1. While considerable attention has been devoted to how precipitation modulates net primary productivity in arid and semiarid 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. 

2. 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 three-year period.  

3. As predicted, leaf litter decomposed significantly (nearly six-fold) faster aboveground than did root litter belowground (P < 0.001). Surprisingly, root litter decomposed significantly faster than leaf litter aboveground (P < 0.001), and aboveground decomposition was not strongly affected by soil resource additions. Belowground 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, demonstrated large differences between the contribution of the above- and belowground biomass to soil organic matter (SOM) pools and a long residence time of undecomposed root litter.

4. Synthesis. Litter position clearly emerged as the predominant variable determining C turnover in this semiarid steppe ecosystem, with litter quality and soil resources having significant, but more modest, effects. The near complete independence of aboveground 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 aboveground litter to SOM formation. These results have mechanistic implications for the distinct functionality of litter decomposition above- and belowground in semiarid ecosystems, and how these differential controls may alter the C balance due to future changes in climate and land use.