Effects of straw application on nitrate leaching in fields in the Yellow River irrigation zone of Ningxia, China

A five-year field experiment was conducted to investigate the effects of straw application on nitrate leaching loss. Treatments included soil that was not treated (control), soil treated with straw at a low rate (4,500 kg ha−2, T1) and soil treated with straw at a high rate (9,000 kg ha−2, T2). Nitrate-nitrogen leaching in the 10, 20, 30, 60, and 90 cm soil layers was measured using the resin-core method. The results indicated that straw application could reduce soil nitrate leaching losses in the 0–30 cm layer. In this layer, the nitrate leaching values for T1 (13.76 kg ha−2) and T2 (13.74 kg ha−2) were both significantly lower than those of the control (15.76 kg ha−2) (P < 0.05); the soil nitrate leaching losses decreased by 12.71% and 12.84% for those two treatments, respectively. However, no significant differences in losses were observed (P > 0.05) between T1 and T2. The effects of straw application were apparent only in the ploughing layer (30 cm-depth soil layer). In the deeper layers (60 and 90 cm), no significant differences were observed between the treatments and the control, and the same results were observed in the topsoil layers (10 and 20 cm).

maintain a high carbon/nitrogen ratio and are used to reduce soil soluble nitrogen release in a short time frame. In Europe, OM applications are limited to 110-140 kg ha −2 per season; these rates can markedly decrease nitrate losses 13 . Manures replace some chemical fertilizers and can help control soil nitrogen release 14 . Losses from annual nitrate leaching can be 4.4-5.6 times greater in conventional plots than in organic plots as well as in integrated plots located between conventional and organic plots. Therefore, compared with conventional practices, organic and integrated fertilization practices more actively and efficiently support denitrifier communities, shift the balance of nitrogen emissions and nitrate losses, and cause less environmental harm 15 . Nitrates mainly originate from the mineralization of nonleguminous soil organic matter (SOM); only one-quarter of nitrates originate from leguminous SOM 16 . OM applications can reduce inorganic nitrate leaching, and wheat straw is better than either sawdust or lignite powder 17 . Many studies have revealed environmental advantages of compost regarding reducing both mineralization rates and the potential for nitrate leaching and delaying the change of organic nitrogen to nitrate 18 . Mamo determined that nitrate leaching could be reduced by substituting compost for commercial inorganic fertilizer in vegetable fields 19 . Brinton reported lower potential nitrate leaching from soils amended with compost rather than uncomposted manure in a corn field 20 . The application of rice straw compost can reduce nitrate concentrations in test plots, which implies that the incorporation of rice straw reduces the nitrate concentration not only in surface water but also in percolated water at a depth of 10 cm; the nitrate concentration was much lower at a depth of 20 cm than at 10 cm in the test plots 21 . In another study, although only approximately 10% of the applied 15 N-Iabelled fertilizer remained in the 0-30 cm layer of the control and plant manure plots, more than 25% of the applied 15 N remained in the pig compost plot 22 . An inverse relationship between dissolved organic carbon (DOC) and nitrate concentrations in streams in Japan is closely related to excess nitrogen availability together with a carbon deficit in the soil environment 23 . In a multi-site, four-year study, the application of organic materials to soils enhanced the immobilization of nitrogen into microbial biomass and therefore reduced inorganic nitrogen concentrations at all test sites in all 4 years of the study 24 . If the goal is to reduce nitrate losses, organic fertilizer application rates should not exceed 175 kg N ha −2 in nitrate-vulnerable zones. However, at the Rothamsted Experimental Station, the index was 276 kg N ha −2 25 . Bird guano application resulted in more nitrate losses when the application amount exceeded 11.2 × 10 3 kg ha −2 26 .
In the Ningxia irrigation zone, the SOM is usually deficient; the values range from 9.2 to 14.5 g kg −1 , and the average value is 10.2 g kg −1 . Nitrogen fertilizer application rates can reach 301 kg ha −2 , which is 1.6 times greater than the national average. In the surface water in this region, 61-66% of the TN and 76-81% of the nitrite originates from farmland sources. Approximately 7 billion m 3 y −1 of water is drawn from the Yellow River, and 2.5 billion m 3 y −1 enters the Yellow River again by percolation and filtration from upland fields. The average irrigation rates are 15-45 × 10 3 m 3 ha −2 for rice, 3.3-4.5 × 10 3 m 3 ha −2 for wheat/corn (intercropping), and 1.5 × 10 3 m 3 for winter irrigation and spring irrigation ha −2 27 . These data suggest that the Ningxia irrigation zone is a major region of nonpoint source pollution. The objective of the present field experiments was to prove that straw application affects the control of water pollution, and the long-term goal is to extensively improve straw application in China, especially within irrigation zones.

Materials and Methods
Study zone. The field trial (106°17′52″E, 38°07′26″N) was located in Lingwu County in the Ningxia Autonomous Region. Tvhis area is part of the upstream irrigation zone of the Yellow River ( Fig. 1) and is in a temperate arid zone, which consists of low rainfall and a dry climate. The rainy season is from July to September and accounts for 70% of the total annual precipitation. Snow seldom occurs in winter. The average annual precipitation is only 193 mm y −1 , and the evaporation rate is 1,763 mm y −1 . There are only approximately 150-164 frostfree days. The average annual temperature is 8.9 °C, and the active accumulated temperature ≥10 °C ranges from 3,200-3,400 °C. The average elevation above sea level is 1,130 m, and there are between 2,800 and 3,100 annual daylight hours in this area. The water used for agriculture in the area relies on gravity flow systems. The soil type is classified as anthropogenic-alluvial with low fertility ( Table 1). The topsoil is highly saline and has a high pH, and the area is in a single-harvest zone. The main crops are rice, corn, and spring wheat, and the typical cropping system is a rice-corn rotation.
Experimental design. Each plot was insulated by brick walls lined with plastic film to prevent water exchange; these walls were 40 cm high above the ground and 80 cm deep beneath the soil. There were 3 treatments: no straw application (CK: 0 kg ha −2 ), semi-straw (SS) application (T1: 4,500 kg ha −2 ), and total straw (TS) application (T2: 9,000 kg ha −2 ). The plot area was 200 m 2 and was replicated 3 times. The field trials were conducted from 2009 to 2013. The crop was rice each year, with the exception of 2011, during which the crop was winter wheat. The straw was manually cut to 10 cm pieces and then ploughed into 0-25 cm of the topsoil before winter irrigation.
The fertilizer application rates in the rice production field were as follows: urea, 300 kg N ha −2 ; triple superphosphate, 105 kg P 2 O 5 ha −2 ; and potassium chloride, 60 kg K 2 O ha −2 . Half of the nitrogen and all of the total phosphate and potash were applied as basic fertilizers, and the other 50% of the nitrogen was applied as a topdressing at the seeding (end of May), tillering (end of June), and booting (end of July) stages at a proportion 3:1:1. The time of the first irrigation in the field was approximately mid-May each year, and the last irrigation occurred in early July of each year. The irrigation rate was 15,000 m 3 ha −2 during the rice growing period. Rice transplanting was conducted in mid-May, and harvesting occurred in late September (approximately 120 days).
The fertilizer application rates in the wheat field were as follows: urea, 225 kg N ha −2 ; triple superphosphate, 150 P 2 0 5 kg ha −2 ; and potassium chloride, 90 kg K 2 O ha −2 . Half of the nitrogen and all of the phosphate and potash were applied as basic fertilizers. The remaining 50% the nitrogen was applied as a top dressing at a ratio of 3:1:1 at the seeding (early March), elongation (early May), and earing (early June) stages. The irrigation scheme was as follows: winter irrigation, 1,350 m 3 ha −2 (end of October); greening, 900 m 3  Methods. Soil nitrate losses were determined using a resin-core device (Fig. 2, China National Patent No. 201020282864.4, 2010). This method has been used previously to determine soil nitrogen mineralization 12,28,29 . The resin-core device is composed of a stainless steel pipe (76 mm diameter × 0.82 mm thickness), a resin bag (60-mesh nylon net, 8 cm long × 8 cm wide), 2 pieces of an aluminium plastic material plate (74 mm diameter with 13 eyes (3 mm diameter)), a cover, a handle and an antiskid axis. The resin used was a strongly alkaline anion exchange resin (trademark #717), and the pretreatment was in accordance with the method for pretreating ion exchange resins (GB/T 5476-1996). There were 4 holes in the pipe body for reducing the pressure differential inside and outside of the pipe.
The end of the pipe was a 10 cm-long wedge surface for easy insertion into the soil. A 2 cm-long aluminium plastic plate was above the wedge. The antiskid axis can be fixed to prevent the aluminium plastic plates from dropping. The cover and the handle were convenient for placing and inserting the pipe body in the soil. There  were 5 types of resin-core devices; the types were 22, 32, 42, 72, and 102 cm long. Initially, the resin-core devices were inserted just below the soil surface. The pipes were later removed, and the soil in the wedge and 2 cm above the wedge was collected. However, the aluminium plastic plates, resin bags (16 g of resin per bag), aluminium plastic plates, and antiskid axis were later attached in sequential order. The devices were ultimately retrieved, the old resin bags were removed, and new resin bags were installed for the next test stage. The old resin bags were subsequently refrigerated. Three types of devices were placed in each plot in a line and separated by 2 m, and 3 replications were situated along the diagonal of each plot. The nitrate in the resin bags was extracted with potassium solution (1 mol L −1 KCl), and measured using ultraviolet spectrophotometry. The nitrate loss was calculated using the following equation: The data were analysed using SPSS 19 and Excel 2010. One-way analysis of variance (ANOVA) was used to test for significant (α = 0.05) differences among the different treatments.

Results
Nitrate leaching losses. Figure 3 shows the trends of nitrate leaching. Nitrate leaching losses in each treatment decreased as the soil depth increased. Additionally, T1 lost more than the control did, and T2 lost more than T1 did. Nitrate leaching losses in the rice field (2010, 2012, and 2013) were greater than the losses in the winter wheat field (2011). The trends for each of the 3 years in the rice fields were very similar. These results indicated that straw application was beneficial for reducing nitrate leaching. Table 2 shows the nitrate leaching losses. Straw application reduced nitrate leaching losses in the 30 cm layer. The treatment groups and the control group significantly differed (P < 0.05), with the exception that no obvious differences between T1 and T2 in 2011 (P > 0.05) were observed. The nitrate leaching losses ranged from 14.06-18.12 kg ha −2 in the rice field and from 10.83-11.95 kg ha −2 in the winter wheat field. Above the 30 cm layer, no significant differences were observed between the treatment groups. Below the 30 cm layer, no significant differences (P > 0.05) were observed among T1, T2, and the CK. The results for the 4 years show that straw application could reduce soil nitrate leaching losses in the 30 cm layer, the losses in both T1 (13.76 kg ha −2 ) and T2 (13.74 kg ha −2 ) significantly differed from those in the control (15.76 kg ha −2 ) (P < 0.05), and soil nitrate leaching losses decreased 12.71% and 12.84% in T1 and T2, respectively.

Characteristics of soil nitrate leaching losses. Soil nitrate leaching losses and crops. The results
( Table 2) for rice (13.41, 13.06, and 12.95 kg ha −2 ) and winter wheat (10.21 kg ha −2 ) showed that the soil profiles in the fields significantly differed (P < 0.05); the nitrate leaching losses were more substantial in the rice field than in the winter wheat field. If the goal for the region is to reduce nitrate leaching, then the area devoted to rice production should be reduced. No significant differences (P > 0.05) were observed between years (2010, 2011, and 2013).   . No significant differences (P > 0.05) were observed between the 10 cm and 20 cm layers or between the 60 cm and 90 cm layers. Straw mainly stayed within the top 20 cm of soil; no significant differences were observed between the layers up to 20 cm. Nitrate leaching losses in the 60 cm and 90 cm layers were not impacted by straw applications because of the short duration of the trial period. Therefore, the critical layer for nitrate leaching was the 30 cm layer, which was also the bottom of the soil ploughing layer.
Process of soil nitrate leaching loss. Figure 4 shows the ratios of nitrate leaching losses for different periods. In the rice field, the key period of soil nitrate leaching occurred before the end of June (45-50 days/130-140 growing days) (Fig. 4a.); nearly 80% of the nitrate leached above the 30 cm layer, and almost 50-55% of the nitrate leached below the 30 cm layer. In the winter wheat fields, nearly 80% (Fig. 4b) of the nitrate leaching losses from each soil layer occurred before early June, and almost 70% (Fig. 4c) of the nitrate leaching losses from each aggregate layer occurred during the four months prior to October. The key periods of soil nitrate leaching losses occurred during the early growth stages of rice and winter wheat; for rice, the key period occurred at end of June (tillering phase).
Another key period for winter wheat occurred at the beginning of June (earing phase).

Discussion
Soil nitrate concentration. The soil nitrate concentration is the key factor influencing nitrate leaching losses. Figure 5 shows the course of changes in the soil nitrate concentration in the 0-30 cm layer at different time points. In the rice field, the annual mean of the soil nitrate concentration was clearly higher (P < 0.05) in May and June than in other months (Fig. 5a). In the winter wheat field, the most critical period of leaching was during April (Fig. 5b), and the nitrate concentration was much higher during this time than during the other two months of measurements (P < 0.05). Not surprisingly, the periods of high nitrate concentrations were also periods of high nitrate leaching. The nitrate concentration in the treatments was lower than that in the control treatments in the rice field. Compared with that in the control treatment, the nitrate concentration in the T1 and T2 treatments both significantly differed (P < 0.05) in April, May, June, and July; no significant differences were observed between the controls and the treatments in August or September. However, significant differences were observed between the T1 and T2 treatments at all time points (P > 0.05). In the winter wheat field, the nitrate concentrations in the treatments and the controls significantly differed in April. However, the control treatment and the T1 treatment were not significantly different in May, but the T1 and T2 treatments were significantly different in May. The results show that straw application was beneficial for reducing the soil nitrate concentrations during the early stages of rice growth. However, in the middle and at the end of the rice growing stage, the soil nitrate leaching did not increase despite the soil nitrate concentration increasing slightly. Although the straw application constituted an additional source of nitrogen, the nitrate leaching losses did not increase. Wang demonstrated that nitrate and nitrite concentrations clearly decrease after straw applications to paddy fields and that these concentrations are negatively related to straw application rates 30 . The processes of soil nitrogen mineralization and fixation occur simultaneously; more fixation occurs relative to mineralization during the early stages of straw decomposition 31 .  Nitrate in the soil leachate. Figure 6 shows the change in nitrate concentrations in the soil leachate of the 30 cm layer in rice fields. The nitrate concentrations in the soil leachate were higher in May and mid-June than at the end of June and the end of July, and there was a decreasing trend of nitrate concentration over time. The nitrate concentrations in T1 and T2 did not significantly differ (P > 0.05) from those in the control, with the exception of the concentrations in May. Gollany determined that straw applications could clearly reduce the nitrate concentrations in soil leachates and could reduce nitrate leaching losses 33-35 . Straw application can reduce the nitrate and nitrite concentrations in surface water and in percolating water in paddy fields because straw enhances crop nitrogen absorption; thus, inorganic nitrogen concentrations would decrease, and nitrogen leaching losses would also decrease 36 . Keeney noted that straw applications increase soil organic carbon and lead to soil fertilizer nitrogen fixation 37 . Diao reported that straw applications can reduce the nitrate concentration in the soil leachate at the final stage of the rice growing period; the nitrate concentrations were similar to those in the control in the early stages of the rice growing period, but the nitrite concentration of the soil leachate improved 38 . A reason for the improved nitrite concentration was that nitrate was fixed by the straw 39 , while organic acids from the straw decomposition restricted the nitrite, enabling its conversion to nitrate 40 . However, Zhang reported that straw Soil TN changes. Soil TN was improved by straw application (determined in 2013). The TN values in the control, T1, and T2 treatments were 0.86, 0.89, and 0.90 g kg −1 , respectively, in the 0-10 cm layer; compared with the control values, the T1 and T2 values improved by 3.25% and 4.31%, respectively. In the 0-20 cm layer, the TN values in the control, T1, and T2 treatments were 0.78, 0.82, and 0.89 g kg −1 , respectively; compared with the control values, the T1 and T2 values improved by 4.83% and 13.89%, respectively. In the 0-30 cm layer, the TN values of the control, T1, and T2 were 0.64, 0.66, and 0.69 g kg −1 , respectively; compared with the control values, the T1 and T2 values improved by 2.76% and 6.83%, respectively. The TN improved by 0.5 g kg −1 in response to 8 years of continuous applications of wheat and corn straw in North China 43 . Straw application clearly increased the TN and SOM in a single-cropping rice system and in a double-cropping rice system 44 . Organic nitrogen is the key form of soil nitrogen; inorganic nitrogen is only 1% of the soil TN. Organic nitrogen can be absorbed by crops under conditions of soil microbial decomposition and mineralization 45 and is a kind of natural controlled-release fertilizer that is controlled by soil microbes. The carbon/nitrogen ratio of straw is usually greater than that of soil microbes, so soil microbes can absorb inorganic nitrogen and convert it to microbial nitrogen 46 . Furthermore, soil inorganic nitrogen concentrations decrease, and nitrate losses are reduced.

SOM.
Compared with that in the CK (13.78 g kg −1 ), the SOM in T1 and T2 increased by 0.89 g kg −1 and 1.24 g kg −1 , respectively, in the 0-30 cm layer at the end of September in 2013.
The decomposition of straw consumes soil inorganic nitrogen, decreases the accumulation of alkali-hydrolysable nitrogen in the short term and reduces the risk of nitrogen leaching. SOM contributes to improved soil water-stable aggregates and absorbs more nitrate and nitrite 47,48 ; in the present study, the macroaggregates in T1 and T2 increased by 7.4% and 12.8%. Straw application can reasonably maintain the soil carbon/nitrogen ratio and reduce soil nitrogen leaching losses 49 . Straw application can enhance nitrogen supplies and reduce fertilization rates, nitrogen losses, and nitrogen pollution problems in agricultural ecosystems 50 . In Northeast China, straw application can improve the SOM and TN and benefit the formation of macroaggregates, which can adsorb soil nutrients and reduce losses 51 . The SOM was shown to increase by 4.9 g kg −1 when wheat and corn straw was applied continuously for 8 years 43 . Straw application can supply nitrogen to the soil, reducing fertilizer nitrogen rates, so nitrogen pollution of agricultural ecosystems could be somewhat relieved 50 . Straw application can maintain the balance between soil carbon and nitrogen, reduce soil nitrogen leaching, and improve soil structure, especially in high-fertilizer-input fields. Straw application has not reached 50% in China; there is a large gap that contrasts with developed countries.
Cropping patterns. Soil nitrogen leaching losses were affected by cropping patterns, including crops and cropping systems. Nitrate leaching losses were greater in the rice field than in the winter wheat field; the primary reason was the presence and quantity of irrigation water. Nitrate leaching losses increased as irrigation increased. More nitrate leaching losses occurred at the seedling stage because of the small roots; soil nitrate could not be absorbed rapidly and completely at this stage. To alleviate these losses, a new fertilization technique named "the technique of backward nitrogen fertilizer" was developed; in this technique, the fertilizer rate is reduced at the early stage but shifted to the middle stage, so that the total dose is not changed 52 . The top 4 of 12 cropping patterns, which include winter wheat-rice, spring wheat-Chinese cabbage, winter wheat-silage corn and winter wheat-oil sunflower, can clearly reduce residual soil nitrate levels, so multiple cropping and intercropping systems could reduce the nitrate leaching losses and protect shallow groundwater 53 . Traditional cropping systems consist of a long fallow period from early October to early March or mid-May (150 or 220 days, respectively), and the nitrate leaching losses consist of pure nitrogen (0.69-0.81 kg ha −2 ) 44 . However, Yang determined that long-term straw application caused nitrate to leach into deep soil layers 32 . The main reasons supporting this phenomenon may be the short growing season and asynchrony between the crop growth period and precipitation; nitrate that has accumulated during a long period can be easily leached, so this time scale should be considered during long-term planning.
Crop yields. In the present study, straw application increased crop yields. Compared with those in the control treatment, the rice yields in the T1 and T2 treatments increased by 7.79% and 14.56% in 2010, by 8.17% and 10.35% in 2012, and by 9.45% and 9.26% in 2013, respectively; the mean rice yields in T1 and T2 were 9.24% and 10.37% higher than the mean yields in the control. The winter wheat yields in T1 and T2 increased by 10.11% and 11.51%, respectively, in 2011. Zhang reported that rice straw application increases rice yields and that, compared with a total application, SS application leads to stronger yield gains 54 . Zhong reported that TS and SS applications can increase barley yields by 16.68% and 12.28%, respectively (P < 0.05) 55 . Zeng reported that crop yields increased by 1.7-145.8% (mean of 15.17%) in response to straw application at a rate of 1,500-9,000 kg ha −2 (mean of 4,611 kg ha −2 ) in China 56 . Li analysed the effects of straw application rates on crop yields and reported that wheat and corn yields decreased to 470 kg ha −2 (7.16%, P > 0.05) and 60 kg ha −2 (0.91%, P > 0.05), respectively, by straw application at a rate of 3,000 kg ha −2 ; in addition, the wheat yield decreased by 262 kg ha −2 , and the corn yield increased by 113 kg ha −2 in response to straw application at a rate of 6,000 kg ha −2 57 . In general, a reduction in yield usually occurs in dry years, especially in arid regions and in non-irrigated zones. Yield reductions seldom occur in irrigated zones, so long-term straw applications benefit crop yields.
Extension trials of rice straw applications. In 2012, an extension trial was conducted near the initial trial plots. The trial consisted of 3 treatments: a CK and SS and TS applications. The area of each treatment was 1/15 ha 2 . Compared with that in the CK, the soil nitrate in the SS and TS decreased by 12.13% and 18.02%, respectively, in the 0-30 cm soil layer. The soil ammonium concentration increased by 48.24% in the SS and by 65.84% in the TS treatments, but the concentration was only approximately one-tenth of the nitrate concentration. In addition, ammonium leaching losses were rare. In 2013, a more large-scale extension trial was conducted in Qingtongxia, Ningxia, at a site located 30 km west of the initial plots. The test area was 2.3 ha 2 (1.9 ha 2 received straw applications, and 0.4 ha 2 served as controls). Compared with the control, straw application reduced nitrate leaching losses by 15.44%, and the rice yield improved by 7.23%. Shallow groundwater. Leached nitrates lost in the 90 cm soil layer have a greater chance to infiltrate water because the groundwater level is only 120-200 cm high during the planting season. The amount of leakage water was approximately 100-120 cm in the rice fields each year during the study, and the nitrate leaching losses were 8-8.69 kg pure nitrogen ha −2 y −1 . If the leakage water and leached nitrates enter the groundwater together, the nitrogen concentration in the leachate would be 0.67-0.87 mg L −1 , which is almost equal to 2.96-3.85 mg L −1 of the nitrate leachate. The nitrate concentration exceeded 10 mg L −1 in some places, and it was highest during the planting season. Zhao demonstrated that nitrate and organic nitrogen concentrations in the groundwater ranged from 11.85-26.12 and 0.64-5.89 mg L −1 , respectively, and Zhao demonstrated that nitrate and organic nitrogen concentrations in the groundwater ranged from 11.85-26.12 and 0.64-5.89 mg L −1 , respectively 58 . Ju noted that greenhouse vegetable farming caused soil nitrate accumulations and leaching: the groundwater nitrate concentration was 9-274 mg L −1 , and the soil nitrate concentration was 270-5038 kg ha −2 in the 0-90 cm soil layer in SCIEntIfIC RepoRts | (2018) 8:954 | DOI:10.1038/s41598-017-18152-w Huimin County, Shandong Province, where 99% of the groundwater had nitrate concentrations greater than 10 mg L −1 59 . Song reported that soil nitrate leaching losses from greenhouse vegetables reached 152-347 kg ha −2 in the 0-100 cm soil layer in Shouguang County, Shandong Province and that those losses were closely related to groundwater pollution 60 .

Conclusion
Straw application can reduce soil nitrate leaching losses. Nitrate leaching losses decreased by 12.71% to 12.84% in the Ningxia irrigation zone, an area in which TS application is recommended first because of low the SOM and low soil nutrient levels. Straw application improved the SOM by 2.76% to 6.83% and the TN by 0.89-1.24 g kg −1 in the 0-30 cm layer. As such, nitrogen fertilizer application rates could be reduced in the long term, and this approach could also reduce soil nitrogen leaching. Straw application could also increase crop yields. There are 0.7 billion tons of crop straw produced annually in China. This material is equivalent to 3.5 million tons of fertilizer nitrogen, 8 million tons of fertilizer potash, and 0.8 million tons of fertilizer phosphate. However, the rate of straw application is less than 50% 61 . For these reasons, more attention should be paid to both additional testing and to educating producers about the benefits of straw application in China.