Development of a critical nitrogen dilution curve based on leaf dry matter for summer maize

Accurate diagnosis of nitrogen (N) fertilizer required for crop growth can serve as a guide for N management by improving N use efficiency and grain yields. The critical N concentration (Nc), the minimum N required for maximal crop growth has been widely used to determine crop N status. Nc dilution curves have been determined in several crops including summer maize on plant dry matter (DM) basis, yet no attempt has been made to determine the Nc dilution curve on the basis of leaf dry matter (LDM) in summer maize. The present study aimed to determine a Nc dilution curve based on LDM for in-season assessment of crop N status in summer maize. Six field experiments were performed with four summer maize cultivars using varied N fertilizer rates ranging from 0 to 320 kg N ha−1. The leaf Nc curve was described by the equation: Nc = 3.45LDM−0.22, when LDM ranged from 1.18 to 3.45 t ha−1. For LDM <1.18 t ha−1, the constant Nc = 3.33% was used. The newly developed curve effectively distinguishes N-limiting from non-N-limiting treatments under different environmental conditions. In the present study, the N nutrition index (NNI) ranged from 0.49 to 1.16 under different N rate treatments. The correlation between NNI and relative yield (RY) was a significantly positive, while the correlation between NNI and the agronomic N use efficiency (AE) was significantly negative. The newly developed leaf Nc dilution curve not only determines the crop N status, but also elucidates the yield and AE changes in response to different N rate treatments in summer maize. The projected results of the study will provide accurate N status diagnosis at critical growth stages and guidance for precision N management in summer maize, thus contributes towards the sustainability of intensive maize cropping systems in China.