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Vertebrate herbivore excrement is thought to influence nutrient cycling, plant nutrition, and growth; however, its importance is rarely isolated from other aspects of herbivory, such as trampling and leaf removal, leaving questions about the extent to which herbivore effects are due to feces. We hypothesized that as a source of additional nutrients, feces would directly increase soil N concentrations and N2O emission, alleviate plant, and microbial nutrient limitations, resulting in increased plant growth and foliar quality, and increase CH4 emissions. We tested these hypotheses using a field experiment in coastal western Alaska,USA, where we manipulated goose feces such that naturally grazed areas received three treatments:feces removal, ambient amounts of feces, or double ambient amounts of feces. Doubling feces marginally increased NH4 +-N in soil water, whereas both doubled feces and feces removal significantly increased NO3--N; N2O flux was also higher in removal plots. Feces removal marginally reduced root biomass and significantly reduced productivity (that is, GPP) in the second year, measured as greater CO2 emissions. Doubling feces marginally increased foliar chemical quality by increasing %N and decreasing C:N. Treatments did not influence CH4 flux. In short, feces removal created sites poorer in nutrients, with reduced root growth, graminoid nutrient uptake,more »and productivity. While goose feces alone did not create dramatic changes in nutrient cycling in western Alaska, they do appear to be an important source of nutrients for grazed areas and to contribute to greenhouse gas exchange as their removal increased emissions of CO2 and N2O to the atmosphere.« less

Rapid warming in northern ecosystems over the past four decades has resulted in earlier spring, increased precipitation, and altered timing of plant–animal interactions, such as herbivory. Advanced spring phenology can lead to longer growing seasons and increased carbon (C) uptake. Greater precipitation coincides with greater cloud cover possibly suppressing photosynthesis. Timing of herbivory relative to spring phenology influences plant biomass. None of these changes are mutually exclusive and their interactions could lead to unexpected consequences for Arctic ecosystem function. We examined the influence of advanced spring phenology, cloud cover, and timing of grazing on C exchange in the Yukon–Kuskokwim Delta of western Alaska for three years. We combined advancement of the growing season using passive-warming open-top chambers (OTC) with controlled timing of goose grazing (early, typical, and late season) and removal of grazing. We also monitored natural variation in incident sunlight to examine the C exchange consequences of these interacting forcings. We monitored net ecosystem exchange of C (NEE) hourly using an autochamber system. Data were used to construct daily light curves for each experimental plot and sunlight data coupled with a clear-sky model was used to quantify daily and seasonal NEE over a range of incident sunlight conditions.more »Cloudy days resulted in the largest suppression of NEE, reducing C uptake by approximately 2 g C m⁻²d⁻¹regardless of the timing of the season or timing of grazing. Delaying grazing enhanced C uptake by approximately 3 g C m⁻²d⁻¹. Advancing spring phenology reduced C uptake by approximately 1.5 g C m⁻²d⁻¹, but only when plots were directly warmed by the OTCs; spring advancement did not have a long-term influence on NEE. Consequently, the two strongest drivers of NEE, cloud cover and grazing, can have opposing effects and thus future growing season NEE will depend on the magnitude of change in timing of grazing and incident sunlight.

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