher absorption rate, which easily absorb more N fertilizer and soil indigenous N. Therefore, NO− 3 -N concentrations in the soil surface water among all treatments during the tassel fertilization period re-

NO− 3 -N DYNAMICS AND N BUDGETS

mained at low levels. The concentration of NO− 3 -N in the late stage during the rice season in 2008 showed an increasing trend compared to 2007, which may have been due to the soil redox potential resulting from the drying at the later growth due to precipitation (Fig. 1). During paddy field drying, the soil showed an increased redox potential and stronger nitrification (Huang et al., 2007), which easily contributed to the accumulation of NO− 3 -N in the soil. For the wheat seasons, the NO− 3 -N concentrations in the leachates among N treatments were significantly higher than N0 and increased with increasing N rates, which suggests that part of the NO− 3 -N in water samples for the wheat seasons arose from seasonal fertilizer application. During the wheat seasons, no soil surface water was maintained and there was no irrigation; therefore, the soil had a higher redox potential that easily contributed to the accumulation of NO− 3N. Therefore, NO− 3 -N concentrations in leachates for the wheat seasons were much higher than those in the rice seasons. Before wheat regeneration, NO− 3 -N content had been maintained at a high level due to the high redox potential of the upland, easier nitrification, high input of N fertilizer, small biomass in the seedling stage, underdevelopment of the root system and low uptake of N. With the crop growth, the biomass of all treatments showed an S-type distribution in the rice seasons, and it showed a parabolic distribution in the wheat seasons. This was different from the results by Liu et al. (2004), which reported that the dry weight of wheat was an S-type distribution in the North China Plain. The differences in the results may be due to different soil conditions and crop genotypes. The variations of above-ground biomass for rice seasons and wheat seasons were all closely related to the dynamics of NO− 3 -N, especially in the late growth stage of crops, which suggests that fertilization at the peak of N uptake is an effective measure to reduce N loss and increase N use efficiency. High N application rates (70% of total inputs) and low assimilation rates in the seedling stage contributed to the accumulation of N in the plough layer, and in the case of heavy rain during this time, NO− 3 -N concentrations around rivers would rapidly increase. Therefore, imperative management practices need to be undertaken at the wheat seedling stage because surface applied fertilizer is easily lost. The N3 treatment for rice and wheat showed no significant differences compared to N2 treatment, which indicated that no real fertilizer efficiencies were produced in N rates over the N2 level. The additional N would easily enter into surface water or groundwa-

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ter by leaching, ammonia volatilization or runoff and contributed to a decline in the water quality. A great deal of researches have shown that the accumulation and transport of soil N were affected by crop uptake, N management, precipitation, irrigation, soil characteristic, N transformation and other factors (Ottman and Pope, 2002; Wu et al., 2010). N application rates accounted for the highest proportion in the total inputs under R-W rotations, while the soil indigenous supply ranged from 158 to 199 kg N ha−1 , which was far more than the N uptake of N0 (Table III). However, yield-increasing effects by applying N fertilizer were still observed, probably due to the competition between mineral N uptake of the crop and N loss. In addition, the heavy plow pan at a depth of 15– 20 cm in paddy soil resulted in low N use efficiency. Liu et al. (2003a) reported high rates of N mineralization existed when the soil temperature and moisture were favorable, which is similar to our results. Minimizing the limiting factors that affect crop growth, such as the deficiency of phosphorus and potassium, drought and flood, would improve the absorptive capacity of crops for mineral N, which would improve N use efficiency and reduce N loss. A study by Tian et al. (2009) held that the residual N fertilizer from the previous season had low bioavailability in the following season, which suggests that it is difficult to be utilized by crops. The residual N was the major part of N surplus in the wheat seasons, while it accounted for less than 50% during the rice seasons. After two years of R-W rotations, the residual N increased with increasing N rates; however, the apparent N losses were higher than the residual N. The apparent N loss of N1 occupied 58.1% of the N surplus, N2 occupied 64.0% and N3 occupied 70.6%, which endangered the water and atmosphere. These were different from the situations in winter wheat-summer maize rotations where the residual N occupied two-thirds of the total N surplus, which showed the large impact of the climate on the N surplus in the soil (Ju et al., 2002). Rainfall intensity and intervals between precipitation and fertilization had a great impact on N losses in the farmlands. Douglas et al. (1998) showed that if heavy rainfall occurred in a short time after fertilization, the amount of N loss was much higher than during other periods. Therefore, no fertilizing before a heavy rain was an effective method to reduce apparent N loss. An important direction of recommended fertilization in foreign nations was to maintain the effective nutrient content in the soil at a set level through regulation of fertilizers in recent decades, which would not only ensure higher yields but also decrease environmental pollution (Raun

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et al., 1999). In addition to the yield-increasing effects, rational application of N fertilizer mainly reflected on the level of residual N. Therefore, many European and American countries required that residual N between 0–90 cm depths should not be higher than 50 kg N ha−1 (van der Ploeg et al., 1997). However, after two years of R-W rotations in the study, residual N was maintained at a high level, especially in the N3 treatment, which left 241 kg N ha−1 of residual N in the soil and was far beyond the threshold of 50 kg N ha−1 . Considering the aim of increasing yields, N fertilizer rates in this region could meet the needs of crops at the N2 level, meaning the best fertilization N rates for rice and wheat in the Taihu Lake region were 225 and 169 kg N ha−1 , respectively. CONCLUSIONS The NO− 3 -N concentrations at different depths for the rice seasons remained at a lower level and only showed a rising trend during the late drying period. The NO− 3 -N concentrations in leachates for the wheat seasons were much higher than those for the rice seasons, especially in the wheat seedling stage. The concentrations of NO− 3 -N at a soil depth of 80 cm were far below the drinking water quality criteria regulated by the WHO for both rice and wheat seasons. As for the crop growth, the above-ground biomass showed an S-type distribution in the rice seasons and a parabolic distribution in the wheat seasons, and both were closely related to variations of NO− 3 -N in the water samples. The above-ground biomass in R-W rotations increased with increasing N applications; however, no significant biomass for rice and wheat were produced in N application rates higher than N2. After two years of R-W rotations, the apparent N loss of N1 accounted for 58.1% of the N surplus, N2 accounted for 64.0%, and N3 accounted for 70.6%. With the increase of N rates, the percentage of N uptake occupying the total N outputs decreased from 63.9% to 46.9%. The high amount of apparent N loss contributed to the waste of fertilizers and the pollution of water bodies. Therefore, avoiding fertilization before rainstorm, crediting fertilizer applications, and synchronizing fertilization with the peak of N uptake by the crops were effective measures to reduce N loss. REFERENCES Alam, M. M., Ladha, J. K., Foyjunnessa, Rahman, Z., Khan, S. R., Harun-ur-Rashida, Khana, A. H. and Bureshe, R. J. 2006. Nutrient management for increased productivity of rice-wheat cropping system in Bangladesh. Field Crop. Res. 96: 374–386.

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