Recovery of fertilizer-derived inorganic-¹⁵N in a vegetable field soil as affected by application of an organic amendment

To examine the effect of organic amendment application on the fate of inorganic-N accumulated in a vegetable field soil during conversion from inorganic to organic input, a pot experiment using ¹⁵N-labeled soil was conducted. The soil was labeled with ¹⁵N through addition of urea-¹⁵N (98 atom % ¹⁵N) and was then incubated for 1 year resulting in inorganic soil-N concentration and ¹⁵N abundance of 211 mg kg⁻¹ soil and 4.950 atom %, respectively. Chinese cabbage [Brassica campestris (L.) Samjin] plants were grown in the labeled soil for 30 and 60 days after application of organic amendment at the rates of 0 (control), 200, 400, and 600 mg N kg⁻¹ soil. Although organic amendment application did not show any significant effect on the uptake efficiency of inorganic-N by Chinese cabbage during the first 30 days, it significantly (P < 0.05) increased inorganic-N uptake efficiency as well as total-N uptake and dry matter yield at the end of the 60-day growth period. Application of the organic amendment also increased microbial immobilization of inorganic-N in both growth periods. Between 30 and 60 days of growth, however, the amount of immobilized N from the inorganic-¹⁵N pool decreased, indicating re-mineralization of previously immobilized N. Although the amount of inorganic-¹⁵N lost was virtually the same among treatments at day 30, increased immobilization of inorganic-¹⁵N caused by organic amendment application led to the higher retention of inorganic-N in the soil and less loss of N at day 60 as compared to the control. These results indicate that increased immobilization by organic amendment application in the early growth season and the subsequent gradual re-mineralization may play an important role in increasing plant uptake of inorganic-¹⁵N, while minimizing N loss.