Worlds apart: Location above- or below-ground determines plant litter decomposition in a semi-arid Patagonian steppe

While considerable attention has been devoted to how precipitation modulates net primary productivity in arid and semi-arid ecosystems, the emergence of multi-faceted controls on carbon (C) turnover suggests that there is much to be understood with respect to the mechanistic controls on plant litter decomposition. In the Patagonian steppe, we conducted a long-term factorial experiment, evaluating the importance of position, litter quality, tissue origin and soil resources on rates of C turnover under natural field conditions. Leaf and root litter of dominant grass species were placed in litterbags in different positions, on the soil surface and buried at 5-cm depth, with soil treatments of labile C, nitrogen (N) and their combination (C + N) over a 3-year period. As predicted, leaf litter decomposed significantly (nearly sixfold) faster above-ground than did root litter below-ground (p < 0.001). Surprisingly, root litter decomposed significantly faster than leaf litter above-ground (p < 0.001), and above-ground decomposition was not strongly affected by soil resource additions. Below-ground decomposition was largely determined by the interaction of litter quality and soil resource availability. Determining a C balance by integrating biomass allocation and primary productivity from this field site, combined with the data from this study, suggests large differences between the contribution of the above- and below-ground biomass to soil organic matter (SOM) pools and a long residence time of undecomposed root litter. Synthesis. Litter position clearly emerged as the predominant variable determining C turnover in this semi-arid steppe ecosystem, with litter quality and soil resources having significant, but more modest, effects. The near complete independence of above-ground litter decomposition from soil resources and rapid decomposition of surface litter, coupled with the counterintuitive relationships with litter quality, suggests that, in the long term, C loss from photodegradation may result in a minimal contribution of above-ground litter to SOM formation. These results have mechanistic implications for the distinct functionality of litter decomposition above- and below-ground in semi-arid ecosystems, and how these differential controls may alter the C balance due to future changes in climate and land use.