Linking soil moisture with chemical quality of soil organic matter to evaluate belowground carbon storage in savannas

Belowground carbon storage is a function of soil organic carbon content and soil respiration, and both of these factors are profoundly influenced by soil moisture. Soil organic matter (SOM) comprises a broad pool, including labile plant residues, microbial biomass and more biologically recalcitrant humic substances. Water extractable organic carbon (WEOC) represents the active fraction of SOM that becomes available for decomposition during wetting, and the chemical quality of WEOC provides information about its biodegradability. Under conditions of decreasing soil moisture, both the SOC pool and soil respiration are expected to decrease, but depending on the rates of both the net effect on soil carbon storage is unclear. Therefore, in the semiarid savanna ecosystem, the influence of WEOC chemical quality on SOM mineralization and soil respiration may be extremely important. In this study, we address this notion by comparing SOC content, C:N ratios, and SOM quality at two savanna sites in the Kalahari Desert, Botswana with contrasting soil moisture content. We present new results showing higher SOC content and more humified SOC in the wetter savanna site. Evidence from UV-vis absorbance and fluorescence spectroscopy also suggests that at the drier site, there is great contrast with respect to the degree of humification in soils beneath and between canopy than at the wetter site. The WEOC at both sites also contains a substantial amount of amino acid-like fluorescence (Fig. 1) that may be derived from microbial biomass. Our SOM characterization at these two sites is being applied in SOC dynamics model validation. We will also discuss the implications of our findings for belowground C sequestration in light of projected climate change for the region. Fig 1. Representative fluorescence excitation emission matrices from WEOC under A. mellifera canopy (top) and bare ground (bottom) at the dry savanna site.