The role of soil organisms controlling aboveground litter decomposition in the semiarid Patagonian steppe, Argentina

Litter decomposition in low rainfall ecosystems often demonstrates higher rates of mass loss than would be predicted based on climatic parameters, and little correlation with mean annual rainfall and standard indices of litter quality.  As a result, several alternatives have been suggested regarding important controls on litter mass loss in arid and semiarid ecosystems independent of the direct effects of precipitation.  Recent studies have shown that photodegradation of aboveground plant material might account for a fraction of aboveground carbon loss in semiarid ecosystems, but soil fauna, particularly macrofauna such as termites and ants, have been mentioned as key players affecting dynamics of mass loss and nutrient release in deserts.  Our objective in this study was to quantify the relative importance of macro-, meso-, microfauna and microflora on litter decomposition in a semiarid Patagonian steppe. We conducted a manipulative experiment of soil faunal exclusion in the field using litterboxes with litter of a highly palatable grass species, Bromus pictus. The litterboxes had variable mesh openings that excluded different groups of soil organisms based on size class (10mm, 2mm, and 0.01mm), and were placed beneath shrub canopies of Adesmia campestris.  We found that the exclusion of the macro- and mesofauna had no effect on litter mass loss, with similar rates in the three types of mesh sizes (P = 0.486).  On the other hand, the reduction of fungal activity through the application of a fungicide significantly reduced both fungal colonization and litter decomposition (P = 0.002).  These results suggest that in contrast to studies from hot deserts, soil macrofauna do not appear to play a disproportionate role in aboveground litter breakdown in this semiarid system, but fungal decomposition may predominate in the biotic fraction of litter decomposition.