Land use change impact on soil carbon cycling and elemental budget.

摘要

Soil carbon cycling and elemental budget are known to be sensitive to land use types. Here we focused on the carbon translocation and long-term accumulation of elements in two adjacent forest and agricultural soils that share virtually identical environments. Soil mixing rates were calculated based on 210Pb radioactivity. The mixing velocities in A horizon range from 0.1 to 0.7 cm yr-1 in the forest soil and from 1 to 10 cm yr-1 in the agricultural soil. The turnover times of organic carbon by mixing in A horizons are decadal in the forest but seasonal in the agricultural soil, indicating that the agricultural soils' low carbon content is partially due to the loss of the labile carbon pool of which decomposition is facilitated by the mixing. In another translocation mechanism, podzolization, the amounts of organically complexed and inorganic amorphous and pedogenic crystalline Fe and Al are all significantly lower in the agricultural soil. For a given organically complexed Fe and Al, smaller amounts of organic carbon are complexed in the agricultural soil. The reduction of organic carbon content and the increase of soil pH via liming in the agricultural soil have reduced the podzolization. A geochemical mass balance approach was employed in order to quantify the mass losses or gains via chemical weathering over the course of soil formation under the two land use types. Zirconium is used as an index element. The two soils and their underlying parent materials have identical X-ray diffraction-based mineralogy, supporting the assumptions that the two soils are developed in-situ from identical parent materials and that aeolian inputs can be ignored. Calculated fractional mass losses or gains show that the agricultural soil has net mass gains in Ca, Mg, Na and P relative to its parent material, while the forest soils show losses or less gain in the nutrient elements. Heavy metals such as Cr, Cu, Ni, Pb, Zn and Co are more enriched in the agricultural soil than in the forest soil. This showed that the accumulation of nutrient elements and incorporation of heavy metals by agricultural activities.