Intercropping Increases Soil Extracellular Enzyme Activity: A Meta-Analysis

Intercropping has been shown to increase the productivity of agroecosystems, but the belowground mechanisms for this have not been well elucidated. The soil microbial community produces extracellular enzymes that are responsible for degrading plant residues and maintaining nutrient cycles in the soil. While the effects of plant diversity on enzyme activities have been studied, the results are often highly variable, both within and between studies. Therefore, to determine whether intercropping enhances nutrient cycling by increasing extracellular enzyme activity and to identify sources of variation in enzyme rates, we performed a meta-analysis drawing on 969 observations across 100 studies from around the world. We find that intercropping significantly increases enzyme activities by an average of 13% (P < 0.001) and that the intercropping effect varies by enzyme category, the type of plant used as the main crop or intercrop, and other experimental and environmental factors. Increased quantity and quality of plant residue inputs is a likely driver of the intercropping effect. The larger intercropping effect on increasing enzyme activity in nutrient-poor soils, together with differences between plant types, supports the view that soil nutrient status mediates patterns of microbial nutrient acquisition strategies. Future research should be aimed towards capturing regional variation within North American and Europe and towards identifying the specific drivers of the microbial response to increased plant diversity.