Natural 15N abundance of plant and soil inorganic-N as evidence for over-fertilization with compost

To test the hypothesis that N isotope composition can be used as evidence of excessive compost application, we measured variation in patterns of N concentrations and corresponding δ15N values of plants and soil after compost application. To do so, a pot experiment with Chinese cabbage (Brassica campestris L. cv. Maeryok) was conducted for 42 days. Compost was applied at rates of 0 (SC0), 500 (SC1), 1000 (SC2), and 1500 mg N kg−1 soil (SC3). Plant-N uptake linearly increased with compost application (r2 = 0.956, P < 0.05) with an uptake efficiency of 76 g N kg−1 of compost-N at 42 days after application, while dry-mass accumulation did not show such linear increases. Net N mineralized from compost-N increased linearly (r2 = 0.998, P < 0.01) with a slope of 122 g N kg−1 of compost-N. Plant-δ15N increased curvilinearly with increasing compost application, but this increase was insignificant between SC2 and SC3 treatments. The δ15N of soil inorganic-N (particularly NO3−–N) increased with compost application. We found that plant-δ15N reflected the N isotope signal of soil NO3−–N at each measurement during plant growth, and that δ15N of inner leaves and soil NO3−–N was similar when initial NO3− in the compost was abundant. Therefore, we concluded that δ15N of whole plant (more obviously in newer plant parts) and soil NO3−–N could reveal whether compost application was excessive, suggesting a possible use of δ15N in plants and soil as evidence of excess compost application.