Controls over soil microbial biomass responses to carbon amendments in agricultural systems: A meta-analysis

Soil microbial biomass (MB) facilitates key ecosystem functions such as soil aggregation and nutrient cycling and makes a substantial contribution to soil organic matter. While agricultural conversion drastically reduces MB, the use of organic amendments is an effective way to rebuild depleted MB. Yet, little is known about broad-scale, global controls over MB responses to organic inputs. We used a meta-analysis to identify the degree to which soil properties, agricultural management, and geographic location regulate MB response (carbon, Cmic; nitrogen, Nmic; and C:N ratio, C:Nmic) to animal manure-based inputs relative to inorganic fertilizers. We show that organic amendments increased Cmic by 36% and Nmic by 27% across all observations. The chemistry of amendments and their application rates were the strongest regulators of Cmic but edaphic properties were also important. C:Nmic averaged 8.6 and was not influenced by organic amendments under any conditions, providing evidence that the physiological requirements of microbes, rather than management or environmental factors, constrain their elemental stoichiometry. Our study indicates that even small quantities of organic amendments can be used to rapidly restore MB across a range of cropping systems but specific responses depend upon the type and rate of inputs as well soil characteristics.