The Effects Of Nitrogen On Soil Nutrients And Key Functional Genes Of Microbial Carbon And Nitrogen Cycling In Young Phoebe Bournei Plantation

Nitrogen is an important nutrient,but plant growth and microbial metabolism are limited by nitrogen in terrestrial ecosystem.Nitrogen application can alleviate this limitation.Reasonable nitrogen application can improve plantation productivity and increase forest carbon and nitrogen sequestration.In this study,The 10-year-old Phoebe bournei plantation was used as experimental object with five nitrogen application levels.The nitrogen application per plant of each treatment was 0g(N0),100g(N1),200g(N2),300g(N3)and 400g(N4).The changes of soil chemical properties,soil stoichiometric characteristics,soil enzyme activity and soil microbial carbon and nitrogen cycle function genes under different nitrogen additions were analyzed to clarify the impact of nitrogen addition on soil C and N cycling and its mechanism,so as to provide basic data and theoretical basis for the management of soil carbon sequestration and sink increase and the stability of soil ecosystem.The main results are as follows:(1)Nitrogen addition improved the growth status of Phoebe bournei plantation and the nutrient content of surface soil,but more than 300g/plant nitrogen addition would reduce the content of soil organic carbon,DBH-increment and height volume growth of Phoebe bournei plantation would also decrease,and there was a threshold effect of nitrogen addition.The pH of rhizosphere and bulk soil decreased with the increase of nitrogen addition,and reached the minimum value under N4 treatment.Except for the contents of total nitrogen,hydrolytic nitrogen and total magnesium in rhizosphere soil,the contents of other soil nutrients first increased and then decreased with the increase of nitrogen addition,reaching the maximum under N3 treatment.The contents of total nitrogen,hydrolytic nitrogen and total magnesium in bulk soil increased with the increase of nitrogen addition,and reached the maximum under N4 treatment.(2)C:N of rhizosphere and bulk soil decreased after nitrogen addition treatment,and reached the minimum value under N4 treatment.bulk soil C:P increased with the increase of nitrogen addition,and reached the minimum value under N3 treatment.After nitrogen addition,N:P in bulk soil increased and reached the maximum value under N4 treatment.(3)Nitrogen addition had different effects on the activities of different functional enzymes.The activities of sucrase and urease in rhizosphere and bulk soil first increased and then decreased with the increase of nitrogen addition,and reached the maximum under N3 treatment.The order of soil invertase and urease activities was N3>N2>N1>N0>N4.However,the acid phosphatase activity of bulk and rhizosphere soil decreased first and then increased with the increase of nitrogen addition,and reached the minimum value under N3 treatment.The catalase activity of bulk soil increased first and then decreased with the increase of nitrogen addition,and reached the maximum under N3 treatment.The catalase activity of rhizosphere soil decreased with the increase of nitrogen addition,and reached the minimum value under N4 treatment.(4)Nitrogen addition significantly changed the community composition of the key functional groups of soil microbial C,N,P cycling.N2 treatment(200g/plant)and N3 treatment(200g/plant)significantly increased the relative abundance of readily decomposable carbon degradation genes(such as sga,abf A,manB),ammoniation genes(urecC),denitrification genes(nosZ1,nirK3),nitrogen fixation genes(nifH),These positive effects were mainly related to the increase of soil organic carbon,total nitrogen and available phosphorus,as well as the decrease of soil c:n and soil c:p ratio.When the nitrogen application rate was higher than 300g/plant,the relative abundance of key functional genes of microbial C,N and P cycling was inhibited.This was because the increase of soil nutrient content did not promote the abundance of microbial functional genes enough to offset the negative impact of the decrease of soil pH on the abundance of microbial functional genes.(5)Based on the plant soil microbial system,the abundance and composition of microbial functional genes also have a strong ability to explain the contents of soil C and N nutrients.Multiple stepwise regression showed that amyA,amyX,pgu,mnp and frdA genes were the key genes affecting soil organic carbon content;nirKl,nirS1,nirS2,nosZ2,nasA and napA were the key genes affecting soil total nitrogen content;amoAl,nirKl,nirS2,nirS3,ureC,gdhA,hzsB and nasA genes were good predictors of soil hydrolytic nitrogen content.The above results showed that different nitrogen additions would have different effects on soil chemical properties,soil C,N,P stoichiometric characteristics,enzyme activities and microbial functional genes,and then change the soil C,N cycling process.At present,N2 and N3 treatments in the study area improved soil nutrient status,increased soil enzyme activity and microbial functional gene abundance,and promoted the transformation of soil carbon and nitrogen.