Effects Of Chinese Milk Vetch Planting And Co-incorporation With Rice Straw On Fertilizer Eduction And Their Regulating Mechanisms Of Biological Nitrogen Fixation

It is crucial to maximize the ecological service value of leguminous green manure for fertilizer reduction and soil fertility improvement in rice?Oryza sativa L.?poruduction.In the present study,multiple site-year“rice and Chinese milk vetch?Mv,Astragalus sinicus L.?”rotation systems were selected to investigate the impacts of Mv planting and incorporation or in combination with rice straw return on fertilizer reduction in rice production.Meanwhile,the regulating roles of organic amendments and fertilizer inputs in biological nitrogen?N?fixation of Mv were examined using ¹⁵N natural acundance method.Further,the diazotroph diversity and community structure in soil at Mv flowering stage was characterized based on nifH gene qPCR and high-throughput sequencing to reveal the mechanisms of diazotroph responses to different organic additions.The main results were as follows:?1?The effects of Mv planting and incorporation on fertilizer reduction in paddy soil differed between rice cropping systems.Compared to the full rate of conventional fertilization(F₁₀₀)in rice-winter fallow rotation,yield of early rice was maintained or increased by Mv incorporation in combination with 20%⁴0%fertilizer reduction,so was the N and K uptake.No more than 20%reduction of fertilizer inputs could be adopted for the late rice.Grain yield increased with increasing rate of Mv incorporation under either 20%or 40%reduction of fertilizer in early rice,and better yield performance was found in treatments with 30.0³7.5 t ha^-1 of Mv incorporation,while late rice yield decreased at different extents under 20%or 40%fertilizer reduction when compared to the F₁₀₀.?2?Co-incorporatoin of rice straw?Rs?and Mv residue benefited for the rice yield and soil fertility improvement.Co-incorporation in combination with 20%fertilizer reduction i.e.conventional straw return F₈₀RsMv and high stubble return F₈₀RhMv,had comparable yield with F₁₀₀(except for the 5%yield losses in F₈₀RhMv of late rice).On average,of the treatments with either Rs or Mv amendment plus F₈₀,grain yield decreased by 5.5%and 5.1%in RsF₈₀ of early rice and MvF₈₀ of late rice,respectively.F₈₀RsMv treatment resulted in 8%¹4 and 30%⁵3%more N uptake than F₁₀₀ at maturity in early and late rice,respectively.Treatment F₈₀RhMv resulted in relatively lower N and K uptake than F₈₀RsMv in some cases,but they all had greater fertilizer N,P and K use efficiency than F₁₀₀00 by 0.6-1.7,8.0-11.6,and 4.4-6.4 percent unit.Meanwhile,the combination treatments helped maintain the contents of soil organic matter,total N and available N.?3?Organic amendments and fertilizer application in rice seasons had signicant impacts on biological N fixation of Mv plants.Fresh biomass of Mv at flowering stage ranged from 9.0 to 24.8 t ha^-1.The percentage of N derived from biological fixation(N_fix%)in Mv was 59%⁸5%,corresponding to the N fixation amount of 24⁵5 kg ha^-1.The N_fix%in FRs and FRh treatment was 59%⁶8%,which was significantly lower than that in CK?85%?,while N fixation amount in FRh was greater than others.Increasing N inputs to the straw incorporation decreased N_fix%and N fixation amount of Mv plants,while such responses were weak in the glucose-included treatments,indicating impacts of N addition on Mv N fixation vaired with the availability of C sources.?4?Diazotroph abundance and community structure were reshaped by rice straw return and fertilizer application.Copy number of nifH gene and the relative abundance of predominant diazotroph genera were significantly decreased by straw return alone,but increased to a moderately higher level by the fertilizer and straw combinations.?5?Diazotroph community responded differentially to organic amendments i.e.rice straw?Rs?and glucose?Glc?depending upon the availability of C sources.In both examined soils?pH=5.93 and 7.92?,mineral N addition suppressed nifH gene copy numbers in the Rs-included treatments,but not in the Glc-included groups.Compared to the Rs-included soils,glucose addition rsulted in less?-diversity of diazotroph.Meanwhile,diazotroph community structure was cluseterd into different groups by the C sources,while marginally affected by N levels.These results suggested that the responses of diacotroph community to N supply were regulated by the C availability.Glucose addition decreased the relative abundance of Bradyrhizobium compered to straw incorporation?16.0%²5.9%vs.19.5%³8.2%?,while significantly increased the abundance of the second most dominant genus Geobacter?12.8%²3.7%vs.4.1%⁸.7%?.In addition,the diazotroph diversity and community structure were less responsive to straw,glucose and mineral N addition in acid soil than in the alkaline soil.Overall,the study suggests that optimizing the management of Mv planting,straw return and fertilizer application is the key to maintain soil diazotroph ecological function,enhance biological N fixation of Mv plants,and achieve the goal of fertilizer reduction and green development in rice production.