Effects Of Nitrogen And Phosphorus Addition On Soil Organic Carbon And Its Driving Mechanism In Chinese Fir Plantations

The dynamic change of carbon(C)source and sink of plantation is of great significance to global climate change.Nitrogen(N)and phosphorus(P)are essential elements for plant growth and limit forest ecosystem productivity.N and P additions change the soil nutrient supply and the balance between nutrients,thus affecting plant growth and nutrient return to soil,soil microbial community and function,decomposition of soil organic matter,and corresponding changes in soil C reserves and components in forest ecosystems.For plantations,litter return and root input improve soil characteristics and nutrient availability as the stand grows and develops.There is a lack of systematic studies on whether N addition,P addition and NP addition change soil organic C(SOC)and its component content in plantations of different ages,and the effects of soil nutrient supply,microbial community structure and extracellular enzyme activity on SOC and its components.In this study,N and P addition control test platform was used to study the response characteristics of SOC and its component content in organic layer and mineral layer of Chinese fir plantation with different growth cycle(age series)to N and P addition.The effects of N addition,P addition and NP addition on soil N,P content and stoichiometric ratio of Chinese fir plantation were analyzed,and the differences of soil nutrient changes among Chinese fir plantations of different ages were clarified.The effects of N,P and NP supplementation on soil microbial biomass,community structure and extracellular enzyme activity of different forest ages were analyzed.Enzyme stoichiometric ratio model and vector model were used to investigate the limiting characteristics of soil microbial C and nutrient in Chinese fir plantations of different ages and their responses to N and P additions.The effects of soil physical and chemical properties,microbial community characteristics and extracellular enzyme activity on SOC dynamics were elucidated.The main findings are as follows:(1)SOC,particulate organic C(POC)and mineral-associated organic C(MAOC)contents in organic layer all showed an increasing trend with the increase of forest age.In the absence of nutrient addition(control group),SOC and POC contents in organic layer of Chinese fir plantation increased by 130%and 200%from 7 to 36 years,respectively,and MAOC contents increased by 89%from 7 to 29 years,mainly due to the accumulation of SOC promoted by nutrient return in litter.There was no significant difference in SOC,POC and MAOC contents among different forest ages,indicating that SOC and its components accumulated slowly in mineral layers.Except for the addition of N in 29 years and P in 36 years,the short-term addition of N and P had no significant effect on SOC and MAOC of organic layer.Short-term N addition,P addition and NP addition increased POC content in young(7 years)and middle-aged(15 years),indicating that nutrient addition in young and middle-aged forests was conducive to the accumulation of new C.In the organic layer,alkyl C increased significantly under the simultaneous addition of N,P and NP at 7 years,which further proved that nutrient input promoted the accumulation of new C in young forests.(2)Except for the simultaneous addition of N and NP in Chinese fir plantation in 29 years,the short-term addition of N and P had no significant effect on the total N(TN)content of soil in organic layer.In subtropical areas,the exogenous N input would be lost in the form of leaching or absorbed by plants.Total P(TP)content increased significantly with the addition of P and NP in organic layer,increasing by 15-83%and 29-121%,respectively,mainly because the input of P can increase TP content through the adsorption and precipitation of soil and promote the decomposition of litter.The contents of nitrate N,ammonium N and available P in organic layer control group showed an increasing trend with the increase of forest age.The N:P and C:P ratios of soil were significantly reduced under the addition of P,and the input of P improved the combination of P and soil organic matter,accelerated the decomposition of organic matter and nutrient release,and alleviated the imbalance of soil element stoichiometric ratio caused by N deposition.(3)The microbial biomass under the simultaneous addition of N,P and NP at 7 years was significantly higher than that of the control group,mainly due to the large nutrient demand of young forests,and the increase of soil available nutrients would promote the propagation of microorganisms.F:B and G+:G-showed no significant change under the simultaneous addition of NP in organic layer,indicating that the anti-interference and recovery of soil ecosystem under the simultaneous addition of NP were better than that under the separate addition of N and P.The ratio of G+:G-decreased with the increase of SOC and nutrient content,indicating that the increase of nutrient content in soil would increase the proportion of G-bacteria,alleviate microbial nutrient stress,and thus facilitate the accumulation of SOC.(4)Acid phosphatase(AP)increased significantly in both organic and mineral layers under N addition except for years 15 and 36,suggesting that N addition may exacerbate P restriction,which in turn requires microorganisms to secrete more AP to improve P availability.Except for the 3 6-year organic layer control group,the soil microorganisms in other treatments were restricted by C and P.When the control group and NP were added at the same time,the microbial C restriction and P restriction were significantly positively correlated.When N and P were added separately,microbial C restriction and P restriction were negatively correlated.Microbial C limit in organic layer reached the maximum at 20 years,followed by 7 years young forest.C loss was caused by human disturbance in the early stage,and increased nutrient demand for tree growth in the middle stage.(5)The amount of PLFAs markers and G+:G-in organic layer increased with the increase of microbial C and P restriction,indicating that microbial community structure affected soil nutrient acquisition during stand development.With the increase of microbial C and P restriction,the metabolism of the microbial community will shift from growth to stability maintenance,and the soil microbial community will balance the soil nutrient cycling process of Chinese fir plantation by increasing the secretion of extracellular enzymes,thus affecting the soil fixation C during the development of the stand.The contents of POC and MAOC in organic layer increased with the increase of TN content and C:N.The accumulation of POC and MAOC was promoted by the increase of soil nutrients and the increase of N mineralization rate under the addition of N and P.The addition of P alleviates the restriction of P and promotes the accumulation of SOC,which can alleviate the restriction of microbial C and P simultaneously.With the addition of N and P,the increase of F:B will increase the degradation of complex compounds such as lignin,and reduce the proportion of alkyl C and aromatic C.In conclusion,this study clarified the response characteristics of SOC and its components,soil nutrients,microbial biomass,microbial community structure,enzyme activity,microbial C and nutrient restriction characteristics of Chinese fir plantations of different ages to N and P addition,and revealed the influence mechanism of SOC components and stability changes under N and P addition conditions.The results are helpful to improve the soil nutrient utilization efficiency of plantation and provide scientific basis for plantation management to cope with global change.