Downward carbon transport in a 2000-year rice paddy soil chronosequence traced by radiocarbon measurements

Abstract

Paddy and non-paddy soils from a chronosequence of 50–2000years of agricultural use, developed on former estuarine sediments of the Yangtze River, were sampled near Cixi, Zhejiang Province, China, in the framework of the Research Unit “Biogeochemistry of paddy soil evolution” of the German Research Foundation (DFG). In addition samples of Yangtze River estuarine sediments were obtained. The parent sediment shows a fairly homogeneous composition with ca. 0.3% TOC and a 14C concentration of ca. 50 pMC. After being diked-in, gradients in soil organic carbon and 14C concentration develop under the influence of vegetation and cultivation. In the non-paddy soil, a 14C gradient with concentration decreasing with increasing depth from modern (>100pMC) to original sediment values around 50pMC is already established after 50years and can also be observed in the older sites. In contrast, the 50years old paddy soil shows organic carbon and 14C enrichment only in the A-horizon and a nearly constant TOC and 14C stock of original sediment below the plough pan. To test the basic approach that the soil profile development started on homogeneous sediment, an isotope and mass balance calculation was used. The results show a quite similar age composition of different sample sites. Paddy rice cultivation quickly leads to a dense plough pan, which seriously reduces, but not totally prevents, downward transport of organic matter. The equilibrium times for TOC and 14C in paddy soil profiles are short (decades) in the topsoil and in the order of centuries in the subsoil, underlining the dynamic character of soil organic carbon.