We compared the ability of several models to describe decomposition patterns in arid sites from the Long-term Intersite Decomposition Team data set, where litter mass loss followed an atypical linear pattern, suggesting a novel driver of decomposition. We hypothesized that this pattern reflected the contribution of abiotic photodegradation. Research suggests that exposure to shortwave (including ultraviolet, UV) radiation increases mass loss from cellulosic and/or lignin pools, but may slow biotic decomposition. Specifically, photodegradation may increase mass loss from non-labile pools through abiotic carbon (C) mineralization, but decrease net mass loss from labile pools by either (1) decreasing decomposition rates via adverse effects of UV on decomposers, or (2) adding C to labile C pools from photochemical breakdown of non-labile C. We developed a set of models to test these hypotheses and compared them using likelihood methods. Models were based on a biotic decomposition model (Adair et al., 2008), which served as a “null” hypothesis of biotic decomposition only. This model consists of three C pools (labile, cellulosic, and lignin) that decompose exponentially at rates modified by climate and litter chemistry. Based on laboratory experiments, we added photodedegradation fluxes to this model as zero order flows from cellulosic or lignin pools.  

Results/Conclusions

The model that best described decomposition in these arid sites (1) increased mass loss from the cellulosic pool, (2) added approximately 50% of the mass lost from this pool to the labile litter pool, and (3) reduced the decomposition rate of the labile pool. This model explained 81% of the variation in litter mass loss data from these sites. In contrast to the best model, the biotic null hypothesis model explained 71% of the variation in the data and did not capture the linear pattern of decomposition at these sites. These results suggest that mass loss of non-labile C at these arid sites was increased by direct photodegradation of cellulosic pools, but that net mass loss from labile litter was decreased by a combination of negative impacts on decomposers (reducing the decomposition rate of labile C) and the addition of a flow of C into the labile pool from the photodegradation of non-labile litter C.