Coupling Nitrogen and Carbon Cycles in Soil to Increase N Use Efficiency and Decrease Its Loss.

To study the fates of nitrogen fertilizer in different fertile soils, the 15N-labeled fertilizer micro-plots were settled in the three different fertilized treatments (No-F, no fertilizer; NPK, inorganic fertilizers including N, P, and K; and MNPK, combined application of manure and inorganic fertilizer) of two 19-year trials (wheat-fallow, and wheat-maize rotation) located at the south edge of the Loess Plateau, China. The MNPK soil had significantly higher grain yield than the NPK soil wheat-fallow system. After harvesting wheat, more than 30% of added N was leached below 20 cm soil layer in the No-F soil. The nitrogen use efficiency (NUE) of the 15N fertilizer in the No-F soil was only 20%; this compared to a NUE of 61% and 65% in the NPK and MNPK soils, respectively. During the summer fallow, 5-33% of the added 15N was lost from the different soils. The residual 15N use efficiency by the second winter-wheat was 13% in the No-F soil, 6% in the NPK soil, and 8% in the MNPK soil. These were equivalent to 9.0%, 2.0% and 2.2% of applied 15N. The total 15N recovery rates in the MNPK and NPK soils (84.5% and 86.6%, respectively) were both significantly higher than that in the No-F soil (59%) after two growing seasons. In the wheat-maize system, the NUE by wheat was 62% in the MNPK soil, significantly higher than that in the NPK (50%) and No-F soil (only 13%). More than 50% of 15N fertilizer was leached below 20 cm soil layers in the No-F soil. After harvesting wheat, 88%, 45%, and 38% of the added 15N-labeled fertilizer was remained in the 0-100 cm profile of No-F, NPK, and MNPK soils, respectively. The fertilizer 15N was mainly lost from these soils during the maize growing season. We concluded that coupling nitrogen and carbon transformations in soil by the combined application of manure and inorganic fertilizers improves synchrony between N supply and crop demand, thus reducing N loss.