Apparent variations in nitrogen runoff and its uptake in paddy rice under straw incorporation

Muhammad Amjad BASHlR ,ZHAl Li-mei ,WANG Hong-yuan ,LlU Jian ,Qurat-Ul-Ain RAZA ,GENG Yu-congAbdur REHlM,,LlU Hong-bin

1 Key Laboratory of Nonpoint Source Pollution Control,Ministry of Agriculture and Rural Affairs/Institute of Agricultural Resources and Regional Planning,Chinese Academy of Agricultural Sciences,Beijing 100081,P.R.China

2 College of Agriculture,Bahadur Sub-Campus Layyah,Bahauddin Zakariya University,Mutan 60800,Pakistan

3 School of Environment and Sustainability,University of Saskatchewan,Saskatoon,SK S7N 3H5,Canada

4 Department of Soil Science,Faculty of Agricultural Sciences and Technology,Bahauddin Zakariya University,Multan 60800,Pakistan

Abstract Straw incorporation is a widespread practice to promote agricultural sustainability. However,the potential effects of straw incorporation with the prolonged time on nitrogen (N) runoff loss from paddy fields are not well studied. The current study addresses the knowledge gap by assessing the effects of straw incorporation on the processes influencing N runoff patterns and its impacts on crop yield,N uptake,total N (TN),and soil organic matter (SOM). We conducted field experiments with rice (Oryza sativa L.)-wheat (Triticum aestivum L.) rotation,rice-tobacco (Nicotiana tabacum L.)rotation,and double-rice cropping in subtropical China from 2008 to 2012. Each rotation had three N treatments: zero N fertilization (CK),chemical N fertilization (CF),and chemical N fertilization combined with straw incorporation (CFS). The treatment effects were assessed on TN runoff loss,crop yield,N uptake,soil TN stock,and SOM. Results showed that TN runoff was reduced by substituting part of the chemical N fertilizer with straw N in the double rice rotation,while crop N uptake was significantly (P＜0.05) decreased due to the lower bioavailability of straw N. In contrast,in both rice-wheat and rice-tobacco rotations,TN runoff in CFS was increased by 0.9-20.2% in the short term when straw N was applied in addition to chemical N,compared to CF. However,TN runoff was reduced by 2.3-19.3% after three years of straw incorporation,suggesting the long-term benefits of straw incorporation on TN loss reduction. Meanwhile,crop N uptake was increased by 0.8-37.3% in the CFS of both rotations. This study demonstrates the challenges in reducing N runoffloss while improving soil fertility by straw incorporation over the short term but highlights the potential of long-term straw incorporation to reduce N loss and improve soil productivity.

Keywords: straw return,nitrogen runoff,water pollution,rice yield,nitrogen uptake

1.lntroduction

Crop residues are essential by-products of crop harvests(Liuet al.2017;Liet al.2021). Annually,the production of crop residues worldwide amounts to 4 Gt,of which more than 16% is produced in China (Lal 2005;Quet al.2012). With the increase in rice cultivation,the problem of rice straw generation and management has also arisen. For every mega-gram of paddy production,about 1-1.5 mega-grams of rice straw are produced (Migo-Sumaganget al.2020). This excessive straw generates problems related to proper disposal and seriously impacts the environment as per other agricultural wastes(Kalaivanan and Hattab 2016;Razaet al.2021). A common strategy used to dispose of crop residues is straw incorporation,i.e.,mixing the shredded straw into soils during tillage operations (Chenet al.2014;Zhaoet al.2021). Traditionally farmers used to incorporate straw into the soil to improve soil fertility status in China.However,for the last few years,straw burning has been adopted to save labor,which causes environmental pollution and depletion of nutrient resources (Zhanget al.2016). Therefore,straw incorporation’s agronomic and environmental impacts on soil,plant,and atmosphere are long-lasting research priorities.

Rice is a staple food for roughly one-third of the world’s population;in China,rice accounts for almost 40% of the national gross grain production (Panet al.2017;Bashiret al.2019a,2021). Even though rice yields have been substantially increased by enhanced nutrient management and other management strategies in the past two decades,the challenges of reducing nutrient runoff from paddy soils are increasing in China (Cuiet al.2020). A recent empirical model estimation suggests that nitrogen(N) loss through surface runoff from Chinese paddy fields reaches as high as 1.1 Tg N yr-1,equivalent to around 7%of the total N fertilizer input to the paddy fields (Houet al.2016). A large amount of N loss has detrimental impacts on downstream water quality and causes eutrophication and ecological degradation in the adjacent water bodies (Liet al.2018;Henrysonet al.2020). Therefore,mitigation strategies are needed to reduce N runoff.

The studies have documented considerable knowledge of some aspects,such as the effects on: (i) soil carbon(C) storage (Liet al.2016;Chaudharyet al.2017;Zhanget al.2017),soil N storage (Cucuet al.2014),soil chemical properties (Yuanet al.2014;Zhaoet al.2019),and soil microbial communities (Fenget al.2016;Penget al.2016;Tanget al.2020);(ii) crop N use efficiency(Nguyenet al.2016),water use efficiency (Liuet al.2017),and productivity (Zhanget al.2021);and (iii) greenhouse gas emissions (Yaoet al.2013,2017;Huanget al.2017)and ammonia volatilization (Zhanget al.2020). The study concludes that the increase in soil organic C (SOC)content with the straw return is significantly positively correlated to the decrease in N leaching and runoff (Xiaet al.2016;Zhaoet al.2021). The findings of these studies provided scientific bases for understanding how straw incorporation and decomposition changed soil quality,crop productivity,and soil ecosystem functions.Many of the findings pointed to the positive consequences of straw incorporation.

Given the positive effects of straw incorporation on crop yield and greenhouse gas sequestration,straw incorporation is recommended as an important management practice for rice production in China.However,the application time effects of straw incorporation on N runoff from paddy fields are not well documented,particularly regarding repeated straw incorporations,which can have both short-and long-term effects on soil N and C turnover. This gives rise to the need for the current study. This study aimed to assess the effects of straw incorporation on N runoff from three paddy-cropping systems over five years. In addition,crop yield,N uptake,total N (TN),and soil organic matter(SOM) were monitored to obtain an overall assessment of straw incorporation impacts and shed light on the processes influencing N runoff patterns. The findings of this study would provide scientific insights on crop residue management in paddy production systems and support the development of sustainable agriculture in China and other rice-growing countries. The objectives of the current study were to: (i) identify the short-and long-term impacts of the straw incorporation on soil,environment,and crop productivity;(ii) find the particular processes influencing N runoff losses;and (iii) measure the impacts of straw incorporation of nutrient uptake.

2.Materials and methods

2.1.Description of the experimental site

The field experiments were carried out from 2008 to 2012 in three extensive rice-producing provinces of Sichuan,Fujian,and Hunan in subtropical China. The experiments included three rice rotation systems typical of the study regions: (i) Rice-wheat rotation in Sichuan Province(30.2°N,103.9°E);(ii) Rice-tobacco rotation in Fujian Province (26.7°N,117.5°E);and (iii) Double rice rotation in Hunan Province (28.1°N,113.2°E). All three provinces have a humid subtropical monsoon climate. Even so,the weather varied considerably across the experimental sites. Over the five experimental years,the mean daily temperature ranged from 17.8°C in Sichuan to 19.2°C in Fujian,and mean annual precipitation ranged from 915 mm in Sichuan to 1 693 mm in Fujian (Table 1). Due to weather variabilities,crop seasons somewhat differ with the regions. Sichuan and Fujian had approximately the same length of the rice-growing season from transplant to harvest for 3-3.5 mon,much longer than 2.5 mon for early rice and 3 mon for late rice in Hunan. Moreover,in Sichuan and Fujian,winter wheat or tobacco grows for 4 or 6 mon of the year,while in Hunan,the paddy fields are fallowed for up to 6.5 mon. All three experimental sites had medium loamy soil with similar bulk densities,while the Hunan soil had the highest fertility in terms of SOM and TN contents,followed by Fujian soil and then Sichuan soil (Table 1).

2.2.Experimental design

In each of the 15 field experiments (three sites by five years),a randomized complete block design was used(n=3;plot size=25-30 m2). Three treatments combining N fertilizer and crop residue management practices were implemented to examine their effects on N runoff.Specifically,the treatments were: the control null N fertilization (CK),conventional N fertilization in accordance with farmers’ practice (CF),and N fertilization combined with straw incorporation (CFS) (Table 2). Moreover,chemical phosphorus (P) and potassium (K) fertilizers were applied at consistent rates between the treatments within an experimental site but at different rates across the sites. The rates of P and K fertilizers represented farmers’ practices in the respective study provinces.The chemical N,P,and K fertilizers used were urea(46% N),superphosphate (16% P2O5),and potassium sulphate (50% K2O),respectively,and were applied as the broadcast method.

Table 1 A summary of experimental conditions for the three rice rotation systems1)

Table 2 Fertilizer and straw return treatments for the three rice rotation systems

For the CFS treatment in all experiments and crop seasons except early rice in Hunan,the crop residues (rice/wheat/late rice straw) were chopped and incorporated into the soil before crop sowing or transplanting. For the rice-wheat/tobacco rotations,the CFS treatment received the same rate of chemical N fertilizer as CF;thus,straw return in CFS resulted in a higher total N application rate than CF. For the double rice rotation,in contrast,the application rate of chemical N fertilizer was reduced in CFS to maintain the same total N rate as in CF.

2.3.Monitoring of runoff,crop,and soil

To monitor surface runoff,each field plot was surrounded by cemented ridges (20 cm in height and 12 cm in width)on all sides and 30-40 cm in the ground to prevent water and nutrient transport between plots. A cemented pool(5-6 m in length,0.8-1.2 m in width,and 0.8-1 m in height) was installedin situat the edge of each plot tocollect the runoff,with the top of the pool 20 cm above the ground. The pool was connected to the plot through a polyvinyl chloride pipe (5 cm in diameter),with the top of the pipe equaled altitude with the top of the ridge. To prevent the entry of rainwater or small field animals,the pool was covered with a stainless-steel plate all the time except when water samples were collected.

The surface runoff caused by rainfall was collected for each plot after every runoff event. The water depth of runoff was measured in each pool and was used to calculate the volume of runoff water. Runoff water samples were collected with 500 mL clean plastic bottles,placed in a portable refrigerator,and immediately transported to the laboratory for testing. After collecting the samples,residual water and sediments in the pool were removed so that the pool was prepared for the next runoff collection (Shanet al.2015).

In conjunction with runoff monitoring,daily meteorological data,including precipitation and air temperature,were recorded at an automatic HOBO-U30 weather station installed on-site (Onset Computer Corporation;MA,USA). Moreover,the yield and aboveground biomass of all crops were measured at harvest. Soil samples were collected at the beginning of the trial in 2008 and annually after harvesting rice from 2008 to 2012. The samples were collected at a depth of 0-20 cm using a 5-cm internal diameter auger.

2.4.Laboratory analysis and calculations

Water samples were analyzed for TN using a continuous flow analyzer (AA3,BRAN+LUEBBE,Germany).In addition,water pH was determined by using a potentiometer. To determine general soil properties(Table 1),soil samples collected from all plots within an experimental field were mixed to prepare a homogenous mixture. Soil texture was obtained by the sedimentation method of the samples and reported according to the United States Department of Agriculture (USDA)classification. Soil bulk density was measured after drying the soil in an oven at 105°C for 24 h. Soil pH was determined at 1:5 soil/water with a potentiometer. Soils were analyzed for individual plots to assess treatment effects on soil C,soil N,and crop N uptake. The content of SOM was determined by the dichromate oxidation method. The concentration of TN in both soil and plant was determined by the Kjeldahl digestion method (KDY-9830,China) (Zhanget al.2017).

Loss of TNviarunoff and contributions of N loss by soil residue N and exogenous N (chemical fertilizer N and straw N) were calculated using the following equations,respectively:

where TN is the total N runoff losses (kg N ha-1);TNC is the TN concentration in runoff (mg L-1);V is the runoffwater volume (L);S is the area of field treatment plot(m2);CR is TN runoff coefficients;TNCK is the annual cumulative TN runoff loss of the CK treatment (kg N ha-1yr-1);CSN is the contribution of residual soil N to the annual cumulative TN runoff loss in a given treatment(%);TNT is the annual cumulative TN runoff loss of the CF or CFS treatment (kg N ha-1yr-1);and CEN is the contribution of exogenous N to the annual cumulative TN runoff loss (%).

2.5.Statistical analysis

Data were analyzed using SPSS 19.0 Statistical Software(SPSS,China). Mean comparisons were performed to test the treatment effects on N runoff loss,crop yield,crop N uptake,SOM,and soil TN,using a one-way analysis of variance (ANOVA). Repeated measures ANOVA was used to test the significance of differences among treatments. When theF-test was significant at a 0.05 probability level,mean differences were analyzed by the least significant difference (LSD) method.

3.Results

3.1.Crop yield and N uptake

Crop yields were significantly (P＜0.05) affected by the treatments (Fig.1). Yields of all crops were increased significantly (P＜0.05) by 21-270% in the CF treatment and by 26-267% in the CFS treatment as compared to the yields in the CK treatment,respectively. The only exceptions were found in the rice-wheat rotation,where the yields of rice and wheat in 2008 and the rice yield in 2012 did not differ significantly between the N treatments and CK. Among the three crop rotations,the greatest increase in yield by N application and straw return was observed in the rice-tobacco rotation.Generally,the two N treatments with and without straw return produced statistically the same crop yields,except that,compared to CF,CFS produced higher wheat yields from 2009 to 2012 in the rice-wheat rotation but lower early rice yield in 2009 and late rice yield in 2012 in the double rice rotation.

Even though CFS and CF had similar effects on crop yields in most of the experiments,their effects on crop N uptake differed more profoundly (Fig.2). Particularly for the rice-wheat rotation during 2009-2012,the crop N uptake in the CFS treatment was 15-37% higher than the N uptake in the CF treatment. For the double rice rotation,in contrast,the crop N uptake in the CFS treatment was significantly reduced in both 2009 and 2010,compared with that in CF (P＜0.05). It implies that straw incorporation substituting equal chemical N might suppress the rice N uptake in the short term.

In the rice-tobacco rotation system,N uptake was high in rice in 2008,2009,2010,and 2011,while it was high in tobacco in 2012. Moreover,for the double rice rotation,straw incorporation improved N uptake in late rice during 2008,2009,2010,and 2012 and in early rice during 2011.Furthermore,the rice-wheat rotation system showed high N uptake in wheat during 2009,2010,and 2012 and in rice during 2008 and 2011 (Fig.3).

3.2.Characteristics of TN runoff

The annual cumulative TN runoff from different experiments and treatments is presented in Fig.4. As affected by rainfall patterns,the TN runoff loss varied greatly between years and between cropping rotation systems. The annual TN runoff ranged from 0.7 to 1.9,1.7 to 16.3,and 1.0 to 10.8 kg N ha-1yr-1in rice-wheat,ricetobacco,and double rice rotation systems,respectively.Moreover,the TN runoff loss was significantly (P＜0.05)affected by treatments. Compared to CK,the CF and CFS treatments significantly (P＜0.05) increased annual TN runoff loss in most experimental years. The relative comparisons between CF and CFS shifted over the five experimental years. During the initial three experimental years (2008-2010),compared with CF,the return of crop straws in CFS increased annual cumulative TN runoff by 0.9-20.2% and 10.1-13.3% in the rice-wheat and ricetobacco rotations,respectively. After the three years of straw return,the annual TN runoff losses in the CFS treatment were reduced by 2.3-8.9% in the rice-wheat rotation and by 5.4-19.3% in the rice-tobacco rotation,as compared to the CF treatment. In contrast,for the double rice rotation,the CFS treatment consistently resulted in less annual cumulative TN runoff than the CF treatment throughout the five experimental years. Even so,the difference in TN loss between the two treatments was significant (P＜0.05) from 2008 to 2011 (by 6.4-33.6%)but insignificant in 2012 (by 2.1%). These results indicate that the long-term straw application has a tremendous influence on reducing runoff losses.

The predominant runoff occurrence periods greatly varied in the three crop rotations (Appendices A-C).Specifically,the runoff events mainly occurred in rice (late May to middle October),tobacco (early March to June),and early rice (May to early July) growing seasons for the rice-wheat rotation,rice-tobacco rotation,and double rice rotation,respectively. In the three seasons,the seasonal precipitation accounted for 70-89%,37-82%,and 19-42% of the respective annual precipitation.Correspondingly,the TN runoff loss in these seasons amounted to 79-100%,58-100%,and 68-100% of the annual TN loss,respectively.

Averaged across five years,the TN runoff coefficients of CFS were 0.09-1.20%,slightly lower than the coefficients of the CF treatment (0.11-1.50%) (Table 3). Residue soil N was a primary source of TN runoff loss from paddy soils in China. Across all rice cropping systems,residue soil N contributed 41.8-86.6% of the annual TN losses,with the highest rate in the rice-wheat rotation. In addition,straw incorporation improved the contribution of exogenous N in all rice cropping systems. In contrast,the soil N contribution was high in double rice cropping system as compared to other rotations (Fig.5).

Table 3 Nitrogen runoff coefficients (runoff N/total N applied)for chemical fertilizer treatments with and without straw incorporation1)

3.3.Changes in SOM and soil TN stock

Over the five experimental years,the CFS treatment significantly (P＜0.05) improved both SOM and soil N stock in all rice rotation systems (Fig.6). The temporal changes in SOM were similar in all treatments from 2008 to 2009,demonstrating no short-term effect of CFS on SOM. After that,however,the treatment effects started to display,following the order: CFS＞CF＞CK. By 2012,the CFS treatment increased SOM by 3.3-14.1% and 10.0-51.8% compared with CF and CK,respectively. In all treatments,the patterns of soil TN were similar to that of SOM. Compared to CF,the soil TN in the CFS treatment was increased by 4.8-28.9% by 2012. This indicates that over two years (long-term application),straw incorporation improves soil fertility.

4.Discussion

4.1.Effect of straw incorporation on crop yield

In all cropping systems,the crop yield increased significantly with straw incorporation. This major increase in crop yield can be associated with the direct nutrient supply from straw as well as the improvement of soil fertility (Wanget al.2015;Zhaoet al.2021). In addition to improving soil quality,residue retention also plays a critical role in moderating soil temperature,ultimately enhancing root and plant growth and thus increasing crop yield and productivity (Sharmaet al.2021). Moreover,residue incorporation contributes to increasing crop yield due to the improved soil physico-chemical properties,available macro-and micro-nutrients and SOC. It also improves soil aggregation,water intake,availability,and retention,root penetration,and soil microbial status. All these entities contribute to maintaining an edaphic environment and enhancing crop yield (Vashishtet al.2021).

4.2.Short-term effects of straw incorporation on N runoff

Surface runoff is primarily determined by precipitation and irrigation patterns. Previously,several studies reported elevated nutrient concentrations in surface runoff from fertilized croplands during rainfall events(Lianet al.2020). Moreover,it is found that applying organic amendments in addition to chemical N fertilizers reduced N runoff by 27.4-36.3% compared to applying chemical N alone (Xieet al.2021). However,our results showed that the effects of short-term (1-3 years) straw incorporation on the reduction of N runoff losses were unsatisfactory. The reason could be the release of higher nutrients at an early and higher decomposition rate of straw,which was reduced at later stages. So,the long-term application helped to control the losses with involved microbial activities (Jinet al.2020).Compared with CF,the annual TN runoff losses were even increased in the CFS treatment with additional straw return in both rice-wheat and rice-tobacco rotation systems. This result was primarily due to the extra straw N input to the soil,which led to additional N release from plant materials and N mineralization in the CFS treatment. The additional N release eventually contributed to elevated TN concentrations in runoff water(Appendix D).

In the double rice system,TN runoff loss in the CFS treatment was reduced throughout the entire experimental period (2008-2012) compared to CF. The decrease in TN runoff was primarily because of the reduction of fertilizer N rate in the CFS treatment. Even though the total N application rate as a sum of fertilizer N and straw N in the CFS treatment was the same as the total N application in the CF treatment,which solely was fertilizer N,the straw N in CFS is expected to have lower bioavailability to crops particularly over the short term (Zhanget al.2018). Indeed,the crop N uptake in the CFS treatment was significantly decreased (P＜0.05) during the early experimental years (2009-2010) compared to that in the CF treatment. Two reasons could have contributed to the reduction of crop N uptake in the CFS treatment. First of all,as a kind of organic material with high C/N,the decomposition of straw can lead to competition between microorganisms and crops for the uptake of N (Jinet al.2020). Also,the release of N from crop straws associated with decomposition and mineralization may be too slow to supply sufficient N to the seasonal crops (Zechmeister-Boltensternet al.2015;Jinet al.2020). For example,it was observed that after 4-5 years of field application,up to 20% of straws could remain on the soil surface without decomposition (Lvet al.2011). As years of straw incorporation increase,more straws are available for decomposition. This is likely the reason for the low crop N uptake in the early experimental years but increased N uptake in the later years.

4.3.Long-term effects of straw incorporation on N turnover

Contrary to the short-term effects,the TN runoff loss was reduced by straw incorporation in the later experimental years for all sites,suggesting the efficacy of straw return in reducing TN runoff over the long term. The reduction of TN loss by straw incorporation can further help to improve soil quality and agricultural productivity (Liuet al.2021). Following decomposition of straw over the long term,SOM and crop N uptake were greatly increased,which contributed to reducing N loss and improving crop productivity. In the current study,five years of straw incorporation increased SOM and soil TN stock by 3.3-14.1% and 4.8-28.9%,respectively. Similar results were obtained by other research. For example,a meta-analysis reported an average 12% increase in SOM and TN by straw incorporation across a large array of croplands (Liuet al.2014;Berhaneet al.2020). In general,long-term straw incorporation can enhance the formation of macroaggregates while decreasing the proportion of microaggregates. This can help to improve SOM and TN stock because macro-aggregates often have higher C and N densities than micro-aggregates (Xueet al.2020). It has been suggested that straw incorporation improves soil aggregation and enhances SOC stabilization (Zhanget al.2017). In turn,physical soil conditions are also effectively promoted with increased SOM content,as indicated by improved soil water-holding capacity,total porosity,and bulk density (Bashiret al.2019b). Therefore,the SOC sequestration in cropping systems has been considered a cost-effective and eco-friendly strategy for sequestrating soil N and reducing N loss (Soong and Cotrufo 2015;Khanet al.2016),and straw returning to soil is an effective measure to improve SOC. Additionally,crop N uptake was greatly affected by straw incorporation over the long term. With the same rates of chemical N fertilizer,straw incorporation increased the N uptake by crops,especially in the rice-wheat rotation. As a result,the long-term straw return can increase SOM accumulation and crop N uptake,which improves soil N immobilization potential and crop N uptake,eventually reducing risks of N loss in the rice cropping systems in subtropical China.

4.4.Sources of N in runoff from paddy systems

Residue soil N was a primary source of TN runoff loss from paddy soils (Langdaleet al.2018). In this study,residue soil N contributed 41.8-86.6% of TN runoff,while the exogenous N contributed 13.4-58.6% (Fig.5)in CF treatments. Notably,runoff is different from other pathways of N loss. For instance,with regard to N losses via ammonia volatilization,the contribution of residue soil N was only up to 17.3% (Chenet al.2014). The reasons for the high contribution rate of residue soil N were: (i)large amounts of N accumulation in the topsoil resulting from historically high N inputs (Zhaoet al.2016),and (ii)rainfall-induced soil erosion that results in large amounts of sediment-bound N loss (Chenet al.2015). To reduce N runoff loss,it is recommended that mitigation measures should be taken not only for N loss from seasonal N applications (e.g.,by reducing N application rates) but also for the loss from residue soil N. Meanwhile,the contribution of residue soil N to TN runoff in the ricewheat rotation was much greater than its contribution in the other two rotation systems. This might be due to the much lower SOM content (7.3 g kg-1) in the rice-wheat rotation system than elsewhere (15.9-37.7 g kg-1),which has unfavorable soil physical conditions,such as soil water-holding capacity,total porosity,and bulk density.The results indicated that maintaining high SOM through long-term straw incorporation might be an effective measure to reduce TN runoff losses resulting from residue soil N in paddy soils.

Chemical fertilization is a common practice in the agroecosystem,which has generated serious concerns about N losses. Long-term straw incorporation can be an effective technique to overcome the chemical fertilization rate and boosts the integrated agroecosystem. This will be not just eco-friendly but also cheap and more beneficial for the farming community. In the field of resources management and environmental research,the process of farmland N runoff mainly depends on the water-soluble N content of farmland during rainfall or drainage. Although the water-soluble N contents of farmland are affected by other processes,including the long-term process of SOC and N conversion,these processes are lacking in the current study;hence,it is recommended to consider the processes for future research studies.

5.Conclusion

Results indicated that short-term straw incorporation might increase N runoff loss and reduce crop N uptake in paddy soils in subtropical China. After three years of straw incorporation,the TN runoff was reduced,and N uptake by crops was increased,suggesting the longterm efficacy of straw return on reducing TN runoff. Straw incorporation also improved SOM,an important indicator of soil fertility. Moreover,residue soil N was the primary source of TN runoff loss from paddy soils,contributing about 41.8-86.6% of TN loss across different rice rotation systems. This study demonstrates the challenges in reducing N runoff with straw incorporation while improving soil fertility over the short term but highlights the benefits of long-term straw incorporation on N runoff reduction and soil productivity enhancement. We acknowledge that limitations are present related to examining the relationships among straw retentions and transformation rates of N-species,interactions associated with microbial communities,and influences of specific functional genes or N-cycling.

Acknowledgements

This research was financially supported by the National Key Research and Development Program of China(2021YFD1700901),the National Natural Science Foundation of China (31972519),the earmarked fund for China Agriculture Research System (CARS-01-33),and the Agricultural Science and Technology Innovation Program of Chinese Academy of Agricultural Sciences(2060302-05-956-1).

Declaration of competing interest

The authors declare that they have no conflict of interest.

Appendicesassociated with this paper are available on http://www.ChinaAgriSci.com/V2/En/appendix.htm