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

AUSTIN, AMY T., MARINA GONZALEZ-POLO, LUCÍA VIVANCO Y OSVALDO E. SALA

Montréal, Canada

Congreso; Ecological Society of America 90th Annual Meeting; 2005

Ecological Society of America

The patchy distribution of vegetation in arid and semiarid ecosystems results 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 temperate Patagonian steppe is the large fraction of aboveground biomass that persists as standing dead material. We conducted an experiment to examine the effect of litter position on patterns of mass loss, and the effect of litter quality of standing dead material using litter types of different age classes and functional groups of vegetation. We decomposed recently senesced (yellow) and standing dead (gray) of shrub (Mulinum spinosum) and grass ((Stipa speciosa) litter in nine different microsites: suspended in the air on windward 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 had been removed, in bare soil, and buried. Position demonstrated a highly significant effect on mass loss after one year for all litter types (P<0.0001), but surprisingly, the fastest decomposition occurred in litter that was suspended in aerial positions or buried (k=0.25 and 0.32 year-1, respectively), intermediate values for mass loss of litter in bare soil and in shrub removal patches (k=0.21 and 0.24 year-1, respectively), and markedly slowest decomposition occurring under shrub patches (k = 0.018 year-1). This evidence supports the idea that abiotic variables, including physical degradation by wind and direct photodegradation, are important in determining rates of decomposition in this ecosystem. The predominance of abiotic controls on decomposition and the large fraction of standing dead material susceptible to abiotic decomposition could result in a small fraction of aboveground biomass entering the soil as organic matter, restricting the carbon sequestration capacity of this semiarid ecosystem.