Vertical Distribution Patterns And Controlling Factors Of Soil Organic Carbon In Poplar Plantations In Coastal Jiangsu

Soils are home to the largest carbon（C）pool in terrestrial ecosystems.They are estimated to contain more C than is stored in the atmosphere and vegetation combined.The long-term retention of soil organic C（SOC）largely depends on the combined action of many biotic and abiotic regulatory factors in soil.Therefore,the vertical distribution of SOC and changes in factors regulating it will have an important impact on C sequestration in terrestrial ecosystems and the global C cycle.Previous studies have shown that plant input,soil physicochemical properties,and microbial activity significantly affect SOC sequestration.However,the trajectories of these effects and differences between soil depths remain unclear.We sampled 18 soil profiles to a depth of 1 m in a poplar plantation at Dongtai Forest Farm to investigate the vertical distribution of SOC and its related physicochemical properties and microbial abundance at each soil depth,the stoichiometric relationship between SOC and total nitrogen（TN）,and the controlling factors of SOC across the soil profile and in the top-（0–30 cm）and subsoil（30–100 cm）layers.The main results are:（1）Variation in SOC storage,soil physicochemical properties and microbial abundance among soil layersSOC stocks decreased with increasing soil depth.SOC at 0–10 cm was 11.48 g kg^-1,8.85times higher than the 1.30 g kg^-1 of the 90–100 cm.66.92%of the SOC were stored in the 0–30cm soil layer.All soil physiochemical properties we measured generally varied greatly along the soil profile.Specifically,soil p H and gravimetrical moisture increased with increasing soil depth.Soil properties,including TN,total phosphorus（TP）,C:N,C:P,N:P,dissolved organic C（DOC）,available nitrogen（AN）,available phosphorus（AP）,and AN:AP,all largely decreased along the soil profile to1 m.The bacterial and fungal abundance（here quantified by microbial gene copy number）decreased significantly with increasing soil depth.We found the fungal-to-bacterial ratio（F:B ratio）ranged from 0.0013 to 0.1006 and observed no significant differences among soil layers to 1 m depth.（2）SOC is tightly coupled to TN in topsoil but decoupled in the subsoil environment.We calculated the significance of the difference between the N-C scaling slope（i.e.,the slope of the relationship between log-transformed N and C）and 1.0.The results revealed significant differences between top-and subsoil with N and C scaled isometrically in topsoil,but not in subsoil.This may indicate that SOC is tightly coupled to TN in topsoil but exhibits decoupling relationships in the subsoil environment.（3）Topsoil SOC content was mainly regulated by soil chemical properties,but subsoil was mainly regulated by microbial properties.Through performing the best subset（BSS）regression analysis,we found that SOC content was co-regulated by soil physiochemical and microbial properties at the site level with soil chemical properties dominant in the topsoil and microbial properties dominant in subsoil.Topsoil SOC increased with soil TN,AP and fungal abundance.Subsoil SOC increased with F:B ratio,fungal abundance,and soil DOC.Overall,our study indicated that SOC content decreased significantly with increasing soil depth.The stoichiometric relationship between SOC and TN in different soil layers was inconsistent.The dominant factors regulating SOC differ along soil layers.The results of this study could help to understand the SOC storage patterns along soil profile and the associated regulatory factors.In the context of global change,it will help us to formulate and develop the subsoil C management strategy.