Rethinking Plant Litter Decomposition Across Global Drylands

The carbon (C) balance in terrestrial ecosystems is primarily determined by inputs from net primary productivity and C outputs to the atmosphere through organic matter. Understanding the global C balance is crucial for predicting their current and future roles of terrestrial ecosystems as C sources or sinks in the context of global change. Drylands, covering nearly 45% of Earth's land surface, contribute significantly to net primary production (NPP) and influence the interannual variability of the terrestrial C sink. However, the controls on plant litter decomposition, a major pathway of C release, remain unclear in these ecosystems. Here, we present a global analysis of plant litter decomposition in drylands, using a dataset from 116 sites across six continents spanning diverse climates and ecosystems. We found that litter decomposition did not correlate with mean annual precipitation (MAP) or aridity at the global scale, challenging the paradigm that the quantity of rainfall received annually is the primary constraint on ecological processes in drylands. Instead, our analysis identifies mean annual temperature (MAT), precipitation-temperature synchrony, precipitation variability, and cloud-cover frequency as key drivers. Specifically, our model predicted faster decomposition rates in drylands with higher MAT, more synchrony between wet and warm seasons (monsoonal climate), larger precipitation variability, and litter with higher N content. Across drylands, decomposition correlated positively with both nitrogen and lignin content, in contrast to the negative lignin-decomposition relationship commonly observed in mesic ecosystems. Because MAT and solar irradiance strongly covary, apparent temperature effects may in part reflect radiation-driven processes, highlighting photodegradation as an important control, particularly in very arid zones. Given the ongoing expansion of drylands, rising temperatures and changes in precipitation variability under climate change, our results underscore the need to refine decomposition models beyond traditional aridity frameworks to accurately predict dryland contributions to the global C balance.