Grassland use intensity and climate as key drivers of soil organic carbon across four continents

Grasslands cover nearly 40% of the Earth’s terrestrial surface and store large quantities of carbon (C) in their soils. However, grassland-use intensification, unsustainable management practices and climate change threaten this important C reservoir. Understanding how different grassland use intensities (GUI) influence soil C stocks is therefore essential to promote C accumulation and improve grassland sustainability. Although many studies have addressed this issue in recent decades, most have been conducted at local or regional scales, limiting our ability to detect general patterns because the response of soil C to GUI is strongly context dependent. Therefore, disentangling management effects from pedoclimatic factors is crucial to improving our understanding of how grassland management influences soil C.To address this knowledge gap, we investigated 46 grasslands across 15 sites located in Argentina, Colombia, Germany, Kenya, New Zealand, Spain and Switzerland, all sampled following a standardized protocol. Soil samples were collected at three depths (0–10, 10–20 and 20–30 cm) to quantify soil organic C and additional soil properties. Information on management practices was compiled for each grassland. Using livestock density (livestock unit grazing days ha−1 yr−1), the number of mowing events per year, and annual nitrogen fertilization (kg N ha-1 yr-1), we calculated the GUI index proposed by Blüthgen et al. (2012) which reflects the combined effects of these management practices.Our sites span a wide climatic gradient, with mean annual temperature ranging from 0.8 to 27.4°C, precipitation from 518 to 2357 mm, and aridity index from 0.42 to 3.49. Tropical and subtropical grasslands were generally characterized by low grazing intensity and little or no N fertilization, whereas temperate sites often combined grazing, mowing and, in some cases, high N fertilizer inputs. As a consequence, we obtained a wide range of GUI index values, from 0 in unmanaged conservation grasslands to values >10 in intensively managed systems in Switzerland and Germany. Preliminary analyses suggest that both the aridity index and the GUI index may play an important role in explaining variation of soil C concentrations across sites, underscoring the importance of GUI in shaping soil C storage. Ongoing analyses incorporating additional explanatory variables (i.e. clay, bulk density, biomass production or soil pH) will provide deeper insights into the drivers of soil C dynamics in grasslands worldwide.AcknowledgementsThis research was developed within the framework of the European Joint Program for SOIL, "Managing and Mapping Agricultural Soils for Enhancing Soil Functions and Services" (EJP SOIL), project CARBOGRASS, funded by the European Union Horizon 2020 research and innovation program (Grant Agreement No. 862695). UPM was funded by Project PCI2023-143386 funded by MCIN/AEI/ 10.13039/501100011033/EU. ILRI was funded by the CGIAR Science Programs Climate Action and Multifunctional Landscapes.ReferenceBlüthgen, N., Dormann, C. F., Prati, D., Klaus, V. H., Kleinebecker, T., Hölzel, N., ... & Weisser, W. W. (2012). A quantitative index of land-use intensity in grasslands: Integrating mowing, grazing and fertilization. Basic and Applied Ecology, 13(3), 207-220.