Controls on soil microbial carbon use efficiency over long-term ecosystem development

Carbon use efficiency—the proportion of substrate carbon that is converted to microbial biomass—is an important control on many ecosystem properties including carbon sequestration and nutrient cycling. Although CUE varies widely across terrestrial ecosystems, a coherent understanding of edaphic controls on CUE is lacking, thereby limiting the accuracy of global carbon models. The objective of this study was to determine how microbial CUE changes with long-term soil development and nutrient availability. Soil was collected across the chronosequence and fertility gradient at Jug Handle State Natural Reserve (the “Ecological Staircase”) in Mendocino County, CA. These soils exhibit a range in pH (3.29–6.59), litter quantity and quality (litter C:N, 32–70), and clay content (14.13–87.30%), while other factors such as modern-day climate, potential biota, and parent material are common to all sites. CUE varied significantly with soil and ecosystem development; the relationship was unimodal over geological time with CUE peaking at intermediate aged soils. Soil organic matter (SOM) content and pH were the most important variables each accounting for 30% of the variation in CUE across sites. Soil pH had a quadratic relationship with CUE, peaking at pH 4.7. Litter carbon to phosphorus (C:P) and nitrogen to phosphorus (N:P) ratios were also significant factors (Pearson’s r = 0.57 and 0.39 respectively). These results demonstrate that CUE changes in complex ways with soil fertility and long-term ecosystem development, and that edaphic factors such as SOM, pH, and litter quality need to be taken into account when predicting CUE for a given system.