Dryland responses to climate change: Assessing the biogeochemical consequences of Syntrichea caninervis mortality resulting from altered precipitation regimes

Background/Question/Methods

Biological soil crusts-a community of mosses, lichens, cyanobacteria, and heterotrophs living at the soil surface of many drylands-are a fundamental component of arid and semiarid ecosystems. These photosynthetic soil communities play critical roles in dryand function; for example, carbon fixation, nitrogen fixation, and soil stabilization – and existing data suggest biocrusts can be quite sensitive to seemingly subtle changes in climate. In particular, previous research on the Colorado Plateau showed dramatic mortality of the common moss Syntrichea caninervis in response to altered precipitation treatments: Increased frequency of 1.2mm monsoonal rainfall events reduced moss cover from >25% to <2% after only one growing season. Yet our understanding of the ecosystem consequences of these large changes to the system remain notably poor. Here we explore how the moss mortality affects belowground biogeochemistry over the course of the lethal stress. Twice weekly for 5 months we added 1.2mm of simulated rainfall to S. caninervis, as well as maintained control mosses. Throughout the experiment, we assessed the soils beneath the moss for multiple forms of carbon, nitrogen and phosphorus; nitrogen mineralization rates; and aspects of moss photosynthetic capacity (F_v/F_m) to explore how belowground biogeochemistry is affected over the course of the mortality event.

Results/Conclusions

As expected, mosses were strongly, negatively affected by the increased frequency of small rainfall events. In concert with declining moss health, we found significant changes to soil biogeochemical cycling. For instance, during the first week of treatments we observed an increase in extractable NH₄⁺ from soils associated with the stressed moss compared with the controls, however, this pattern switched such that soil extractable NH₄⁺ for stressed moss was much lower than controls as moss decline progressed. In contrast, extractable NO₃^- remained elevated in the treated moss relative to controls until the last sampling event when no significant difference was observed. These patterns match well with assessments of nitrification rates, which showed nitrification was consistently elevated in soils beneath stressed moss relative to controls. In addition, the moss physiological decline was associated with a reduction in total available nitrogen and with changes in soil carbon chemistry. Taken together, our data suggest the stress mosses experience in response to altered precipitation results in significant changes to soil biogeochemical cycling. Due to the nature of these shifts, the data have important implications for soil fertility, as well as for the trajectory of biocrust recovery after the loss of a dominant community member.