Land use types with different fertilization management affected isotope ratios of bulk and water-extractable C and N of soils in an intensive agricultural area

This study was conducted to investigate variations in the stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) of agricultural soils under different land uses (i.e., paddy, upland, and orchard) that subjected to different fertilization management (i.e., 15 N-depleted synthetic fertilizer and 13C- and 15 N-enriched livestock manure and compost application). Soil samples were collected from paddy, upland, and orchard fields in an intensive agricultural area, and forest (pine and oak) soils were additionally included as background soils. The C and N concentrations and isotope ratios of both bulk and water-extractable soil fractions were analyzed. The δ13C and δ15N of agricultural soils were higher than those of forest soils, reflecting repeated manure and compost applications (for both δ13C and δ15N) and higher N loss (for δ15N) in agricultural soils. Among agricultural soils, orchard (− 24.2‰ for δ13C and + 10.6‰ for δ15N) and upland (− 25.4‰ and + 9.6‰, respectively) soils which received higher rates of manure and compost were more enriched with 13C and 15N compared with paddy (− 28.0‰ and + 4.9‰, respectively). Such differences in the isotopic compositions among agricultural soils were also found for water-extractable soil fractions. Our study suggests that δ13C and δ15N of agricultural soils are affected by land use types with different fertilization management, particularly application of 13C- and 15 N-enriched livestock manure and compost. The δ13C and δ15N could be used as chemical indicators to evaluate the effects of the application of manure and compost on soil C and N dynamics.