Nitrogen Recommendations for Summer Maize in Northern China Using the Nmin Test and Rapid Plant Tests

A field experiment with a split-plot design was carried out at Dongbeiwang Farm in Beijing Municipality to establish reliable N fertilizer recommendation indices for summer maize (Zea mays L.) in northern China using the soil Nmin (mineral N) test as well as the plant nitrate and SPAD (portable chlorophyll meter readings) tests. The results showed that Nmin sollwert (NS) 60 kg N ha-1 at the third leaf stage and N rate of 40 to 120 kg N ha-1 at the tenth leaf stage could meet the N requirement of summer maize with a target yield of 5.5-6 t ha-1. Sap nitrate concentrations and SPAD chlorophyll meter readings in the latest expanded maize leaves at the tenth leaf stage were positively correlated with NS levels, indicating that plant nitrate and SPAD tests reflected the N nutritional status of maize well. Considering that winter wheat subsequently utilized N after the summer maize harvest, the 0-90 cm soil Nmin (74 kg N ha-1) and apparent N loss (12 kg N ha-1) in the NS60+40 treatment were controlled at environmentally acceptable levels. Therefore NS60+40, giving a total N supply of 100 kg N ha-1, was considered the optimal N fertilizer input for summer maize under these experimental conditions.