Abstract
AbstractTraditional models of decomposition fail to capture litter mass loss patterns in dryland systems. This shortcoming has stimulated research into alternative drivers of decomposition, including photodegradation. Here, we use aboveground litter decomposition data for dryland (arid) sites from the Long‐term Intersite Decomposition Experiment Team data set to test hypotheses (models) about the mechanisms and impacts of photodegradation. Incorporating photodegradation into a traditional biotic decomposition model substantially improved model predictions for mass loss at these dryland sites, especially after four years. The best model accounted for the effects of solar radiation via photodegradation loss from the intermediate cellulosic and lignin pools and direct inhibition of microbial decomposition. Despite the concurrent impacts of photodegradation and inhibition on mass loss, the best photodegradation model increased mass loss by an average of 12% per year compared to the biotic‐only decomposition model. The best model also allowed soil infiltration into litterbags to reduce photodegradation and inhibition of microbial decomposition by shading litter from solar radiation. Our modeling results did not entirely support the popular hypothesis that initial lignin content increases the effects of photodegradation on litter mass loss; surprisingly, higher initial lignin content decreased the rate of cellulosic photodegradation. Importantly, our results suggest that mass loss rates due to photodegradation may be comparable to biotic decomposition rates: Mass loss due to photodegradation alone resulted in litter mass losses of 6–15% per year, while mass loss due to biotic decomposition ranged from 20% per year during early‐stage decomposition to 3% per year during late‐stage decomposition. Overall, failing to account for the impacts of solar radiation on litter mass loss under‐predicted long‐term litter mass loss by approximately 26%. Thus, not including photodegradation in dryland decomposition models likely results in large underestimations of carbon loss from dryland systems.
Highlights
Traditional decomposition models fail to accurately predict aboveground decomposition in arid systems, underestimating mass loss and overestimating nitrogen immobilization (Parton et al 2007, Adair et al 2008)
For short decomposition periods (i.e., < 4 yr), the biotic-only and best photodegradation models both captured the general pattern of aboveground litter decomposition, but observed data and the best photodegradation model predictions increasingly diverged from biotic-only model predictions as the decomposition period lengthened (Fig. 1)
We found that incorporating photodegradation into our decomposition model increased the accuracy of long-term litter mass loss predictions in dryland systems, especially after four years
Summary
Traditional decomposition models fail to accurately predict aboveground decomposition in arid systems, underestimating mass loss and overestimating nitrogen immobilization (Parton et al 2007, Adair et al 2008) This failure has stimulated research into novel drivers of decomposition. Austin and Ballare (2010) found that photodegradation did not occur in a pure cellulose (i.e., lignin-free) substrate, but only occurred when lignin was present and increased with lignin concentration These patterns were consistent with photodegradation of lignin (Austin and Ballare 2010), it has been suggested that lignin induces the breakdown of other compounds via indirect photolysis: The UV (280–400 nm) radiation absorbed by photoreactive lignin may produce free radicals that break bonds in other compounds, such as hemicellulose, in the lignocellulose matrix (Schade et al 1999, Brandt et al 2010, Baker and Allison 2015)
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