Effects Of Nitrogen And Phosphorus Additions On Organic Carbon Fractions And Its Stability Mechanism Along A Soil Profile In A Chinese Fir Plantation

Atmospheric nitrogen（N）deposition and soil phosphorus（P）limitation are prominent environmental problems in subtropical regions of China,and the effects of N and P additions on ecological processes and productivity in Chinere fir（Cunninghamia lanceolata）plantation have been of great concern.Soil is the largest carbon（C）reservoir in terrestrial ecosystems,and small fluctuations in it can cause large impacts on the global C balance.Forest ecosystems are the most important component of terrestrial ecosystems,and their soil organic carbon（SOC）pools play a key role in the global C cycle.SOC fractions are a complex system,and their properties and turnover rates vary widely,exhibiting different storage capacities,stability and ecological functions.A series of field-controlled experiments on the regulation of SOC dynamics by N and P additions have been conducted globally,and some breakthroughs and progress have been made,but the conclusions obtained are inconsistent due to the different forest types,fertilization practices and intensities involved.Most previous studies have focused on the biological processes of SOC input（plant productivity）and SOC output（microbial decomposition）,but not enough attention has been paid to the structure and composition of SOC itself,as well as physical and chemical protection and microbial regulation,especially in deep soils,and little is known about its regulatory mechanisms.To reveal these mechanisms mentioned above,a filed experiment was conducted based on the field experiment platform of fertilization for Chinese fir plantation established by Qianyanzhou experimental station of Chinese Academy of Sciences in 2011.Six treatments including N1(50 kg N·hm^-2·a^-1),N2(100 kg N·hm^-2·a^-1),P(+50 kg P·hm^-2·a^-1),N1+P(50 kg N·hm^-2·a^-1+50 kg P·hm^-2·a^-1),N2+P(100 kg N·hm^-2·a^-1+50 kg P·hm^-2·a^-1)and CK（control）were carried out,and each treatment comprised five completely randomized zone group trials.After 9 years of experimental treatments,the effects of N and P addition on SOC and its active and inert components in the soil profile（0-5,5-10,10-20,20-40,40-60 cm）,as well as the regulatory mechanisms of soil aggregates（physical stability）,Fe/Al oxides and SOC chemical structure（chemical stability）,and soil microbial community structure and function（microbial stability）were investigated.The aim is to understand the basic patterns of soil SOC dynamics in subtropical forests,to investigate the vertical divergence patterns of soil SOC fractions and stability,and to reveal the mechanisms of changes in SOC and its fractions and their stability in subtropical soil profiles under atmospheric N deposition and soil P addition,so as to provide a scientific basis for predicting C dynamics in terrestrial ecosystems and for coping with atmospheric N deposition and mitigating P limitation in subtropical regions.The main results are as follows:（1）SOC and different fractions respond differently to N and P additionsCompared to the control,N addition significantly increased SOC content in the 0-10cm soil layer,while P addition decreased SOC content.High N（N2）addition significantly increased POC（particulate organic carbon）,DOC（dissolved organic carbon）and EOC（readily oxidisable organic carbon）in 0-10 cm soil and decreased MAOC（mineral bound organic carbon）in 0-10 cm soil;P addition significantly decreased POC in 0-10 cm soil.The changes in MAOC under N and P additions were smaller than those of POC,DOC and EOC,indicating that N additions had a greater effect on SOC fractions than P additions and that reactive SOC was more sensitive to N and P additions than recalcitrant SOC.（2）Physical stability of SOC driven by N addition but not P additionCompared with the control,N addition improved the physical protection of macroaggregates in 0-10 cm soils,and N addition significantly increased the proportion of macroaggregates,macroaggregates SOC content,MWD（mean weight diameter）and GMD（geometric mean diameter）,and decreased the proportion of macroaggregates in 0-10 cm soil layer;whereas P addition had no significant effect on any of the aggregates stability indicators,indicating that N but not P was the most important factor affecting the physical stability of SOC in subtropical Chinese fir plantation.（3）Different pathways of N and P additions drive SOC chemical stabilityIn terms of Fe/Al oxides,N addition significantly increased Fe/Al oxide sorption in the0-20 cm soil layer compared to the control,and high N addition increased Fe_o,Fe_p and Al_ocontent in the 0-20 cm soil layer by affecting p H value,with N addition having a greater effect than P addition.In terms of SOC chemical structure,N and P addition had opposite effects,with N addition significantly increasing the relative abundance of aromatic groups in the 0-20 cm soil layer.N addition significantly increased the relative abundance of aromatic groups and the stability of SOC chemical structure in 0-20 cm soil layer;P addition significantly increased the relative abundance of polysaccharides and the degree of SOC decomposition in 0-5 cm soil layer while decreasing the relative abundance of aromatic and aliphatic groups and the stability of SOC chemical structure in 0-20 cm soil layer,indicating that N addition helped to improve the chemical structure of recalcitrant SOC and the stability of SOC structure,while P addition increased the active SOC chemical structure and decrease the recalcitrant SOC chemical structure,thus decreasing SOC stability.（4）N and P jointly regulate the microbial stability mechanism of SOCCompared to the control,N addition reduced soil bacterial Chao1 index andαdiversity in the 0-20 cm soil layer,while P addition did the opposite;N addition altered the community structure of bacteria in the 0-20 cm soil layer,preferentially supporting the growth of soil commensal bacteria,such as Actinobacteria,over oligotrophic bacteria,such as Acidobacteria),consistent with the commensal hypothesis;fungi were more tolerant to N and P addition than bacteria.inhibited BG activity and microbial demand for C in 0-40 cm soil layer,while N addition inhibited NAG and LAP activity and increased AP activity in 0-20 cm soil layer;P addition increased BG,NAG and LAP activity and stimulated microbial demand for C and N in 0-20 cm soil.In addition,N addition significantly reduced ln（BG）:ln（NAG+LAP）and ln（NAG+LAP）:ln（AP）,and ln（BG）:ln（NAG+LAP）:ln（AP）significantly deviated from the global mean,indicating that the study area was N rich and P poor.（5）Physical,chemical and microbial mechanisms combine to explain changes in SOC and its fractionsStructural equation modelling（SEM）indicated that N addition promoted the accumulation and stabilization of POC and SOC by increasing the SOC content of macroaggregates,MWD and GMD;N addition inhibited the loss of DOC and SOC by increasing the content of Fe_o,Fe_p and Al_o;N and P addition also changed DOC and SOC by affecting the relative abundance of aromatic groups,SOC chemical stabilization,and the degree of SOC decomposition In addition,N and P addition affected DOC,EOC and SOC sequestration and decomposition by altering bacterial alpha diversity,Acidobacteria phylum and BG activity.（6）The mechanism of the effect of N and P addition on SOC and fractions is characterised by vertical variation in the profileThe key regulatory indicators affecting SOC and fraction changes in deep soils,such as the content of soil SOC of macroaggregates,Fe/Al oxides,and the relative abundance of aliphatic and aromatic groups,were significantly lower compared to those in surface soils,indicating that the physical and chemical protection of SOC stability was weaker in deep soils.SOC fractions and physical,chemical and microbial stability mechanisms driven by N and P additions are mainly manifested in 0-20 cm soil layer,while deeper soil layer was not sensitive,indicating that deeper soil layer SOC has higher stability.In summary,N addition increased the SOC and fraction content of surface soils,while P addition did the opposite,suggesting that the high N and low P environment in subtropical fir forests is conducive to the sequestration of SOC.N addition promotes the stabilization and sequestration of SOC and its fractions in Chinese fir plantation by increasing the physical protection of soil aggregates and the chemisorption of Fe/Al oxides,while N and P together influence the stability of SOC chemical structure and microbial N and P together affected SOC chemical structure stability and microbial structure and function,thus altering SOC content,with N addition promoting SOC and component stability and sequestration by enhancing SOC chemical structure and stability and inhibiting microbial decomposition,while P addition had the opposite effect.These results suggest that the response of SOC and fractions to atmospheric N deposition and soil P addition in subtropical Chinese fir plantation is driven by a combination of physical,chemical and microbial stability mechanisms.In addition,SOC and fractions and stability mechanisms in subtropical Chinese fir plantation are characterized by significant soil profile changes,and deeper soil SOC fractions and physical and chemical stability mechanisms are not sensitive to changes in the external environment,suggesting that deeper soil depth weakens the influence of physical and chemical stability mechanisms on SOC under N and P addition,and that deeper soils have stronger stability and some potential for C fixation,but physical and chemical protection is weaker.Future research should clarify the characteristics of the coupled physical-chemical-microbial stability mechanisms of SOC in response to N and P addition and their vertical distribution patterns,and consider the whole soil profile,not just the surface soil,for implementing C fixation measures.The results of the above studies can deepen the understanding of SOC stabilization and sequestration in subtropical forest ecosystems and provide some scientific basis for enhancing soil C sink capacity in the context of global change.