Soil organic carbon in dry miombo landscapes decreases with higher grazing intensity, but trees can counteract the effect

Abstract Soil organic carbon is one of the key determinants of soil quality and productivity, contributing to food production and mitigation of climate change. Vegetation, particularly trees, is essential in maintaining and enhancing soil organic carbon. However, there is a lack of knowledge on the ability of trees to enhance soil organic carbon in the presence of high-intensity livestock grazing. Here, we conducted a study in Morogoro Rural District, Tanzania, where we established a 10 × 10 km study site covering part of the Kitulangalo forest reserve and surrounding areas. We identified four main land uses and land cover types: forest reserve, open access forest, cropland under fallow, and cropland under cultivation. We assessed soil organic carbon stocks, livestock grazing intensity, and tree basal area in 149 plots. We also tested the effect of total grazing exclusion in two 12-years-old fenced plots located within the forest reserve. Topsoil organic carbon stocks were higher in land use classes with higher tree cover; At low grazing intensity, the forest reserve had the highest mean topsoil organic carbon (51 ± 8 tones ha −1 ) and that this decreased with decreasing tree cover across land uses, with croplands under cultivation having the lowest value (32 ± 12 tones ha −1 ). We found that soil organic carbon declined with increased grazing intensity, but this decrease was higher in croplands, with a 64% decrease to 12 ± 8 tones ha −1 when comparing the lowest and highest grazing intensities. In contrast, the decrease in forested lands was 18% to 42 ± 17 tones ha −1 . We conclude that increasing livestock grazing intensity has a negative impact on soil organic carbon, particularly in land uses with a low tree cover, and that more trees in the landscape have the potential to counteract the adverse effects of livestock grazing.