Climate, land cover, and parent material drive global patterns of soil C partitioning and persistence

Abstract:

Quantifying how the distribution and persistence of soil organic carbon (C) varies as a function of soil forming factors (climate, organisms/biome, relief, parent material, time) is essential to understanding and predicting the potential vulnerability of soil C to climate change. Much research to date has focused on bulk soils, a heterogenous mixture of soil C pools with different transit times that may have different sensitivities to the drivers of soil formation. Here, we use data from the International Soil Radiocarbon Database (ISRaD) combined with globally gridded data products to produce a global-scale meta-analysis of the relative influence of soil forming factors on the soil organic C abundance (on a per g soil basis) and persistence (∆14C) of soil density fractions (free light, occluded, and heavy). The data include 298 soil profiles across 121 sites and 52 studies. Mean annual temperature (MAT), moisture availability (aridity index=MAP/potential evapotranspiration), land cover, and parent material were the most important drivers. ∆14C in all fractions increased with increasing MAT, but the amount of C declined with MAT in the free light fraction only, implying the occluded and heavy fractions may be less temperature sensitive. ∆14C values decreased as available moisture increased, an effect that was strongest in subsurface layers. The C abundance increased with increasing moisture availability in all fractions, but the fractions responded differently across depths. The fractions also responded differently to land cover and parent material. Overall, the amount and persistence of C are tightly coupled to the factors that control soil development and weathering, such as temperature and moisture, which affect reaction rates and transport, and vegetation, which affects C inputs and soil acidity. The increasing influence of moisture availability as soil depths increased highlights the need for more explicit representations of soil C transport in models and for more experimental and observational studies focused on soil moisture, which is undergoing substantial shifts across much of the globe due to climate change. Furthermore, the different responses of individual fractions to MAT highlights the importance of going beyond bulk pools in order to understand responses to global change.