Effects of Organic Materials and Their Incorporation Depths on Humus Substances Structure and Soil Microbial Communities' Characteristics in a Chinese Mollisol

Organic material incorporation are important agricultural practices, which can influence soil organic carbon (SOC) sequestration and stabilization. However, the response of interaction between SOC structure and soil microbial to organic material incorporation management are still poorly understood. In 2021, we conducted a three years field experiment in Guangrong country, northeastern China. Five treatments were established: conventional tillage (CK), conventional tillage with straw incorporation (T1); subsoil tillage with straw incorporation (T2); subsoil tillage with straw and organic manure incorporation (T3) and subsoiling tillage with organic manure incorporation (T4). Fulvic-like and protein-like components were found in fulvic acid (FA) in a 0-15 cm soil layer, while fulvic-like components in humic acid (HA) were found in 0-15 cm and 15-35 cm soil layers. In the 15-35 cm soil layer, the bacterial, fungal and total phospholipid fatty acid (PLFA) contents were significantly higher by 159.62%, 687.00%, and 139.02% in T3 than CK, respectively. The fungal to bacterial PLFA ratios (F/B) were significantly higher by 97.46% and the Gram-positive bacteria to Gram-negative bacteria PLFA ratios (G(+)/G(-)) were lower by 20.99% in T3 than CK in the 15-35 cm soil layer. Therefore, subsoil tillage with straw and organic manure incorporation could be recommended to improve soil quality in Mollisol.