ZHOU Xing,LIAO Yu-lin ,LU Yan-hong ,Robert M.REES,CAO Wei-dong,NIE Jun ,LI Mei

1 Crop Research Institute,Hunan Academy of Agricultural Sciences,Changsha 410125,P.R.China

2 Soil and Fertilizer Institute of Hunan Province,Hunan Academy of Agricultural Sciences,Changsha 410125,P.R.China

3 Scientific Observing and Experimental Station of Arable Land Conservation (Hunan),Ministry of Agriculture and Rural Affairs,Changsha 410125,P.R.China

4 Scotland’s Rural College,Edinburgh EH9 3JG,UK

5 Key Laboratory of Plant Nutrition and Fertilizer,Ministry of Agriculture and Rural Affairs/Institute of Agricultural Resources and Regional Planning,Chinese Academy of Agricultural Sciences,Beijing 100081,P.R.China

Abstract Improved utilization of rice (Oryza sativa L.) straw and Chinese milk vetch (Astragalus sinicus L.,vetch) has positive effects on rice production.So far,few studies have investigated the productivity of vetch under different residue management practices in double-rice cropping system.The effects of rice straw on the growth and nutrient accumulation of vetch across seven years (2011–2017) and the subsequent effects of rice straw and vetch on two succeeding rice crops in a vetch–rice–rice cropping system,with the vetch established by relay cropping,were examined.The seven-year double-rice experiment consisted of the following treatments:(1) 100% chemical fertilizer (F-F100);(2) only vetch without chemical fertilizer (M-Con);(3) 80% chemical fertilizer plus vetch plus a low-cutting height (low-retained stubble) with the removal of straw (M-F80);(4) 80% chemical fertilizer plus vetch plus a low-cutting height with the retention of straw (M-F80-LR);(5) 80% chemical fertilizer plus vetch plus a high-cutting height (high-retained stubble) with the retention of straw (M-F80-HR);and (6) no fertilizer (F-Con).The yields of the two rice crops after vetch were not affected by either the cutting height of stubble with retention of straw or by the management of straw (retention vs.removal) with low-cutting height of stubble.The yields of the two rice crops after vetch were significantly higher for M-F80-HR than for M-F80-LR,but the relative contributions of the high-cutting height and straw retention to the higher rice yield could not be determined in this study.The yield stability of the double-rice grain in M-F80-HR was also increased,as determined by a sustainable yield index.Significant increases in vetch biomass and nutrient uptake were observed in the fertilized treatments during the rice season compared with the unfertilized treatments.In M-F80-HR plots,improvements in the growing environment of the vetch by conserving soil water content were associated with the highest vetch biomass,nutrient uptake,and yield stability of vetch biomass.These increased nutrient inputs partially replaced the demand for chemical fertilizer and stimulated the rice yields.It can be concluded that retaining higher-cutting stubble residues with straw retention could be the best straw management practice for increasing the vetch biomass and nutrient use efficiency,thereby allowing utilization of high-cutting height with retention of straw and vetch to improve the stability of rice productivity in a double-rice cropping system.

Keywords:double-rice cropping system,Chinese milk vetch,biomass productivity,rice straw,sustainable yield index

1.Introduction

Rice (OryzasativaL.) is the staple food for most of the world’s population and maintaining the sustainable production of rice is therefore critical to global food security(Alexandratos and Bruinsma 2012).China is the largest rice producer in the world and accounted for 28.5% of the global rice production in 2016 (FAO 2016).Rice grain yields have risen dramatically in recent decades,largely due to the increased use of inorganic fertilizer.This has also coincided with a decreased nutrient use efficiency,leading to severe environmental impacts on soil,water and atmospheric quality (Leet al.2010;Liuet al.2010).Declining soil quality,e.g.,soil acidification and structural damage,has already had negative effects on rice yield (Liuet al.2013;Chenet al.2016).The production of larger grain yields with lower environmental impacts is therefore a major challenge for sustainable rice production (Chenet al.2014).Reducing the amount of chemical fertilizer with replenishment of organic materials offers the potential to deliver such improvements.

Improving yield gaps could be achieved in the future through improvements in soil organic matter managementviaadditions of organic materials such as crop residues and green manures (Fanet al.2013).The subsequent use of organic materials as nutrient sources depends on their quality,environmental factors,soil properties,and management practices (Singhet al.2005).A number of studies have demonstrated that the return of crop residues is an effective way of increasing soil organic matter content,maintaining the productivity of soils and improving soil fertility(Malhiet al.2011;Huanget al.2013;Murphyet al.2016).Rice straw,which is characterized by a high C/N ratio and abundant K,Si,often decomposes rapidly in soils but can result in the short-term immobilization of soil nutrients and anaerobism as the microbial population utilizes the available carbon substrates contained in the straw (Yanget al.2017;Zhuet al.2017).Legume green manures by contrast such as Chinese milk vetch (AstragalussinicusL.,hereafter vetch),are characterized by a lower C/N ratio,which often contributes to N mineralization and enhancement in paddy fields (Tejadaet al.2008;Yanget al.2011;Piotrowska and Wilczewski 2012).Residues with a high C/N ratio often decompose more slowly than those with a low C/N ratio or high N content (Magidet al.1997;Trinsoutrotet al.2000).And it has been reported that the decomposition of cereal straw could be accelerated by the addition of green manures(lowering the C/N ratio of the decomposed materials;Mishraet al.2001).Co-incorporation of rice straw and leguminous green manures could therefore potentially improve the synchrony of N supply and demand thereby contributing to increases in crop yield and reductions in N losses from the soil–plant system (Kaewpraditet al.2009).However,it has not been possible to evaluate these potential effects of co-incorporation of rice straw and leguminous green manures due to the lack of any effects on the stability of rice productivity in double-rice cropping systems.Few studies have evaluated the effects of green manure applications with different rice straw management practices on longterm yield.

The incorporation of crop residues is known to deliver multiple soil quality benefits,such as increasing soil fertility,improving soil physical and chemical properties,reducing soil erosion and soil evaporation,and reducing soil sealing and crusting (Gangwaret al.2006;Wilhelmet al.2007;Blanco-Canqui and Lal 2008;Bandyopadhyayet al.2016).In practice,straw management is currently changing from conventional tillage to high-retained stubble because of the popularity of mechanized agricultural operations,which favor reduced tillage,and offer labor-saving approaches.Previous research found that keeping tall stubble following rice production is an effective way to promote the productivity of succeeding legumes (Bandyopadhyayet al.2016).This traditional system conserved soil moisture by increasing height of standing rice stubble,which substantially improved the productivity of succeed crops.In southern China,vetch as cover crop was broadcasted within the standing late rice without any tillage at 20–25 days prior to the harvest of late rice (Lin and Gu 2000).However,the influence of keeping tall stubble on the vetch productivity and thereby co-incorporation of standing late rice residues and vetch on the stability of vetch-early rice-late rice rotation system is not fully understood.Therefore,the objectives of this study were to:1) evaluate the effect of different residue management practices on vetch production and the nutrient supply capacity of the vetch when it was used as green manure,and 2) explore the influence of co-incorporation of rice straw and vetch on the stability of rice productivity under reduced fertilization in a double-rice cropping system over seven consecutive years.

2.Materials and methods

2.1.Site description

Field experiments were performed from 2011 to 2017 at Nan County,Hunan Province,China (29°13´N,112°28´E and altitude 30.0 m).The soil is a typical purple calcareous loamy clay soil derived from lake sediment.The initial chemical properties of 0–20 cm soil layer in 2011 were as follows:pH (H2O),7.70;soil organic matter,47.5 g kg–1;total N,3.28 g kg–1;total P,1.28 g kg–1;total K,22.2 g kg–1;available nitrogen,261 mg kg–1;available phosphorus,15.6 mg kg–1;available potassium,98.0 mg kg–1.The annual mean temperature is 17.6°C and the yearly average precipitation is 1 293 mm at the experimental site.The annual sunshine is about 1 479 h (1 149 h during crop growing period).The monthly precipitation and mean temperature at the experimental site over the experimental years (2011–2017) are shown in Fig.1.

2.2.Crop management and experimental design

The experiments were conducted in cropping system of early rice–late rice rotation in seven successive years(2011–2017).The key dates of early rice (cv.Yuanzao 1),late rice (cv.Huanghuazhan) and vetch of each season in each year including sowing date,transplanting seedling age and harvesting time listed in Table 1.Rice seedlings (both 30-days-old for early rice and late rice) were transplanted at a spacing of 20 cm×20 cm for double rice.

In this study,six treatments were established as follows:(i) F-Con,control (no chemical fertilizer and vetch);(ii)F-F100,sole 100% chemical fertilizer;(iii) M-Con,only vetch without chemical fertilizer;(iv) M-F80,80% chemical fertilizer plus vetch plus low-cutting height (retained stubble of about 5 cm height) with removal of rice straw;(v) M-F80-LR,80%chemical fertilizer plus vetch plus low-cutting height with retention of rice straw;(vi) M-F80-HR,80% chemical fertilizer plus vetch plus high-cutting height (retained subble of about 40 cm height,left after the late rice harvest) with retention of rice straw.All treatments were arranged in a randomized complete block design with three replicates.The plot size was 20 m2(4 m×5 m) separated by a ridge (0.3 m wide and 15 cm aboveground).

Fig.1 The monthly rainfall and mean temperature at the Nan County in Hunan Province,China from 2011 to 2017.

Table 1 The farming operation dates of experimental crops in each year

The recommended fertilizer application rates to rice of the experiment were:150 kg N ha–1(urea,46% N),75 kg P2O5ha–1(superphosphate,12% P2O5) and 90 kg K2O ha–1(potassium chloride,60% K2O) for early rice,and 180 kg N ha–1,45 kg P2O5ha–1and 120 kg K2O ha–1for late rice.It has been reported that the application of vetch plants as green manure could partially reduce the use of chemical fertilizer (by 20–40%) due to its nutrient contents (Zhouet al.2015).The application rate of chemical fertilizer in manure-amended treatments (M-F80,M-F80-LR and M-F80-HR) was therefore reduced by 20%.Half of the urea and all the superphosphate and potassium chloride were applied as basal fertilizer and manually raked into a 5 cm soil depth using a smoothing harrow one day prior to rice transplanting.The remaining nitrogen was top-dressed at the tillering stage of each rice season.

In every harvest season,both early and late rices were cut manually using a sickle so that the remaining stubble after harvest had a height of 5 or 40 cm,thereby simulating traditional low-cutting stubble (i.e.,the traditional method for harvesting) or high-cutting stubble harvesting practices (i.e.,the harvesting method using a full feeding rice combine),respectively.The rice straw in the first four treatments at the harvesting stage (both early and late rice) was removed with low-cutting height (about 5 cm).The early rice straw in M-F80-LR and M-F80-HR with low-cutting height (about 5 cm) was chopped into 5 cm long pieces and uniformly mechanically returned into the top 10 cm depth of the surface soil after the harvest.The late rice straw with lowcutting height in M-F80-LR was uniformly mulched on the field surface,while in M-F80-HR,a high-cutting height of stubble was retained (about 40 cm height,the cut straw was uniformly spread on the field as a mulch).

Vetch (A.sinicusL.Xiangzi 1) was manually broadcast at a uniform density of 30 kg ha–1,and directly seeded without tillage before the harvest of late rice in early October each year and it grew throughout the interval in the doublerice cropping system (except in the F-Con and F-F100 treatments).It was cultivated without additional fertilization.The fresh vetch was harvested manually by cutting the soil surface in each plot,measured,and then ploughed back.It was mixed mechanically within the top 20 cm of the surface soil,which was thenflooded up a depth of 5–7 cm in April,ten days prior to the transplanting of early rice.

2.3.Plant sampling and analysis

Grain yield and yield components of double-rice cropping systemAt the grain maturity stage,five representative rice plant hills from an area of 1 m2in each plot (total of 25 hills)were collected to measure yield components,and rice grains were separated from straw using a plot thresher for each whole plot,the grains were separately sun-dried,and rice yield was determined.The panicle number of each area was recorded as the efficient panicle number per 1 m2.Panicles were hand-threshed,and filled and unfilled grain in a spikelet were separated by submerging in tap water.They were then dried to a constant weight in an oven at 60°C.Filled and unfilled grains were used to calculate filled grains per panicle and filled grain percentages.1 000-grain weight was determine by three subsamples of the1 000 filled grains.

Biomass,growth parameters and nutrient content of vetchAt the full bloom stage,two 1-m2areas of fresh vetch were sampled diagonally from each plot to determine fresh aboveground and root biomass.Five representative vetch plants from 1 m2in each plot were collected to measure their growth parameters.The growth parameters were plant height,number of compound leaves per plant,apparent leaf area,branches per plant and plant weight.Plant samples were dried at 60°C to a constant weight after drying at 105°C.The plant materials were used to determine the C concentration by the potassium dichromate oxidation method,N concentration by a modified Kjeldahl digestion method,P concentration by the molybdovanadate method,and K concentration by flame photometry (Lu 2000).

Soil water content of 0–20 cm soil layerSoil water content was measured gravimetrically from vetch plots,using a core sampler with a diameter of 3 cm and a length of 20 cm for the topsoil (0–20 cm).The soil water content was recorded over two days in successive vetch seasons after each notable rainfall.

2.4.Statistical analyses

The productivity of vetch biomass and double-rice cropping system was calculated using a sustainable yield index (SYI).The SYI was calculated to counteract any annual variation in yield for each treatment throughout the test period.The SYI was calculated as follows (Singhet al.1990):

whereis the mean yield or vetch biomass of a practice across the years,Ymaxis the maximum yield or vetch biomass data obtained from the treatment in any year,σn–1is the standard deviation,which measured the variation and stability in yield caused by soil and climatic factors (Singhet al.1990;Efthimiadouet al.2010).The yield stability of various agricultural management practices in cropping systems is estimated through SYI (Shekhawatet al.2016;Choudharyet al.2018).An SYI of 1.0 suggests an ideal situation that can sustain the maximum crop yield,while any deviation from 1.0 implies a loss of yield stability (Bhindhu and Gaikawad 1998).

Data obtained from various treatments were subjected to analysis of variance (ANOVA) using the SPSS 19.0(IBM SPSS Statistics Version 19).The difference among treatments were tested using Duncan’s test (P<0.05).Determination of differences between treatments,years and their interaction were performedviatwo-way ANOVA.Linear regression analyses of vetch biomass and growth parameters (plant height,number of compound leaves per plant,apparent leaf area,branches per plant,plant weight) were performed to determine their relationships across years.The relationships among the rice yield and yield properties,vetch biomass input and nutrient inputs were evaluated by regression analysis using the Pearson correlation analyses in the SPSS 19.0.,Sigmaplot 12.5 and Excel 2007.

3.Results

3.1.Rice productivity under different treatments

Grain yields of rice plantsThe grain yields of the double rice over the period 2011–2017 showed significant‘treatment×year’ interactions in the early rice,not in the late rice and double rice.In addition,grain yields showed significant responses to treatments and year (Table 2).

The mean rice yields observed in the control (F-Con)treatment suggests that the soil of the experimental field was capable of generating average grain yields in earlyand late rices of about 3.3 and 5.6 Mg ha–1,respectively(Table 2).By comparison,the fertilized treatments produced significantly (P<0.05) greater yields,except for late rice in the M-Con treatment.Yields of the two rice crops after vetch were not affected by either the cutting height of stubble in the treatments with retention of straw (M-F80-HRvs.M-F80-LR) or the management of straw (retentionvs.removal) in the treatments with low-cutting height of stubble (M-F80-LRvs.M-F80),but yields of the two rice crops after vetch were significantly (P<0.05) higher for M-F80-HR than M-F80.Rice yield were higher for M-F80-HR than for F-F100,with early rice higher by 9.1% and late rice by 8.3%.There was no significant difference between the yields of the two rice crops in M-F80-LR and F-F100,indicating that the incorporation of vetch and rice straw with low-cutting height could maintain rice yield by reducing the application of chemical fertilizer.

Table 2 Mean grain yields influenced by various treatments over seven years (2011–2017)

In addition,the yield response trend of early rice with various treatments was similar to that of late rice (Table 2 and Fig.2).However,the range of variation of early rice was more obvious (yield in manure-amended treatments increased by 3.6 to 9.1% compared to the F-F100) than late rice (increased by 1.4 to 8.3%).The yields of two rice crops with the same straw management practice (straw removal) were not affected by vetch (F-F100vs.M-F80),suggesting no decrease in rice yield with 20% less fertilizer by application of vetch.

Furthermore,the grain yields of the double-rice cropping system fluctuated over the experimental period.The grain yield gap of early rice between F-Con and M-Con progressively narrowed over the study period;but the gap in the synergistic treatment (M-F80-HR or M-F80-LR) and conventional fertilizer treatment (F-F100) displayed the reverse trend (Fig.2).Comparable trends in yield during the seven years between F-F100 and the vetch treatment suggested that vetch might maintain high rice yields.

Rice yield componentsA combined analysis of variance using data obtained over the experimental period of 2011–2017 indicated that the effects of year and treatment were significant for rice grain yield components except for 1 000-grain weight (Table 3).In addition,only the effects of the interaction of treatment and year were significant for the filled grains per panicle of yield components in early and late rice.When compared with the grain yields of unfertilized treatments (F-Con or M-Con),the yield changed in fertilized treatments mainly due to the changes in the panicle number,filled grains per panicle and filled grain percentage.However,yield increases in the vetch-amended treatments (M-F80,M-F80-LR and M-F80-HR) were mainly due to the increase of filled grains per panicle.

Fig.2 Sequential changes of rice grain yields under various treatments from 2011 to 2017.F-Con,control (no chemical fertilizer or vetch);F-F100,sole 100% chemical fertilizer;M-Con,only vetch without chemical fertilizer;M-F80,80% chemical fertilizer plus vetch plus low-cutting height with removal of rice straw;M-F80-LR,80% chemical fertilizer plus vetch plus low-cutting height with retention of rice straw;M-F80-HR,80% chemical fertilizer plus vetch plus high-cutting height with retention of rice straw.Error bars represent the standard error of mean (n=3).

Table 3 Yield components in double-rice cropping system under different treatments over seven years (2011–2017)

Stability of double-rice cropping system productivityThe SYI provided a consistent framework for comparing treatments because it allowed an assessment of individual treatment performance over the study period.The SYI for the double-rice cropping system revealed that there was a higher sustainability for late rice than for early rice in all plots,due to the erratic weather conditions in the early rice season (Table 4).The SYI for early and late rice was the highest in the M-F80-HR plot,whereas the lowest SYI was in the F-Con plot.The SYI for vetch-amended plots tended to be higher than that of the F-F100 treatment.These results implied that the effective management of straw combined with milk vetch incorporation has the potential to promote sustainable rice grain yields in the double-rice cropping system of Central-South China.

3.2.Vetch biomass productivity

Biomass production of vetch in full-bloom stageDuring the 7-year field study,the weather conditions and particular rainfall at the experimental site varied considerably (Fig.1).These resulted in large inter-annual variations in the vetch biomass production (Fig.3).While treatments also caused significant impacts on vetch aboveground biomass,plant height,number of leaves,branches per plant and plant weight (P<0.05) (Table 5).However,there were no significant ‘treatment×year’ interactions which indicated that the treatment in different years independently affected the vetch biomass productivity.

The mean biomass yields of vetch were 15.9–24.2 Mg ha–1on a fresh weight basis (the mean moisture content is 85%;Table 5).Fertilization treatments in the rice season significantly increased the biomass yields of vetch by 15–52% compared with the only vetch treatment (M-Con).This implies that the fertilizer applied in the rice season had a positive residual effect on vetch production.The highest mean biomass yield was observed in the high-cutting stubble with retention of straw plots (M-F80-HR),which was 29% higher than low-cutting height with removal of straw(M-F80,P<0.05);M-F80-HR also resulted in a significantly higher vetch biomass production than the low-cuttingheight with retention of straw (M-F80-LR,by 32%,P<0.05).Furthermore,there was no significant difference between M-F80-LR and M-F80.

Table 4 Sustainable yield index (SYI) for vetch biomass cultivated with different utilization patterns of rice straw and grain yield in double-rice cropping system over seven years (2011–2017)

Fig.3 Sequential changes of vetch biomass yield in full-bloom stage under different treatments in the vetch–rice–rice rotation system from 2011 to 2017,Central-South China.M-Con,only vetch without chemical fertilizer;M-F80,80% chemical fertilizer plus vetch plus low-cutting height with removal of rice straw;M-F80-LR,80% chemical fertilizer plus vetch plus low-cutting height with retention of rice straw;M-F80-HR,80% chemical fertilizer plus vetch plus high-cutting height with retention of rice straw.Error bars represent the standard error of mean (n=3).

Table 5 The mean biomass production and growth parameters at full-bloom stage of Chinese milk vetch (vetch) cultivated with different treatments over seven years in the vetch–rice–rice rotation system,Central-South of China (2011–2017)

As far as vetch biomass productivity is concerned,the SYI was the highest in the M-F80-HR plot,while the lowest SYI was in the M-Con treatment (Table 4).No distinct trend of SYI was observed between M-F80-LR and M-F80.It indicated that the effective management of stubbles and incorporated residues could maintain stable production for Chinese milk vetch.

Growth parameters of vetch at full-bloom stageThe parameters describing vetch growth over seven years showed no significant ‘treatment×year’ interactions at full-bloom stage (Table 5).But growth parameters of vetch (except apparent leaf area and branches per plant)changed significantly in different years and treatments.By comparison with M-Con treatment,the M-F80-LR and M-F80 treatments showed significant higher plant height,more compound leaves and greater plant weight,by 19,41 and 22%,and 23,30 and 30%,respectively,while no significant differences of the three parameters were observed between M-F80-LR and M-F80.Meanwhile,these three parameters in the M-F80-HR treatment increased significantly than the M-Con treatment by 50,76 and 74%,respectively,even significantly higher than that in M-F80-LR and M-F80.

3.3.Milk vetch nutrient enrichment

At the full bloom stage of the vetch prior to being returned to the soil as green manure,more than 80% of the C,N,P2O5and K2O accumulation had occurred in the aboveground biomass (Table 6).The C,N,P2O5and K2O accumulation of the experiment changed over various years,but again there was no significant ‘treatment×year’ interaction for biomass nutrient accumulation.In addition,different treatments had a significant effect on the accumulation of C,N,P2O5and K2O relative to the M-Con (Table 6).

The accumulation of C,N,P2O5and K2O (Table 6)clearly indicate the higher nutrient content of M-F80-HR treatment by comparison to M-Con.The biomass of vetches at the flowering stage in the M-F80-HR treatment were approximately 25.8 Mg ha–1(fresh weight),which contained 105 kg N ha–1,12 kg P2O5ha–1and 100 kg K2O ha–1,however,this was insufficient to fully satisfy the recommended levels of nutrition (N–P2O5–K2O=150–33–75 kg ha–1) for early rice in a double-rice cropping system,and was well below what would be required by later rice crop.However,the accumulation decreased significantly with decreasing vetch biomass productivity in M-F80-LR and M-F80.

3.4.Correlation between rice grain yield,yield components and vetch biomass input properties

By analyzing the correlation between rice grain yield,yield components and vetch biomass input properties (Table 7),we found that the grain yield of early rice and late rice was positively correlated with the vetch biomass input and nutrient inputs.The increase in grain yield could be explained by the observation that the increasing nutrient inputs from the vetch simulated the increase of the filled grains per panicle and panicle number in the double-rice cropping system.

4.Discussion

4.1.Effect of incorporation of vetch and rice straw on double-rice cropping system productivity

The capacity of cover crops to accumulate nutrients that are then made available to subsequent crops throughstraw decomposition is an important feature of cover crops in rotation systems.Legume cover crops are particularly important in this context (e.g.,Chinese milk vetch),because they can fix atmospheric N with rhizobia.Consequently,when compared with the conventional fertilizer treatment (FF100),the vetch-amended treatment with 20% less fertilizer(M-F80) was able to maintain the productivity of the doublerice cropping system even though the fertilizer application rate was reduced (Table 2).In the experiment,the lowest mean nutrient accumulation of pure vetch without fertilizer in the rice season (M-Con) reached 81 kg N ha–1,9 kg P2O5ha–1and 33 kg K2O ha–1.This demonstrated that nutrients such as N,P and K and other minerals contained in the vetch biomass application could enhance the availability of nutrient for rice production (Zhouet al.2015).In some studies,the effects of rice straw on the subsequent crop production were found to be significantly negative due to the high C/N ratio of the straw (Yanget al.2017;Zhuet al.2017).This provides evidence of high rates of microbial immobilization,with soil microbial populations competing for ammonium-N(Muhammadet al.2011;Takakaiet al.2018).The oxygen demand associated with this process often leads to the production of toxic substrates and anaerobic conditions due to microbes were switched away from oxygen,which serves as a terminal electron acceptor.In the experiment,the use of vetch under rice straw with 20% less chemical fertilizer produced a much higher level of rice productivity than the pure chemical fertilizer,particularly where tall rice stubbles was retained,which in turn,significantly improved the grain yield of the double-rice cropping system (Table 2).The higher nutrient content provided by the vetch was able to support production of the tall rice stubble and could partially replace the chemical fertilizers,thus improving rice productivity.The combination of fresh vetch with low C/N ratios and standing rice stubbles with high C/N ratios helped to reduce N immobilization and N losses from the application of pure rice straw (Kaewpraditet al.2009).Moreover,adding rice straw delayed decomposition of the vetch and may therefore have resulted in a more stable N supply over the year compared with the pure rice straw and vetch because of the complementarity of the two materials(Kaewpraditet al.2009).

Table 6 Annual mean biomass nutrient accumulation of milk vetch cultivated under different treatments

However,there were some differences in rice yield between the two residue treatments (M-F80-LR and M-F80-HR).Kaewpraditet al.(2009) found that rice yield could be improved by adding 2.5 Mg ha–1rice straw to 5 Mg ha–1legume crop (groundnut).It is possible that such changes in yield may occur as a consequence of improved synchrony between the release and uptake of nutrients.In a study that incorporated a mixture of legume and cereal straw,increased nutrient recovery was observed in a subsequent maize crop,which was ascribed to the improved synchrony (Myerset al.1994).However,the impacts of green manures on soil microbial activity is complex and can influence the nature and abundance of ammonia oxidizing bacteria which play a key role in soil N transformations in rice cropping systems (Gaoet al.2018).Further research is therefore required to fully understand the influence of rice straw and legume cover crop mixtures in different proportions on the growth and yield of rice.

The sustainability of a cropping system is dependent on the combination of soil,climate and cropping system(Mannaet al.2005).The yield trend is therefore complex phenomenon and may not be determined precisely on the basis of the fertilizer treatments applied here.Climate change has already had an impact on many agricultural activities.Climate change might affect yield trends due to changes in photo-synthesis and respiration in the vegetative and grain filling periods (Mannaet al.2005;Yujiet al.2009).In this study,the yield of late rice in all treatments decreased due to continuous rain over several weeks in the grain filling period in 2014 (Fig.2).Farm yields are usually influenced by seasonal rainfall,particularly in dry-farming regions,where the amount of rainfall received at critical growth stages of the crop.In addition to the soil,climate and cropping system,changes in yield trends in long-term experiments could occur because of the adversity of other factors such as imbalanced fertilizer use,cultivar,and insect pest.

4.2.Effect of rice straw management methods on the productivity of succeeding Chinese milk vetch

Previous studies have demonstrated that the application of rice straw with low rice stubble can have a larger effect on a succeeding legume (mung bean) production than other planting methods (removal) in rice-based lowland areas in the Mekong region(Bunnaet al.2011).The main reason for this is that retention of rice straw has the potential beneficial effect of reducing soil surface crusting and evaporation of moisture in the study area (Van den Berg and Lestari 2001).But the results of this study showed that rice straw mulched on the field surface (M-F80-LR) had no significant effects on succeeding vetch growth in a double-rice cropping system when compared with a conventional system of vetch cultivation without rice straw (M-F80;Table 5).Although the retention of rice straw provides insulation for vetch growth in winter,it may also cause shading and therefore have a negative effect on vetch at the seeding stage,which having more slender stems that lodge easily (Jianget al.2001).In the experiment,the vetch biomass production in the treatment where high stubble was maintained (M-F80-HR)was significantly greater than that of the conventional cultivation of vetch without rice straw (M-F80) and vetch with a low stubble retention of rice straw (M-F80-LR).This was likely due to the direct effect of higher temperature regulation and moisture status under high stubbles,which increased root proliferation and thus enhanced availability of nutrients to succeeding crop roots (Bandyopadhyayet al.2016).This led to a higher plant biomass yield and better growth parameters (Table 5).

In previous studies,both the retention of tall rice stubbles and retention of rice straw with low stubbles have been shown to have a positive effect on mitigating soil moisture stress and increasing productivity of the succeeding crop (Bunnaet al.2011;Bandyopadhyayet al.2016).Therefore,the average moisture content of the topsoil(0–20 cm) during the vetch growing period in winter was observed for two days following periods without rainfall (over 5 days;Fig.4).The results showed that under the short stubbles with no rice straw (M-Con and M-F80),soils were exposed earlier during the growth of vetch and deep cracks were then created during the mature phase of the late rice,which led to greater evaporation of residual soil moisture.Although low-cutting stubbles with retention of rice straw (M-F80-LR) has been showed to prevent soil water loss (Mulumba and Lal 2008;Zhaoet al.2014),in this study there were no consistent effects.This was mainly because of surface mulching,which shaded the soil allowing to retain more moisture for vetch at the seeding stage (Huanget al.2007),but then about 50–60% of the returned straw was decomposed 2 months after harvest of the late rice (Liet al.2006;Guoet al.2018) and it gradually lost the capacity to retain moisture.Retaining tall rice stubble conserves large amounts of soil moisture (Bandyopadhyayet al.2016).This prevents soil water loss by reducing evaporation and thus helps to retain moisture during the succeeding crop growth phase.As expected,keeping tall rice stubbles plots (M-F80-HR) significantly and consistently increased soil water content of the topsoil (Fig.4).One reason for these differences was that a taller rice stubble residue creates a more complex micro-climate which could directly affect the distribution of surface soil moisture (Huet al.2015).

4.3.The stability of vetch–rice–rice rotation system

Fig.4 The average soil water content of topsoil (0–20 cm) in different treatments was recorded for 2 days in two successive vetch seasons.M-Con,only vetch without chemical fertilizer;M-F80,80% chemical fertilizer plus vetch plus low-cutting height with removal of rice straw;M-F80-LR,80% chemical fertilizer plus vetch plus low-cutting height with retention of rice straw;M-F80-HR,80% chemical fertilizer plus vetch plus high-cutting height with retention of rice straw.Error bars represent the standard error of mean (n=3).Lower case letters in the same time indicate significant differences between treatments at P<0.05.

The SYI is considered to be an important indicator for measuring the yield stability of crop production system (Srinivasaraoet al.2012;Liet al.2016).High SYI values generally indicate that the system is in a more sustainable stage.In the experiment,fertilizer application in the rice season improved the SYI of vetch biomass yields compared with the no-fertilizer treatment in the rice season (Table 4).Increases in the SYI of vetch biomass yields were mainly attributed to the residual effects from fertilizer applied in the rice season (Panet al.2012).The highest SYI of vetch biomass yields was observed in the treatment in which tall late rice stubble was retained.Retaining tall late rice stubble improves the environment for vetch growth,particularly in relation to moisture status and temperature (Bandyopadhyayet al.2016),resulting in a positive effect on the sustainability of vetch biomass production.In addition,the SYI of rice grain yields of doublerice cropping system indicated that there was a higher yield stability for late rice than for early rice in all plots,which was due to the erratic weather conditions in the early rice season.Fertilizer application improved the SYI of doublerice grain when compared with treatments without fertilizer application.The SYI of double-rice grain increased by applying organic and inorganic fertilizer,especially where co-incorporation of vetch and tall rice stubbles was included(M-F80-HR;Table 4).It is possible that adding rice straw might delay decomposition of vetch and therefore result in a more stable N supply throughout the year because of the complementarity of the two materials (Kaewpraditet al.2009).Rice straw and incorporated vetch biomass were able to improve the system’s soil N-supplying capacity thereby enhancing the yield stability of vetch–rice–rice rotation system.

5.Conclusion

Chinese milk vetch (vetch) was once commonly used as a green manure in China.However,its use has virtually ceased due to the minimal direct economic benefits,increase in labor costs,and increased use of chemical fertilizer.The study found that improved rice straw management during the vetch phase of a vetch–rice–rice rotation could help to enhance the stability.Maintaining a high-cutting height of late rice stubble with the retention of straw may conserve enough soil moisture to improve the growing environment for vetch in winter.This would result in significant increases in the total biomass and nutrient retention by vetch,thereby retaining more nutrients within the system.The increased nutrient input and co-incorporation of high-cutting height of late rice stubble with the vetch stimulated the double-rice growth and significantly increased the double-rice grain yield.Thus,effective management of rice straw by the maintenance of high-cutting stubble could improve the value of vetch and promote the production of a double-rice cropping system.Such systems are required for the use of vetch by farmers in the future.

Acknowledgements

This work was supported by the earmarked fund of China Agriculture Research System (CARS-22),the Key R&D Projects in Hunan Province,China (2017NK2051),the National Key R&D Program of China (2017YFD0301504 and 2018YFD03006),and the Hunan Agricultural Science and Technology Innovation Fund Project,China (2018zd06).