Interactive effects of multiple global change factors on soil nitrogen transformations in a California annual grassland

Background/Question/Methods

Anthropogenic disturbances such as elevated nitrogen (N) deposition, land-use change, and invasion by exotic plants have substantial impacts on terrestrial ecosystems. Specifically, many studies have concluded that each of these global change factors individually has the ability to alter microbial activity and soil N dynamics in grasslands, which can have important consequences for plant community composition, water quality, and greenhouse gas emissions. However, despite the fact that these disturbances often co-occur, very few studies have investigated their interactive effects on the aboveground and belowground components of an ecosystem. We examined the response of microbial communities and soil N cycling to the interaction of three factors that currently affect California grasslands: N deposition, grazing (simulated by clipping), and changes in vegetation . The vegetation treatments include exotic forage annuals (e.g. Avena fauta, Bromus hordeaceus), invasive plants (Aegilops triuncialis, Taeniatherum caput-medusae), native species (e.g. Bromus carinatus, Elymus glaucus, Leymus triticoides). The study is located in experimentally established grasslands near Davis, CA. The experiment is a full factorial arranged in a randomized complete block experimental design. Samples were collected once every 12 weeks, starting in October 2011, to determine seasonal and treatment effects.

Results/Conclusions

We found that soils associated with invasive plants had significantly higher microbial biomass, lower nitrification potential rates, and lower denitrification potential rates than native soils. When spring grazing was simulated in mixtures of invasive species, exotic forage annuals, and native plants, denitrification potentials were significantly increased. However, this shift in denitrification potential rates did not occur when spring grazing was combined with elevated N deposition. In soils associated with mixtures of invasive and native species, the soil microbial biomass increased only when spring grazing co-occurred with elevated N deposition. These results suggest that the effects of N deposition, grazing, and changes in vegetation on soil microorganisms and N dynamics are non-additive; hence, prediction of grassland functional responses to multiple global change factors cannot be accurately made from their observed responses in single factor studies.