Effect Of Nitrogen And Phosphorus Addition On Soil Enzyme Activity And Enzyme Stoichiometry Ratios With Stand Age In Chinese-fir Plantations

Chinese fir plantations are important fast-growing timber forests in southern China,and play an important role in timber production and in maintaining the regional resource environment and ecological security.Atmospheric nitrogen(N)deposition disrupts the N balance of forest ecosystems and may exacerbate phosphorus(P)limitation,thus affecting soil nutrient cycling and leading to changes in forest productivity.Soil enzyme activity and enzyme stoichiometric ratios can provide good insight into nutrient limitation in soil carbon(C)nutrient cycling and microbial growth and metabolism,but the response of soil enzyme activity and its stoichiometric ratio to N and P addition in fir plantations of different ages is unclear.In this study,young(7a),middle-aged(20a)and mature(33a)stands of Chinese fir were selected based on a controlled trial of N and P nutrient addition starting in 2018,including control(CK),N addition(100 kg N hm2 a-1),P addition(50 kg P hm2 a-1)and NP co-addition(100 kg N hm2 a-1+50 kg P hm2 a-1)to investigate the effects of short-term N and P additions on enzyme activities and enzyme stoichiometry ratios associated with the carbon,nitrogen and phosphorus cycle in fir forests,and to provide a scientific basis for soil nutrient management for productivity improvement in fir plantations under the background of nitrogen deposition.The main results showed that:(1)In the organic layer,soil organic carbon(SOC),total nitrogen(TN)and inorganic nitrogen(NO3-+NH4+)contents increased with increasing stand age,while total phosphorus(TP)and effective phosphorus(AVP)contents were lowest in middleaged stands and highest in mature stands.N addition did not significantly affect TN contents in all stands of fir,but significantly increased inorganic N contents,increased C:N in mature stands,decreased C:P in middle-aged stands,and decreased N:P in middle-aged and mature stands.P and NP additions significantly increased TP and AVP in all stands and decreased C:P and N:P in all stands.In the mineral soil layer,soil C and nutrient contents did not differ significantly between stands,N additions significantly decreased soil C:P and N:P in young stands,P additions significantly decreased N:P in young stands and increased C:P in mature stands.NP addition significantly reduced C:P and N:P in young stands.(2)In organic soils,β-1,4-glucosidase(BG),β-1,4-N-acetylglucosaminidase+Lleucine aminopeptidase(NAG+LAP)and acid phosphatase(AP)activities increased with increasing stand age.P and NP additions significantly increased BG and NAG+LAP in all stands,but decreased AP activity in all stands.In the mineral soil layer,BG and AP increased with increasing stand age and NAG+LAP decreased with increasing stand age.N addition significantly increased BG and AP activity in all stands and NAG+LAP activity in middle-aged and mature stands.N and NP addition significantly increased BG,NAG+LAP and AP activity in all stands.(3)In the organic layer,enzyme C:N and enzyme C:P increased with increasing stand age,while enzyme N:P did not differ significantly between stand ages.N and P additions significantly reduced enzyme C:N for all stand ages,NP additions significantly reduced enzyme C:N for mature stands,and N,P and NP additions increased enzyme C:P and enzyme N:P for all stand ages.In the mineral soil layer,enzyme C:N and C:P increased with increasing stand age and enzyme N:P decreased with increasing stand age.N addition significantly increased enzyme C:N for all stand ages and significantly decreased enzyme N:P for young and middle-aged stands,while P and NP addition significantly increased enzyme C:N and enzyme C:P for all stand ages and enzyme N:P for middle-aged and mature stands,but decreased enzyme N:P for young stands.(4)In the organic layer,vector length(VL)increased with increasing stand age,and N addition reduced VL in all stands,indicating that N addition alleviated soil microbial C limitation in fir forests.The enzyme vector angle(VA)did not differ significantly between stand ages and was>45°,indicating that soil microbial metabolic activity in fir forests was limited by P.P and NP additions significantly reduced VA in all stands,indicating that P additions alleviated microbial P limitation.In the mineral soil layer,both VL and VA increased with stand age.N,P and NP additions increased VL for all stand ages,indicating that nutrient additions exacerbated microbial C limitation in mineral soils.N additions increased VA in young and middle-aged stands,indicating exacerbation of soil microbial P limitation.P and NP additions significantly reduced VA in mature stands,indicating alleviation of soil microbial P limitation.(5)The results of the redundancy analysis(RDA)showed that the effective phosphorus and ammonia N content of the organic soil layer were the main drivers of variation in enzyme activity and enzyme stoichiometry ratios in young stands,explaining 37.6%and 35.4%,respectively.Soil C:P explained 45.7%of the variation in enzyme activity and enzyme stoichiometry ratios in the middle-aged forest.NitrateN,N:P,ammonia-N,effective phosphorus content,and C:N were the main factors affecting enzyme activity and enzyme stoichiometry ratios in mature stands,explaining 42.5%,27.7%,9.2%,8.8%and 4.2%of the variation,respectively.In the mineral soil layer,N:P,C:N and water content explained more of the variation in soil enzyme activity and enzyme stoichiometry ratio and were the main factors affecting the variation in soil enzyme activity and enzyme stoichiometry ratio in young,middle-aged and mature forests.These results suggest that N and P additions altered soil nutrient status,extracellular enzyme activity and their stoichiometric ratios,and in particular that N additions exacerbated the P limitation of soil microorganisms.Therefore,P inputs should be considered in the management of fir plantations in the context of future increased N deposition in order to alleviate microbial P limitation and promote soil nutrient cycling to improve stand productivity.