Climate change and biological invasion are among the most significant drivers of environmental change that alter plant assemblage and biodiversity, and potentially lead to major shifts in soil microbial communities and ecosystem functions. There is an extensive literature on ecological effects of individual stressors (climate or invasion). However, interactive effects of multiple stressors (e.g., antagonistic or synergistic) occurring simultaneously have not been well understood, and most prior efforts alluded to effects on native plant communities. In this study, we conducted a long-term experiment to manipulate (1) invasion of Imperata cylindrica (cogongrass) – a globally problematic C4 grass that threats the survival and growth of native plants, and (2) chronic drought (projected to increase in frequency and intensity in the Southeast U.S. over the coming decades) simulated by rainout shelters in a longleaf pine forest. We ask, how drought and grass invasion interact to affect soil carbon and nitrogen pools and dynamics, and how such effects vary by soil depths? Soil samples (at two depths, 0-5 and 5-15 cm) were collected after 4 four years of experiments to measure total and inorganic C and N pools, and laboratory and in situ methods were used to quantify C and N mineralization rates.
Results/Conclusions
Our preliminary results showed that there is no significant effects between grass invasion and drought on total soil C and N pools across both depths (i.e., 0-5 and 5-15 cm) of soils, after four years of experiments. However, we found significant interactive effects (P<0.01) on soil inorganic N pools (including nitrate and ammonia), and effects are stronger for nitrate at surface soils. Specifically, at 0-5 cm soils, under drought treatment, soil nitrate concentration was ~2.7x lower in invaded plots (compared to no invasion), whereas ~4.1x higher in invaded plots under ambient conditions. Such results are perhaps partly due to stronger competition for available nutrients from invasive grasses under drought with low soil moisture that reduces available nutrients. In contrast, with no-drought condition, high productivity and associated high litter inputs from cogongrass invasions might lead to higher nutrient inputs for decomposition and mineralization. Further results and analyses on the mineralization rates of C and N will provide additional insights on how C and N processes and transformation are susceptible to drought and cogongrass invasion. Our results demonstrate that grass invasion could interact with altered climate to affect ecosystem functions, and could help predict ecological consequences of multiple, future global environmental changes.