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.

The aim of this study was to analyze the characteristics of changes in soil acidity and nutrient content after the conversion of paddy fields with different parent material development into dryland and vegetable fields in Qiyang, a typical county in the red soil area, so as to provide a scientific basis for the rational use of land to prevent acidification in the area.

18 sites were selected and collected from paddy fields and adjacent dryland and vegetable fields to analyze the changes of soil pH, exchangeable acid, exchangeable salt-based ions, organic matter, cation exchange, and nutrient content and their interrelationships.

The pH of all the soils developed with alkaline parent material was significantly higher than that of the soils developed with acidic parent material. Soil pH decreased by 0.48 units after the conversion of acidic parent material developed paddy fields to vegetable fields; soil pH decreased by 0.74 and 0.53 units after the conversion of alkaline parent material developed paddy fields to drylands and vegetable fields, respectively. The bilinear model fit analysis showed that the soil exchangeable aluminum content increased rapidly when the soil pH was below 5.88, 5.78 and 5.63 in the paddy field, dryland and vegetable field, respectively, and the increment of soil exchangeable aluminum content increased by 1.09, 2.33 and 2.93 cmol₍₊₎·kg^-1 by one pH unit, respectively. Soil organic matter and total nitrogen content decreased by 11.06 and 0.42 g·kg^-1, respectively, in the acidic matrices developed paddy fields converted to drylands, while no significant changes were observed in vegetable fields; soil organic matter and total nitrogen content decreased significantly in the alkaline matrices developed paddy fields converted to drylands and vegetable fields, by 13.88-17.28 and 0.57-0.71 g·kg^-1, respectively. The total and effective phosphorus content of the soil increased significantly from 0.41-0.48 g·kg^-1 and 26.79-28.69 mg·kg^-1 after the conversion of the paddy field with alkaline parent material to dryland and vegetable field, respectively. Correlation analysis showed that soil pH was significantly and positively correlated with soil exchangeable calcium and magnesium, cation exchange, organic matter content and total nitrogen content (P<0.01); soil exchangeable acid and aluminum were significantly correlated with effective phosphorus content (P<0.05) and negative correlation (P<0.01) with soil exchangeable calcium and magnesium, cation exchange, organic matter and total nitrogen.

Soil organic matter and total nitrogen content decreased significantly after the conversion of acidic and alkaline parent material developed paddy fields to drylands, while soil total phosphorus and effective phosphorus content tended to increase. Soil acidification was observed after the conversion of paddy fields to vegetable fields for acidic parent materials or to drylands and vegetable fields for alkaline parent materials; the increased nitrification and increased leaching of salt-based ions from paddy fields to drylands and vegetable fields might be one of the main reasons for soil acidification.