Diverse Microbial Carbon as a Source of Soil Organic Matter

Soil microorganisms are known to contribute a substantial amount of C to soil, but little is known of the importance of microbial biochemistry as a factor influencing soil organic matter (SOM) dynamics, or mechanisms controlling the stability of diverse microbial C in soil. This research assessed the potential for stabilization of microbial cells and extracellular products (EPS) in two contrasting forest ecosystems.;While there were some differences in the chemical composition of diverse microbial cell isolates and EPS, both across groups and for microbes of the same group but isolated from different origins, microbial cells exhibited similar turnover rates in soils; and any differences that were observed did not correlate with measured chemical properties of microbial cells as determined by elemental analyses (C and N) or Py-GC-MS. Results suggest that, unlike plant residues, the source and quality of microbial C does not substantially influence its stability in soil. Chemical characterization showed some differences between four biochemically diverse microbial groups from California and Puerto Rico and their native soils; but greater similarities. Results suggest that microbial cells and products accumulate in soils and contribute substantially to the overall soil organic matter (SOM) signature. Additionally, results showed that diverse microbial groups and EPS exhibit similar chemical complexity as SOM, suggesting that the production of microbial biomass and turnover does not reduce the biochemical complexity of SOM as previously hypothesized.;Site specific effects for input microbial residues were observed in both whole soils and soil fractions, suggesting different mechanisms operate at each site to influence the turnover of specific microbial C. Site differences likely occurred due to different native soil microbial communities, which likely differ in enzymatic expression patterns or substrate utilization preferences. However, numerous edaphic and climatic factors differ between sites, and the effects of specific factors controlling the turnover of microbial isolates are difficult to fully differentiate. Site differences in C cycling were further highlighted by chemical characterization of soils, which showed unique compounds and distinct SOM signatures for California and Puerto Rico soils. Differences in the chemical composition of native SOM at both sites support different processes and mechanism controlling soil C. Substantial climatic differences between the two study sites resulted in greater overall decomposition rates in PR than CA, as expected. The abundance of decomposition byproducts suggest that primary litter inputs likely have little effect on the overall chemical of SOM; rather, decomposition products (the chemistry and total amount of stabilized vs. unstabilized C) likely have a greater influence on the overall SOM composition. These results were supported by characterization of soil C fractions, which showed greater differences in the composition of mineral- associated SOM than for SOM less associated with soil minerals. Results suggest that the distinct microbial communities at the two study sites correspond to unique biogeochemical processes which result in different types of compounds available to stabilize with soil minerals.;At both sites, association with soil minerals through direct complexation or intra-aggregate occlusion increased the stability of microbial C. Microbial C turnover in operationally defined soil physical fractions support different mechanisms controlling the turnover of microbial C at the two study sites. In particular, results suggest microbial C with lesser association with soil minerals was more susceptible to effects by climate; as differences in the overall turnover of microbial C were more pronounced in the FLF and OLF in California soil fractions. The pronounced effect of microbial residue quality in CA, with lower associations with the mineral matrix (i.e. the FLF and OLF), suggests that these fractions may be sensitive indicators of microbial residue decomposability in addition to plant litter decomposability as previously hypothesized. However, the usefulness of these fractions for predicting litter quality inputs may be sight-specific. For example, our results showed that in CA, uncomplexed microbial C showed a greater sensitivity to any effect of inherent recalcitrance of microbial inputs, whereas in PR this effect did not occur.;Greater stability of microbial C in aggregate-occluded fractions in CA than in PR also supports different mechanisms operating at each site, likely associated with the dominating fungal community in CA which could have redistributed the input 13C into aggregates. Mineral-complexed microbial C was relatively stable in PR, where the native microbial community is dominated by bacteria, which typically reside in water films on mineral surfaces. Results suggest that, in addition to potential differences in enzyme expression patterns or unique substrate preferences and discrimination across sites influencing the stability of diverse microbial C residues applied to soils, the physiology of the native living microbial communities likely played an important role in the uptake and redistribution of input microbial 13C in different SOM fractions.;This research highlights important interactions of factors operating at different scales to affect C turnover from diverse microbial groups and stabilization and chemistry of native SOM, and associated stabilization mechanisms. Moreover, the effect of microbial residue quality on its turnover was dependent on site, as well as its location in the mineral matrix and relative interaction with soil minerals. Additionally, the interaction of microbial residue quality and mineral association on microbial C stabilization differed between the two sites, with results showing greater overall discrimination in PR, but suggest that in CA relative association (or lack-thereof) with mineral matrix had a greater influence on the retention of diverse microbial residues. Our results suggest that unique effects across sites controlling differences in the stabilities of specific microbial C inputs, and associated stabilization mechanisms, may be as important as macromolecular structure and recalcitrance.