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Control mechanisms of soil particle size on the quality of soil organic matter


The quality of soil organic matter (SOM), which measures the resistance of SOM to biological degradation, is an important factor controlling soil turnover rates and soil carbon sequestration. The main goals of this study were to evaluate how soil particle size affects the quality of SOM, the mechanism responsible for the relationship between particle size and SOM quality, and the potential impact of the relationship on carbon dynamics and sequestration. We used stable C and N isotopic ratios (δ13C and δ15N) to measure the relative quality of nine physically separated particle size fractions from soil collected in Hanover, NH, Suffolk County, Long Island, NY and the northeastern coast of Newfoundland mainland province (formerly Labrador), Canada. Radiocarbon dating, X-ray diffraction (XRD), scanning electron microscopy (SEM) imagery and X-ray microanalysis were used to investigate the mechanism of the observed quality-particle size relationship. At a given depth, Hanover soil yielded a strong nonlinear relationship between δ13C or δ15N and particle size, where low isotopic ratios were associated with large and small size fractions, while relatively high ratios with intermediate particles. When combined with the radiocarbon results, it shows that clay particle fractions contain high quality SOM that is disproportional to the radiocarbon age. This result suggests that SOM associated with clay-sized particles are physically protected from microbial decomposition, such that stable isotope ratios cease to change after aggregation while C-14 continues to decay. Both XRD and SEM studies confirmed the role of clay minerals (particularly vermiculite) for mineral protection of SOM. We explain the nonlinear relationship between δ13C or δ15N and particle size to be a result of the competing processes that cause reducing particle size of SOM with increasing the degree of degradation, and increasing mineral protection by aggregate structures with decreasing particle size. In contrast to the Hanover soil, the Newfoundland soils showed no nonlinear relationship between quality and particle size. This was predicted by our conceptual model, because there was little clay in these colder soils. The Long Island soil showed only a weak nonlinear relationship between quality and particle size, despite this site being slightly warmer than Hanover. However, these results are also consistent with differences in observed soil mineralogy, namely a lack of clay minerals capable of bonding with and protecting organic molecules. Our results imply that measuring the bulk quality of SOM is not sufficient for predicting carbon sequestration, because OM of the same quality but associated with different soil microstructure has different accessibility by soil microbes and may responds to climate change with different sensitivities. The results also suggest that processes that disturb the soil structure may also result in changes in soil respiration rates.