Grassland and savanna systems are one of the largest reservoirs of terrestrial carbon on earth (storing 50% more than forests1 and 15-30% of global reserves). Despite the recent evidence suggesting that in some habitats grasslands may play a larger role than forests in future sequestration of stable soil carbon, the management and restoration of savannas and grasslands for purposes of carbon sequestration has been less emphasized than forest systems (e.g., under-emphasized in the 2021 UN Climate Change Conference – COP26).
Although it is clear grassland systems are strongly regulated by two key disturbances, herbivory (by megaherbivores) and fire, we know much less about how they interact to influence system response, including soil carbon accrual. We propose that recent paradigm shifts in soil ecology are likely helpful in explaining how single vs multiple disturbance patterns influence soil carbon accrual, namely the importance of: a) deep carbon accrual beyond topsoil (i.e. > 30cm), b) root growth and turnover, and c) the fundamental importance of microbial biomass which capture and store carbon in their bodies (in form of necromass).
This collaborative project – using three long-term studies (on two continents) – seeks to understand how fire and grazing interact to affect soil carbon. The Kenya Long-term Exclosure Experiment (KLEE) serves as one of these sites, where we have been implementing controlled burns within different herbivore exclosures since 2013. A subset of plots is burned annually and a second set is burned every 3 years. We have been measuring how herbivores and fire interact to affect herbaceous plant composition, tree growth and mortality, microbial community composition, root growth, and soil carbon. We are using minirhizotrons (root viewing tubes) to describe belowground root processes for trees and grasses and using metagenomics to describe the microbial composition in soils. Our goal is to integrate these data to better understand soil carbon dynamics. Our preliminary results indicate interesting interactions between how fire and herbivory influence the percent carbon in the soil. Fire seems to increase carbon in sites from which herbivores are excluded, but decreases carbon in sites that are grazed. These types of ecological surprises suggest that additional attention should address the specific mechanisms that underlie these interactions.
This work arises out of the long-term collaborative partnerships between the following individuals and institutions. Ryan Sensenig (University of Notre Dame), Duncan Kimuyu (Karatina University), Mary Ngugi (PhD student, University of Nairobi), John Mischler (Goshen College), and Truman Young (UC Davis). None of it would be possible without the outstanding contributions of the MRC field assistants and supporting staff, including the amazing burn team that gathers each March to implement the burns!