Effects of Organic Materials on Soil Microbial Biomass and Its Acidity Regulation Mechanism

This study explored the effects of the application of organic materials on soil pH, microbial biomass and microbial entropy, so as to provide a scientific basis for the improvement of arable land quality.

Based on the meta-analysis, 1296 sets of data from 224 literatures around the world were integrated to comprehensively analyze the differences in soil pH, microbial biomass carbon (MBC), nitrogen (MBN), microbial entropy carbon (qMBC), and nitrogen (qMBN) under the conditions of organic material application measures (type, application amount, and application years), climate factors (annual average temperature, precipitation), and initial soil properties (pH value, organic carbon content).

After the application of organic materials, the soil pH value increased significantly by 0.23 units, increased by 4.0%, while MBC, MBN, qMBC and qMBN increased by 46.8%, 54.0%, 16.4% and 14.9%, respectively. Compared with chemical fertilizer, soil pH, MBC, MBN, qMBC and qMBN increased by 5.9%, 53.4%, 13.2%, 79.9% and 38.2%, respectively. The effect of increasing soil pH was the best when the amount of manure application was more than 10 t·hm^-2·a^-1 and the application period exceeded 10 years; however, the soil microbial biomass index could be significantly improved when the amount of fertilizer was more than 40 t·hm^-2·a^-1 and the application period was 3-10 years. Under the climate conditions of annual average temperature >16 ℃ and annual precipitation >1200 mm, the effect of applying organic materials on improving soil pH was better, especially in soils with initial soil pH value of 4.5-5.5 and organic carbon content (SOC) >12 g·kg^-1; however, under the climate conditions of 8-16 ℃ annual average temperature and annual precipitation <600 mm, the application of organic materials had better effect on the improvement of MBC, MBN, qMBC and qMBN in weakly alkaline soil (pH>7.5) with SOC≤12 g·kg^-1. The random forest model showed that the effects of applying organic materials on soil pH, MBC, MBN and microbial entropy were affected by soil initial pH and SOC, application measures of organic materials and climate conditions, respectively. The partial least squares path model further proved that the application measures of organic materials, climate conditions and initial soil properties affect microbial biomass and microbial entropy by adjusting pH value.

Organic materials most effectively elevated soil pH in acidic with high-SOC soils (pH≤5.5, SOC>12 g·kg^-1), which enhanced microbial biomass and microbial quotients in alkaline with low-SOC soils (pH>7.5, SOC≤12 g·kg^-1). The regulation of soil pH, microbial carbon and nitrogen, and microbial quotients was primarily controlled by initial soil properties, organic material application measures, and climatic conditions.