Using stable nitrogen isotope to indicate soil nitrogen dynamics under agricultural soil erosion in the Mun River basin, Northeast Thailand

Abstract

Soil erosion has threatened food security by decreasing the area of arable lands and disturbing soil nutrient cycling, especially nitrogen (N) cycling. However, there is still a challenge to indicate soil N dynamics by N stable isotope composition (δ15N) under an erodible environment. In the present study, the six soil sites from forest lands and agricultural lands were selected to collect soil samples in the Mun River basin, the largest agricultural region in Thailand. The contents of soil organic nitrogen (SON), C/N ratios, and δ15N values of SON in soil profiles were analyzed to identify soil N transformation processes under agricultural soil erosion. Moreover, the relationships of SON and δ15N with soil erodibility K factor were determined by linear regression analysis. Fine particles in the paddy land were largely lost under intensive soil erosion during a short-term abandonment (1 ~ 5 years). Agricultural soil erosion has reduced SON content by 3.8 ~ 4.9 times. The δ15N values of SON in the paddy soils ranged from –5‰ to 5‰, which was attributed to the application of manure and synthetic fertilizer and the δ15N fractionations in the alteration processes of the redox environment. In the abandoned paddy lands, δ15N values of SON in the soils above the illuvial horizon were 4‰ higher than those in the soils below the horizon. The results indicated the discrepant soil N processes at the two soil layers, i.e., soil N mineralization in the upper layer produces 15N-depleted inorganic N and leads to 15N enrichment in organic residues; the inorganic N was leached into the lower layer and re-assimilated by microbes, resulting in 15N-depleted SON. The results suggest that agricultural soil erosion significantly reduces soil N level. However, microbial re-assimilation of inorganic N in the deep soils can mitigate soil N loss.