Summer fallow increases loss of residual nitrogen fertilizer in dryland of the Loess Plateau: a 15N-labeled method.

Summer fallow is very common in dryland agriculture to conserve rainwater and replenish soil fertility. However, bare land and intensive rainfall during summer fallow might result in a potential risk of N loss. We used a 15N-labelling method to study the loss of residual N fertilizer during summer fallow and its use by next wheat in the Loess Plateau. Our study included three treatments: without the addition of N (N0W0), with the addition of 50 kg ha-1 N (NW0) and with the addition of 50 kg ha-1 N plus 35% more water (NW). The N fertilizer (K15NO3) in solution was injected into the soil at a depth of 35 cm of the polyvinyl chloride (PVC) columns in field. The fates of 15N were followed after summer fallow and in the next season's wheat (Triticum aestivum L.). The summer fallow of this study was a dry summer; however, fertilizer 15N was still leached down to 40-cm depth for the NW0 treatment; and for the NW treatment, the peak of 15N fertilizer was approximately 20 cm deeper. After summer fallow, the loss of the initially applied 15N was 26% in the soil profile for the NW0 treatment; and for the NW treatment, it increased to 37%. Soil 15N abundance in 0-20 cm of the NW0 and NW treatments was higher than the N0W0 treatment, indicating the upward movement of 15N in summer fallow. After the next wheat harvest, 15N uptake by wheat in the NW treatment decreased from 21.0 to 18.6% compared to the NW0 treatment. High rainfall during summer fallow increased residual N loss during summer fallow but decreased its use by the next crop.