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

Abstract

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 δ 15N 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 ( r 2 = 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 ( r 2 = 0.998, P < 0.01) with a slope of 122 g N kg −1 of compost-N. Plant- δ 15N increased curvilinearly with increasing compost application, but this increase was insignificant between SC2 and SC3 treatments. The δ 15N of soil inorganic-N (particularly NO 3 −–N) increased with compost application. We found that plant- δ 15N reflected the N isotope signal of soil NO 3 −–N at each measurement during plant growth, and that δ 15N of inner leaves and soil NO 3 −–N was similar when initial NO 3 − in the compost was abundant. Therefore, we concluded that δ 15N of whole plant (more obviously in newer plant parts) and soil NO 3 −–N could reveal whether compost application was excessive, suggesting a possible use of δ 15N in plants and soil as evidence of excess compost application.