Dynamics of organic matter in soils formed on limestone and sandstone in the karst area in southwestern China: δ 13C and δ 15N approaches

Abstract

To better understand biogeochemical processes associated with the dynamics of soil organic matter (SOM), the δ 13C and δ 15N values were analyzed in the SOM of limestone soil (developed on limestone) and sandstone soil (developed on sandstone), as well as in dominant plant species and litter samples collected on the studied soil profiles in karst areas of Southwest China. In general, the content of soil organic carbon (SOC) and soil organic nitrogen (SON) is highest in the surface soil layer and decreases downward in both types of soil profiles. In addition, the content of SOM (including both SOC and SON) in the limestone is much higher than in sandstone soil. The higher pH values, as well as the content of calcium, magnesium, and clay minerals, are probably the reasons for the higher SOM content in the limestone soil compared to the sandstone soil.In the SOM of the upper layers of yellow sandstone and limestone soil δ 13C values range from 2.6‰ to 6.3‰ and δ 15N values from 5.1‰ to 8.1‰, which is larger than in litters. Yellow sandstone soils have smaller differences between δ 13C and δ 15N values of SOM from litter and topsoil than the limestone soil. The δ 13C and δ 15N values of SOM increase with depth in yellow sandstone soil profiles, but not in the limestone soil profiles. An increase in the values of δ 13C and δ 15N with a decrease in the SOM content in the yellow sandstone soil significantly substantiates the decomposition of SOM, which causes higher values of δ 13C and δ 15N values in the deeper soil horizons. By contrast, the limestone soil has complex depth profiles of δ 13C and δ15N, which can be mainly attributed to land-use changes and organic and mineral interactions between soil accumulations.