Linkages between extracellular enzyme activities and the carbon and nitrogen content of grassland soils

• Positive link between extracellular enzyme activities and soil C and N content.
• Evidence of how grassland management may affect enzyme activity.
• Enzyme activity may not be simply a reflection of SOM concentration.
• Changes in enzyme activity could influence soil C and N in managed grasslands.

Important biochemical reactions in soils are catalyzed by extracellular enzymes, which are synthesized by microbes and plant roots. Although enzyme activities can significantly affect the decomposition of soil organic matter and thus influence the storage and cycling of carbon (C) and nitrogen (N), it is not clear how enzyme activities relate to changes in the C and N content of different grassland soils. Here we address whether the activity of C-acquiring (β-1,4-glucosidase, BG) and N-acquiring (l-leucine aminopeptidase (LAP) and β-1,4-N-acetyl-glucosaminidase (NAG)) enzymes is linked to changes in the C and N content of a variety of human-managed grassland soils. We selected soils which have a well-documented management history going back at least 19 years in relation to changes in land use (grazing, mowing, ploughing), nutrient fertilization and lime (CaCO3) applications. Overall we found a positive relationship between BG activity and soil C content as well as between LAP + NAG activity and soil N. These positive relationships occurred across grasslands with very different soil pH and management history but not in intensively managed grasslands where increases in soil bulk density (i.e. high soil compaction) negatively affected enzyme activity. We also found evidence that chronic nutrient fertilization contributed to increases in soil C content and this was associated with a significant increase in BG activity when compared to unfertilized soils. Our study suggests that while the activities of C- and N-acquiring soil enzymes are positively related to soil C and N content, these activities respond significantly to changes in management (i.e. soil compaction and nutrient fertilization). In particular, the link between BG activity and the C content of long-term fertilized soils deserves further investigation if we wish to improve our understanding of the C sequestration potential of human-managed grassland soils.