Understanding Effects of Grazers on Soil Organic Carbon (SOC)
Another aspect of my PhD work was looking at how large grazers affect soil carbon storage. Soils of grasslands represent a large potential reservoir for storing CO2, and thus may contribute to mitigating climate change, but this potential likely depends on how grasslands are managed for large mammal grazing. Many studies have measured how grazing affects soil organic carbon (SOC) but effects have been found to be both strongly negative and strongly positive. To synthesize these data and draw more general conclusions regarding grazer effects on SOC, Mark Ritchie and I conducted a literature review and meta-analysis of studies from across the globe that measured grazer effects on SOC.
This work expanded on previous reviews to test the hypotheses that, after controlling for study duration and sampling depth, grazer effects would shift from negative to positive with decreasing precipitation, increasing fineness of soil texture, a transition from dominant grass species with C3 to C4 photosynthesis, and decreasing grazing intensity. We found that the six variables of soil texture, precipitation, grass type, grazing intensity, study duration, and sampling depth together explained 85% of a large variation (±150 g m−2 yr−1) in grazing effects, and our best model included significant interactions between precipitation and soil texture (p = 0.002), grass type, and grazing intensity (p = 0.012), and study duration and soil sampling depth (p = 0.020).
We found that specifically, an increase in mean annual precipitation of 600 mm resulted in a 24% decrease in grazer effect size on finer textured soils, while on sandy soils the same increase in precipitation produced a 22% increase in grazer effect on SOC. Increasing grazing intensity increased SOC by 6–7% on C4-dominated and C4–C3 mixed grasslands, but decreased SOC by an average 18% in C3-dominated grasslands. We discovered these patterns despite a lack of studies in natural, wildlife-dominated ecosystems, and tropical grasslands. Our results show that grazer effects on SOC are highly context-specific and suggest that grazers in different regions might need to be managed differently in order to help mitigate greenhouse gas emissions.
The below figures highlight two of the most significant results from our review.
This work expanded on previous reviews to test the hypotheses that, after controlling for study duration and sampling depth, grazer effects would shift from negative to positive with decreasing precipitation, increasing fineness of soil texture, a transition from dominant grass species with C3 to C4 photosynthesis, and decreasing grazing intensity. We found that the six variables of soil texture, precipitation, grass type, grazing intensity, study duration, and sampling depth together explained 85% of a large variation (±150 g m−2 yr−1) in grazing effects, and our best model included significant interactions between precipitation and soil texture (p = 0.002), grass type, and grazing intensity (p = 0.012), and study duration and soil sampling depth (p = 0.020).
We found that specifically, an increase in mean annual precipitation of 600 mm resulted in a 24% decrease in grazer effect size on finer textured soils, while on sandy soils the same increase in precipitation produced a 22% increase in grazer effect on SOC. Increasing grazing intensity increased SOC by 6–7% on C4-dominated and C4–C3 mixed grasslands, but decreased SOC by an average 18% in C3-dominated grasslands. We discovered these patterns despite a lack of studies in natural, wildlife-dominated ecosystems, and tropical grasslands. Our results show that grazer effects on SOC are highly context-specific and suggest that grazers in different regions might need to be managed differently in order to help mitigate greenhouse gas emissions.
The below figures highlight two of the most significant results from our review.
This work with Mark Ritchie was published in Global Change Biology and can be found here: Effects of grazing on grassland soil carbon: A global review.