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Improving agronomic practices to reduce nitrate leaching from the rice–wheat rotation system
Agriculture, Ecosystems and Environment 195 (2014) 61–67
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Short communication
Improving agronomic practices to reduce nitrate leaching from the rice–wheat rotation system Yansheng Cao a,b, * , Yuhua Tian a , Bin Yin a , Zhaoliang Zhu a a b
State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, 71 Beijing East Road, Nanjing 210008, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 November 2013 Received in revised form 14 May 2014 Accepted 16 May 2014 Available online xxx
Winter wheat–summer rice rotation with unique dry–wet cycle water regime is one of the most general cropping systems in the Yangtze Delta, which might favor NO3-N leaching into shallow groundwater due to high fertilizer-N input with low N use efficiency. To alleviate the detrimental impacts caused by N leaching, it is necessary to apply improved management practices. Therefore, the objectives of the present study are to investigate characteristics of N leaching under drained upland crop-flooded rice rotation and illustrate the effectiveness of an improved agronomic practice for mitigating NO3-N leaching. N leaching measured in situ for the control (CK), conventional (CT), and improved (IT) treatments for one rice–wheat rotation. Improved fertilization and crop cultivation managements have been applied to the IT treatment in an integrated manner, including low N fertilization rate, high fertilization frequency, and dense planting. During the rice-growing season, NO3–-N and dissolved organic N (DON) were the predominant forms of N in percolation water, and contributed over 25% and 59% to TN, respectively. NO3–-N and TN concentrations in percolation water and leaching losses did not differ significantly (p > 0.05) between treatments. N leaching occurred mainly during the early stage of rice seedling transplanting. During the wheat-growing season, N in percolation water was predominantly in the form of NO3–-N, and accounted for over 64.5–82.9% of TN. N fertilization significantly increased NO3–-N and TN concentrations in percolation water. Nitrate and TN leaching losses from the IT treatment (1.15 and 1.38 kg N ha1, respectively) were smaller than the CT treatment (1.34 and 1.76 kg N ha1, respectively). The majority of N leaching losses for wheat occurred during the period from late February to final harvest. The IT treatment exhibited no significant effect on wheat yield, but significantly (p < 0.05) increased rice yield (8.60 Mg ha1 compared to 8.06 Mg ha1) relative to the CT treatment. Compared with the conventional management practice, the improved management practice appeared to be effective in reducing N leaching and increasing grain yield under the rice–wheat rotation. ã 2014 Elsevier B.V. All rights reserved.
Keywords: Nitrate Leaching Rice Wheat
1. Introduction Increasing NO3–-N concentrations in shallow groundwater and surface waters continue to be of international concern with respect to human health and eutrophication. The rising concentrations of NO3–-N in shallow groundwater and surface waters are closely associated with the intensification of agricultural production with increasing application of inorganic N fertilizers (Kumazawa, 2002; Ju et al., 2006).
* Corresponding author at: State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, 71 Beijing East Road, Nanjing, China. Tel.: +86 21 86881543. E-mail address: [email protected] (Y. Cao). http://dx.doi.org/10.1016/j.agee.2014.05.020 0167-8809/ ã 2014 Elsevier B.V. All rights reserved.
The Yangtze Delta, within latitude 30 200 N and 32 480 N and longitude 119 240 E and 121540 E, is the most developed economic area and the most intensive agricultural region in China (Fang and Mu, 2009). Its grain cropping system is characterized by large inputs of synthetic N fertilizer but low N use efficiency (Ju et al., 2009). Winter wheat–summer rice rotation is one of the most common farming patterns in the area (Liang et al., 2011) and unique water management practice with a dry–wet cycle is adopted. The water regime is likely to result in favorable conditions for NO3–-N leached into shallow groundwater. The aerobic soil environment is kept for almost the entire wheat season, which contributes to microbial nitrification (Liu et al., 2010). This significant accumulation of NO3–-N within the soil after the wheat plants is harvested is susceptible to leaching when the soil is reflooded before transplantation of rice seedling in the following season. The NO3–-N concentrations in shallow groundwater have
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been found to exceed the maximum permissible concentration recommended by USEPA (10 mg NO3–-N L1) or by WHO (11.3 mg NO3–-N L1) for drinking water in the area (Zhu and Chen, 2002; Chen et al., 2010). Algal bloom has also been observed recently in the Taihu Lake located in the central Yangtze Delta (Xie et al., 2008). One of the keys to controlling NO3 leaching is to select a package of management practices that can best control NO3 accumulation in the soil profile (Power et al., 2001). Various improved management practices that better match spatial and temporal N supply with plant N demand can significantly increase N use efficiency (Peng et al., 2002; Tilman et al., 2002; Dobermann, 2007; Ju et al., 2009; Peng et al., 2009), which is likely to regulate the accumulation of NO3 in the soil profile, thereby controlling NO3 leaching. The objectives of the present study are to: (a) examine the temporal variation of N concentrations in percolation water and the characteristics of N leaching in situ; (b) evaluate the effectiveness of an improved management practice combining improved fertilization with crop cultivation for mitigating N leached from the plant root zone. 2. Materials and methods
treatments were 19.6 kg P ha1 and 49.8 kg K ha1, respectively, in both seasons. In the rice season, N fertilizer was applied in three splits for the CT treatment and in four splits for the IT treatment. P fertilizer was only basal-applied for both treatments. K fertilizer was only basal-applied for the CT treatment and was applied in two splits for the IT treatment. In the wheat season, N fertilizer was applied in three splits for both treatments, while both P and K fertilizers were only basal-applied. The detailed fertilization scheme in both seasons for different treatments is shown in Table 2. Basal fertilizers were incorporated into the topsoil and the additional fertilizers were homogeneously broadcast onto the surface soil by hand. The surface water was principally maintained at a depth of 5 cm by irrigation except during mid-season aeration (over 1 week) and final drainage, which was performed ca. 1 week before rice harvesting. No irrigation was applied for both treatments during the wheat-growing season. Pesticide and herbicide application were the same for all treatments. Each plot was 40 m2 in area and earthen banks covered with plastic film were constructed between each plot to prevent lateral water movement. Summer rice was transplanted and harvested on June 16 and October 24, 2009, respectively. Winter wheat was sown and harvested on November 23, 2009 and June 6, 2010, respectively.
2.1. Experimental site summarization 2.3. Collection and measurement of percolation water The field experimental site was situated in Suzhou City, Jiangsu Province, China. The climate is characterized by a humid subtropical monsoon. The annual mean temperature was 15.5 C, with an annual mean precipitation of 1038 mm. The soil of the experimental site is a gleyed paddy soil evolved from lacustrine deposits, which is classified as Hydragric Anthropy. The main physic-chemical properties of the 0–20 cm soil layer are presented in Table 1. The experimental site has been cultivated with the summer rice (Oryza sativa L.)–winter wheat (Triticum aestivum L.) rotation, where rice was planted from June to October each year and wheat was planted from November each year to May the following year for the past 10 years. The typical farm management practices were equivalent to the CT treatment. Meteorological data were collected from the weather station which was situated in the experimental site. 2.2. Experimental design and farm management practices The field experiments were conducted for a rice–wheat rotation. Three treatments with four replicates were designed in a randomized complete block, included control (no N fertilizer applied, CK), conventional (CT), and improved (IT). Other farm management practices except N fertilization were the same between the CK and CT treatments in the rice and wheat seasons, while planting density and times and rate of fertilization in the rice season and rate of fertilization in the wheat season were different between the CT and IT treatments. Rice seedlings were transplanted into well-puddled soils at the spacing of 20 cm 20 cm and 20 cm 15 cm for the CT and IT treatments, respectively. Wheat was direct seeded at a density of 225 kg ha1 for all treatments. The N fertilization rates in the form of urea (N 46%) were 300 kg N ha1 for the CT treatment and 225 kg N ha1 for the IT treatment in the two seasons. The application rates of P (as superphosphate P2O5 16%) and K fertilizer (as potassium chloride K2O 60%) for all
A porous cup (2 mm pore size) was set up into the soil at an angel of ca. 10 in each plot, with the center of the cup at a depth of 90 cm. The porous cup was pumped out 24 h before each sampling time which was once every 10–15 days. Samples were stored in the refrigerator at 4 C till analysis for NH4+-N, NO3–-N, and TN concentrations with the indophenol blue method (Novozamsky et al., 1974), ultraviolet spectrophotometric method (HJ/T 3462007), and alkaline potassium persulfate digestion-ultraviolet spectrophotometric method (GB 11849-89), respectively. DON was difference between TN and mineral N (NH4+- and NO3–-N). The method used in the present study has its limitation as it is difficult to determine the actual volume of water leached. While this can be estimated using indirect methods such as a lysimeter experiment during the rice growth season. During the rice growth season, the rate of percolation was determined using a lysimeter (a height of ca. 1.5 m and an internal diameter of ca. 1.1 m) experiment and undisturbed soil column with water sampled at the bottom of the lysimeter. The irrigation scheme in the lysimeter experiment was the same as that in the field experiment. However, the lysimeter experiment is unsuitable for the measurement of the percolation water during the wheat growth season and so this was calculated from the mass balance of water as follows: L ¼ ðP þ IÞ ðR þ ETÞ where L is the amount of percolation water in mm, P is the amount of precipitation in mm, I is the amount of irrigation water in mm, R is the amount of surface runoff in mm, and ET is the amount of evapotranspiration in mm, which is calculated according to the method reported by Wang et al. (2012). The ET can be expressed as follows: ET ¼ K c K w ETo where Kc is the crop coefficient, Kw is the soil moisture coefficient, which can be calculated according to the method of Wang et al.
Table 1 The main physic-chemical properties of the surface soil (0–20 cm). Items
pH
Soil organic matter (g kg1)
Total N (g kg1)
Total P (g kg1)
Olsen P (mg kg1)
Extractable K (mg kg1)
Cation exchange capacity
Value
7.36
35.0
2.09
0.93
5.00
121.3
17.7
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Table 2 The scheme of chemical fertilizer management for different treatments in the rice and wheat seasons. Code Rice season Basal fertilizer 2009/6/16
Wheat season 1st additional fertilizer 2009/6/23
19.6P + 39.6K – 180N + 19.6P + 49.8K 60 N 112.5N + 19.6P + 24.9K 22.5 N
CK CT IT
2nd additional fertilizer 2009/7/27
3rd additional fertilizer 2009/8/13
Basal fertilizer
1st additional fertilizer 2010/3/11
2009/11/23
– – 45N + 24.9K
– 60 N 45 N
19.6P + 39.6K – 180N + 19.6P + 49.8K 30 N 135N + 19.6P + 49.8K 22.5 N
2nd additional fertilizer 2010/3/21 – 90 N 67.5 N
Units of N, P, and K fertilizers are kg N ha1, kg P ha1, and kg K ha1, respectively.
(2012) ETo is the reference crop evapotranspiration in mm day1, which is calculated using the modified Penman–Monteith method (Allen et al., 1998). The Kw and ETo can be expressed as follows: Kw ¼
Sw u w u c uw
ETo ¼
0:408DðRn GÞ þ g ð900=T þ 273ÞU 2 VPD D þ g ð1 þ 0:34U2 Þ
at the crop surface in MJ m2 day1, G is the soil heat flux density in MJ m2 day1, g is the psychrometric constant in kPa C1, T is the mean daily air temperature at 2 m height in C, U2 is the wind speed at 2 m height in m s1, VPD is the saturation vapor pressure deficit in kPa. After rice was harvested, a drain ditch with a depth of ca. 12 cm and a width of ca. 15 cm was dug within each plot to protect the wheat plant from water logging. A runoff collector, ca. 10 cm below the ground, was set up in each plot. The surface runoff from individual plot was carried to a corresponding collection pool constructed outside the plots via buried pipes. All the collectors, collection pools, and buried pipes have been
where Sw is the mean soil moisture in mm, uc is the field capacity in mm, uw is the wilting moisture in mm, D is the slope of the saturation vapor pressure curve in kPa C1, Rn is the net radiation
CK CT IT
CK CT IT
4 -1
3
2
1
0
6/15
3
2
1
0
6/30
7/15
7/30
8/14
8/29
9/13
9/28
10/13
6/15
Sampling date (mm/dd) CK CT IT
4
6/30
7/15
7/30
8/14
8/29
9/13
9/28
10/13
9/13
9/28
10/13
Sampling date (mm/dd) CK CT IT
4 -1
Total N concentration (mg N L )
-1
Dissolved organic N concentration (mg N L )
Nitrate N concentration (mg N L )
-1
Ammonium N concentration (mg N L )
4
3
2
1
0
6/15
3
2
1
0
6/30
7/15
7/30
8/14
8/29
Sampling date (mm/dd)
9/13
9/28
10/13
6/15
6/30
7/15
7/30
8/14
8/29
Sampling date (mm/dd)
Fig. 1. Variations of NH4+-N (upper left), NO3–-N (upper right), DON (lower left), and TN (lower right) concentrations in percolation water from different treatments during the 2009 rice-growing season.
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set up several years ago. The amount of runoff was recorded after each runoff event. N leaching losses were calculated by multiplying the arithmetic average value of N concentrations in percolation water by the volume of percolation water. At crop maturation, the above ground parts of the plants within a 5-m2 area in each plot were harvested to determine grain yield. The grains were threshed from the straw, oven-dried at 80 C for 24 h and weighted. 2.4. Statistical analysis of data Statistical analyses of the data were performed using the SPSS 12.0 analysis software package. ANOVA was used to test whether the differences in different forms of N leaching losses and grain yields among all treatments were significant at the 0.05 probability level. When the F-test was statistically significant (p < 0.05), mean comparisons were made using the least significant difference (LSD) at the 0.05 probability level. 3. Results and discussions 3.1. N leaching in rice season As shown in Fig. 1, NH4+-N concentrations in percolation water for all treatments were low (<0.3 mg N L1) and nearly unchanged and not significantly different between treatments throughout the rice-growing season. The soil in the present study is a permanentcharge paddy soil and has a negative charge which limits NH4+ leaching. NO3–-N concentrations in percolation water had a great temporal variability (Fig. 1). For all treatments, NO3–-N concentrations reached peak values at 6 days after rice seedling transplanting, and then dropped gradually. Approximately 1 month later, NO3–-N concentrations remained at relatively low levels till the crops were harvested, a pattern similar to that described by Wang et al. (2004). A buildup of high NO3–-N concentrations in percolation water during the early stage of rice growth might have been due to cultivation accelerating the mineralization of the soil organic N (Francis et al., 1995) or the initial field flooding for rice resulted in bypass flow (Zheng et al., 2003; Cao et al., 2006). However, during the mid-late stage of rice growth, surface water which is present during almost the entire season created an anaerobic soil environment inhibiting nitrification, resulting in much lower NO3–-N concentrations. DON concentrations in percolation water for all treatments were roughly constant over the first 70 days, after which those dropped rapidly and remained at relatively low levels (Fig. 1). NO3–-N and TN concentrations in percolation water among all treatments did not differ significantly (p > 0.05) suggesting direct losses from applied fertilizer were minimal as highlighted by Di and Cameron (2002). This observation might have been ascribed to two reasons. On the one hand, the N applied was largely consumed by plant uptake or N losses through NH3 volatilization or nitrification–denitrification processes taking place under the
flooding conditions (Bouman et al., 2002). An unpublished 15N tracing experiment conducted at the same site showed that NH3 volatilization or nitrification–denitrification accounted for more than 40% of applied N. Low NO3–-N available for leaching under irrigated rice is also corroborated by the low levels of NO3–-N in surface water (data not shown). On the other hand, the percolation rate in the present study was low. In the irrigated rice production systems in the area, the puddling before rice seedling transplanting results in the formation of a more compact subsoil layer at 15– 20 cm depth that has a relatively stable low leaching rate, which contributes to maintain soil saturation and the saturated hydraulic conductivity (Di and Cameron, 2002 Zhou et al., 2009). This can also explain why the fertilizer-derived N leaching was very low in the present experiment. NO3–-N was one of the predominant forms of N in percolation water, and contributed over 25% to TN (Table 3). In addition to NO3–-N, a large fraction of N was detected in the forms of DON, accounting for 59.6–64.1% of TN (Table 3). This suggests that DON greatly contributed to N leaching during the rice-growing season. Field experiments conducted in the Taihu Lake region and Dongting Lake region in China all detected a considerable amount of DON in percolation water from rice fields (Zhu et al., 2000; Zhao et al., 2009; Ji et al., 2011). DON in percolation water could originate mainly from the soil N. The N retained in the soil which was mostly in organic forms might have been leached into groundwater during the dynamic adsorption–desorption processes in the soil matrix (Di and Cameron, 2002; Ji et al., 2011). The magnitude of N leaching is dependent on two main factors: N concentrations in percolation water and drainage volume (Liang et al., 2011). During the early stage of rice growth, a high amount of N accumulated in percolation water coincides with a period of high drainage, thence resulting in significantly high N leaching. However, N leaching appeared to be lower during the mid-late stage of rice growth because of the decreased N concentrations in conjunction with a relatively low permeability. Similar results have also been obtained in other studies on rice fields (Wang et al., 2004; Zhou et al., 2009). This indicates that the early stage of rice growth is obviously crucial for mitigating N leaching. The mean rate of percolation was estimated to be 5.0 mm day1, which was comparable to the mean percolation rates of divers rice fields in Jiangsu Province (ranging from 4.86 to 5.58 mm day1) (Cai, 1997). N leaching losses were listed in Table 3. The differences in different forms of N leaching (NH4+-N, NO3–-N, DON, and TN) among all treatments were almost negligible. NH4+-N leaching was minimal, less than 1 kg N ha1 for all treatments. NO3–-N and DON leaching were ca. 2 and 5 kg N ha1, respectively. Approximately 8 kg N ha1 of TN was lost through leaching, accounting for 2.7– 3.5% of total N inputs into paddy fields, which were consistent with the findings obtained in the same area (Zhu et al., 2000; Zhu and Chen, 2002). 3.2. N leaching in wheat season NH4+-N concentrations in percolation water from all treatments remained at the same levels as those during the rice-growing
Table 3 N leaching losses from different treatments during the rice-growing season. NO3-N loss
Code NH4+-N loss
CK CT IT
DON loss
TN loss
Amount (kg N ha1)
Percentage Amount (kg N ha1) of TN (%)
Percentage of TN (%)
Amount (kg N ha1)
Percentage Amount (kg N ha1) of TN (%)
Percentage of total N applied (%)
0.77a 0.74a 0.82a
9.88 9.28 10.4
26.3 31.1 25.5
4.97a 4.75a 5.08a
63.8 59.6 64.1
– 2.66 3.52
2.05a 2.48a 2.02a
Means followed by the same letter within the same column were not significant at the 0.05 level.
7.79a 7.97a 7.92a
3
0.1
0.0
11/20 12/15 1/9
2/3
2/28 3/25 4/19
1
0
11/20 12/15 1/9
Fig. 2. Variations of season.
2/3
CK CT IT
-1
2
Sampling date (d) NH4+-N
Total N concentration (mg N L )
0.2
65
3
CK CT IT
-1
0.3
Nitrate N concentration (mg N L )
CK CT IT
-1
Ammonium N concentration (mg N L )
Y. Cao et al. / Agriculture, Ecosystems and Environment 195 (2014) 61–67
2
1
0
2/28 3/25 4/19
11/20 12/15 1/9
Sampling date (d)
2/3
2/28 3/25 4/19
Sampling date (d)
(left), NO3 -N (medium), and TN (right) concentrations in percolation water from different treatments during the 2009–2010 wheat-growing
season from late November to early February and then dropped rapidly to the end of the wheat season (Fig. 2). This may be due to a combination of plant uptake of NH4+-N, conversion to NO3 or loss through volatilization. Similar to during the rice-growing season, NH4+-N concentrations from all treatments did not differ significantly (p > 0.05). These results indicate that, regardless of which agricultural management regime was adopted, almost no fertilizer-derived NH4+-N was leached under the rice–wheat rotation in the area. The temporal variation of NO3–-N concentrations in percolation water exhibited an opposite trend to NH4+-N, being almost negligible for treatments from late November to early February, and then exhibited an increasing trend (Fig. 2). For the N treatments, NO3–-N concentrations reached peaks within 2 weeks following the second additional fertilization and then decreased gradually. For the CK treatment, NO3–-N concentrations roughly remained at relatively low levels from late Feb. to the end of the wheat season, but slightly higher than those from late Nov. to early Feb. This suggests that the variation of NO3–-N concentrations responded well to the second additional fertilization. Temperature and N fertilization could be the dominating factors to the variations of NO3–-N concentrations. The soil temperatures during the period from late Nov. to early Feb. were relatively low, which delayed the hydrolysis of urea and accompanying nitrification (Liang et al., 2011), thus resulting in extremely low NO3–-N concentrations. The subsequent increase in NO3–-N concentrations was mainly due to the increased soil nitrification rate caused by the elevated air and soil temperatures and more substrate released from urea hydrolysis for microbial nitrification (Liang et al., 2011). More attention should be paid to this observation. N accumulation resulting from nitrification during the late stage of wheat growth might have been a key source of NO3–-N leaching when soils become re-flooded before rice seedling transplanting in the following season (Luo et al., 2011). The variation of TN concentrations in percolation water showed the same pattern as NO3–-N (Fig. 2).
The increase in NO3–-N and TN concentrations in percolation water was mainly associated with N fertilization. The average NO3–-N and TN concentrations for the CK treatment were 0.26 and 0.41 mg N L1, respectively, which were increased by 2.6–3.1 and 2.0–2.6 times for the N treatments. The stimulatory effect of N fertilization on NO3–-N and TN concentrations has also been shown in the previous studies (Li et al., 2007; Zhang et al., 2011). This indicates that excessive N fertilization can result in a high potential for N leaching. By reducing the N rate, the average NO3–-N and TN concentrations for the IT treatment were decreased by 15.7% and 21.6% compared with the CT treatment, respectively. Although NO3–-N concentrations in percolation water in the rice and wheat seasons were below the criterion of 10 mg N L1, NO3–-N leaching might have been a potential threat to the local shallow groundwater. This is because NO3–-N was readily leached into the groundwater system as a result of the relatively shallow groundwater level (usually <1 m) in the area (Liang et al., 2011). Moreover, long-term excessive N fertilization could accelerate NO3–-N accumulation in shallow groundwater (Owens et al., 1992). NO3–-N is the predominant form of N in percolation water, accounting for 64.5–82.9% of the TN (Table 4), which is comparable to the previous published results (Zhu et al., 2000; Zhao et al., 2009). Compared with that in the rice season, more N in percolation water in the wheat season was found in the form of NO3–-N mostly due to higher nitrification capacity in the soil profile or NO3–-N leaching through large soil cracks. From late November to early February, N leaching was small due to the extremely low N concentrations and little rainfall. However, the relatively heavier rainfall triggered much infiltration, and N concentrations were much higher, thence resulting in significant N leaching from late February to the end of the wheat season. No significant differences in NH4+-N and DON leaching losses were observed among all treatments (Table 4). N fertilization significantly increased NO3–-N and TN leaching. NO3–-N and TN leaching were significantly lower (p < 0.05) for the IT treatment compared with the CT treatment. Only ca. 0.6% of the N applied was
Table 4 N leaching losses from different treatments during the wheat-growing season. NO3–-N loss
Code NH4+-N loss 1
Amount (kg N ha CK CT IT
0.17a 0.13a 0.18a
DON loss
TN loss
) Percentage Amount (kg N ha1) Percentage Amount (kg N ha1) Percentage Amount (kg N ha1) Percentage of total N applied (%) of TN (%) of TN (%) of TN (%) 24.4 7.59 13.2
0.44a 1.34c 1.15b
64.5 77.0 82.9
0.08a 0.27b 0.05a
Means followed by the same letter within the same column were not significant at the 0.05 level.
11.1 15.4 3.97
0.68a 1.76c 1.38b
– 0.59 0.61
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Table 5 N fertilization rates and grain yields for different treatments. Code
CK CT IT
Rice season
Wheat season
Rate (kg N ha1)
Grain yield (Mg ha1)
Rate (kg N ha1)
Grain yield (Mg ha1)
0 300 225
5.63a (0.29) 8.06b (0.09) 8.60c (0.13)
0 300 225
3.17a (0.08) 5.99b (0.05) 5.89b (0.11)
Means followed by the same letter within the same column were not significant at the 0.05 level.
lost through leaching which was different from the findings of Liang et al. (2011), but was in the same range (0.5–4.2% of the N applied) obtained by Xing and Zhu (2000) and Tian et al. (2007). 3.3. Grain yield N fertilizer was reduced from 300 kg N ha1 season1 for the CT treatment to 225 kg N ha1 season1 for the IT treatment. However, 25% reduction in the fertilization rate had no significant effect on the wheat yield for the IT treatment compared with the CT treatment (Table 5). The reason for this observation is that the relatively high soil fertility in the area might have mitigated the effect of N fertilizer (Qiao et al., 2012). The N supply from soil indigenous and applied N could be greatly matched by crop N demand by reducing the fertilization rate coupled with increasing the fertilization frequency (Peng et al., 2002; Tilman et al., 2002). Moreover, under the denser plant root systems due to increased planting density, plants could effectively use N fertilizer. These factors resulted in a significant increase in the rice yield for the IT treatment than the CT treatment (8.60 vs. 8.06 Mg ha1). 4. Conclusions NO3–-N is prone to leaching under the wheat–rice rotation with dry–wet cycle water regime. N leaching occurred mostly during the period from late February to final harvest in the wheat season and during the early stage of rice seedling transplanting in the rice season. The IT treatment had no significant difference in N leaching compared with the CT treatment in the rice season, while it significantly decreased N leaching in the wheat season. The IT treatment had no significant effect on the wheat yield compared with the CT treatment, but significantly increased the rice yield. Thus, the improved management practice can be considered effective in reducing N leaching and increasing grain yield under the wheat–rice rotation compared with the conventional management practice. However, the long-term effects of the IT practice should further be investigated in the future. Acknowledgements This study was financially supported by the National Basic Research Program of China (Grant No. 2009CB118603). We sincerely give thanks to Dr. Liqiang Zhou and Mrs. Yujing Peng for their support. References Allen, 1998 Allen, R.G., Pereira, L.S., Raes, D., Smith, M., et al., 1998. Crop evapotranspiration: Guidelines for computing crop water requirements. Irrigation and Drainage Paper NO. 56. FAO, Rome, pp. 301. Bouman, B.A.M., Castañeda, A.R., Bhuiyan, S.I., 2002. Nitrate and pesticide contamination of groundwater under rice-based cropping systems: past and current evidence from the Philippines. Agric. Ecosyst. Environ. 92, 185–199. Cai, G.X., 1997. Ammonia volatilization. In: Zhu, Z.L., Wen, Q.X., Freney, J.R. (Eds.), Nitrogen in soils of China. Kluwer Academic Publishers, Dordrecht/Boston/ London, pp. 193–213.
Cao, W., Hong, H., Zhang, Y., Chen, N., Zeng, Y., Wang, W., 2006. Anthropogenic nitrogen sources and exports in a village-scale catchment in Southeast China. Environ. Geochem. Health 28, 45–51. Chen, X., Wo, F., Chen, C., Fang, K., 2010. Seasonal changes in the concentrations of nitrogen and phosphorus in farmland drainage and groundwater of the Taihu Lake region of China. Environ. Monit. Assess. 169, 159–168. Di, H.J., Cameron, K.C., 2002. Nitrate leaching in temperate agroecosystems: sources, factors and mitigating strategies. Nutr. Cycl. Agroecosyst. 46, 237–256. Dobermann, A., 2007. Nutrient use efficiency – measurement and management. Fertilizer Best Management Practices. IFA, Paris, pp. 1–27. Fang, S.X., Mu, Y.J., 2009. NOx fluxes from several typical agricultural fields during summer–autumn in the Yangtze Delta, China. Atmos. Environ. 43, 2665–2671. Francis, G.S., Haynes, R.J., Williams, P.H., 1995. Effects of the timing of ploughing-in temporary leguminous pastures and two winter cover crops on nitrogen mineralization, nitrate leaching and spring wheat growth. J. Agric. Sci. 124, 1–9. Ji, X.H., Zheng, S.X., Shi, L.H., Liu, Z.B., 2011. Systematic studies of nitrogen loss from paddy soils through leaching in the Dongting Lake area of China. Pedosphere 21, 753–762. Ju, X.T., Kou, C.L., Zhang, F.S., Christie, P., 2006. Nitrogen balance and groundwater nitrate contamination: comparison among three intensive cropping systems on the North China Plain. Environ. Pollut. 143, 117–125. Ju, Ju, X.T., Xing, G.X., Chen, X.P., Zhang, S.L., Zhang, L.J., Liu, X.J., Cui, Z.L., Yin, B., Christie, P., Zhu, Z.L., Zhang, F.S., et al., 2009. Reducing environmental risk by improving N management in intensive Chinese agricultural systems. Proceedings of the National Academy of Sciences 106, 3041–3046. Kumazawa, K., 2002. Nitrogen fertilization and nitrate pollution in groundwater in Japan: Present status and measures for sustainable agriculture. Nutr. Cycl. Agroecosyst. 63, 129–137. Li, X., Hu, C., Delgado, J.A., Zhang, Y., Ouyang, Z., 2007. Increased nitrogen use efficiencies as a key mitigation alternative to reduce nitrate leaching in north china plain. Agric. Water Manage. 89, 137–147. Liang, Liang, X.Q., Xu, L., Li, H., He, M.M., Qian, Y.C., Liu, J., Nie, Z.Y., Ye, Y.S., Chen, Y.X., et al., 2011. Influence of N fertilization rates, rainfall, and temperature on nitrate leaching from a rainfed winter wheat field in Taihu watershed. Phys. Chem. Earth 36, 395–400. Liu, C., Zheng, X., Zhou, Z., Han, S., Wang, Y., Wang, K., Liang, W., Li, M., Chen, D., Yang, Z., 2010. Nitrous oxide and nitric oxide emissions from an irrigated cotton field in Northern China. Plant Soil 332, 123–134. Luo, L.G., Itoh, S., Zhang, Q., Yang, S., Zhang, Q., Yang, Z., 2011. Leaching behavior of nitrogen in a long-term experiment on rice under different N management systems. Environ. Monit. Assess. 177, 141–150. Novozamsky, I., Eck, R.V., Schouwenburg, J.C.V., Walinga, I., 1974. Total nitrogen determination in plant material by means of the indophenol-blue method. Neth. J. Agric. Sci. 22, 3–5. Owens, L.B., Edwards, W.M., Van Keuren, R.W., 1992. Nitrate levels in shallow groundwater under pastures receiving ammonium nitrate or slow-release nitrogen fertilizer. J. Environ. Qual. 21, 607–613. Peng, S., Tang, Q., Zou, Y., 2009. Current status and challenges of rice production in China. Plant Prod. Sci. 12, 3–8. Peng, S.B., Huang, J.L., Zhong, X.H., Yang, J.C., Wang, G.H., Zhou, Y.B., Zhang, F.S., 2002. Research strategy in improving fertilizer nitrogen use efficiency of irrigated rice in China. Sci. Agric. Sinica 35, 1095–1103. Power, J.F., Wiese, R., Flowerday, D., 2001. Managing farming systems for nitrate control: a research review from management systems evaluation areas. J. Environ. Manage. 30, 1866–1880. Qiao, J., Yang, L., Yan, T., Xue, F., Zhao, D., 2012. Nitrogen fertilizer reduction in rice production for two consecutive years in the Taihu Lake area. Agric. Ecosys. Environ. 146, 103–112. Tian, Y.H., Yin, B., Yang, L.Z., Yin, S.X., Zhu, Z.L., 2007. Nitrogen runoff and leaching losses during rice–wheat rotations in Taihu Lake region, China. Pedosphere 17, 445–456. Tilman, D., Cassman, K.G., Matson, P.A., Naylor, R., Polasky, S., 2002. Agricultural sustainability and intensive production practices. Nature 418, 671–677. Wang, D.J., liu, Q., Lin, J.H., Sun, R.J., 2004. Optimum nitrogen use and reduced nitrogen loss for production of rice and wheat in the Yangtse Delta region. Environ. Geochem. Health 26, 221–227. Wang, X.M., Shen, S.H., Han, X.M., Xu, Y.H., 2012. Study on evapotranspiration of the winter wheat measured by large-scale weighting lysimeter. Meteorol. Environ. Sci. 34, 14–18 (in Chinese). Xie, Y., Xiong, Z., Xing, G., Yan, X., Shi, S., Sun, G., Zhu, Z., 2008. Source of nitrogen in wet deposition to a rice agroecosystem at Tai lake region. Atmos. Environ. 42, 5182–5192.
Y. Cao et al. / Agriculture, Ecosystems and Environment 195 (2014) 61–67 Xing, G.X., Zhu, Z.L., 2000. An assessment of N loss from agricultural fields to the environment in China. Nutr. Cycl. Agroecosyst. 57, 67–73. Zhang, J.S., Zhang, F.P., Yang, J.H., Wang, J.P., Cai, M.L., Li, C.F., Cao, C.G., 2011. Emissions of N2O and NH3, and nitrogen leaching from direct seeded rice under different tillage practices in central China. Agric. Ecosyst. Environ. 140, 164–173. Zhao, X., Xie, Y.X., Xiong, Z.Q., Yan, X.Y., Xing, G.X., Zhu, Z.L., 2009. Nitrogen fate and environmental consequence in paddy soil under rice–wheat rotation in the Taihu lake region, China. Plant Soil 319, 225–234. Zheng, X., Huang, Y., Wang, Y., Wang, M., 2003. Seasonal characteristics of nitric oxide emission from a typical Chinese rice–wheat rotation during the nonwaterlogged period. Global Change Biol. 9, 219–227.
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Short communication
Improving agronomic practices to reduce nitrate leaching from the rice–wheat rotation system Yansheng Cao a,b, * , Yuhua Tian a , Bin Yin a , Zhaoliang Zhu a a b
State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, 71 Beijing East Road, Nanjing 210008, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 November 2013 Received in revised form 14 May 2014 Accepted 16 May 2014 Available online xxx
Winter wheat–summer rice rotation with unique dry–wet cycle water regime is one of the most general cropping systems in the Yangtze Delta, which might favor NO3-N leaching into shallow groundwater due to high fertilizer-N input with low N use efficiency. To alleviate the detrimental impacts caused by N leaching, it is necessary to apply improved management practices. Therefore, the objectives of the present study are to investigate characteristics of N leaching under drained upland crop-flooded rice rotation and illustrate the effectiveness of an improved agronomic practice for mitigating NO3-N leaching. N leaching measured in situ for the control (CK), conventional (CT), and improved (IT) treatments for one rice–wheat rotation. Improved fertilization and crop cultivation managements have been applied to the IT treatment in an integrated manner, including low N fertilization rate, high fertilization frequency, and dense planting. During the rice-growing season, NO3–-N and dissolved organic N (DON) were the predominant forms of N in percolation water, and contributed over 25% and 59% to TN, respectively. NO3–-N and TN concentrations in percolation water and leaching losses did not differ significantly (p > 0.05) between treatments. N leaching occurred mainly during the early stage of rice seedling transplanting. During the wheat-growing season, N in percolation water was predominantly in the form of NO3–-N, and accounted for over 64.5–82.9% of TN. N fertilization significantly increased NO3–-N and TN concentrations in percolation water. Nitrate and TN leaching losses from the IT treatment (1.15 and 1.38 kg N ha1, respectively) were smaller than the CT treatment (1.34 and 1.76 kg N ha1, respectively). The majority of N leaching losses for wheat occurred during the period from late February to final harvest. The IT treatment exhibited no significant effect on wheat yield, but significantly (p < 0.05) increased rice yield (8.60 Mg ha1 compared to 8.06 Mg ha1) relative to the CT treatment. Compared with the conventional management practice, the improved management practice appeared to be effective in reducing N leaching and increasing grain yield under the rice–wheat rotation. ã 2014 Elsevier B.V. All rights reserved.
Keywords: Nitrate Leaching Rice Wheat
1. Introduction Increasing NO3–-N concentrations in shallow groundwater and surface waters continue to be of international concern with respect to human health and eutrophication. The rising concentrations of NO3–-N in shallow groundwater and surface waters are closely associated with the intensification of agricultural production with increasing application of inorganic N fertilizers (Kumazawa, 2002; Ju et al., 2006).
* Corresponding author at: State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, 71 Beijing East Road, Nanjing, China. Tel.: +86 21 86881543. E-mail address: [email protected] (Y. Cao). http://dx.doi.org/10.1016/j.agee.2014.05.020 0167-8809/ ã 2014 Elsevier B.V. All rights reserved.
The Yangtze Delta, within latitude 30 200 N and 32 480 N and longitude 119 240 E and 121540 E, is the most developed economic area and the most intensive agricultural region in China (Fang and Mu, 2009). Its grain cropping system is characterized by large inputs of synthetic N fertilizer but low N use efficiency (Ju et al., 2009). Winter wheat–summer rice rotation is one of the most common farming patterns in the area (Liang et al., 2011) and unique water management practice with a dry–wet cycle is adopted. The water regime is likely to result in favorable conditions for NO3–-N leached into shallow groundwater. The aerobic soil environment is kept for almost the entire wheat season, which contributes to microbial nitrification (Liu et al., 2010). This significant accumulation of NO3–-N within the soil after the wheat plants is harvested is susceptible to leaching when the soil is reflooded before transplantation of rice seedling in the following season. The NO3–-N concentrations in shallow groundwater have
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been found to exceed the maximum permissible concentration recommended by USEPA (10 mg NO3–-N L1) or by WHO (11.3 mg NO3–-N L1) for drinking water in the area (Zhu and Chen, 2002; Chen et al., 2010). Algal bloom has also been observed recently in the Taihu Lake located in the central Yangtze Delta (Xie et al., 2008). One of the keys to controlling NO3 leaching is to select a package of management practices that can best control NO3 accumulation in the soil profile (Power et al., 2001). Various improved management practices that better match spatial and temporal N supply with plant N demand can significantly increase N use efficiency (Peng et al., 2002; Tilman et al., 2002; Dobermann, 2007; Ju et al., 2009; Peng et al., 2009), which is likely to regulate the accumulation of NO3 in the soil profile, thereby controlling NO3 leaching. The objectives of the present study are to: (a) examine the temporal variation of N concentrations in percolation water and the characteristics of N leaching in situ; (b) evaluate the effectiveness of an improved management practice combining improved fertilization with crop cultivation for mitigating N leached from the plant root zone. 2. Materials and methods
treatments were 19.6 kg P ha1 and 49.8 kg K ha1, respectively, in both seasons. In the rice season, N fertilizer was applied in three splits for the CT treatment and in four splits for the IT treatment. P fertilizer was only basal-applied for both treatments. K fertilizer was only basal-applied for the CT treatment and was applied in two splits for the IT treatment. In the wheat season, N fertilizer was applied in three splits for both treatments, while both P and K fertilizers were only basal-applied. The detailed fertilization scheme in both seasons for different treatments is shown in Table 2. Basal fertilizers were incorporated into the topsoil and the additional fertilizers were homogeneously broadcast onto the surface soil by hand. The surface water was principally maintained at a depth of 5 cm by irrigation except during mid-season aeration (over 1 week) and final drainage, which was performed ca. 1 week before rice harvesting. No irrigation was applied for both treatments during the wheat-growing season. Pesticide and herbicide application were the same for all treatments. Each plot was 40 m2 in area and earthen banks covered with plastic film were constructed between each plot to prevent lateral water movement. Summer rice was transplanted and harvested on June 16 and October 24, 2009, respectively. Winter wheat was sown and harvested on November 23, 2009 and June 6, 2010, respectively.
2.1. Experimental site summarization 2.3. Collection and measurement of percolation water The field experimental site was situated in Suzhou City, Jiangsu Province, China. The climate is characterized by a humid subtropical monsoon. The annual mean temperature was 15.5 C, with an annual mean precipitation of 1038 mm. The soil of the experimental site is a gleyed paddy soil evolved from lacustrine deposits, which is classified as Hydragric Anthropy. The main physic-chemical properties of the 0–20 cm soil layer are presented in Table 1. The experimental site has been cultivated with the summer rice (Oryza sativa L.)–winter wheat (Triticum aestivum L.) rotation, where rice was planted from June to October each year and wheat was planted from November each year to May the following year for the past 10 years. The typical farm management practices were equivalent to the CT treatment. Meteorological data were collected from the weather station which was situated in the experimental site. 2.2. Experimental design and farm management practices The field experiments were conducted for a rice–wheat rotation. Three treatments with four replicates were designed in a randomized complete block, included control (no N fertilizer applied, CK), conventional (CT), and improved (IT). Other farm management practices except N fertilization were the same between the CK and CT treatments in the rice and wheat seasons, while planting density and times and rate of fertilization in the rice season and rate of fertilization in the wheat season were different between the CT and IT treatments. Rice seedlings were transplanted into well-puddled soils at the spacing of 20 cm 20 cm and 20 cm 15 cm for the CT and IT treatments, respectively. Wheat was direct seeded at a density of 225 kg ha1 for all treatments. The N fertilization rates in the form of urea (N 46%) were 300 kg N ha1 for the CT treatment and 225 kg N ha1 for the IT treatment in the two seasons. The application rates of P (as superphosphate P2O5 16%) and K fertilizer (as potassium chloride K2O 60%) for all
A porous cup (2 mm pore size) was set up into the soil at an angel of ca. 10 in each plot, with the center of the cup at a depth of 90 cm. The porous cup was pumped out 24 h before each sampling time which was once every 10–15 days. Samples were stored in the refrigerator at 4 C till analysis for NH4+-N, NO3–-N, and TN concentrations with the indophenol blue method (Novozamsky et al., 1974), ultraviolet spectrophotometric method (HJ/T 3462007), and alkaline potassium persulfate digestion-ultraviolet spectrophotometric method (GB 11849-89), respectively. DON was difference between TN and mineral N (NH4+- and NO3–-N). The method used in the present study has its limitation as it is difficult to determine the actual volume of water leached. While this can be estimated using indirect methods such as a lysimeter experiment during the rice growth season. During the rice growth season, the rate of percolation was determined using a lysimeter (a height of ca. 1.5 m and an internal diameter of ca. 1.1 m) experiment and undisturbed soil column with water sampled at the bottom of the lysimeter. The irrigation scheme in the lysimeter experiment was the same as that in the field experiment. However, the lysimeter experiment is unsuitable for the measurement of the percolation water during the wheat growth season and so this was calculated from the mass balance of water as follows: L ¼ ðP þ IÞ ðR þ ETÞ where L is the amount of percolation water in mm, P is the amount of precipitation in mm, I is the amount of irrigation water in mm, R is the amount of surface runoff in mm, and ET is the amount of evapotranspiration in mm, which is calculated according to the method reported by Wang et al. (2012). The ET can be expressed as follows: ET ¼ K c K w ETo where Kc is the crop coefficient, Kw is the soil moisture coefficient, which can be calculated according to the method of Wang et al.
Table 1 The main physic-chemical properties of the surface soil (0–20 cm). Items
pH
Soil organic matter (g kg1)
Total N (g kg1)
Total P (g kg1)
Olsen P (mg kg1)
Extractable K (mg kg1)
Cation exchange capacity
Value
7.36
35.0
2.09
0.93
5.00
121.3
17.7
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Table 2 The scheme of chemical fertilizer management for different treatments in the rice and wheat seasons. Code Rice season Basal fertilizer 2009/6/16
Wheat season 1st additional fertilizer 2009/6/23
19.6P + 39.6K – 180N + 19.6P + 49.8K 60 N 112.5N + 19.6P + 24.9K 22.5 N
CK CT IT
2nd additional fertilizer 2009/7/27
3rd additional fertilizer 2009/8/13
Basal fertilizer
1st additional fertilizer 2010/3/11
2009/11/23
– – 45N + 24.9K
– 60 N 45 N
19.6P + 39.6K – 180N + 19.6P + 49.8K 30 N 135N + 19.6P + 49.8K 22.5 N
2nd additional fertilizer 2010/3/21 – 90 N 67.5 N
Units of N, P, and K fertilizers are kg N ha1, kg P ha1, and kg K ha1, respectively.
(2012) ETo is the reference crop evapotranspiration in mm day1, which is calculated using the modified Penman–Monteith method (Allen et al., 1998). The Kw and ETo can be expressed as follows: Kw ¼
Sw u w u c uw
ETo ¼
0:408DðRn GÞ þ g ð900=T þ 273ÞU 2 VPD D þ g ð1 þ 0:34U2 Þ
at the crop surface in MJ m2 day1, G is the soil heat flux density in MJ m2 day1, g is the psychrometric constant in kPa C1, T is the mean daily air temperature at 2 m height in C, U2 is the wind speed at 2 m height in m s1, VPD is the saturation vapor pressure deficit in kPa. After rice was harvested, a drain ditch with a depth of ca. 12 cm and a width of ca. 15 cm was dug within each plot to protect the wheat plant from water logging. A runoff collector, ca. 10 cm below the ground, was set up in each plot. The surface runoff from individual plot was carried to a corresponding collection pool constructed outside the plots via buried pipes. All the collectors, collection pools, and buried pipes have been
where Sw is the mean soil moisture in mm, uc is the field capacity in mm, uw is the wilting moisture in mm, D is the slope of the saturation vapor pressure curve in kPa C1, Rn is the net radiation
CK CT IT
CK CT IT
4 -1
3
2
1
0
6/15
3
2
1
0
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4
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4 -1
Total N concentration (mg N L )
-1
Dissolved organic N concentration (mg N L )
Nitrate N concentration (mg N L )
-1
Ammonium N concentration (mg N L )
4
3
2
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0
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0
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9/13
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10/13
6/15
6/30
7/15
7/30
8/14
8/29
Sampling date (mm/dd)
Fig. 1. Variations of NH4+-N (upper left), NO3–-N (upper right), DON (lower left), and TN (lower right) concentrations in percolation water from different treatments during the 2009 rice-growing season.
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set up several years ago. The amount of runoff was recorded after each runoff event. N leaching losses were calculated by multiplying the arithmetic average value of N concentrations in percolation water by the volume of percolation water. At crop maturation, the above ground parts of the plants within a 5-m2 area in each plot were harvested to determine grain yield. The grains were threshed from the straw, oven-dried at 80 C for 24 h and weighted. 2.4. Statistical analysis of data Statistical analyses of the data were performed using the SPSS 12.0 analysis software package. ANOVA was used to test whether the differences in different forms of N leaching losses and grain yields among all treatments were significant at the 0.05 probability level. When the F-test was statistically significant (p < 0.05), mean comparisons were made using the least significant difference (LSD) at the 0.05 probability level. 3. Results and discussions 3.1. N leaching in rice season As shown in Fig. 1, NH4+-N concentrations in percolation water for all treatments were low (<0.3 mg N L1) and nearly unchanged and not significantly different between treatments throughout the rice-growing season. The soil in the present study is a permanentcharge paddy soil and has a negative charge which limits NH4+ leaching. NO3–-N concentrations in percolation water had a great temporal variability (Fig. 1). For all treatments, NO3–-N concentrations reached peak values at 6 days after rice seedling transplanting, and then dropped gradually. Approximately 1 month later, NO3–-N concentrations remained at relatively low levels till the crops were harvested, a pattern similar to that described by Wang et al. (2004). A buildup of high NO3–-N concentrations in percolation water during the early stage of rice growth might have been due to cultivation accelerating the mineralization of the soil organic N (Francis et al., 1995) or the initial field flooding for rice resulted in bypass flow (Zheng et al., 2003; Cao et al., 2006). However, during the mid-late stage of rice growth, surface water which is present during almost the entire season created an anaerobic soil environment inhibiting nitrification, resulting in much lower NO3–-N concentrations. DON concentrations in percolation water for all treatments were roughly constant over the first 70 days, after which those dropped rapidly and remained at relatively low levels (Fig. 1). NO3–-N and TN concentrations in percolation water among all treatments did not differ significantly (p > 0.05) suggesting direct losses from applied fertilizer were minimal as highlighted by Di and Cameron (2002). This observation might have been ascribed to two reasons. On the one hand, the N applied was largely consumed by plant uptake or N losses through NH3 volatilization or nitrification–denitrification processes taking place under the
flooding conditions (Bouman et al., 2002). An unpublished 15N tracing experiment conducted at the same site showed that NH3 volatilization or nitrification–denitrification accounted for more than 40% of applied N. Low NO3–-N available for leaching under irrigated rice is also corroborated by the low levels of NO3–-N in surface water (data not shown). On the other hand, the percolation rate in the present study was low. In the irrigated rice production systems in the area, the puddling before rice seedling transplanting results in the formation of a more compact subsoil layer at 15– 20 cm depth that has a relatively stable low leaching rate, which contributes to maintain soil saturation and the saturated hydraulic conductivity (Di and Cameron, 2002 Zhou et al., 2009). This can also explain why the fertilizer-derived N leaching was very low in the present experiment. NO3–-N was one of the predominant forms of N in percolation water, and contributed over 25% to TN (Table 3). In addition to NO3–-N, a large fraction of N was detected in the forms of DON, accounting for 59.6–64.1% of TN (Table 3). This suggests that DON greatly contributed to N leaching during the rice-growing season. Field experiments conducted in the Taihu Lake region and Dongting Lake region in China all detected a considerable amount of DON in percolation water from rice fields (Zhu et al., 2000; Zhao et al., 2009; Ji et al., 2011). DON in percolation water could originate mainly from the soil N. The N retained in the soil which was mostly in organic forms might have been leached into groundwater during the dynamic adsorption–desorption processes in the soil matrix (Di and Cameron, 2002; Ji et al., 2011). The magnitude of N leaching is dependent on two main factors: N concentrations in percolation water and drainage volume (Liang et al., 2011). During the early stage of rice growth, a high amount of N accumulated in percolation water coincides with a period of high drainage, thence resulting in significantly high N leaching. However, N leaching appeared to be lower during the mid-late stage of rice growth because of the decreased N concentrations in conjunction with a relatively low permeability. Similar results have also been obtained in other studies on rice fields (Wang et al., 2004; Zhou et al., 2009). This indicates that the early stage of rice growth is obviously crucial for mitigating N leaching. The mean rate of percolation was estimated to be 5.0 mm day1, which was comparable to the mean percolation rates of divers rice fields in Jiangsu Province (ranging from 4.86 to 5.58 mm day1) (Cai, 1997). N leaching losses were listed in Table 3. The differences in different forms of N leaching (NH4+-N, NO3–-N, DON, and TN) among all treatments were almost negligible. NH4+-N leaching was minimal, less than 1 kg N ha1 for all treatments. NO3–-N and DON leaching were ca. 2 and 5 kg N ha1, respectively. Approximately 8 kg N ha1 of TN was lost through leaching, accounting for 2.7– 3.5% of total N inputs into paddy fields, which were consistent with the findings obtained in the same area (Zhu et al., 2000; Zhu and Chen, 2002). 3.2. N leaching in wheat season NH4+-N concentrations in percolation water from all treatments remained at the same levels as those during the rice-growing
Table 3 N leaching losses from different treatments during the rice-growing season. NO3-N loss
Code NH4+-N loss
CK CT IT
DON loss
TN loss
Amount (kg N ha1)
Percentage Amount (kg N ha1) of TN (%)
Percentage of TN (%)
Amount (kg N ha1)
Percentage Amount (kg N ha1) of TN (%)
Percentage of total N applied (%)
0.77a 0.74a 0.82a
9.88 9.28 10.4
26.3 31.1 25.5
4.97a 4.75a 5.08a
63.8 59.6 64.1
– 2.66 3.52
2.05a 2.48a 2.02a
Means followed by the same letter within the same column were not significant at the 0.05 level.
7.79a 7.97a 7.92a
3
0.1
0.0
11/20 12/15 1/9
2/3
2/28 3/25 4/19
1
0
11/20 12/15 1/9
Fig. 2. Variations of season.
2/3
CK CT IT
-1
2
Sampling date (d) NH4+-N
Total N concentration (mg N L )
0.2
65
3
CK CT IT
-1
0.3
Nitrate N concentration (mg N L )
CK CT IT
-1
Ammonium N concentration (mg N L )
Y. Cao et al. / Agriculture, Ecosystems and Environment 195 (2014) 61–67
2
1
0
2/28 3/25 4/19
11/20 12/15 1/9
Sampling date (d)
2/3
2/28 3/25 4/19
Sampling date (d)
(left), NO3 -N (medium), and TN (right) concentrations in percolation water from different treatments during the 2009–2010 wheat-growing
season from late November to early February and then dropped rapidly to the end of the wheat season (Fig. 2). This may be due to a combination of plant uptake of NH4+-N, conversion to NO3 or loss through volatilization. Similar to during the rice-growing season, NH4+-N concentrations from all treatments did not differ significantly (p > 0.05). These results indicate that, regardless of which agricultural management regime was adopted, almost no fertilizer-derived NH4+-N was leached under the rice–wheat rotation in the area. The temporal variation of NO3–-N concentrations in percolation water exhibited an opposite trend to NH4+-N, being almost negligible for treatments from late November to early February, and then exhibited an increasing trend (Fig. 2). For the N treatments, NO3–-N concentrations reached peaks within 2 weeks following the second additional fertilization and then decreased gradually. For the CK treatment, NO3–-N concentrations roughly remained at relatively low levels from late Feb. to the end of the wheat season, but slightly higher than those from late Nov. to early Feb. This suggests that the variation of NO3–-N concentrations responded well to the second additional fertilization. Temperature and N fertilization could be the dominating factors to the variations of NO3–-N concentrations. The soil temperatures during the period from late Nov. to early Feb. were relatively low, which delayed the hydrolysis of urea and accompanying nitrification (Liang et al., 2011), thus resulting in extremely low NO3–-N concentrations. The subsequent increase in NO3–-N concentrations was mainly due to the increased soil nitrification rate caused by the elevated air and soil temperatures and more substrate released from urea hydrolysis for microbial nitrification (Liang et al., 2011). More attention should be paid to this observation. N accumulation resulting from nitrification during the late stage of wheat growth might have been a key source of NO3–-N leaching when soils become re-flooded before rice seedling transplanting in the following season (Luo et al., 2011). The variation of TN concentrations in percolation water showed the same pattern as NO3–-N (Fig. 2).
The increase in NO3–-N and TN concentrations in percolation water was mainly associated with N fertilization. The average NO3–-N and TN concentrations for the CK treatment were 0.26 and 0.41 mg N L1, respectively, which were increased by 2.6–3.1 and 2.0–2.6 times for the N treatments. The stimulatory effect of N fertilization on NO3–-N and TN concentrations has also been shown in the previous studies (Li et al., 2007; Zhang et al., 2011). This indicates that excessive N fertilization can result in a high potential for N leaching. By reducing the N rate, the average NO3–-N and TN concentrations for the IT treatment were decreased by 15.7% and 21.6% compared with the CT treatment, respectively. Although NO3–-N concentrations in percolation water in the rice and wheat seasons were below the criterion of 10 mg N L1, NO3–-N leaching might have been a potential threat to the local shallow groundwater. This is because NO3–-N was readily leached into the groundwater system as a result of the relatively shallow groundwater level (usually <1 m) in the area (Liang et al., 2011). Moreover, long-term excessive N fertilization could accelerate NO3–-N accumulation in shallow groundwater (Owens et al., 1992). NO3–-N is the predominant form of N in percolation water, accounting for 64.5–82.9% of the TN (Table 4), which is comparable to the previous published results (Zhu et al., 2000; Zhao et al., 2009). Compared with that in the rice season, more N in percolation water in the wheat season was found in the form of NO3–-N mostly due to higher nitrification capacity in the soil profile or NO3–-N leaching through large soil cracks. From late November to early February, N leaching was small due to the extremely low N concentrations and little rainfall. However, the relatively heavier rainfall triggered much infiltration, and N concentrations were much higher, thence resulting in significant N leaching from late February to the end of the wheat season. No significant differences in NH4+-N and DON leaching losses were observed among all treatments (Table 4). N fertilization significantly increased NO3–-N and TN leaching. NO3–-N and TN leaching were significantly lower (p < 0.05) for the IT treatment compared with the CT treatment. Only ca. 0.6% of the N applied was
Table 4 N leaching losses from different treatments during the wheat-growing season. NO3–-N loss
Code NH4+-N loss 1
Amount (kg N ha CK CT IT
0.17a 0.13a 0.18a
DON loss
TN loss
) Percentage Amount (kg N ha1) Percentage Amount (kg N ha1) Percentage Amount (kg N ha1) Percentage of total N applied (%) of TN (%) of TN (%) of TN (%) 24.4 7.59 13.2
0.44a 1.34c 1.15b
64.5 77.0 82.9
0.08a 0.27b 0.05a
Means followed by the same letter within the same column were not significant at the 0.05 level.
11.1 15.4 3.97
0.68a 1.76c 1.38b
– 0.59 0.61
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Table 5 N fertilization rates and grain yields for different treatments. Code
CK CT IT
Rice season
Wheat season
Rate (kg N ha1)
Grain yield (Mg ha1)
Rate (kg N ha1)
Grain yield (Mg ha1)
0 300 225
5.63a (0.29) 8.06b (0.09) 8.60c (0.13)
0 300 225
3.17a (0.08) 5.99b (0.05) 5.89b (0.11)
Means followed by the same letter within the same column were not significant at the 0.05 level.
lost through leaching which was different from the findings of Liang et al. (2011), but was in the same range (0.5–4.2% of the N applied) obtained by Xing and Zhu (2000) and Tian et al. (2007). 3.3. Grain yield N fertilizer was reduced from 300 kg N ha1 season1 for the CT treatment to 225 kg N ha1 season1 for the IT treatment. However, 25% reduction in the fertilization rate had no significant effect on the wheat yield for the IT treatment compared with the CT treatment (Table 5). The reason for this observation is that the relatively high soil fertility in the area might have mitigated the effect of N fertilizer (Qiao et al., 2012). The N supply from soil indigenous and applied N could be greatly matched by crop N demand by reducing the fertilization rate coupled with increasing the fertilization frequency (Peng et al., 2002; Tilman et al., 2002). Moreover, under the denser plant root systems due to increased planting density, plants could effectively use N fertilizer. These factors resulted in a significant increase in the rice yield for the IT treatment than the CT treatment (8.60 vs. 8.06 Mg ha1). 4. Conclusions NO3–-N is prone to leaching under the wheat–rice rotation with dry–wet cycle water regime. N leaching occurred mostly during the period from late February to final harvest in the wheat season and during the early stage of rice seedling transplanting in the rice season. The IT treatment had no significant difference in N leaching compared with the CT treatment in the rice season, while it significantly decreased N leaching in the wheat season. The IT treatment had no significant effect on the wheat yield compared with the CT treatment, but significantly increased the rice yield. Thus, the improved management practice can be considered effective in reducing N leaching and increasing grain yield under the wheat–rice rotation compared with the conventional management practice. However, the long-term effects of the IT practice should further be investigated in the future. Acknowledgements This study was financially supported by the National Basic Research Program of China (Grant No. 2009CB118603). We sincerely give thanks to Dr. Liqiang Zhou and Mrs. Yujing Peng for their support. References Allen, 1998 Allen, R.G., Pereira, L.S., Raes, D., Smith, M., et al., 1998. Crop evapotranspiration: Guidelines for computing crop water requirements. Irrigation and Drainage Paper NO. 56. FAO, Rome, pp. 301. Bouman, B.A.M., Castañeda, A.R., Bhuiyan, S.I., 2002. Nitrate and pesticide contamination of groundwater under rice-based cropping systems: past and current evidence from the Philippines. Agric. Ecosyst. Environ. 92, 185–199. Cai, G.X., 1997. Ammonia volatilization. In: Zhu, Z.L., Wen, Q.X., Freney, J.R. (Eds.), Nitrogen in soils of China. Kluwer Academic Publishers, Dordrecht/Boston/ London, pp. 193–213.
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