Frequent defoliation of perennial legume-grass bicultures alters soil carbon dynamics

Purpose Perennial forage systems (e.g., pastures and hayfields) cover a significant land area worldwide and are important for soil carbon storage; however, the degree to which these plant communities can be managed to accumulate and store soil carbon is not well understood. Additionally, less is known about how forage defoliation mediates C dynamics or if the effects of defoliation depend on plant community composition. Methods To address these questions, we quantified soil C pools and C-degrading enzyme activity in a three-year experiment where bicultures of alfalfa-orchardgrass and red clover-orchardgrass were managed under treatments of contrasting defoliation frequency (3 vs. 5 cuts per year) and severity (i.e., cutting height; 10 cm vs. 5 cm residual forage height). Results We found that more frequent defoliation resulted in greater permanganate oxidizable C (POX-C), particulate organic carbon (POC), mineral-associated organic C (MAOC), and the activity of β-glucosidase (BG) and cellobiohydrolase (CBH). The effects of defoliation frequency on total organic C (TOC) varied depending on biculture composition. Alfalfa-orchardgrass bicultures, characterized by a lower legume-grass proportion, had greater POX-C and lower hot-water extractable organic carbon (HWEOC) compared to red clover-orchardgrass which had a higher legume-grass proportion. We also found that BG activity and POX-C were positively correlated with MAOC and TOC. In contrast, we observed no effects of defoliation severity on any measured soil parameters. Conclusion Our study highlights that defoliation frequency and relative abundance of legumes to grass are both potential management levers for increasing SOC storage in legume-grass agroecosystems.