PSXIII-6 Weed encroachment reduces soil organic carbon stock in bermudagrass pastures

Weed encroachment might reduce stocking rate and overall animal performance in grazing systems. In addition, weeds can affect soil organic carbon because of changes in primary productivity and belowground biomass compared with well managed pastures. This study assessed different levels of spiny pigweed (Amaranthus spinosus L.) encroachment on bermudagrass [Cynodon dactylon (L.) Pers.] pastures. Spiny pigweed was drilled onto bermudagrass pastures to create three levels of encroachment: weed free, medium encroachment, and high encroachment. The weed-free pastures were treated with herbicide to control weed encroachment, and the medium had strips where weeds were controlled and strips uncontrolled. The high encroachment had no weed control. Treatments (weed free, medium, and high) were allocated in a randomized complete block design. The spiny pigweed was drilled in 2020 and grazing occurred from 2021 to 2023. Soil samples were collected in October 2023 to assess soil organic carbon (C) stock by determining soil bulk density and soil C concentration in the 0- to 5- and 5- to 15-cm layers. Results indicated that weed-free and medium-encroached pastures had greater soil organic carbon stock (20.2 Mg C/ha) compared with the weed-infested pastures (17.2 Mg C/ha). Soil C concentration was similar, but soil bulk density was lower in the weed-infested pastures, affecting soil carbon stocks.