Effects of soil disturbance on aggregation and mineral-associated carbon and nitrogen storage

The specifics of soil organic matter storage and aggregate dynamics are not fully understood. However, it is generally agreed upon that physical disturbance of soil, often in the form of agricultural tillage, reduces soil aggregation. In turn, decreased aggregation causes a loss of soil carbon and nitrogen, both in the form of particulate organic matter and mineral-associated carbon and nitrogen. Highly disturbed soils such as those tilled conventionally (CT) have lower aggregation and organic matter than less-disturbed soils such as no-till (NT) agricultural soil and forest soil. In this study I investigate these processes, focusing on the variation in mineral-associated organic matter and aggregate fraction distribution in different land use types. I propose a soil disturbance gradient consisting of a single soil series (the Hagerstown silt loam) under three fertilized and planted land use types: CT, NT, and undisturbed urban lawn (URB) as a fertilized, but disturbance-free equivalent to forest soil. Samples of a silt loam were taken from the three land use types and fractionated into size classes, distinguishing between microaggregates and fines within macroaggregates, and those outside of macroaggregates. My data consistently follow a pattern of aggregation and carbon/nitrogen content with URB as the maximum, NT as the intermediate, and CT as the minimum. These data support the concept of a soil disturbance gradient. Urban grassland soil is a novel addition to the conventional disturbance gradient. In order to prove its validity as a no-disturbance end member it was compared to forest soil, a standard for undisturbed land use. NT and CT aggregation data were consistent with the literature, with macroaggregation decreasing significantly with disturbance. Macroaggregates, rather than microaggregates, were enriched in mineral-associated organic matter (carbon and nitrogen), a finding that differs from previous studies. These results suggest that the dynamics of mineral-associated organic matter differ from those of particulate organic matter