Effects of Biochar Application on Soil Organic Carbon in Degraded Saline-sodic Wetlands of Songnen Plain, Northeast China

Biochar amendment is considered as an efficient practice for improving carbon storage in soils. However, to what extent that biochar application promotes organic carbon in saline-sodic soils remains poorly understood. By comparing soil organic carbon (SOC) contents change before and after biochar addition, we deciphered the driving factors or processes that control SOC change in response to biochar application. A limited increase in SOC was observed, about by 1.16%-12.80%, even when biochar was applied at the rate of 10% of bulk soil weight. Biochar application enhanced soil dissolved organic carbon (DOC) significantly by up to 67%. It was estimated that about 50% SOC was allocated to small macroaggregates (250-2000 mu m, CPOC), and SOC in silt and clay-sized particles (< 53 mu m) decreased obviously after biochar addition. Microbial biomass increased with biochar amendment, of which actinomycetes (ACT), fungus (FUN), protozoon (PRO), and bacteria with straight-chain saturated fatty acids (OB) increased remarkably. Multiple linear regression models implied that DOC was governed by ACT and soil N: P ratio, while SOC mostly depended on CPOC. The principal component analysis and the partial least square path model (PLS-PM) indicated that biochar addition aggravated nitrogen limitation in saline-sodic soils, and effects of microorganisms on regulating SOC greatly depended on nitrogen bioavailability. Biochar application had vastly changed interactions between environmental factors and SOC in saline-sodic soils. Effects of nutrients on SOC shifted to great inhibition from strong stimulation after biochar addition, meanwhile, aggregation was the only factor presenting positive effects on SOC. How to eliminate nutrient limitation and better soil aggregation process should be considered in priority when biochar was used to improve SOC in saline-sodic soils.