Despite the importance of soil carbon cycling in the response of water-limited ecosystems to global change processes, our understanding of this ecosystem component is still in development. Compounding this uncertainty, ecosystems may experience multiple environmental and structural shifts as several global change processes occur simultaneously. For example, as land cover changes with the encroachment of woody-plants into semi-arid grasslands, the changing proportion of herbaceous to shrub cover affects the distribution of nutrients and soil microclimates. At the same time, these systems experience changes in climate that may bring alterations in the timing and quantity of precipitation inputs. Using a series of sites that represent a space for time substitution of the changing distributions of shrubs and grasses, we are investigating how woody-plant encroachment alters ecosystem structure and the dynamics of carbon and water in riparian and upland systems in the San Pedro River Basin, AZ. Discontinuities between rates of evapotranspiration and net ecosystem exchange at the intermediate shrubland stage suggest that heterogeneity in plant cover alters water and carbon balances during the encroachment dynamics of these systems. Here we test the hypothesis that woody plant cover affects soil responsiveness to temperature and moisture through an alteration of soil properties. We compared potential C-mineralization rates that were assayed in short-term incubations of soils from grassland, shrubland, and woodland sites.

Results/Conclusions

Reflecting soil physical and chemical properties among the sites, the riparian shrubland had intermediate rates of potential C-mineralization relative to the riparian grassland and mature woodland. Soils at the shrubland site had the least responsiveness to changes in both soil temperature and soil moisture, while the grassland site showed the most responsiveness to soil temperature and moisture. Patterns of potential C-mineralization at the upland sites were complicated by physiographic site characteristics, but showed similar trends in responsiveness to plant cover types. Our results suggest that during the transition from grasslands to more mature woodlands, soil C dynamics show a non-linear response where the sensitivity of C-mineralization to soil temperature and moisture initially decreases, followed by some recovery of this sensitivity as the shrubs mature. Better understanding of such non-linear dynamics of soil processes will improve predictions of ecosystem responses and potential feedbacks to interacting global change processes, such as climate change, land cover change, and alterations in biodiversity.