Ecoenzymatic stoichiometry as a temporally integrated indicator of nutrient availability in soils

The extent to which soil enzyme activity in assessing soil nutrient availability is useful has been controversial. In this review, we discuss the utility of ecoenzymatic stoichiometry (i.e. the ratio of nutrient- to carbon (C)-acquiring enzyme activities) on the basis of the resource allocation model for ecoenzyme synthesis. Both the selection of appropriate enzymes and the balance between relative amounts of substrates and enzymes in the soil are decisive factors in utilizing the ecoenzymatic stoichiometry. Ecoenzymatic stoichiometry can evaluate the availability of nitrogen (N), phosphorus, and sulfur in many soils in which the enzyme catalytic reactions are substrate-limited but not enzyme-limited. However, the ecoenzymatic stoichiometry approach does not seem to be applicable in soils where microbes are limited by factors other than nutrient availability, such as low temperature, where the enzyme catalytic reactions are enzyme-limited. Certain enzymes, such as N-acetyl-β-glucosaminidase and protease, appear to be insensitive to soil N availability, because they release compounds containing both N and C which serve as sources for both N and C/energy. We propose the use of enzymes such as L-asparaginase and urease as N-acquiring enzymes that release a compound containing N but not C (i.e. NH4+) as the hydrolysis product. Ecoenzymatic stoichiometry can be considered as an indicator of long-term (weeks) temporally integrated soil nutrient availability, rather than instantaneous availability, for plants as well as microbes, because of (i) the long-term persistence of extracellular enzymes in soils; (ii) a significant correlation between ecoenzymatic stoichiometry and the measurements reflecting the quantity of long-term available nutrients in soil; and (iii) a significant correlation between ecoenzymatic stoichiometry and plant nutrient uptake. This review also identifies challenges in assessing microbial nutrient limitation using ecoenzymatic stoichiometry. With a comprehensive understanding of underlying mechanisms and limitations, ecoenzymatic stoichiometry can be used as a biologically relevant indicator of nutrient availability in combination with other approaches such as conventional chemical extraction methods and the nutrient addition approach.