Surface soil organic carbon in temperate and subtropical oriental oak stands of East China

Abstract

Forest ecosystems contain large amounts of soil organic carbon (SOC), which is a major component of biogeochemical cycles that may be sensitive to environmental change. We used a combination of nuclear magnetic resonance (NMR) spectroscopy and elemental and isotopic composition to examine the influence of soil properties and climatic factors on the quantity and degree of decomposition of SOC for organic and surface mineral horizons in seven oriental oak (Quercus variabilis Blume) forest sites arranged across a 11o latitudinal gradient in East China. Lacking Oa horizons, the two southernmost sites contained lower amounts of SOC in the forest floor horizon, but otherwise, latitudinal trends were not consistent. The SOC stock in the 0–10 cm mineral horizon exhibited no clear trend along the gradient and had a negative association with clay + silt content. Based on a higher alkyl/O-alkyl (A/O) ratio and alkyl/methoxyl (A/M) ratio, the SOC at the 0–10 cm depth appeared to be relatively more decomposed in three of the four southern subtropical sites. However, the degree of SOC degradation also decreased strongly with increasing soil pH (R2 = 0.90, P = 0.001). Soil organic carbon exhibited increases in δ13C and δ15N and decreases in the C/N ratio with depth for all the seven sites, indicating an increase in its extent of decomposition. Our analysis indicated that the A/M ratio from NMR provided the best indication of the extent of SOC degradation along the latitudinal transect, whereas the elemental and isotopic composition better reflected patterns with soil depth.