Soil microbial physiology plays a fundamental role in soil organic carbon sequestration and stabilization. Heterotrophic soil microorganisms take up organic carbon, which is partitioned between respiration and growth, with the latter having the potential to be stabilized in soils. This partitioning factor – microbial carbon use efficiency (CUE) – is a critical parameter in soil biogeochemistry models. Several agricultural management practices, including crop rotation, have been shown to have the potential to increase soil organic carbon storage, but the underlying mechanisms are yet to be understood. Crop rotations are characterized by greater diversity of plant litter inputs and higher litter quality, and have been shown to sustain soil quality and productivity by enhancing soil carbon, nitrogen, and microbial biomass.
Here we assess how crop rotational diversity affects soil microbial physiological parameters across five long-term experiments in Central U.S. and Canada along a soil-climate gradient. Specifically, we analysed microbial carbon use efficiency, growth and organic carbon uptake in maize-based crop rotations ranging from monocultures to five crops in rotation. To determine microbial CUE, we measured gross microbial growth using incorporation of 18O into microbial DNA. The method is direct (growth is measured as DNA replication, not incorporation of carbon into biomass) and substrate-independent.
Results/Conclusions
We found that microbial CUE, ranging between 0.1 and 0.4, significantly varied between sites, whereas organic carbon uptake rates of the microbial communities were similar across the five sites, despite different organic carbon availabilities. These results indicate that the rate of microbial biomass production was driven by how organic carbon taken up was partitioned between anabolic and catabolic processes and not by the soil organic matter decomposition flux.
Microbial growth rates increased with increasing crop rotational diversity. However, microbial biomass turnover was not affected because higher growth rates were accompanied by higher microbial biomass carbon pools. Microbial CUE, in turn, did not show a clear response pattern to crop rotational diversity; Compared to simple rotations, CUE increased in the mid-diverse crop rotations but not in the most diverse rotations. At the field scale, microbial CUE may be controlled by changes in soil properties and microbial community structure induced by the type and sequence of crops included in the rotation. Therefore, we will further explore environmental and microbial community controls on microbial CUE and growth at both the regional and field scale.