Soil microbial growth and carbon-use efficiency: ecological control mechanisms

During the decomposition of organic matter (OM), microorganisms use the assimilated carbon (C) for biomass production or respiration, and the fraction of growth to total assimilation defines the microbial carbon-use efficiency (CUE). Therefore, microbial CUEs have direct consequences for the balance of C between atmosphere and soil, and is as such a central parameter to represent the global C cycle well in Global Cycling Models (GCMs). Despite its enormous leverage this factor remains critically underexplored. Based on the physiology of cultured microorganisms, it is anticipated that (H1) high nutrient availabilities will increase microbial CUE, (H2) that higher quality substrate will increase microbial CUE, (H3) that microbial communities more dominated by fungi will have higher CUE, and (H4) that microbial CUE will decrease in response to environmental stress. We combined extensive field surveys with experimental treatments in microcosms to assess our hypotheses. We sampled temperate forest soils, temperate agricultural soils, and subarctic forest soils, encompassing a wide range of soil pHs (4.0-7.1), nutrient availabilities (10<soil C/N<33), and soil OM qualities (7-fold differences in respiration per SOM). We also surveyed environmental pollution gradients where metallurgy had contaminated soil with high heavy metal concentrations in boreal forest and temperate grassland sites. We also subjected selected soils to microcosm experiments where soil pH (liming), mineral N (50 kg N ha^-1), OM quality (plant litter), or heavy metal stress were manipulated and the resulting bacterial and fungal growth, respiration, and CUE were monitored over the course of 2 months.

 

Fungal-to-bacterial growth ratios (F:B) ranged from 0.02 to 0.44 across the studied ecosystems, and that the fungal dominance was higher in soils with lower C:N ratio and higher C-quality. CUE ranged from 0.03 to 0.30, and values clustered most strongly according to site rather than level of soil N. CUE was higher in soil with high C:N ratios and high C-qualities. However, within each land-use type, a high mineral N-content did result in lower F:B and higher resulting CUE. In the microcosm experiments, plant litter addition stimulated the growth of fungi more than bacteria, while increasing soil pH stimulated bacteria more than fungi. Mineral N additions inhibited bacterial growth and stimulated fungal growth. This resulted in microbial CUE estimates in real time that ranged from ca 0.05 to 0.55, and where increased pH and litter increased values while mineral N supplements decreased values. Long-term exposure to heavy metals decreased microbial CUE, but only marginally, even at very high rates of metal exposure. Short-term exposure to metal stimulated microbial CUE in soil from contaminated sites, while CUE was reduced in soil with no history of metal contamination. In conclusion, a higher site soil C-quality coincided with lower F:B and higher CUE across the surveyed sites, while a higher N availability did not. A higher site N availability resulted in higher CUE and lower F:B within each site, while mineral N supplements in the microcosm induced the opposite response, suggesting that site-specific differences associated with fertility such as the effect of plant communities, overrode the influence of mineral N-availability.