Fate of nitrogen-15 as influenced by soil and nutrient management history in a 19-year wheat–maize experiment

Abstract

High soil organic matter content may improve synchronization between N supply and crop demand. To test this hypothesis, we compared the fate of 15N-labeled fertilizer in soil with different management history. The soils had received no fertilizer (No-F soil), inorganic N, P, and K fertilizer (NPK soil), or manure plus N, P, and K fertilizer (MNPK soil) as part of a 19-year long-term fertilization trial. The N use efficiency (NUE) of wheat (Triticum aestivum L.) was 62% in the MNPK soil, higher than that in the NPK soil (50% NUE), and in the No-F soil (13% NUE). At wheat harvest, 38% of the fertilizer 15N remained in the 0–100cm depth of the MNPK soil, significantly less that the amount of fertilizer 15N that remained in the NPK soil (45%) or in the No-F soil (88%). More than 50% of the fertilizer 15N in the No-F soil had leached below the 20cm depth by wheat harvest, significantly more than in the NPK or MNPK soils. The amount of immobilized 15N at wheat stem elongation was significantly (P<0.05) greater in the MNPK soil than in the NPK soil. The mineralization of immobilized 15N between stem elongation and flowering was also significantly higher in the MNPK soil than in the NPK soil (P<0.05). The succeeding maize (Zea mays L.) crop took up 9% of the fertilizer 15N in the No-F soil, 6% of the fertilizer 15N in the NPK soil, and 2% of the fertilizer 15N in the MNPK soil. Combined soil profile and crop removal analyses at wheat harvest accounted for nearly 100% of the fertilizer 15N for all three soils. However, only 45% of the fertilizer 15N added to the No-F soil could be accounted for at maize harvest, significantly less than the recovery rate in the NPK (83%) and MNPK (85%) soils (P<0.01). These results indicate that the fertilizer 15N was mainly lost from these soils during the maize growing season. We conclude that the combined application of manure and inorganic fertilizers improves synchrony between N supply and crop demand, thus reducing N losses from agriculture.