Plant N uptake (NA, kg ha-1) was calculated as follows: NA (kg·ha−1) = crop dry matter (kg ha−1) × crop N concentration (g g−1)(1) The N use efficiency (NUE) was calculated as follows : NUE ( %)=UMN+USN−UM0−US0AMN+ASN*100 %(2) where UMN (or USN) (kg N ha-1) is total N accumulation by maize (or soybean) with N application, UM0 (or US0) is total N accumulation by maize (or soybean) without N application, and AMN (or ASN) is the amount of N supplied during the growing season. The NUE of maize (or soybean) was calculated from formula (2) in which USN (or UMN), US0 (or UM0) and ASN (or AMN) were considered zero.