Jianwei Zhang,Weiwei Li,Yan Zhou,Yanfeng Ding,Lei Xu,Yu Jiang,Ganghua Li*

National Engineering and Technology Center for Information Agriculture/Key Laboratory of Crop Physiology and Ecology in Southern China/Jiangsu Collaborative Innovation Center for Modern Crop Production,Nanjing Agricultural University,Nanjing 210095,Jiangsu,China

Keywords:Long-term straw incorporation Rice yield Stability Rice-wheat system PAR

ABSTRACT Straw incorporation is a global common practice to improve soil fertility and rice yield.However,the effect of straw incorporation on rice yield stability is still unknown,especially under high fertilization level conditions.Here,we reported the effect of straw returning on rice yield and yield stability under high fertilization levels in the rice–wheat system over nine years.The results showed that straw incorporation did not significantly affect the average rice yield of nine years.Straw incorporation reduced the coefficient of variation of rice yield by 25.8% and increased the sustainable yield index by 8.2%.The rice yield positively correlated with mean photosynthetically active radiation (PAR) of rice growth season and the effects of straw incorporation on rice yield depended on the PAR.Straw incorporation increased the rice yield by 5.4% in the low PAR years,whereas it did not affect the rice yield in the high PAR years.Long-term straw incorporation lowered soil bulk density but improved the soil organic matter,total N,available N,available P,and available K more strongly than straw removal.Our findings suggest that straw incorporation can increase rice yield stability through improving the resistance of rice plant growth to low PAR.

1.Introduction

Rice (Oryza sativaL.) is one of the most important staple food,feeding more than 50% of the global population [1].China is the largest rice-producing country,accounting for 27.4% of global rice production [2].The global rice demand is expected to increase by 28%in 2050 due to population growth and economic development[1].Yet,rice yields have stagnated in 35%of global rice paddies and 22% of the rice–wheat rotation areas in Asia [3,4].Long-term experiments indicate that depletion of soil fertility may be one of the most important factors that caused the rice yield stagnation[5,6].Furthermore,because the intensity and frequency of unfavorable weather (i.e.,heat stress,cold stress,and low photosynthetically active radiation (PAR)) [7–10] may increase in the future due to climate change,rice yield stability being greatly threatened[8,11].Therefore,it is urgent to establish sustainable practices for improving soil fertility and rice yield stability.

Straw incorporation is a common practice to improve soil fertility of rice paddies and increase rice yield[12–14].The global metaanalysis showed that straw incorporation increased the soil organic matter by 12.8%,total N by 11.1%,available P by 12.2%,and available K by 13.0%[14].The other meta-analysis showed that straw incorporation increased rice yield by 5.2%in China because it stimulated soil fertility [12].However,most of the observations were from normal fertilization rate conditions.Indeed,under high fertilization level conditions,nutrients are not the limiting factor for rice plant growth because of optimizing the fertilizer input[13,15,16].The positive effect of straw incorporation on rice yield may be reduced under high fertilization level conditions.Yet,the effect of long-term straw incorporation on rice yield under high fertilization levels is still unclear,especially in rice–wheat systems.

Recently,numerous studies have investigated the effect of straw incorporation on crop yield stability [17–21].Verhulst et al.showed that straw returning benefited maize yield stability in rainfed semi-arid highland [17].Li et al.also found that straw returning combined with NPK fertilizer lowered the coefficient of variation(CV)of wheat yield and increased sustainable yield index(SYI),compared with NPK fertilizers [20].Zhang et al.indicated that straw incorporation increased yield stability of maize and winter wheat in a maize-wheat rotation system [19].However,to the best of our knowledge,no experiments have so far assessed the effect of straw incorporation on rice yield stability.Moreover,the underlying mechanisms of higher crop yield stability under straw incorporation are still unclear.

The Rice-wheat system is one of the most popular cropping systems in China,with an area of about 7.4 Mha and average annual grain production of 8 t ha-1rice and 5.5 t ha-1wheat[22,23].Thus,we conducted a 9-year field experiment to investigate the effect of straw incorporation on rice yield and yield stability under high fertilization level conditions in the rice–wheat system.We also analyzed the relationship between rice yield and climatic factors to find the reason for higher crop yield stability under straw incorporation.To the best of our knowledge,this is the first study to report on the effects of straw incorporation on yield stability.

2.Materials and methods

2.1.Experimental design and field management

We established this experiment at the Danyang Experimental Station (119°10′,34°36′),Jiangsu province,China,in October 2009.The site is a typical wheat-rice double-cropped area in the middle and lower Yangtze River Region.At the beginning of the field experiment,the topsoil (0–20 cm) had an organic matter of 17.2 g kg-1,total N of 1.0 g kg-1,available P of 13.6 mg kg-1,and available K of 93.5 mg kg-1.Air temperature,PAR,and precipitation during the experimental period were obtained from the weather station (2009ET,WatchDog,USA) at the experiment site,and the mean value in each rice season is summarized in Table 1.

The experiment was arranged in a completely randomized design,including two treatments with three replicates:straw removal (-S) and full straw incorporation (+S).Each plot was 7.0 m long and 4.5 m wide.For the+S treatment,all wheat and rice straw were chopped into about 5 cm long pieces before applied to the subsequent crop.Wheat straw was incorporated into the 0–15 cm soil layer by plowing in late May,while rice straw was spread evenly over the field surface after wheat sowing in mid-November.

In this study,mineral fertilizers were applied according to high fertilization level practice,i.e.,N fertilizer as urea,P fertilizer as superphosphate and K fertilizer as potassium chloride at the rate of 300 kg N ha-1,150 kg P2O5ha-1and 240 kg K2O ha-1in the rice season,and at the rate of 225 kg N ha-1,105 kg P2O5ha-1,and 105 kg K2O ha-1in the wheat season,respectively.The fertilization regime and method for rice and wheat were the same.Fertilizer-N was split-applied as basal,tillering,spikelets-promoting and spikelets-protecting fertilizers with a splitting ratio of 4:2:2:2,and fertilizer-P and fertilizer-K were applied as basal fertilizer and topdressing fertilizer at the jointing stage with a ratio of 5:5.The basal fertilizers were broadcast and incorporated one day before plowing at the time of rice transplanting and wheat sowing.

In each year,the rice variety Wuyunjing 23(Oryza sativaL.)was transplanted by hand in mid-June with a plant spacing of 30.0 cm×13.3 cm,and winter wheat(Yangmai 16(2010–2013)and Yangmai 20 (2014–2018),Triticum aestivumL.) was sown in mid-November at a seed rate of 225 kg ha-1.Other crop management was carried out following the farmer practices.

2.2.Measurements and data analysis

2.2.1.Rice yield and yield stability

At maturity stage in each rice season,a 5 m2area in the center of each plot was harvested to determine grain yield on the standard moisture content of 13.5%.

Rice yield stability was evaluated by comparing CV and SYI between two treatments,which were calculated based on the following equations [20].

whereYmeanis the mean of rice yield during 2010–2018 for a treatment,Ysdis the yield standard deviation,andYmaxis the maximum rice yield over 2010–2018 for each treatment.

2.2.2.Selected soil properties

Soil samples (0–20 cm) were gathered using a 5-point random sampling method after the rice harvest.Soil organic matter and total N were measured by the potassium dichromate oxidation method and the Kjeldahl method,respectively [25,26].Available N,available K,and available P were determined by continuous flow analyzer after extraction with 0.01 mol L-1CaCl2(1:10),flame photometry after extraction with 1 mol L-1NH4OAC (1:10),and spectrophotometry after extraction with 0.5 mol L-1NaHCO3,respectively [27].Soil bulk density for the 0–20 cm layer was determined in undisturbed soil samples,according to Robertson et al.[28].

2.3.Statistical analysis

We analyzed data on rice yield,CV,SYI,and soil properties by independent samplet-test using SPSS 17.0.We also analyzed the data on rice yield by two-way analysis of variance (ANOVA) (that is,straw and PAR).Differences between treatments were considered significant atP<0.05.The linear fitting analysis was used by Origin 2017 software.

3.Results

3.1.Yield and yield stability

Straw incorporation did not affect the mean rice yield of 9-year(Fig.1A).However,it increased the yield stability significantly over 2010–2018 (Fig.1B and C).Compared with -S treatment,the +S treatment decreased the among-year CV of yield by 25.8% and increased SYI by 8.2%,respectively.Straw incorporation increased the numbers of spikelets per panicle significantly,whereas trended to reduce the panicle numbers.Grain weight and seed setting rate were similar in the -S treatment and the +S treatment.Straw incorporation had no significant effect on the average biomass across nine years (Table 2).

Table 1 Meteorological factors during the 2010–2018 rice season.

Table 2 The effect of straw returning on yield,yield components (No.of panicles,No.of spikelets per panicle,Grain weight,and Seed setting rate),dry matter accumulation and harvest index in H-PAR and L-PAR years.

3.2.Relationship between yield and weather conditions

The relationship between rice yield and the average air temperature,mean maximum temperature,mean minimum temperature,and precipitation was weak(Fig.2A,B,C and E).However,the rice yield of all treatments significantly and positively correlated with PAR below a certain threshold value from 2010 to 2018(R2=0.29*) (Fig.2D).We further investigated the effect of PAR on the treatment effect(i.e.,Y+S/Y-S,the ratio of yield in+S to that in-S)(Fig.3A).The results showed that Y+S/Y-Slinearly decreased with increasing PAR (R2=0.65*),and a positive effect was only observed when PAR was lower than 7.0 MJ m-2d-1during the experimental years.Therefore,we took a PAR value of 7.0 MJ m-2d-1as a threshold and roughly divided the experimental years into two categories:high PAR (H-PAR) years (2013,2015,2016,and 2018) and low PAR (L-PAR) years (2010,2011,2012,2014,and 2017).

Fig.1.Effect of different long-term treatments on average yield(A),coefficient of variance(B)and sustainable yield index(C).-S,straw removal;+S,full straw incorporation;** and * indicate significant differences between treatments at P <0.01 and P <0.05,respectively.

3.3.Response of yield,yield compounds,biomass and harvest index to different PAR years

Regardless of treatments,the average rice yield in the H-PAR years was 11.3%higher than in the L-PAR years due to significantly increasing the number of panicles and grain weight (Fig.3B;Table 2).Straw incorporation increased rice yield by 5.4% in the L-PAR years,but it did not affect the rice yield in the H-PAR years.Compared with -S treatment,the +S treatment significantly increased the number of spikelets per panicle and dry matter accumulation in the L-PAR years.However,there was no considerable difference in yield components,dry matter accumulation,and harvest index between two straw incorporation treatments in the HPAR years (Table 2).

Fig.2.Linear fitting analysis between the rice yield and the average temperature (A),mean maximum temperature (B),mean maximum temperature (C),PAR (D) and precipitation(E)during the rice growing season.All of the individual values represent the rice yield of both treatments in 2010–2018.PAR,photosynthetically active radiation during the rice season.

3.4.Soil properties

Our fertilizer management benefited soil properties greatly,regardless of treatments,while long-term straw incorporation increased the soil fertility more strongly than straw removal(Table 3).Compared with -S,the content of soil organic matter,total N,available N,available P and available K in +S was significantly increased by 10.7%,6.1%,22.0%,8.6%,and 40.1%,respectively,whereas soil bulk density was decreased by 16.3% after nine years of continuous straw returning (in 2018).

Table 3 Comparison of soil properties at 0–20 cm depth in two treatments (-S:straw removal,+S:full straw incorporation) in 2018.

4.Discussion

Our results showed that long-term straw incorporation did not affect the rice yield under high fertilization levels conditions,which was in agreement with previous experiments[15,16].However,many studies indicated that long-term straw incorporation significantly increased rice yield because it can improve soil fertility[12–14,29].In our study,straw incorporation also increased soil organic matter,total N,available N,available P and available K.Yet,the fertilizer rates were very high in this experiment.In the -S treatment,soil properties improved greatly and the soil available P and available K concentrations were more than 35 mg kg-1and 110 mg kg-1in 2018,indicating that the P and K were not the limiting factors for rice plant growth[30,31].Moreover,the N application rate,timing and frequency can ensure an adequate supply of N for rice growth [12,13,15].Thus,the positive effect of straw incorporation on rice yield through improving soil fertility may be limited in our study.

Indeed,straw incorporation also has some negative effects on rice plant growth.Crop straw incorporation under anaerobic soil conditions can increase phytotoxic substances (e.g.,organic acids and reducing substances) [32],thus inhibiting rice plant growth.Also,in situ retention of wheat straw with a high C:N ratio may lead to microbial N immobilization and a temporary decrease of available N for rice plant growth [33–36].Anaerobic decomposition of wheat straw under continuous rice cropping may result in the enrichment of soil phenols,which can reduce the soil N availability [37].The +S treatment trended to reduce the panicle numbers in this study,suggesting straw incorporation may decrease N supply to the rice plant at the early growth stage.Taken together,we speculate that the positive effects of straw incorporation on rice yield was possibly offset by its negative effects.

Fig.3.The relationship between Y+S/Y-S and PAR(photosynthetically active radiation)in the corresponding year(A)and the effect of straw returning on rice yield in H-PAR and L-PAR years (B).Y+S/Y-S,the ratio of yield in full straw incorporation (+S) to that in straw removal (-S);H-PAR,high PAR;L-PAR,low PAR.**,significant at P <0.01.

Our study indicated that straw incorporation increased the rice yield stability,corroborating several previous studies [18,20,38].Why straw incorporation increased the crop yield stability? This might because straw incorporation was an efficient approach to improve topsoil quality,which is favorable for plant roots growth and thereby increase the ability of water and nutrient absorption,leading to stable and increase of crop growth [19,24].Our results showed that straw incorporation increased the rice yield more strongly in the low PAR year,indicating that straw incorporation can stimulate the resistance to insufficient sunlight.Indeed,plants would be easily suffered in N and K deficiency under insufficient light stress and adding supernumerary nutrients (e.g.,N,K) could improve crop N and K nutrition status,which was beneficial for photosynthesis[39–42].Pan et al.found that N addition could alleviate the detrimental effects of shading on rice grain yield and total nutrient accumulation[39].Cong et al.showed that N fertilization compensated the weak photosynthesis of oilseed rape in the low PAR year [40].K application also had a similar effect as N application under the shading stress condition and contributed to the accumulation of photosynthetic products and transportation [41].In our experiment,appropriate soil structure and higher soil fertility under +S treatment were conducive to provide more nutrients for rice plants,which may lead to higher resistance to low PAR than-S treatment.It deserves further investigation to understand the mechanism of straw incorporation,improving rice resistance to insufficient light.

Because global warming often increases the frequency of heavy cloud cover and rainfall,the PAR is projected to decrease [7,9].Actually,the PAR in rice season has decreased by about 8.9% in the lower Yangtze River Region from 1981 to 2009[43].The strong resistance to low PAR in our long-term straw incorporation may imply that it can increase the adaptability of rice production to climate change.Moreover,compared to open-field burning,straw incorporation is helpful in increasing the air quality and enhance the active radiation of sunlight to crop[44,45].We might speculate that apart from adding chemicals,the incorporation of straw could be an environmentally-friendly measure to cope with low PAR stress.Furthermore,our study showed that straw incorporation stimulated soil fertility,suggesting that the fertilizer input can be reduced [12].A lower fertilizer application rate can not only save chemical input costs but also reduce the environmental risk caused by the loss of inorganic fertilizer[12,46].Long-term straw addition also increased the soil organic matter by 10.6%and reduced the soil bulk density by 18.1% in our study,both of which are in favor of preserving soil health and increasing soil productivity [13,17].Therefore,we believe that straw returning management could contribute to a more resilient agronomic system to biotic and abiotic stresses other than low PAR.Taken together,straw incorporation is a sustainable practice for improving soil fertility and rice yield stability,which can benefit agriculture sustainability and food security.

In conclusion,long-term straw incorporation did not significantly affect the average rice yield in our study;however,it could reduce the CV of rice yield and increase the SYI over 2010–2018.We found a straw×PAR interaction,whereby straw incorporation increased rice yield more strongly in the low PAR year.Straw incorporation improved soil organic matter,total N,available N,available P,and available K significantly.Our findings indicate that straw incorporation is a sustainable practice for improving rice yield stability and soil fertility,and suggest that straw incorporation can increase the adaptability of rice production to climate change.Moreover,under the straw incorporation condition,the proportion of fertilizer application should be appropriately increased in the early stage and reduced in the middle and late stage to alleviate the negative effects of the deficiency of N during the rice growth early periods

CRediT authorship contribution statement

Jianwei Zhang and Weiwei Li:wrote the manuscript.Jianwei Zhang,Yan Zhou,and Lei Xu:and performed this experiment and analyzed the data.Yu Jiang:critically revised the article.Yanfeng Ding and Ganghua Li:helped designed the experiments and financed this work.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Funding was provided by the National Key Research and Development Program of China (2017YFD0301203,2017YFD0300100,and 2018YFD0300803) and Jiangsu Agriculture Science and Technology Innovation Fund (CX(18)1002).