Effects of agricultural land management and livestock grazing intensification on ecosystem carbon and nutrient stocks

Agricultural land management intensification and livestock grazing can significantly affect ecosystem biogeochemical cycling, yet their interactive effects on carbon (C), nitrogen (N), and phosphorus (P) stocks and stoichiometry across different ecosystem pools remain poorly understood, especially in (sub)tropical systems. Using a nested experimental design in humid subtropical grasslands in south-central Florida, we investigated how agricultural land management intensity (intensively managed or IM vs. semi-natural or SN pastures) and cattle grazing intensity (high vs. low) affect C, N, and P stocks and their stoichiometric ratios in aboveground plant biomass and two soil layers (surface 0-5 cm and subsurface 5-15 cm). Our results demonstrated that agricultural management intensification increased C, N, and P stocks in aboveground biomass and subsurface soil, while high-intensity grazing reduced nutrient stocks in vegetation. Significant interactive effects between management and grazing intensities showed in stoichiometric ratios: high-intensity grazing decreased C:P and N:P ratios in aboveground biomass, and C:N ratio in subsurface soil in SN pastures but increased these ratios in IM pastures. The sensitivity of C and nutrient stocks to edaphic properties varied with soil depth and management intensity, with C and nutrient stocks in plant biomass and subsurface soil showing higher sensitivity to soil properties in IM pastures, while surface soil stocks were more sensitive in SN pastures. Our findings emphasize the need for management strategies that consider both soil depth and site-specific conditions when determining appropriate grazing stocking rates and land management practices to maintain ecosystem nutrient stocks and ensure sustainable grassland management.