Enzyme additions as a tool to assess the potential bioavailability of organically bound nutrients

Potential enzyme activities in soil and water samples are measured by addition of an excess amount of suitable substrate and subsequent determination of product release. If the approach is reversed and an excess of enzyme is added, substrate availability becomes rate-limiting and the maximum release of product indicates the availability of a given substrate in a sample. This approach has been used in a range of studies using phosphatase enzyme additions to soil, manure and sediment extracts, soil suspensions, and lake and sea water (n = 41). Significantly fewer studies have used enzymes from the carbon, nitrogen and sulfur cycles (n = 14). In this review, the methodological aspects of enzyme additions are discussed using examples from studies in which enzymes from the phosphorus cycle were used. A meta-analysis performed for various soil extracts and water samples revealed that the majority of studies (75th percentile) indicate availability of organic phosphorus to enzymatic hydrolysis of up to 60%, with crude phytase preparations showing the lowest substrate specificity and greatest release of phosphorus. Compared to addition of enzymes from the phosphorus cycle, lower substrate degradation was generally achieved by addition of enzymes from the carbon, nitrogen and sulfur cycles to soil suspensions and soil organic matter extracts. Enzyme additions can be a valuable tool in process research, provided all the necessary controls are included and assay conditions optimized to ensure that the reaction reaches completion. Recommendations for the development of a standard protocol are made.