Empirical evidence that soil carbon formation from plant inputs is positively related to microbial growth
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Plant-carbon inputs to soils in the form of dissolved sugars, organic acids and amino acids fuel much of heterotrophic microbial activity belowground. Initial residence times of these compounds in the soil solution are on the order of hours, with microbial uptake a primary removal mechanism. Through microbial biosynthesis, the dissolved compounds become dominant precursors for formation of stable soil organic carbon. How the chemical class (e.g. sugar) of a dissolved compound influences stabilization in field soils is unknown and predictions from our understanding of microbial metabolism, turnover and identity are contradictory. We show that soil carbon formation, from chronic amendments of dissolved compounds to fertilized and unfertilized grasslands, is 2.4-times greater from a sugar than an amino acid. Formation rates are negatively correlated with respiration rates of the compounds, and positively correlated with their recovery in microbial biomass. These relationships suggest that the efficiency of microbial growth on a compound is positively related to formation rates of soil organic carbon. Fertilization does not alter these findings, but together nitrogen and phosphorus additions reduce soil carbon formation. Our results highlight the need to consider both nutrient enrichment and global-change induced shifts in the form of dissolved root inputs to soils to predict future soil carbon stocks and hence phenomena such as climate warming and food security to which these stock sizes are intimately tied.
KeywordsSoil organic carbon Soil carbon formation Microbial biomass Root exudation Low molecular weight carbon compounds Dissolved organic carbon
The work was supported by the U.S. Department of Energy’s Office of Science (BER) and the U.S. National Science Foundation through the Coweeta Long Term Ecological Research Program. Thanks to Brian Kloeppel and Greg Zausen for field assistance; Pete Raymond, Cynthia Kallenbach and Stuart Grandy for manuscript comments.
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