XIE Jun ,Blagodatskaya EVGENIA ,ZHANG Yu ,WAN Yu ,HU Qi-juan ,ZHANG Cheng-ming,WANG Jie,ZHANG Yue-qiang,SHI Xiao-jun

1 College of Resources and Environment,Southwest University,Chongqing 400716,P.R.China

2 Department of Soil Ecology,UFZ-Helmholtz Centre for Environmental Research,Halle 06120,Germany

3 Agro-Technological Institute,RUDN University,Moscow 117198,Russia

4 School of River and Ocean Engineering,Chongqing Jiaotong University,Chongqing 400074,P.R.China

5 Research Faculty of Agriculture,Hokkaido University,Sapporo 060-8589,Japan

Abstract Crop straw return after harvest is considered an important way to achieve both agronomic and environmental benefits.However,the appropriate amount of straw to substitute for fertilizer remains unclear. A field experiment was performed from 2016 to 2018 to explore the effect of different amounts of straw to substitute for fertilizer on soil properties,soil organic carbon (SOC) storage,grain yield,yield components,nitrogen (N) use efficiency,phosphorus (P) use efficiency,N surplus,and P surplus after rice harvesting. Relative to mineral fertilization alone,straw substitution at 5 t ha-1 improved the number of spikelets per panicle,effective panicle,seed setting rate,1 000-grain weight,and grain yield,and also increased the aboveground N and P uptake in rice. Straw substitution exceeding 2.5 t ha-1 increased the soil available N,P,and K concentrations as compared with mineral fertilization,and different amounts of straw substitution improved SOC storage compared with mineral fertilization. Furthermore,straw substitution at 5 t ha-1 decreased the N surplus and P surplus by up to 68.3 and 28.9%,respectively,compared to mineral fertilization. Rice aboveground N and P uptake and soil properties together contributed 19.3% to the variation in rice grain yield and yield components.Straw substitution at 5 t ha-1,an optimal fertilization regime,improved soil properties,SOC storage,grain yield,yield components,N use efficiency (NUE),and P use efficiency (PUE) while simultaneously decreasing the risk of environmental contamination.

Keywords: rice,SOC storage,yield components,N surplus,P surplus

1.Introduction

Rice is a vital food crop that supplies calories for more than half of the world’s population;moreover,60% of the population in China consumes rice as a main staple food (Patelet al.2010;Xionget al.2013;Carrijoet al.2017). If rice consumption per capita stays at the current level,China will need to produce appropriately 20%more rice by 2030 to satisfy domestic needs due to the rapid growth of the population (Penget al.2009;Zhanget al.2018). Because of agricultural restructuring,rural industrialization,urbanization,and economic reforms,it will be difficult to expand the area of rice cultivation in the future (Khanet al.2009). To meet current and future food demands,it is crucial to improve rice yield by increasing its nutrient use efficiency on agricultural land currently used for rice production (Muelleret al.2012;Xieet al.2016).

Nitrogen (N) and phosphorus (P) are considered to be the most important limiting factors on rice yield in agricultural fields (Cassmanet al.2003;Konget al.2014).To ensure high crop yields,Chinese farmers tend to apply large amounts of N and P fertilizers (Cuiet al.2008;NBSC 2010;Zhang Y Let al.2016). However,excessive N inputs reduce both the N use efficiency (NUE) and N recovery efficiency (NRE) of crops below 30% (Liuet al.2008;Yanget al.2013) without increasing crop yield. In addition,less than 20% of fertilizer P can be recovered by crops during the growing season because most fertilizer P applied to soil is converted into unavailable forms that crops cannot absorb (MacDonaldet al.2011;Wanget al.2017). Large inputs of N and P fertilizers lead to environmental contamination,for example,air pollution,water eutrophication and soil degradation (Gallowayet al.2008;Chenet al.2010;Zhang Met al.2017). To solve these problems,a “Zero Increase Action Plan” for national fertilizer use was announced by the Ministry of Agriculture of China in 2015,aiming to decrease fertilizer use and the environmental costs of the fertilizers that are used by 2020 (Liuet al.2016). Therefore,many practices have been adopted for rice production with the aim of increasing soil fertility,nutrient use efficiency,and crop yield while simultaneously reducing environmental risk.These practices include the use of slow-release fertilizers,mineral fertilizers incorporated with manure fertilizers,and straw return.

In China,cereal production increased 3.9-fold from 1961 to 2011 (Zhanget al.2011;Cuiet al.2014). Crop straw yields have increased remarkably along with the increase in cereal production (Zhanget al.2011). In 2015,appropriately 1.04 billion tons of crop straw were produced in China,constituting nearly one-third of global production (Liet al.2018). In general,Chinese farmers in the 1980s burned crop straw after the crop harvest,which led to the emission of a number of air pollutants to the environment and caused severe environmental hazards (Zhang X Yet al.2016;Zhouet al.2017). Returning crop straw to the field directly after harvesting is an optimal method of decreasing environmental contamination (Huet al.2016). Straw return is considered an environmentally friendly practice that has a positive impact on crop yield and soil fertility (Liet al.2018). Moreover,straw return increases impede nutrient losses through leaching and runoff (Maurya and Lal 1981;Buerkertet al.2000;Shafiet al.2007;Chahal and Van Eerd 2018). However,the optimal amounts of straw required for fertilizer replacement remain unclear,and the relationships among the different amounts of straw substitution for fertilizers,soil properties,rice grain yield and yield components,and rice aboveground N and P uptake are still uncertain. We carried out a three-year field experiment to explore optimal straw amounts for fertilizer substitution and the effects of straw substitution on rice production. We hypothesized that straw substitution for fertilizers would improve soil fertility and simultaneously increase crop yields. This study could provide information for better rice production management with regard to: (i) soil properties and soil organic carbon (SOC) storage;(ii) rice grain yield and yield components;(iii) N and P use efficiency;and (iv) N and P surplus in rice cropping.

2.Materials and methods

2.1.Site description

A field experiment was performed in a purple soil(entisol) from 2016 to 2018 at the experimental station of Southwest University (106°26´E,30°26´N,266.3 m above sea level),Chongqing,China. The climate of the experimental area is characterized as a typical subtropical monsoon climate,with a mean annual temperature of 18.3°C and rainfall of 1 115.3 mm.An automatic weather station was installed at our experimental field to monitor air temperature and precipitation (Fig.1). At the beginning of the experiment in 2016,the soil had a pH of 7.84 and 16.9 g kg-1of SOC. The total N,phosphorus (P),and potassium (K)were 1.65,0.67,and 31.8 g kg-1,respectively. The available N (alkaline hydrolysis N),P (Olsen-P) and K(exchangeable K) were 141.7,7.5,and 104.8 mg kg-1,respectively.

2.2.Fertilization and treatments

Rice (Oryzasativacv.y2-you) was grown as a single crop each year. It was transplanted between May 1 and 4 and harvested between August 19 and 24. Five treatments were applied in a completely randomized arrangement,and each treatment had three replicates.The experimental treatments were: (1) no fertilization(CT);(2) mineral N,P,and K fertilization (NPK);(3)2.5 t ha-1straw substitution for fertilizers (LNPKS);(4)5 t ha-1straw substitution for fertilizers (MNPKS);and(5) 7.5 t ha-1straw substitution for fertilizers (HNPKS).The area of each plot was 12 m2,and the plots were separated by cement barriers to prevent nutrient and water movement between them. Rice straw was chopped to approximately 0.1-m length after being airdried,and then it was returned to the soil before the rice season. Mineral N,P,and K were applied as urea,superphosphate,and potassium sulfate. The N,P,and K application rates in the NPK treatment were 180 kg N ha-1,75 kg P2O5ha-1,and 75 kg K2O ha-1,annually. N fertilizer was applied in two splits,namely,basal (50%)and topdressing (50%). The P and K fertilizers were applied as basal fertilizers. The NPK,LNPKS,MNPKS,and HNPKS treatments had the same application rates for N and P,at 180 kg N ha-1yr-1and 75 kg P2O5ha-1yr-1,respectively. If straw supplied K fertilizer didn’t reach 75 K2O kg ha-1yr-1under the LNPKS,MNPKS,and HNPKS treatments,we applied mineral K fertilizers as supplements. The C,N,P,and K concentrations in the rice straw were 37.95,0.66,0.19,and 3.17%,respectively,which were averaged values from 10-year determinations.

2.3.Laboratory analyses

After the rice harvest each year,we collected the rice grain and rice straw,and the grain and straw were airdried,ground and passed through a 0.15-mm sieve before they were used for the determination of total N and P concentrations. The total N and P concentrations were determined by micro-Kjeldahl method and vanadomolybdate yellow method (Jackson 1969;Nelson and Sommers 1982),respectively.

After the rice harvest each year,we collected soil samples from five spots in each plot in an “S”-shaped configuration at a depth of 0-20 cm with an auger.These samples were then pooled into one composite sample per plot. Soil physicochemical properties were determined with air-dried,finely ground soil samples that passed through 2.0-and 0.25-mm sieves. The soil pH was determined in a soil:water=1:2.5 (w/v) suspension(Lu 2000). The SOC was measured by the oxidation method using vitriol acid potassium dichromate oxidation(Pageset al.1982). The soil available N,P,and K were measured using the methods outlined by Lu (2000) and Pageet al.(1982). SOC storage was calculated using the following equation (Huanget al.2012):

S=C×BD×H×0.1

where S is the SOC stock (t ha-1);C is the SOC concentration (g kg-1);BD is the soil bulk density (g cm-3),which was determined by the ring knife method (Vaeziet al.2017);and H is the soil sampling depth (cm),i.e.,20 cm in this study.

2.4.Calculations

Effective panicle,number of spikelets per panicle,seed setting rate,and 1 000-grain weight were used as the representative rice yield components (Donget al.2011).Based on the methods reported by Liuet al.(2010),Hartmannet al.(2015) and Xinet al.(2017),the NUE and the fertilizer P use efficiency (PUE) were calculated using the following equations:

NUE=(UN-UN0)/FN×100

PUE=(UP-UP0)/FP×100

where UNand UPare the total N and P uptake of plants under fertilization (kg ha-1),respectively;UN0and UP0are the total N and P uptake of plants without N and P fertilization (kg ha-1),respectively;and FNand FPare the amounts of N and P fertilizer applied (kg ha-1).

The N surplus and P surplus were calculated using the following equations (Groenigenet al.2004;Xinet al.2017):

N surplus=N fertilizer application-Crop N uptake

P surplus=P fertilizer application-Crop P uptake

where N and P fertilizer applications are the total amounts of N and P applied as mineral fertilizer and straw,N and P uptake are the crop aboveground N and P uptake during the whole growing season.

2.5.Statistical analyses

One-way ANOVA was used to test for significant differences between the soil and plant parameters.The significance of treatment effects was tested with Duncan’s new multiple range test atP＜0.05. All statistical tests were performed in SPSS version 16.0(SPSS Inc.,Chicago,IL,USA). The figures and tables were made in Excel 2016 and Origin 18.0. A redundancy analysis (RDA) and variation partitioning analyses (VPAs)for the relationships among rice grain yield and yield components,rice aboveground N uptake and P uptake,and soil properties were performed with CANOCO 5.0 Software.

3.Results

3.1.Soil properties and SOC storage

Compared to purely mineral fertilization,straw substitution exceeding 2.5 t ha-1significantly increased the soil available N,P,and K concentrations in the three years (Fig.2-A-C;Appendix A). The greatest increases occurred with straw substitution at 7.5 t ha-1.Moreover,the SOC under straw substitution at 7.5 t ha-1was significantly greater by 5% compared to that under mineral fertilization. In contrast,soil pH was not different between the different treatments.

Relative to purely mineral fertilization,different amounts of straw substitution improved the SOC storage over the three years (Fig.3),and the effect under straw substitution at 7.5 t ha-1was the strongest. However,the SOC storage under straw substitution at 5.0 and 7.5 t ha-1was not significantly different over the three years.SOC storage under straw substitution at 5 and 7.5 t ha-1was 2.6 and 3.6% greater,respectively,than that under straw substitution at 2.5 t ha-1.

3.2.Rice yield components,grain yield,and straw yield

Straw substitution at 5 t ha-1significantly increased the number of spikelets per panicle,effective panicle,seed setting rate,and 1 000-grain weight by 5.6,15.5,6.4,and 2.5%,respectively,in the three years compared to the values under mineral fertilization (Fig.4-A-D;Appendix B).Moreover,straw substitution at 2.5 and 5.0 t ha-1increased rice grain yield by 12.9 and 23.6%,respectively,compared with that under mineral fertilization,and grain yield was increased the most under straw substitution at 5 t ha-1(Fig.4-E). Furthermore,straw substitution at 2.5 and 5.0 t ha-1increased rice straw yield compared with that under mineral fertilization over the three years,and straw substitution at 5.0 t ha-1had the most substantial effect (Fig.4-F).Simultaneously,except for seed setting rate,rice yield components,grain yield,and straw yield had no difference between mineral fertilization alone and straw substitution at 7.5 t ha-1(Fig.4-A-F).

3.3.Aboveground uptake and use efficiency of N and P

Compared to the mineral fertilization treatment,straw substitution at 5.0 t ha-1increased rice aboveground N uptake by 14.3% (Table 1;Appendix C) and improved the NUE of rice by 11.8%. Compared to mineral fertilization,different amounts of straw substitution improved rice aboveground P uptake in the three years (Table 1). Straw substitution at 5 t ha-1had the greatest effect,which improved rice aboveground P uptake by 19.7%. Straw substitution at 2.5 and 5 t ha-1significantly increased the PUE of rice compared with that under mineral fertilization alone,with the greatest effect again being at 5.0 t ha-1.

Table 1 Aboveground uptake and use efficiency of N and P under different amounts of straw substitution in rice production (mean values for the years 2016 to 2018)

3.4.N surplus and P surplus

The N surplus at the straw substitution rate of 5 t ha-1was significantly lower (by 68.3%) than that under mineral fertilization alone (Fig.5-A). Different amounts of straw substitution decreased the P surplus compared with that under mineral fertilization alone,over the three years. The decrease in the P surplus due to straw substitution at 5 t ha-1was greater than that under all other straw substitution treatments,remarkably decreasing the P surplus by up to 28.9% compared with that under mineral fertilization.

3.5.The contribution of rice aboveground N and P uptake,and soil properties to the variation in rice grain yield and yield components

The soil available N,P,K,and organic C concentrations were positively correlated with rice grain yield,effective panicle,seed setting rate,and the number of spikelets per panicle (Fig.6-A). The rice aboveground N uptake and P uptake were positively correlated with the grain yield,effective panicle,seed setting rate,and the number of spikelets per panicle. The VPA results indicated that soil properties contributed 7.2% to the variation in rice grain yield and yield components,while rice aboveground P uptake and N uptake contributed 6.3 and 3.4%,respectively (Fig.6-B). Rice aboveground N uptake,rice aboveground P uptake,and soil properties together contributed 19.3% of the variation in the rice grain yield and yield components.

4.Discussion

4.1.Effects of different amounts of straw substitution on soil properties and soil C sequestration

Straw substitution exceeding 2.5 t ha-1significantly increased soil available N,P,and K concentrations above those of the mineral fertilization treatment(Fig.2-A-C). Moreover,straw substitution at 7.5 t ha-1significantly increased the SOC concentration above that of the mineral fertilization treatment (Fig.2-E).These results may be attributed to the decomposition of organic materials and the gradual release of nutrients into the soil (Iqbalet al.2021). However,no difference was found in soil pH under different straw substitution treatments (Fig.2-D). This result is consistent with the results of Zhanget al.(2018),who found no difference in pH between mineral fertilization and organic material treatments in the Taihu Lake region. All straw substitution treatments increased SOC storage compared to mineral fertilization (Fig.3). Adding organic amendments to the soil helps to increase SOC storage (Wang and Dalal 2006;Kunduet al.2007;Zhang Tet al.2017) by improving soil biological properties and microbial community activity and increasing soil aggregate stability (Suet al.2006;Liuet al.2017). The SOC storage under straw substitution at 5 and 7.5 t ha-1exceeded that under straw substitution at 2.5 t ha-1(Fig.3),which is consistent with the different amounts of returned straw.

4.2.Effects of different amounts of straw substitution on crop yield

Straw substitution at 5 t ha-1increased the number of spikelets per panicle,effective panicle,seed setting rate,and 1 000-grain weight values above those of the mineral fertilizer alone treatment (Fig.4-A-D). These results are consistent with those of Moeet al.(2019),who found that organic material combined with mineral fertilizer had a positive effect on rice growth. Straw substitution at 2.5 and 5 t ha-1significantly increased rice grain yield by 12.9 and 23.6%,respectively,compared with that under mineral fertilization (Fig.4-E). The improved crop yield under the straw substitution treatments is attributed to three factors: 1) the N immobilized in rice straw may have been released slowly throughout the rice-growing season(Malhiet al.2011),this process reduced soil NO3--N leaching outside of the root zone (Yanget al.2015,2018).In contrast,mineral fertilization provided more available N at the early stage of rice growth,which was the time of the rainy season in Chongqing (Fig.1). The heavy rainfall would have accelerated N runoff and leaching under mineral fertilization treatments,resulting in less N available for later growth and yield formation;and 2)straw return adds available P to the soil directly while also solubilizing P in the soil,thus promoting higher crop yields(Xinet al.2015,2017).

The soil available N,P,and K concentrations under straw substitution exceeding 2.5 t ha-1were higher than those under mineral fertilization (Fig.2-A-C),and SOC storage was also improved (Fig.3). Our findings are consistent with Yanget al.(2015) and Malhiet al.(2011),who suggested that soil properties improved under straw return,which benefited crop yield. Simultaneously,the RDA results demonstrated that SOC correlated with rice grain yield (Fig.6-A),which is consistent with the findings of Liuet al.(2021),who found that SOC under straw return was positively correlated with crop yield. The RDA results indicated that soil properties contributed 7.2% to the variation in the rice grain yield and yield components(Fig.6-B).

However,some findings of previous studies differ from our results. A meta-analysis that gathered results of 45 long-term (more than 10 years in duration) experiments under wheat,maize,and rice cropping systems and explored the different effects on crop yields with or without straw return,revealed that although straw return improved crop yield by 7%,crop yield under straw return in some experiments on the North China Plain decreased by 0.6-7.1% (Wanget al.2015). This discrepancy might result from the differences among various soil types and climatic environments that affect the cycling of straw-derived N under field conditions (Muet al.2016).

4.3.Effects of different amounts of straw substitution on plant N uptake,NUE and N surplus

The increased aboveground N uptake and NUE in rice(Table 1) under straw substitution at 5 t ha-1can be attributed to the immobilization of N in the straw which subsequently provided N slowly throughout the rice growth period (Malhiet al.2011). Straw return enhanced soil N retention by decreasing N leaching,which favors plant growth (Yanget al.2016;Zhouet al.2020). Having higher aboveground biomass enhances crop N uptake,and aboveground biomass is strongly correlated with N uptake(Azeezet al.2006;Abbastet al.2010;Abbasiet al.2012,2013). Our RDA results also indicated rice aboveground N uptake correlated with rice grain yield (Fig.6-A). The RDA results demonstrated that rice aboveground N uptake contributed 3.4% to the variation in rice grain yield and yield components in this study (Fig.6-B).

Soil properties are important factors that increase the N uptake and NUE of crops (Bouwman and Boumans 2002). The improved soil properties improved plant N uptake and NUE in our study. In contrast,mineral fertilizer may accelerate N losses through leaching (Fowleret al.2013;Chenet al.2014;Zhanget al.2014),which may have decreased rice N uptake and NUE. Moreover,rice aboveground N uptake under straw substitution at 5 t ha-1was higher than that under straw substitution at 7.5 t ha-1(Table 1),which might be attributed to the lower amount of N released under straw substitution at 7.5 t ha-1at important rice growth stages,and the inability to provide more available N during the period of rice growth(Choiet al.2004;Iqbalet al.2021). Straw substitution at 5 t ha-1decreased the soil N surplus by 68.3% compared with that under mineral fertilization (Fig.5-A),which indicated that straw substitution at 5 t ha-1might decrease the risk of environmental contamination compared with mineral fertilization.

4.4.Effects of different amounts of straw substitution on plant P uptake,PUE and P balance

All straw return treatments increased the rice aboveground P uptake by up to 19.7% compared to that under mineral fertilization over the three years (Table 1).The different straw substitution treatments provided more available P than mineral fertilization (Fig.2-B). The reason for this difference is that the straw decomposed under the high temperatures (Fig.1) during the ricegrowing season and supplied more nutrients for rice growth (Xinet al.2017),thereby improving crop P uptake. The straw return may have a priming effect on the activity of P with low solubility,which is important for enhancing plant P uptake (Xinet al.2015). The RDA results demonstrated that rice aboveground P uptake contributed 6.3% to the variation in rice grain yield and yield components (Fig.6-B). Relative to mineral fertilization,all straw substitution treatments decreased the soil P surplus by up to 28.9% over the three years(Fig.5-B). Straw incorporation has been shown to decrease P losses compared with those under mineral fertilization (Feiet al.2020). Under mineral fertilization,the amount of P remaining in the soil may increase the risk of environmental contamination (Xinet al.2017).

PUE indicates the P recovery efficiency from applied P resources,which strongly affects nutrient cycling. The different amounts of straw substitution enhanced PUE by up to 11.8% compared with that under the mineral fertilization treatment (Table 1). These results are consistent with Guoet al.(2018),who demonstrated that straw incorporation has the associated benefit of elevating PUE. However,our results showed that PUE ranged from 26.4 to 38.2% over the three years (Table 1). Our increases in PUE are lower than the results obtained in studies by Guoet al.(2018) and Xinet al.(2017) of longterm straw return,in which PUE ranged from 34 to 72%and 53.7 to 61.7%,respectively. This discrepancy can be attributed to the effect of residual soil P in the soil in shortterm experiments (Xiaet al.2013).

5.Conclusion

We conclude that straw substitution at 5 t ha-1is an ecologically friendly practice for rice production that improves soil properties,SOC storage,rice grain yield and yield components,NUE,and PUE and reduces the risk of environmental contamination. Rice aboveground N and P uptake and soil properties together contributed 19.3% to the variation in the rice grain yield and yield components.

Acknowledgements

This work was supported by the earmarked fund for China Agriculture Research System (CARS-22;Green manure).

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