Objective To examine the spatial heterogeneity of soil physicochemical and microbial properties at the county scale and their application in soil comprehensive quality assessment, and offer a theoretical foundation for sustainable use of cultivated land.

Method Surface soil comprehensive samples of farmland from 47 monitoring units in Gaoyao District, Zhaoqing City, Gunagdong Province were collected. The spatial heterogeneity of soil physicochemical properties such as pH, and clay, organic matter, total nitrogen, alkali-hydrolyzed nitrogen, total phosphorus contents, as well as soil microbial characteristics including soil respiration, mircrobial biomass, and fungi biomass/bacteria biomass were analyzed combining geostatistics and ArcGIS-related techniques. By employing principal component analysis, correlation analysis and general indicator of soil quality (GISQ) method, we elucidated the influence of different factors on the comprehensive quality of farmland.

Result The nugget coefficients of soil pH, as well as clay, organic matter, dissolved organic carbon, total nitrogen, alkali-hydrolyzed nitrogen, total phosphorus contents ranged from 25% to 75%, indicating moderate spatial autocorrelation, and was affected by both structural and random factors. Among the soil microbial indicators, the nugget coefficient of soil respiration was 29.4%, indicating moderate spatial autocorrelation, but the nugget coefficients of the total soil microbia biomass, as well as fungi, actinomycetes, bacteria biomass were all greater than 75%, indicating weak spatial autocorrelation and poor spatial structure, and influenced by random factors such as human activities. Soil microorganisms were the primary driving factors of soil quality differentiation of farmland in Gaoyao District, especially the total soil mricrobial biomass, as well as bacterial, fungi, actinomycete biomass. Physicochemical properties such as soil organic matter, total nitrogen and alkali-hydrolyzed contents also had considerable impact on farmland soil quality. Additionally, organic matter, total nitrogen and alkali-hydrolyzed contents were significantly and positively correlated (P<0.05). The soil quality of farmland in Gaoyao District was generally in a good level, the spatial pattern was presented as northern hilly area > eastern plain area > central plain area > southern hilly area.

Conclusion At the county scale, the spatial structure of soil physicochemical properties is relatively stable, and soil respiration is a suitable indicator for analyzing microbia spatial variability. Soil microbia biomass and its structure exhibit significant spatial heterogeneity at the county scale. The combined application of soil physicochemical and microbial indicators in soil quality evaluation can more comprehensively reflect the changes in the farmland quality.