Soil extracellular enzyme activity increases during the transition from conventional to organic farming

There is an increasing interest in developing agricultural management practices that support a more nature-based, sustainable food production system. In organic systems, extracellular enzymes released by soil microorganisms are important regulators of the cycling and bioavailability of plant nutrients due to the lack of synthetical inputs. We used a chronosequence coupled with a paired field approach to evaluate how potential activity of hydrolytic soil extracellular enzymes changed over time (0–69 years) during the transition from conventional to organic agriculture in two types of soils, marine clay and sandy soils. Organic management generally enhanced the activity of enzymes related to the C cycle, particularly in sandy soils, and increased the proportion of C-related enzymes relative to N- and P-related enzymes. Differences in soil extracellular enzyme activity between organic and conventional farming increased with time since conversion to organic farming for α-β-glucosidase, xylosidase, phosphomonoesterase, 4-N-acetylglucosaminidase, arylsulphatase, and the ratio of C:N enzymes. In some cases, the divergence in enzyme activity was driven by enhanced activity with time in organic fields, but in others by reduced activity over time in conventional fields. Our findings suggest that organically managed soils with higher enzyme activity may have a greater potential for organic matter breakdown, residue decomposition, and higher rates of cycling of C and nutrients. However, these positive effects may take time to become apparent due to legacy effects of conventional management.