COS 35-3 - Soil-litter mixing accelerates decomposition and facilitates soil aggregate formation in a Chihuahuan Desert grassland

Tuesday, August 9, 2011: 2:10 PM
Ballroom F, Austin Convention Center
Daniel B. Hewins, Department of Biology, New Mexico State University, Las Cruces, NM, Heather L. Throop, Biology Department, New Mexico State University, Las Cruces, NM, Steven R. Archer, School of Natural Resources and the Environment, University of Arizona, Tucson, AZ and Gregory S. Okin, Department of Geography, UCLA, Los Angeles, CA

Vegetation assemblages have been shown to affect patterns of eolian mediated soil flux in drylands. In the Chihuahuan Desert, encroachment by woody plants and land management practices (e.g., livestock grazing) have led to changes in vegetation community structure, namely reduced cover by native grasses, and consequently soil erosion has increased.  We tested the hypothesis that reduced grass cover promotes litter decomposition a strong regulator of carbon and nutrient cycling by increasing rates of soil-litter mixing.  Our experiment in the northern Chihuahuan Desert in plots where grass cover treatments have been established (0, 50, 75, and 100%).  Previous experiments in these plots have shown these manipulations affect rates of soil flux.


Following 12 months of field exposure, we found a positive, linear relationship (r2= 0.32) between soil-litter mixing (% ash) and litter mass loss. Stepwise regression eliminated grass cover manipulation and litterbag placements as significant variables in modeling our data (α=0.15).  Model selection using Akaike information criteria confirmed the univariate model to be the best predictor when modeling mass loss (AIC difference >100 for models not containing %ash; >2 for those that do). This lack of grass removal treatment effect may be a result of litterbag design as well as the highly heterogeneous nature of localized soil flux. At our 1 and 3-month litterbag collections, visual degradation of leaf pigment was observed. By 6 months, a tightly-adhering soil film thoroughly coated litter material. By 12 months of field exposure, soil aggregates >2mm in diameter had formed within litterbags. Scanning electron micrographs showed fungal hyphae throughout soil-liter films and aggregates. We used energy dispersive X-ray spectroscopy to generate chemical maps that characterize the atomic composition of abiotic and biotic components of soil-litter films. We suggest that soil-litter film and aggregate formation likely relies on soil-litter mixing and precipitation inputs that facilitate decomposition as a carbon source and microbial metabolism as a source of extracellular compounds or carbonate. Ultimately, soil aggregates as a result of the decomposition of organic matter may have strong implications for stabilization of surface soils and carbon. Microbially derived carbon based extracellular compounds or biomineralized carbonates are likely protected within soil aggregates, and soil aggregates may develop into large particles that are less erodible than individual particles. Our results confirm findings of previous work in the Sonoran Desert that show soil-litter mixing accelerates decomposition and shows that soil-litter mixing may also have implications for carbon storage and soil stabilization.

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