Soil-litter mixing promotes decomposition and soil aggregate formation on contrasting geomorphic surfaces in a shrub-invaded Sonoran Desert grassland

Aims: Shrub encroachment into grasslands alters organic carbon pools and patterns of decomposition. Here, we quantify the influence of plant life-form patches occuring on contrasting geomorphic substrates on (1) soil-litter mixing (SLM) and litter decomposition and 2) the development of soil aggregates and soil-microbial films on litter. Methods: A factorial experiment in a shrub-invaded arid grassland tested the hypotheses that SLM would promote decomposition, and that development of soil films and aggregates would be inversely related to grass cover and positively related to soil surface clay content. Litterbags containing shrub (Prosopis velutina) leaf litter were deployed on a sandy Holocene-age soil and on a clayey Pleistocene-age soil under and away from shrubs on areas with low and high grass cover. Results: SLM accelerated decomposition, with highest rates on sandy soils in inter-shrub areas with low grass cover. Decomposition on clayey soils was slower, despite more rapid development of soil films. Soil-microbial film formation in bare ground placements exceeded that in shrub or grass placements and was greater on sandy Holocene-age soils compared to clayey Pleistocene-age soils. Conclusions: Vegetation structure and geomorphology should be considered when modeling decomposition dynamics in systems with low plant cover and high rates of aeolian and fluvial soil movement.