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

Purpose This study was conducted to investigate variations in the stable isotope ratios of carbon (δ 13 C) and nitrogen (δ 15 N) 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 13 C- and 15 N-enriched livestock manure and compost application). Materials and methods 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. Results and discussion The δ 13 C and δ 15 N of agricultural soils were higher than those of forest soils, reflecting repeated manure and compost applications (for both δ 13 C and δ 15 N) and higher N loss (for δ 15 N) in agricultural soils. Among agricultural soils, orchard (− 24.2‰ for δ 13 C and + 10.6‰ for δ 15 N) and upland (− 25.4‰ and + 9.6‰, respectively) soils which received higher rates of manure and compost were more enriched with 13 C and 15 N 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. Conclusions Our study suggests that δ 13 C and δ 15 N of agricultural soils are affected by land use types with different fertilization management, particularly application of 13 C- and 15 N-enriched livestock manure and compost. The δ 13 C and δ 15 N could be used as chemical indicators to evaluate the effects of the application of manure and compost on soil C and N dynamics.