Soil Microbial Community-Level Physiological Profiling as Related to Carbon and Nitrogen Availability Under Different Land Uses

The goal of this work was to assess soil microbial respiration, determined by the assay of community-level physiological profiling in an oxygen-sensitive microplate (O2-CLPP), in response to endogenous C and several individual C substrates in the soils with different organic C contents (as a function of soil type and management practice). We also used the O2-CLPP to determine the respiratory response of these soils to endogenous C and amended C substrates with N addition. A respiratory quotient (RQ) was calculated based on the ratio of the response to endogenous soil C vs. each C-only substrate, and was related to total organic carbon (TOC). For assessing N availability for microbial activity, the effect of N supplementation on soil respiration, expressed as Nratio, was calculated based on the response of several substrates to N addition relative to the response without N. Soils clustered in 4 groups after a principal component analysis (PCA), based on TOC and their respiratory responses to substrates and endogenous C. These groups reflected differences among soils in their geographic origin, land use and C content. Calculated RQ values were significantly lower in natural forest soils than in managed soils for most C-only substrates. TOC was negatively correlated with RQ (r = −0.65), indicating that the soils with higher organic matter content increased respiratory efficiency. The N addition in the assay in the absence of C amendment (i.e., only endogenous soil C present) had no effect on microbial respiration in any soil, indicating that these soils were not intrinsically N-limited, but substrate-dependent variation in Nratio within soil groups was observed.