Weed encroachment affects soil organic carbon stock in bermudagrass pastures

Abstract Weed encroachment indicates pasture degradation and is a major challenge for adequate pasture management. Additionally, it might compromise the soil organic carbon (SOC) stock, affecting global climate change. Common bermudagrass [ Cynodon dactylon (L.) Pers.] is a worldwide grown forage species, but broadleaf weed species often encroach during the warm season without appropriate weed management. Spiny pigweed ( Amaranthus spinosus L.) is one of the widely encountered weeds in pastures across the globe. We quantified the SOC in bermudagrass pastures encroached with different levels of spiny pigweed after 3 years. Different methods (fixed depth vs. equivalent soil mass [ESM] approaches) were evaluated for SOC estimates. The treatments were paddocks free of weeds (weed‐free), paddocks with weeds established in alternated strips (weed‐strips), and paddocks with weeds spread throughout the whole area (weed‐infested) in a randomized complete block design with four replicates. Bermudagrass herbage accumulation rate (HAR) and belowground responses were evaluated as explanatory variables. Spiny pigweed encroachment reduced the bermudagrass HAR and belowground biomass accumulation. The fixed depth method overestimated the SOC in the topsoil layer, while differences between ESM approaches were negligible. The SOC concentration was greater in weed‐free than weed‐infested areas. The cumulative SOC stock (0–15 cm) estimated on an ESM basis was 15% lower in weed‐infested pastures (33.6 Mg ha −1 ) compared to weed‐free or weed‐strips (39.5 Mg ha −1 ). Spiny pigweed encroachment in bermudagrass pastures significantly impacts the soil in the short term, leading to lower SOC stocks compared to weed‐free pastures.