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 delta N-15 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 (SO). 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-delta N-15 increased curvilinearly with increasing compost application, but this increase was insignificant between SC2 and SO treatments. The delta N-15 of soil inorganic-N (particularly NO3--N) increased with compost application. We found that plant-delta N-15 reflected the N isotope signal of soil NO3--N at each measurement during plant growth, and that delta N-15 of inner leaves and soil NO3--N was similar when initial NO3- in the compost was abundant. Therefore, we concluded that delta N-15 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 delta N-15 in plants and soil as evidence of excess compost application. (C) 2009 Elsevier Ltd. All rights reserved.