Soil carbon storage controlled by interactions between geochemistry and climate

Soils are an important site of carbon storage1. Climate is generally regarded as one of the primary controls over soil organic carbon1,2, but there is still uncertainty about the direction and magnitude of carbon responses to climate change. Here we show that geochemistry, too, is an important controlling factor for soil carbon storage.Wemeasured a range of soil and climate variables at 24 sites along a 4,000-km-long north–south transect of natural grassland and shrubland in Chile and the Antarctic Peninsula, which spans a broad range of climatic and geochemical conditions. We find that soils with high carbon content are characterized by substantial adsorption of carbon compounds onto mineral soil and low rates of respiration per unit of soil carbon; and vice versa for soils with low carbon content. Precipitation and temperature were only secondary predictors for carbon storage, respiration, residence time and stabilization mechanisms. Correlations between climatic variables and carbon variables decreased significantly after removing relationships with geochemical predictors. We conclude that the interactions of climatic and geochemical factors control soil organic carbon storage and turnover, and must be considered for robust prediction of current and future soil carbon storage. Soil organic carbon (SOC) is one of themost important terrestrial C pools, with spatially variable but large annual C exchanges with the atmosphere1,2. Single and interactive effects of climatic and biotic factors on SOC dynamics have been studied intensively at various spatial and temporal scales, but are still poorly represented in current Earth SystemModels (ESMs; refs 3–6).Generally, climatic factors have been regarded as primary controls in empirical and modelling approaches1,2,7. Consequently, ESMs predict a significant contribution of SOC to future climate change. However, recent model-based observations indicate that geochemical factors very likely play crucial roles in SOC turnover and large uncertainties remain8,9. These uncertainties are explained partly by ESMs poorly representing the current (observed) global SOC distribution1 and partly by inadequate parameterization of the temperature sensitivity of SOC, microbial carbon use efficiency, and mineral surface sorption of organic matter1,8,9. The latter indicates shortcomings in the current approaches that focus on climatic controls and neglect other factors, such as the geochemistry of soils and effects of soil mineralogy on carbon stabilization10,11. The geochemistry of the reactive mineral phase of a soil depends on the composition of its parent material and weathering status. Weathering is crucially driven by the time since the onset of weathering, resilience of minerals to weathering, vegetation cover, local climate, and hydrologic conditions. Owing to the difference in scales between climate research and geochemical-related soil research (mostly large and local scale, respectively), the interactions of these key factors for SOC dynamics have rarely been assessed. Here, we contribute to the debate on the importance of various environmental factors for global SOC dynamics by assessing the degree of direct versus indirect effects of geochemical and climatic factors on SOC stocks (SOCStock), concentrations (SOC%), specific potential respiration rates (SPR) and SOC fractions along an approximately 4,000 km north–south transect across Chile and the Antarctic Peninsula spanning a large range of climatic and geochemical parameters. All sites were under grassland and/or shrubland vegetation12 to keep C input as similar as possible (Supplementary Table 2). However, differences in net primary productivity (NPP) could not be excluded; biomass production along the transect ranged between 800 to 4,500 kg dry weight ha−1 yr−1, with extreme values of 300 kg ha−1 yr−1 in hot arid climates and up to 8,000 kg ha−1 yr−1 in temperate humid climates13,14. Our data reveal strong connections between geochemical and climatic drivers of global SOC dynamics, demonstrating a need to consider their interactions when addressing current spatial patterns of SOC storage and turnover, as well as future global responses of SOC to climate change. We sampled topsoil (0–10 cm) at 24 sites, in triplicate, along this transect (Supplementary Table 1). To study the effects of geochemistry on the selected SOC variables (SOC%, SOCStock, SPR and SOC fractions), we determined the following key geochemical characteristics of the soil samples: base saturation (BS) of the potential cation exchange capacity (CECpot); soil texture; contents of silicon (Si) and abundant metals (Fe, Mn, Al); Si/Al ratio; the total reserve in ‘base’ cations (TRB); and geochemical soil fertility parameters (pHKCl, P, K) (Supplementary Table 3). We used mean annual precipitation and temperature as indicators of climatic conditions (see Supplementary Fig. 3). The predicting variables used here are thus proxies for environmental conditions controlling SOC dynamics (for a detailed discussion see Supplementary Information). We assume that geochemical