Impacts Of Long-Term Nutrient Management Modes On Rice Yield And Its Mechanisms In The Rice-Wheat System

Improvement of rice yield per unit area is an effective pathway to ensure food security under the unsustainable increase of planting area,with the growing population.Optimized nutrient management could achieve an increase in rice yield.However,previous studies have focused more on the short-term effects of nutrient optimization management,whereas the long-term impact of nutrient management under global climate change was litter known.Therefore,based on a long-term experiment,this study compared the effects and mechanisms of different nutrient-optimized management modes on rice yield in the middle and lower reaches of the Yangtze River,which can provide a basis for selecting long-term nutrient management mode.Long-term experiments were conducted in a typical rice-wheat rotation area in the middle and lower reaches of the Yangtze River（Danyang,Jiangsu Province）from 2009 to2018.Treatments included a control treatment of convention fertilizer mode（CF）,three optimized nutrient management modes（OPs）:optimized fertilizer mode（OF,fertilizer postponing based on the same fertilizer rate as the CF treatment）,optimized fertilizer mode combined with rice and wheat annual straw incorporation（OFS）,optimized reduced fertilizer mode（ORF,reduce the fertilizer rate based on the OF treatment）.Long-term effect of nutrient management on rice yield and its components,the accumulation and distribution of dry matter,the absorption and utilization of nutrients,and their response to different weather types.Simultaneously,soil structure,fertility,and organic carbon pool were compared and analyzed.This study aimed to explore a suitable nutrient management model to improve and stabilize the rice yield in the rice-wheat system in the lower reaches of the Yangtze River.Furthermore,in the 2016 rice season,a pot experiment using¹⁵N-labeled wheat residue was conducted with conventional irrigation（CI）and alternate wetting/drying irrigation（AWD）both with and without N fertilization.This study aimed to quantify the distribution of straw-derived N（straw-N）in the rice-soil system and clarify the suitable water and fertilizer model with straw incorporation.The main reasons were as follows:1.Long-term optimization nutrient management modes can achieve the improvement and stability of rice yield,of which OFS treatment is the best.The OF,OFS,ORF treatments increased the 9-years average rice yields by 5.0%,8.0%,and 0.7%,and the sustainable yield index（SYI）by 4.4%,13.0%,and 2.1%,respectively,compared with the CF treatment.Due to an increase and stability in the number of spikelets per panicle and dry matter accumulation after heading.Correlation analysis showed that the OF,OFS,and ORF treatment had lower R-values between yield and total spikelets(0.47^*,0.41^*and 0.50^**),but higher R-values between yield and 1000-grains weight(0.57^**,0.54^**,and 0.77^**),over the CF treatment(0.84^**and 0.29).It suggested that grain weight was a major factor in the impact of rice yield in the OPs treatment.Rice yield was easily affected by average temperature,maximum temperature,and photosynthetically active radiation（PAR）after heading.Under high temperature-PAR years after heading(AHH_T-PAR,20.8℃,25.7℃and 7.0 MJ m^-2d^-1),the OF treatment had the highest rice yield,followed by the OFS treatment;under low temperature-PAR years after heading(AHL_T-PAR,19.3℃,24.2℃and 4.7 MJ m^-2d^-1),the OFS treatment was significantly higher than other treatments;there was the similar yield between the CF and ORF treatment under both types of years.Compared with other yield components,the change of grain weight only showed different treads under the two types of years,due to the high total spikelet number of OPs under AHL_T-PARwas more not conducive filled grain than under AHH_T-PAR.The OFS treatment could alleviate the reduction of grain weight causing by unfavorable weather after heading,because an increase in dry matter accumulation after heading and the proportion of panicle weight/total dry matter accumulation（PDM%）at maturity stage（MS）in the OF treatment was higher than the OFS and ORS treatments under AHL_T-PAR.It indicated that OFS mode could mitigate the negative effect of AHL_T-PARon the accumulation and distribution of dry matter,leading to the highest total dry matter accumulation and panicle weight at MS.2.OFS can increase and stabilize the absorption and utilization of nutrients by rice plants.The utilization of N,P,and K fertilizer in the OPs treatment was significantly higher than the CF treatment,and the total N,P,and K accumulation in the OF and OFS treatment was improved considerably;there was the same trend under the two types of years.The distribution of dry matter in the panicle depended on the nutrient status（N,K）of the vegetative organs.PDM%at MS was significantly negatively correlated with more than1.22%of the N%in the stem-leaf of MS,and had a logarithmic relationship（P<0.001）with the K%in the stem-leaf of MS.The N%of the stem-leaf of MS in the OF treatment under AHL_T-PARwas 1.28%（exceeded the threshold value）,leading to the significantly higher N distribution ratio in the stem-leaf of MS than OFS treatments.It indicated that OFS with high K%in stem-leaf could prevent the risk of unfavorable-delayed senescence under AHL_T-PAR,and reduce the loss of yield.3.OFS mode contributes more to soil quality improvement,which is conducive to the increasing and stabling of rice yield.Compared with the CF treatment,the OF treatment had no change on soil structure;the OFS treatment was significantly decreased bulk density and increased 1-2 mm water-stable aggregate mass proportion,mean weight diameter（MWD）,and mean geometric diameter;the ORF treatment significantly reduced the stability of soil aggregate.The long-term effects of nutrient management（CF,OF,OFS,and ORF）on the content of total organic matter（TOM）,total N,total P,available P,and available K were positive,making soil integrated fertility index（IFI）increase after 9-year,with decreasing order of OFS>OF>CF>ORF.Redundancy analysis showed the MWD and TOM were the two essential parameters to change IFI and SYI.There was the closest relationship between available K content and rice yield stability.It suggested that stabilizing soil structure and improving soil fertility is conducive to the sustainability of rice production.Compared with the CF treatment,the OF treatment significantly increased the organic carbon content（SOC）in the particle size of<0.25 mm and humic acid.The total SOC and its fractions,soil carbon pool management index,and soil invertase activity in the surface layer of the OFS treatment were higher than other treatments;the microbial biomass carbon content in the ORF treatment was the lowest.Correlation analysis showed that yield was significantly positively correlated with the SOC of bulk soil and 1-2 mm aggregate,easily oxidizable organic carbon,and soil invertase activity in the surface layer.It suggested that the increased quantity and quality of SOC were conducive to the increase of rice yield.4.Model of alternate wetting/drying irrigation combined with N fertilizer addition can increase the effective use of straw-N in the rice-soil system.The whole-plant straw-N recovery rate and the soil residue rate were 9.2-11.9%and10.2-13.8%,and 33.5-43.1%and 33.7-70.2%at panicle initiation stage（PI）and mature stage（MS）,respectively;there was no interaction between water management and N fertilization.Compared to CI,AWD did not affect whole-plant straw-N absorption and significantly changed the distribution of straw-N in various plant parts,such as increasing the straw-N accumulation in roots at PI and decreasing it at MS.N fertilization addition markedly promoted the transfer of straw-N to the plant,but reduced the contribution rate of N uptake by the plant.Furthermore,AWD or N fertilization addition allowed more straw-N to remain in the soil,and a positive interaction effect on the straw-N loss mitigation was found.Based on the above results,rice-wheat annual straw incorporation combined with optimization fertilization is a suitable nutrient management mode for the increasing and stabilizing of rice yield in the rice-wheat system in the lower reaches of the Yangtze River,and simultaneously,combining with AWD is beneficial to the efficient utilization of resources and soil quality improvement.