Effect Of Nitrogen And Phosphorus Addition On Soil Aggregate Carbon And The Coupling Mechanism Of Carbon And Phosphorus In An Evergreen Broad-leaved Forest

Forest soils store the most organic carbon（C）in terrestrial ecosystems.In the global C cycle,Soil C pool acts as both a source and a sink of C.Nitrogen（N）and phosphorus（P）are the major elements limiting subtropical ecosystems.Under the global change of N deposition,the increasing soil N input leads to the intensification of P limitation in the region,the mechanism of stabilization and sequestration of soil organic carbon（SOC）under the change of N and P availability is not clear.Furthermore,soil aggregates,as the basic unit of soil structure,are the functional carriers of soil carbon sequestration.The changes of their stability and C sequestration capacity under the limit of N deposition and p are worthy of attention.At present,the effect of N and P addition on soil aggregate C and the coupling mechanism of C and P are not clear.This study was based on continuous N application(100kg N hm^-2 a^-1),P(50 kg P hm^-2 a^-1),P+N(50 kg P hm^-2 a^-1+100 kg N hm^-2 a^-1)and control in a subtropical evergreen broad-leaved forest,four treatments,were applied in a randomized block trial with five replicates,0-10 cm soil samples were collected during the growing season after 6 years of treatment,physical and chemical properties,particle size composition of aggregates,hydrolase activity（96-well microplate method）,soil P fractions of each aggregate size（Hedley continuous extraction method）,SOC content and chemical composition of each aggregate size（Fourier infrared spectroscopic functional group method）were determined,the responses of surface soil aggregate C and its stability to N and P addition and the coupling effects of C and P were analyzed to provide data support for understanding the mechanism of N and P addition on soil aggregate C.The results of the study are as follows:（1）N addition significantly increased the total nitrogen（TN）and SOC content in whole soil and soil aggregates,and significantly decreased the relative content of small aggregates;The fractal dimension（D）,the SOC contribution of 0.25-0.053 mm agglomerates and the SOC stability of agglomerates>2 mm,2-0.25 mm and>0.053 mm alcohol-phenol,>2 mm and 2-0.25 mm agglomerates were improved The contents of aliphatic and SOC decomposition,Na OHCO₃-Po and HCl-Po in aggregates>2 mm,2-0.25 mm and>0.053mm,<0.053 mm and 0.25-0.053 mm were decreased.Pearson correlation analysis showed that TN was the key driving factor affecting the distribution of SOC content in soil aggregates,indicating that N addition had a positive effect on the stability and retention of SOC.（2）P addition significantly decreased the relative contents of AP and the small aggregate in the bulk soil,and increased the contents of TP,Olsen-P and P fractions in the bulk soil and aggregates.P addition had no significant effect on SOC and SOC functional group content of whole soil and aggregates.Pearson correlation analysis showed that TP and Olsen-P were the key drivers of P content.P addition improves P supply availability and alleviates P limitation in soil.P as a soil P pool can store and fix a large amount of P.When P and N were added at the same time,total soil porosity,C,N invertase activity（βG,NAG and LAP）,enzyme ecological stoichiometry(E_C:P and E_N:P),relative content of macroaggregates,mean mass diameter（MWD）,mean geometric diameter（GMD）,SOC,and soil water content were significantly increased,aggregate TP and each P component,aggregate TN,>2 mm,2-0.25 mm and>0.053 mm aggregate alcohol phenol decreased the fractal dimension D and SOC decomposition degree,2-0.25 mm and>0.053 mm aggregate SOC contribution rate,each aggregate aliphatic and>2 mm,2-0.25 mm and>0.053 mm aggregate SOC decomposition degree.The results showed that the addition of P and N could increase the content of SOC by increasing soil TN,TP and P fractions,which affected the proportion of macroaggregates and the stability of soil aggregates.Pearson correlation analysis showed that alcohols and phenols were significantly correlated with readily decomposable P,and SOC structure stability was significantly correlated with the three total P fractions in the microaggregates,it is shown that the more easily decomposed state P promotes the retention of active SOC,and P component is the key factor driving the structural stability of SOC.In summary,the effects of N and P addition on soil physical and chemical properties,enzyme activities,particle size composition of aggregates,and C and P contents of aggregates were multi-dimensional,and no simple rules could be followed.N increased the SOC content of the whole soil by increasing the SOC of each particle size,while n and P increased the SOC content of the whole soil by increasing the stability of soil aggregate structure and decreasing the decomposition degree of soil aggregate P addition alleviated the P limitation of soil in this area,and only significantly promoted the accumulation of SOC in clay and silt.The results of this study can deepen the understanding of the response and mechanism of C accumulation to N and P addition in subtropical evergreen broad-leaved forest,and provide scientific and technological support for the“Double carbon”strategy,however,it is necessary to further strengthen the study of soil C dynamics and microbial regulation mechanism in deep soil.