Effect of fertilization on soil microorganisms in paddy rice systems – A meta-analysis

Soil microorganisms are considered a sensitive indicator of soil health and quality. In cropping systems, soil microorganisms are strongly affected by crop management, including the application of fertilizers. While studies in natural ecosystems have generally found that increased nitrogen (N) inputs decrease microbial biomass, microorganisms in soils under upland crops often benefit from mineral fertilizer input. Paddy rice soils, being flooded for part of the season, are dominated by different carbon (C) and N cycle processes and microbial communities than soils under upland crops. The objective of this study was to explore the effect of fertilizer on soil microorganisms in paddy rice systems in a meta-analysis of the peer-reviewed literature. Across all studies (n = 55), the addition of mineral fertilizer significantly increased microbial biomass carbon content (MBC) by 26% in paddy rice soils. Mineral fertilizer applications also increased soil organic carbon content (SOC) by 13%. The higher crop productivity with fertilization likely led to higher organic C inputs, which in turn increased SOC and MBC contents. The time of sampling within a season (pre-plant rice, in-season rice, post-harvest rice, or post-harvest rotational crop) did not significantly affect the response of MBC to mineral fertilizer. The positive effect of mineral fertilizer on MBC content did not differ between cropping systems with continuous rice and systems where paddy rice was grown in rotation with other crops. However, compared with upland cropping systems, the increase in the microbial biomass due to mineral fertilizer application is more pronounced in rice cropping systems, even when rice is grown in rotation with an upland crop. Differences in climate and soil oxygen availability likely explain the stronger response of soil microorganisms to mineral fertilizer input in paddy rice systems. Our analysis suggests that fertilization does not consistently select for specific microbial groups (e.g. gram positive or negative bacteria, fungi, actinomycetes) in paddy rice systems; however, it affects microbial community composition through changes in soil properties. How specific groups of microorganisms respond to mineral fertilization likely depends on environmental factors. Overall, our results suggest that in paddy rice systems the application of inorganic fertilizers increases SOC and MBC contents, both of which are important indicators of soil health.