Summer fallow increases loss of residual nitrogen fertilizer in dryland of the Loess Plateau: a N-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 N-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 (NW), with the addition of 50 kg ha N (NW) and with the addition of 50 kg ha N plus 35% more water (NW). The N fertilizer (KNO) in solution was injected into the soil at a depth of 35 cm of the polyvinyl chloride (PVC) columns in field. The fates of N 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 N was still leached down to 40-cm depth for the NW treatment; and for the NW treatment, the peak of N fertilizer was approximately 20 cm deeper. After summer fallow, the loss of the initially applied N was 26% in the soil profile for the NW treatment; and for the NW treatment, it increased to 37%. Soil N abundance in 0–20 cm of the NW and NW treatments was higher than the NW treatment, indicating the upward movement of N in summer fallow. After the next wheat harvest, N uptake by wheat in the NW treatment decreased from 21.0 to 18.6% compared to the NW treatment. High rainfall during summer fallow increased residual N loss during summer fallow but decreased its use by the next crop.