PS 46-127 - Decomposition and soil aggregate formation in a shrub-invaded Sonoran Desert grassland

Wednesday, August 10, 2011
Exhibit Hall 3, Austin Convention Center
Eva M. Levi1, Steven R. Archer1, Craig Rasmussen2, Heather L. Throop3 and Daniel B. Hewins4, (1)School of Natural Resources and the Environment, University of Arizona, Tucson, AZ, (2)Soil, Water and Environmental Science Department, University of Arizona, Tucson, AZ, (3)Biology Department, New Mexico State University, Las Cruces, NM, (4)Department of Biology, New Mexico State University, Las Cruces, NM

Decomposition of organic matter is a crucial component of global biogeochemical cycles that exerts strong control over soil fertility, fate and residence time of soil carbon and nitrogen, and plant community composition and production. Decomposition dynamics are strongly influenced by land-cover changes that alter the quantity and quality of organic matter inputs via changes in plant growth-form composition. The global phenomenon of shrub encroachment into grass-dominated ecosystems redistributes carbon among terrestrial reservoirs and may influence decomposition by altering patterns of soil-litter mixing and the amount of photo-oxidation by UV radiation. We therefore sought to quantify: 1) the influence of plant growth-form and soil type on rates of soil-litter mixing and decomposition and 2) the development of soil films and aggregates that may shield litter from UV radiation and promote microbial colonization. A litterbag experiment in a shrub-invaded grassland in southern Arizona tested the hypotheses that soil-litter mixing would have a net positive effect on decomposition rates, and development of soil films and aggregates would be inversely related to ground cover and surface soil texture. Litterbags containing shrub (Prosopis velutina) leaf litter were deployed on sandy and clayey soils and situated under and away from shrub canopies and on areas with low and high grass cover. Litter was collected at 0, 1, 6, and 12 months and analyzed for mass loss and C and N content.


Results indicate soil-litter mixing accelerates decomposition as soil accumulation occurs, with higher rates of accumulation and decomposition on sandy soils in areas away from shrub canopies and with low grass cover. In contrast, development of soil films was more rapid and more ubiquitous on clayey soils. The relationship between mass loss and %C remaining was strongly linear. Visible soil films were observed on litter within 1 month, and formation of aggregates had begun within 6 months. Dissecting microscope and scanning electron microscope images of litter show visible signs of degradation and coating by soil-fungal films.  In conclusion, soil-litter mixing is a significant driver of decomposition and aggregate formation in drylands that is strongly mediated by interactions between vegetation cover and soil type:  areas of sandy soil and low vegetation cover experience greater soil accumulation, aggregation, and decomposition, whereas areas with clayey soil show more developed soil films. Soil-litter mixing is the first step in soil aggregation, which appears to be a result of fungal hyphae and microbial exudate interactions with litter and soil particles.

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