Effects Of Nitrogen And Phosphorus Addition On Transformation And Stability Of Straw-C In Soil

Soil carbon（C）pool is essential for ensuring food security and mitigating climate change.Straw returning is considered to be the main means of increasing soil organic carbon（SOC）in farmland ecosystems,but its carbon enhancement effect is also affected by nutrient availability.At present,there is still a lack of data on how nitrogen and phosphorus addition affect straw carbon transformation and stability characteristics,and the relevant mechanism is still unclear,which needs further research.Therefore,this study used low-fertility and high-fertility meadow cinnamon soil as the test soil,adjusted the C:N and C:P of exogenous substances by adding ¹³C-labeled straw,ammonium nitrate and potassium dihydrogen phosphate solution,and set up a total of seven treatments containing three C:N levels（45,20,10）and three C:P levels（173,100,50）.The distribution of straw carbon in different carbon fractions is tracked by ¹³C isotope probe technique,and the effects of nitrogen and phosphorus addition on the transformation and stability of organic carbon under straw addition were explored.The mechanism of nitrogen and phosphorus addition affecting the stability of straw carbon was explored by measuring the characteristics of soil ecological stoichiometry and soil microbial community composition.This study is helpful to provide theoretical basis for the analysis of soil organic matter transformation mechanism and the practical management of farmland soil.The main findings are as follows:（1）Under the condition of straw application,N and P additions reduced soil respiration rate and the cumulative CO₂ release.In both low-fertility and high-fertility soils,it was found that under the condition of straw addition,the decrease in the cumulative CO₂ release by P fertilizer alleviated with the increase of P addition rate.In high-fertility soil,it was found that the more N added,the greater the decrease of cumulative release of CO₂ by N fertilizer.The addition of nitrogen and phosphorus increased the turnover rate of the active carbon pool and decreased the turnover rate of the recalcitrant carbon pool under the condition of straw application.N,P additions can overall improve soil microbial carbon use efficiency（CUE）,with N addition having a significant positive effect on CUE,while P addition only has an increasing effect in the early stages of incubation.The net carbon balance showed that the addition of N fertilizer and P fertilizer can enhance the carbon enhancement effect of straw addition.（2）Compared to the early stage of incubation（30 d）,at the 120-day of incubation,the particulate organic carbon（POC）content of all straw-added treatments in both soils increased,but the proportion of POC derived from straw-C decreased.In low fertility soil,after 120 days of incubation,the mineral-associated organic carbon（MAOC）content of N45P100,N20P173and N20P100 treatments was significantly lower than that of N45P173 treatment,with a decrease of 2.67%,3.82%and 3.50%,respectively.On the contrary,the contribution of straw-C to MAOC in the three treatments was significantly higher than that of N45P173 treatment,with N20P100 treatment having the highest contribution rate.（3）Straw and different gradients of nitrogen and phosphorus addition increased soil microbial biomass C,N and P content to a certain extent.The activities of cellulodiose hydrolase（CBH）,β-N-acetylglucosaminidase（NAG）,and phosphatase（PHOS）decreased with the prolongation of incubation time.The soilβ-Xylosidase（BX）activity decreased first and then increased with time,and the soilβ-glucosidase（BG）activity did not change significantly.The increase of the C:P and C:N threshold ratios showed that the C limitation of soil microorganisms increased gradually with time.（4）In low-fertility soil,the addition of nitrogen and phosphorus fertilizer reduced the Alpha diversity of soil microbial community compared with single straw application.Nitrogen and phosphorus addition had no significant effect on microbial community composition.The dominant bacterial species of the two soils in different periods were Actinobacteriota,Proteobacteria,Acidobacteriota,Firmicutes and Chloroflexi,and the dominant fungal species were Ascomycota,Mortierellomycota and Basidiomycotina and unclassified＿k＿＿fungi.Soil microbial biomass carbon（MBC）content was the most important factor driving soil microbial community structure.MBC content explained 12.67%～42.88%of bacterial community structure and 6.04%～80.29%of fungal community structure.In summary,the addition of nitrogen and phosphorus increased the carbon sequestration of soil organic carbon pool under straw application,accelerated the turnover rate of active carbon pool,reduced the turnover rate of recalcitrant carbon pool,and improved soil microbial carbon use efficiency.Nitrogen and phosphorus addition increased soil microbial biomass,and reduced the Alpha diversity of soil microbial community.MBC content is the main factor driving microbial community structure.