Soil microorganisms vary in the efficiency with which they convert soil organic matter carbon to stable biomass products. This carbon use efficiency (CUE) influences carbon budgets and is used in models to predict soil feedbacks to climate and land use intensity (LUI). Because LUI has the potential to alter soil microbial community structure and function, we hypothesized that it would drive changes in CUE. Specifically, at the cell level, microbial energy requirements decrease relative to assimilation with higher growth rates, so we speculated that higher LUI would favor microbes with higher growth rates, promoting a higher CUE. However, CUE results in the literature are mixed and our predictions would benefit from a broader perspective.
To test our theories of CUE and microbial community with grassland LUI, we sampled soil from three levels of LUI, and replicated this at nine sites across the UK; incorporating variety in soil geophysical factors, climate, and management regimes. LUI was scored with an index that scored grazing, mowing, ploughing and fertilizer amendment across the sites. Soils were incubated with a simple or complex stable isotope-labeled substrate (glucose and microbial necromass): the ratio of isotope recovery in microbial biomass versus soil respiration characterized community-level soil CUE, and the recovery of label in the microbial community ascertained functional groups responsible for this CUE.
Results from our experiment indicated strong differences across the UK in the CUE of simple and complex substrates: glucose (average±standard error = 0.66±0.01) and microbial necromass (0.51±0.02). At the landscape scale, partitioning of the variance in the CUE of the two substrates was split between environmental factors (~20%) and microbial community structure (~2%); dominant environmental controls included soil physical characteristics (glucose) and mean annual precipitation (necromass). Five of our nine sites responded to LUI with changes in CUE, and not in predictable or consistent directions. Ongoing analysis of isotope recovery in microbial community functional groups will be presented.
Surprisingly, LUI was not a dominant control on CUE, despite LUI-associated changes in the edaphic and microbial factors that control CUE at the landscape level. We conclude that our LUI index does not capture the complex interaction between management and soil type. The different CUE of the simple and complex substrate supports the incorporation of substrate complexity into soil process models. These results and our continuing investigation will enhance our understanding of soil ecosystem resilience to UK grassland management decisions.