DISTRIBUTION OF ORGANIC AND METAL ELEMENTS IN AGRICULTURAL AND NON-AGRICULTURAL SOILS

Intensive agricultural activities can bring about major environmental distress, including soil degradation, water eutrophication and the interruption of elemental geochemical cycles. Both agricultural and non-agricultural soils near an upstream portion of the Pearl River in China were sampled to investigate the distribution and speciation of plant essential macronutrients linked to agricultural activities, including organic (C, H, N and S) and metal elements (Ca, Mg, K and Na). The results showed that total C, N and S concentrations were closely correlated in agricultural soils and were significantly higher than those in non-agricultural soils, for most of the sampling sites. Low ratios of C/N and N/S could accelerate microbial decomposition and mineralization of N and S; thus, ensuring that agricultural soil would not be deficient in N and S for crop production. Total Ca, Mg, K and Na concentrations in agricultural soils were slightly higher than in non-agricultural soils. Available concentrations and speciation percentages of these four metal elements were analyzed based on the Bureau Communautaire de Reference (BCR) sequential extraction procedure. The distribution patterns of speciation percentages for the metal elements were similar in both the agricultural and the non-agricultural soils. No fraction of the four metals was significantly correlated with total organic carbon (TOC) or microbial biomass carbon (MBC) content, indicating that their speciation fractions were not affected by cultivation, TOC or MBC. The investigated soils held sufficient available Ca, Mg and K for plants.