Micro-Scale Patterns of Organic Matter Decomposition and Storage Within a Buried Paleosol

Buried soils can contain substantial carbon (C) stocks, and this C is often considered stable and removed from active carbon cycling. However, C in buried soils can be sensitive to change, especially if landscape changes expose buried soils to increased inputs of fresh organic matter (OM) or alters conditions for microbial decomposition. This study investigates C composition and cycling in the Brady paleosol from Nebraska, USA, which was buried by loess deposition during the early Holocene. Samples were collected along an erosional gradient of Brady soil at four different depths representing four different levels of exposure to the modern soil surface. We used ¹³C NMR to determine carbon compound chemistry in two OM fractions isolated using an electrostatic separation method. We also used high-resolution secondary ion mass spectroscopy (NanoSIMS) following a 30-day incubation study with enriched ¹³C litter to investigate micro-scale spatial patterns of transformation and storage of fresh plant OM across the exposure gradient. The NMR results showed that OM composition differed by fraction, and also with depth. The alkyl:O/N-alkyl ratio was greater in the <53 mm mineral and aggregate fraction than in the particulate OM fraction. The alkyl:O/N-alkyl ratio increased in both fractions with depth, indicating a greater degree of decomposition in the more deeply-buried soils. This could indicate a lower proportion of residual ancient organic matter and increased incorporation of recent organic matter inputs in the more exposed soils. Following the incubation experiment, we found evidence that unicellular microbes directly incorporated enriched OM, and that fungal hyphae incorporated only small amounts of enriched OM. Enriched material was found associated with both clay-sized particles and microaggregate structures after 30 days of incubation. These results show that there are microbial communities capable of decomposing fresh litter in these paleosol, and that there are differences in OM decomposition given the degree of exposure to the modern soil surface. A better understanding of differences in OM cycling within a buried soil will help us improve predictions of buried soil C stability and susceptibility to future global change.