Variation characteristics of soil microbial biomass and its eco-stoichiometric ratio during the returning farmland to forests in loess hilly region

To elucidate the pattern of variation of soil microbial biomass and its stoichiometric ratio, the key indicators of soil quality, to reveal the mechanisms of soil physicochemical properties on the variations of soil microbial biomass and it’s stoichiometric ratio during the returning farmland to forests in loess hilly region, and to provide a scientific basis for forest management and soil quality evaluation and ecological benefit assessment of returned farmland in the later stage.

The spatiotemporal substitution method was adopted, different ages of reforested farmland of Larix principis-rupprechtii (10, 18 and 27 years old) were selected as the research objects, taking the existing cultivated land as the control, and soil samples were collected from depths of 0-10 cm, 10-20 cm, and 20-40 cm soil layer to analyze soil physicochemical properties and mass ratio of soil microbial biomass, and soil microbial biomass, eco-stoichiometric ratios, microbial quotient, and microbial stoichiometric imbalance were calculated to reveal the changes of mass ratio of soil microbial biomass, soil microbial biomass, eco-stoichiometric ratios, microbial quotient, and microbial stoichiometric imbalance, as well as the influence of soil physicochemical properties on them during the returning farmland to forests in loess hilly region, and finally, try to explain the influence of soil physicochemical properties on these changes and mechanisms.

1) Returning farmland to forests promoted the clustering of mass ratio of SMB, especially mass ratio of MBC and mass ratio of MBP. Compared with cultivated land, the mass ratio of SMB in different years (10, 18 and 27 years) of reforested farmland increased by-33.45%, 31.33% and 44.83%, respectively and after 27 years of returning farmland to forests, the mass ratio of SMB in the 0-10 cm soil layer was significantly higher than that in the cultivated land, and the mass ratio of MBC and mass ratio of MBP in the 10-40 cm layer returned to the cultivated land level; 2) MBC∶MBN, MBC∶MBP and MBN∶MBP are 11.44-65.52,17.40-32.34 and 0.34-2.16, respectively. The q_MBP of 0-20 cm soil layer, q_MBC of 10-20 cm soil layer and C:P_imb of 0-20 cm soil layer were significantly increased after 27 years of returning farmland to forests. 3) The physicochemical properties of soil were the most correlated with the mass ratio of SMB, eco-stoichiometric ratio, q_MB and microbial stoichiometric imbalance in the soil surface layer (0-10 cm), B_D, P_t, SOC and HN were the most correlated indicators, and TP, pH value were the only two correlated indicators in the 10-40 cm soil layer. The results of PLS-PM showed that the influence degree of soil physicochemical properties on the mass ratio of SMB, eco-stoichiometric ratio, q_MB and microbial stoichiometric imbalance were decreased with the depth of soil layer.

A promoting effect was shown on soil microbial growth and nutrient accumulation after 27 years of returning farmland to forests, and the influence of soil physical and chemical properties on the mass ratio of SMB, eco-stoichiometric ratio, q_MB and microbial stoichiometric imbalance in the surface layer (0-10 cm) is the most significant, and nutrient limitations (carbon, nitrogen, phosphorus) remain constraints to the further restoration of soil quality of returning farmland in the loess hilly region.