Topographic and Climatic Determinants of Soil Organic Carbon Across Landscapes: A Meta‐Analysis

Abstract

ABSTRACT The soil is the greatest natural store of organic carbon, holding between 1500 to 1550 Pg C of organic carbon at the top 100 cm of the soil globally—roughly twice the carbon pool in the atmosphere. The spatial distribution of this stock is controlled by a complex chain of controls: topographic position regulates the hydrological redistribution, microclimate and erosion at the landscape scale and climatic variables, such as mean annual temperature (MAT), mean annual precipitation (MAP), aridity index and seasonality of precipitation, determine the envelope of the topographic effects at the regional and continental scale. Although the primary literature is growing at a very fast rate, no quantitative synthesis has to date concurrently evaluated the complete set of topographic variables, such as slope gradient, aspect, elevation, topographic curvature and the topographic wetness index (TWI), alongside the influences of climate and processes of soil carbon fractionation. This meta‐analysis screened 1339 peer‐reviewed studies (1964–2024) following PRISMA 2020 guidelines; of these, 26 reported extractable correlation coefficients and were included in the quantitative meta‐analysis (slope: k = 15; elevation: k = 11). The study addresses the following questions: (i) to quantify the mean effect sizes and between‐study heterogeneity of topographic and climatic controls on SOC; (ii) to specify the moderating variables that justify the extreme heterogeneity ( I 2 > 98%) and (iii) to define the critical gaps in the research. Fisher r ‐to‐ z meta‐analysis showed that slope gradient has a borderline‐significant positive mean effect on SOC ( μ̂ = 0.238; 95% CI: 0.001–0.474; k = 15; I 2 = 98.1%; p = 0.049) and a non‐significant effect of altitude on SOC ( μ̂ = 0.130; 95% CI: −0.178–0.437; k = 11; I 2 = 98.7%; p = 0.409). Wide heterogeneity of the two analyses indicates dominant expression of moderating roles of ecosystem type, land use, soil texture and mineralogy. The topographic aspect has the strongest topographical dominance: the northward slopes accrete 1.9–3.2 times more SOC compared to southward counterparts at the same altitude. TWI—an index that combines terrain location stance and hydrological connection—emerges as the most effective terrain predictor of SOC in digital soil mapping (DSM) models and explains 15%–38% of the landscape‐scale variance in SOC. The current particulate organic carbon (POC) versus mineral‐associated organic carbon (MAOC) fractionation model shows that erosional summit locations selectively lose labile POC and depositional foot‐slope locations accrue high levels of MAOC by means of organo‐mineral complexation with Fe/Al oxides. In the IPCC AR6 warming scenarios (1.5°C–2.0°C) the change in SOC stocks is expected to be an 8%–12% reduction in the global stocks of the soils in fine‐textured foot‐slope and coarse‐textured summit soils. Such findings offer a quantitative basis of digital soil mapping, land‐use planning and the 4 per mille carbon sequestration programme, which are directly related to the national carbon accounting under NDC schemes in concert with SDG 13 and SDG 15.