Effects Of Exogenous Nitrogen And Phosphorus On Carbon And Nitrogen Dynamics Of Nutrient-limited Soils

As soil limiting nutrient elements,the availability of nitrogen and phosphorus in soil affects the composition and activity of soil microorganisms,thus regulating the mineralization of soil organic carbon and the long-term accumulation of nutrient elements.As an important energy and reaction substrate in soil,the ratio of soil carbon to nitrogen and phosphorus can regulate the decomposition of organic matter.The cycle of soil carbon,nitrogen and phosphorus is closely related to each other,and the cycling processes of C,N and P in soil are coupled and closely related.In this study,¹³C isotope labeling technique was used to carry out indoor incubation experiments.The cornfield soil?carbon limitation?and woodland soil?phosphorus limitation?were selected as research target from Banyue Town,Dangyang City,Hubei Province.We added ¹³C labeled glucose as carbon source,NH₄Cl as nitrogen source and KH₂PO₄as phosphorus source.The treatments were as follows:Control?CK?,Nitrogen?N?,Phosphorus?P?,Nitrogen and phosphorus?NP?,Glucose?C?,Glucose and nitrogen?CN?,Glucose and phospgorus,Glucose with nitrogen and phosphorus?CNP?.The effects of nitrogen and phosphorus addition on soil carbon mineralization,soil nitrogen and phosphorus dynamics,soil microbial composition and enzyme activities were studied in different nutrient-limited soils.The main findings are as follows:?1?The response of soil organic carbon mineralization to exogenous nitrogen input was different in soils with different types of nutrient limitation.Compared with C treatment,the SOC mineralization of the CN treatment decreased by 7.27%in the cornfield soil,while the SOC mineralization of the CN treatment increased by 4.67%in the woodland soil.?2?The input of exogenous phosphorus significantly promoted the soil background organic carbon mineralization.Compared with C treatment,the SOC mineralization of CP treatment in cornfield soil and woodland soil increased by 33.94%and 10.55%,respectively.The mixed application of nitrogen and phosphorus adjusted the stoichiometric ratio of soil elements and alleviated the promoting effect of phosphorus on soil carbon mineralization to a certain extent.compared with CP treatment,the SOC mineralization of cornfield soil and woodland soil CNP treatment decreased by 18.93%and 16.76%,respectively.?3?In the cornfield soil which is more limited by nitrogen,the net nitrogen fixation of nutrient addition treatments is 1.11-5.81 times as much as that of woodland soil.The combined application of small molecular organic matter?glucose?and inorganic nitrogen and phosphorus is beneficial to the fixation of nitrogen in soil.The net nitrogen fixation of cornfield soil and woodland soil with glucose treatments was 1.55 and 2.73 times higher than that without glucose treatments.In P-limited woodland soil,single application of phosphorus increased N₂O emission,showing net nitrogen mineralization,and olsen phosphorus was significantly positively correlated with N₂O flux and ammonium nitrogen content,and negatively correlated with nitrate nitrogen content.It is proved that phosphorus restriction can promote soil nitrification and cause the loss of soil nitrogen.?4?The input of exogenous phosphorus significantly increased the content of olsen phosphorus and MBP.The average content of olsen phosphorus in cornfield soil and woodland soil with phosphorus addition was 1.59 and 3.00 times higher than that without phosphorus addition,and the average MBP content of cornfield soil and woodland soil with phosphorus addition was 2.21 and 2.38 times higher than that without phosphorus addition,respectively.The contents of olsen phosphorus and MBP were significantly correlated with soil CO₂flux,DOC,MH₄⁺-N and NO₃^--N,so the turnover and accumulation of microbial biomass phosphorus and olsen phosphorus in soil were closely related to soil carbon and nitrogen cycle.?5?Exogenous carbon,nitrogen and phosphorus could promote the PLFA content of soil microbial community in different degrees.In carbon-limited cornfield soil,exogenous carbon promoted bacterial and fungal biomass and total PLFA,and significantly decreased the ratio of bacteria to fungi.In phosphorus-limited woodland soil,exogenous phosphorus promoted bacterial,fungal biomass and total PLFA,and significantly reduced the ratio of bacteria to fungi.The redundancy analysis of PLFA content of microbial communities and soil physical and chemical properties showed that bacterial PLFA content was significantly positively correlated with DOC,MBC,MBN and NH₄⁺-N,fungal PLFA content was positively correlated with available phosphorus content,and the ratio of bacteria to fungi was positively correlated with NH₄⁺-N.?6?The addition of carbon source could promote the activities of?-1,4-glucosidase?BG?,?-1,4-N-acetyl glucosaminidase?NAG?and phosphatase?AP?in the two soils in different degrees.The addition of nitrogen sources had no significant effect on the activities of the three enzymes,but there was a significant negative correlation between soil C/N and BG/NAG,and a significant positive correlation between soil C/N and NAG/AP.The activity of soil phosphatase was inhibited by the input of exogenous phosphorus,and there was a significant negative correlation between soil C/P,N/P and BG/AP.The RDA analysis of enzyme activity ratio and soil physical and chemical properties showed that the content of soil olsen phosphorus,ammonium nitrogen,DOC,MBC were the main factors affecting the variation of enzyme activity ratio.The input of exogenous carbon,nitrogen and phosphorus changed the eco-stoichiometric ratio of soil enzymes,and the availability of nutrient elements and soil enzyme activities were coupled.