Controls On Soil Microbial Biomass Carbon and Nitrogen In Agricultural Systems: a Meta-Analysis.

Microbial biomass is typically reduced by agricultural conversion and other types of land use intensification. Incorporating organic amendments, such as animal manures and plant residues, into agricultural soils may increase microbial biomass (MB) C and N and thus have the potential to improve soil quality, nutrient cycling, and aggregate stability. Using a meta-analysis approach, we quantified the effects of organic and inorganic fertilizers on MBC and MBN.  We further identified the environmental variables that were the best predictors of variation in soil MBC and MBN response to organic amendments and determined  if and how microbial C:N ratios may be influenced by these variables. Results were analyzed using a database of 509 observations over a diverse, globally-distributed range of cropping systems.

The overall effect of organic inputs was a 28% increase in MBC and a 12% increase in MBN.  The type of organic input was the strongest predictor (Q_b= 10.72; P<0.001) of change in MBC, followed by climate, soil C concentration, clay content, rate of organic input, and soil pH. For MBN, only the type of organic input and climate regulated the response to organic amendments.  We were also able to look within the categorical variables to understand the factors controlling MBC and N.   For example, different types of organic amendments varied in their effects on microbial biomass from cattle manure, which had the largest effect (+58%, CI: 1.47±-1.7) to poultry manure, which had the lowest (+10%, CI: 1.04±1.21).  The MB C:N ratios (mean 10.28; SE ± 0.64) showed no response to organic amendments and was not influenced by any of the categorical variables.  Though the magnitude of MB response differed among and between variables, it was consistently positive across observations, indicating that the use of manures and plant residues in agriculture increases MB, independently of soil properties, geography or crop management.