Standing dead and litter position determine patterns of decomposition in a semiarid shrub steppe of Patagonia, Argentina

AUSTIN, A. T.; GONZALEZ-POLO, M.; VIVANCO, L.; SALA, O. E.

Montréal, Canadá

Congreso; Annual meeting of the Ecological Society of America; 2005

Ecological Society of America

The patchy distribution of vegetation in arid and semiarid ecosystemsresults in a mosaic of microsites of biotic and abiotic conditions, including the well documented´islands of fertility´ and low nutrient conditions in exposed bare soil areas.In addition, an important part of the dynamics of the vegetation patches in the temperatePatagonian steppe is the large fraction of aboveground biomass that persists as standingdead material. We conducted an experiment to examine the effect of litter position onpatterns of mass loss, and the effect of litter quality of standing dead material usinglitter types of different age classes and functional groups of vegetation. We decomposedrecently senesced (yellow) and standing dead (gray) of shrub (Mulinum spinosum) andgrass (Stipa speciosa) litter in nine different microsites: suspended in the air onwindward and leeward sides of a shrub, beneath the shrub, in the shrub-grass ring,suspended in grass tussock, next to tussock grasses, in open soil where the shrub hadbeen removed, in bare soil, and buried. Position demonstrated a highly significant effecton mass loss after one year for all litter types (P<0.0001), but surprisingly, the fastestdecomposition occurred in litter that was suspended in aerial positions or buried (k=0.25and 0.32 year-1, respectively), intermediate values for mass loss of litter in bare soil andin shrub removal patches (k=0.21 and 0.24 year-1, respectively), and markedly slowestdecomposition occurring under shrub patches (k = 0.018 year-1). This evidence supportsthe idea that abiotic variables, including physical degradation by wind and directphotodegradation, are important in determining rates of decomposition in thisecosystem. The predominance of abiotic controls on decomposition and the largefraction of standing dead material susceptible to abiotic decomposition could result in asmall fraction of aboveground biomass entering the soil as organic matter, restrictingthe carbon sequestration capacity of this semiarid ecosystem.