Controls of vertical carbon stable isotope distribution in topsoil: temperature, precipitation and time

Abstract

A crucial ability to evaluate the effects of changes in land-use and global climate is the understanding of carbon (C) storage in soil. The decomposition of organic matter (OM) in soil presents a determining mechanism, due to the impact it has on whether soils function as a sink for C or fuel the atmosphere’s carbon dioxide concentrations. The vertical distribution of C stable isotopes in topsoil serves as a powerful tool to investigate decomposition of OM in soil. It is of particular interest how the decomposition of OM in soil relates to changing mean annual temperature (MAT) and mean annual precipitation (MAP). Therefore, I conducted a field study with comparable confounding variables and MAT or MAP, respectively as changing variable. Relations between the vertical decrease of C concentrations and the increase of δ13C values in soil profiles from litter to mineral soil at 10 cm depth served to approximate decomposition. In contrast to the general assumption of the Van´t Hoff´s kinetic theory, the results suggest a decline of decomposition with increasing MAT. Low soil moisture likely hampered microbial activity under elevated MAT. Approximated decomposition increased across the gradient of MAP. Selective sorption and the downward transport of hydrophilic, 13C enriched compounds with fluxes of soil solution might have dominated the development of δ13C depth profiles under high MAP. The investigation of δ13C depth profiles during land-use change indicated that three decades following afforestation of former cropland are sufficient to develop δ13C depth profiles. On timescales of millennia, aged soils are supposed to sequester large amounts of C with an assumed decrease of decomposition. However, δ13C depth profiles suggested no constant decrease of OM decomposition during 2,870 years of ecosystem development and pedogenesis. Interestingly, δ13C depth profiles were related to the depth distribution of nitrogen stable isotopes, suggesting shared processes shaping δ13C and δ15N vertical depth profiles. Carbon stable isotope distribution in topsoil significantly changed with MAT, MAP and over time. The findings of this work contribute to a better understanding of how decomposition responds to climate change and pedogenesis. In essence, the analysis of δ13C depth profiles in topsoil offers an alternative and reliable method to approximate decomposition of OM in soil.