Global Change Could Amplify Fire Effects on Soil Greenhouse Gas Emissions

Background: Little is known about the combined impacts of global environmental changes and ecological disturbances on ecosystem functioning, even though such combined impacts might play critical roles in shaping ecosystem processes that can in turn feed back to climate change, such as soil emissions of greenhouse gases.[br/] Methodology/Principal Findings: We took advantage of an accidental, low-severity wildfire that burned part of a long-term global change experiment to investigate the interactive effects of a fire disturbance and increases in CO(2) concentration, precipitation and nitrogen supply on soil nitrous oxide (N(2)O) emissions in a grassland ecosystem. We examined the responses of soil N(2)O emissions, as well as the responses of the two main microbial processes contributing to soil N(2)O production - nitrification and denitrification - and of their main drivers. We show that the fire disturbance greatly increased soil N(2)O emissions over a three-year period, and that elevated CO(2) and enhanced nitrogen supply amplified fire effects on soil N(2)O emissions: emissions increased by a factor of two with fire alone and by a factor of six under the combined influence of fire, elevated CO(2) and nitrogen. We also provide evidence that this response was caused by increased microbial denitrification, resulting from increased soil moisture and soil carbon and nitrogen availability in the burned and fertilized plots. [br/] Conclusions/Significance: Our results indicate that the combined effects of fire and global environmental changes can exceed their effects in isolation, thereby creating unexpected feedbacks to soil greenhouse gas emissions. These findings highlight the need to further explore the impacts of ecological disturbances on ecosystem functioning in the context of global change if we wish to be able to model future soil greenhouse gas emissions with greater confidence

Rhizosphere priming effects on soil carbon and nitrogen mineralization

Living roots and their rhizodeposits affect microbial activity and soil carbon (C) and nitrogen (N) mineralization. This so-called rhizosphere priming effect (RPE) has been increasingly recognized recently. However, the magnitude of the RPE and its driving mechanisms remain elusive. Here we investigated the RPE of two plant species (soybean and sunflower) grown in two soil types (a farm or a prairie soil) and sampled at two phenological stages (vegetative and mature stages) over an 88-day period in a greenhouse experiment. We measured soil C mineralization using a continuous 13C-labeling method, and quantified gross N mineralization with a 15N-pool dilution technique. We found that living roots significantly enhanced soil C mineralization, by 27-245%. This positive RPE on soil C mineralization did not vary between the two soils or the two phenological stages, but was significantly greater in sunflower compared to soybean. The magnitude of the RPE was positively correlated with rhizosphere respiration rate across all treatments, suggesting the variation of RPE among treatments was likely caused by variations in root activity and rhizodeposit quantity. Moreover, living roots stimulated gross N mineralization rate by 36-62% in five treatments, while they had no significant impact in the other three treatments. We also quantified soil microbial biomass and extracellular enzyme activity when plants were at the vegetative stage. Generally, living roots increased microbial biomass carbon by 0-28%, β-glucosidase activity by 19-56%, and oxidative enzyme activity by 0-46%. These results are consistent with the positive rhizosphere effect on soil C (45-79%) and N (10-52%) mineralization measured at the same period. We also found significant positive relationships between β-glucosidase activity and soil C mineralization rates and between oxidative enzyme activity and gross N mineralization rates across treatments. These relationships provide clear evidence for the microbial activation hypothesis of RPE. Our results demonstrate that root-soil-microbial interactions can stimulate soil C and N mineralization through rhizosphere effects. The relationships between the RPE and rhizosphere respiration rate and soil enzyme activity can be used for explicit representations of RPE in soil organic matter models. © 2014

Linkages between grazing history and herbivore exclusion on decomposition rates in mineral soils of subalpine grasslands

Background & aims Herbivore-driven changes to soil properties can influence the decomposition rate of organic material and therefore soil carbon cycling within grassland ecosystems. We investigated how aboveground foraging mammalian and invertebrate herbivores affect mineral soil decomposition rates and associated soil properties in two subalpine vegetation types (short-grass and tall-grass) with different grazing histories. Methods Using exclosures with differing mesh sizes, we progressively excluded large, medium and small mammals and invertebrates from the two vegetation types in the Swiss National Park (SNP). Mineral soil decomposition rates were assessed using the cotton cloth (standard substrate) method between May and September 2010. Results Decomposition displayed strong spatio-temporal variability, best explained by soil temperature. Exclusion of large mammals increased decomposition rates, but further exclusion reduced decomposition rates again in the lightly grazed (tall-grass) vegetation. No difference among treatments was found in the heavily grazed (short-grass) vegetation. Heavily grazed areas had higher decomposition rates than the lightly grazed areas because of higher soil temperatures. Microbial biomass carbon and soil C:N ratio were also linked to spatio-temporal decomposition patterns, but not to grazing history. Conclusions Despite altering some of the environmental controls of decomposition, cellulose decomposition rates in the SNP’s subalpine grasslands appear to be mostly resistant to short-term herbivore exclusion.ISSN:0032-079XISSN:1573-503