Decomposition in a dusty world: Soil-litter mixing enhances leaf litter decomposition and soil aggregate formation

Background/Question/Methods Erosional processes strongly affect nutrient availability by redistributing soil and organic matter, and by affecting rates of ecological processes.  Recent research suggests soil-litter mixing (SLM) is an important driver of leaf litter decomposition rates in drylands, where low vegetative cover contributes to high rates of aeolian and fluvial soil movement and leads to strong spatial redistribution and mixing of soil and litter. Presently, decomposition models do not consider these dynamics; and this is one potential reason why these models typically under-predict rates of decomposition in drylands. Mechanistically, SLM may promote microbial colonization while buffering litter microclimate to lengthen the temporal window of opportunity for microbe-mediated decomposition. We present results of comparative litterbag experiments conducted in the Chihuahuan and Sonoran Deserts aimed at exploring SLM as a driver of decomposition under different vegetation cover treatments and soil texture manipulations.  Soil film and soil aggregate development were quantified in situ in relation vegetation surface cover and were observed with scanning electron microscopes (SEM) to detail their abiotic and biotic characteristics.

Results/Conclusions After one year of field exposure, SLM was the strongest driver of mass loss in the Chihuahuan Desert (R²=0.32, P< 0.05), whereas in the Sonoran, vegetation patch type (F=3.03; P<0.05) and SLM (F=53.79; P<0.001) strong predictors of mass loss.  In the Sonoran Desert, vegetation patches appeared to control eolian and fluvial processes, which in turn affected rates of SLM, while in the Chihuahuan mixing appeared to occur at scales finer than those used in field manipulations of plant cover. SLM led to development of films of soil particles, fungal hyphae and extracellular mucilage on litter surfaces. In the Chihuahuan Desert, cover by soil films varied with vegetation patch type and time (P<0.05). Film cover peaked (34 - 55%) at 6 months exposure following the spring winds and summer monsoon rain, then subsequently decreased at 12 months (to 16 - 37%) following a six-month dry and cold period. In the Sonoran, soil film development was strongly influenced by surface soil texture, with litter on clayey soils developing more uniform and stable films more quickly than litter on sandy soils.  SEM images suggest SLM facilitated particle aggregation, which will have implications for soil development, for long-term dynamics of litter C pools, and for the stabilization of surface soils. As a result, SLM should be considered when modeling biogeochemical cycles in drylands. Trade-offs between enhanced microbial decomposition vs. reduced photodegradation with SLM will be discussed.