Decreases in organic C reserves in soils can reduce the catabolic diversity of soil microbial communities

An understanding of the main factors influencing microbial diversity in soils is necessary to predict the effects of current land-use trends on terrestrial diversity. We used microbial catabolic evenness as a measure of one component of soil microbial diversity. Catabolic evenness was assessed by measuring the short-term respiration responses of soil to a range of simple organic compounds. Differences in catabolic evenness between pasture and other land-uses on matched soils were related to differences in organic C pools (total organic C, microbial biomass C, and potentially mineralizable C). This approach enabled comparison of land-use effects on organic C pools in relation to catabolic evenness without the effects of soil type. In general, microbial catabolic evenness was greatest in soils under pasture and indigenous vegetation (range: 19.7–23.3), and least in soils under cereal/maize/horticultural cropping (range: 16.4–19.6). Soils under mixed cropping land-uses had catabolic evenness that ranged between these extremes (range: 17.7–20.5), but under pine forestry there was no characteristic level of evenness (range: 15.1–22.3). Catabolic evenness correlated poorly with the absolute values of soil organic C pools (r2<0.36). However, across a range of paired comparisons between pasture and other land-uses, greater differences in microbial catabolic evenness corresponded with greater differences in organic C (r2=0.76) and, to a lesser degree, with differences in microbial biomass C (r2<0.45) or potentially mineralizable C (r2<0.13). Therefore, land-uses that deplete organic C stocks in soils may cause declines in the catabolic diversity of soil microbial communities. Although the implications of this for microbial processes are unknown, maintenance of soil organic C may be important for preservation of microbial diversity.