Determination of the critical soil mineral nitrogen concentration for maximizing maize grain yield

A critical soil mineral nitrogen concentration (N-min) for guiding fertilizer application and maximizing maize grain yield is needed. A three-year field experiment with three N regimes, unfertilized (N0), optimized N management (Opt.) and conventional N practice (Con.) was performed in maize. The mean soil N-min in 0-60 cm soil profile for N0, Opt. and Con. treatments was 2.0, 6.7 and 8.9 mg kg(-1) at V8-VT growth stages and 2.2, 6.1 and 11.2 mg kg(-1) on average over the whole growth season, respectively. Correspondingly, the soil N supplying capacity (soil N-min content + fertilizer N) of the three N treatments was smaller, identical or greater than the plant N accumulation at different growth stages. The Opt. treatment had significantly higher N use efficiency, N recovery efficiency and N partial factor productivity compared with the Con. treatment, while it did not cause maize yield loss. Compared with the insensitivity of the critical shoot N dilution curve to excessive N application, soil N-min showed strong response to all treatments. We propose a minimum of soil N-min of 6.1 mg kg(-1) at the sowing-V8, 6.7 mg kg(-1) at the V8-VT, and 5.5 mg kg(-1) at the VT-R6 growing stages with an average of about 6 mg kg(-1) of soil N-min in the 0-60 soil depth for maximizing maize yield and N use efficiency in northern China. To maintain this critical N-min value over the whole growth period, N topdressing at V8 and V12 stages was recommended.