Impact of Organic and Chemical Nitrogen Fertilizers on the Crop Yield and Fertilizer Use Efficiency of Soybean–Maize Intercropping Systems

The effect of the mixture (1:1) of chemical and organic nitrogen (N) fertilizer on crop yield quality and N fertilizer use efficiency remains elusive. A nitrogen field experiment was conducted in the growing seasons of 2020 and 2021 to investigate the effects of the mixture of chemical and organic N fertilizer on the crop yield, crop quality and nitrogen fertilizer use efficiency in a maize–soybean intercropping system in China. Four treatments applied at 150 kg N ha−1 were used: no nitrogen fertilizer (CK), chemical N fertilizer (ChemF), mixture (1:1) of chemical and organic N fertilizer (ChemF + OrgF) and organic N fertilizer (OrgF). The results showed that the yield and aboveground N accumulation of both soybean and maize increased with the application of fertilizer. The ChemF + OrgF treatment had lower maize and soybean seed yields than for ChemF treatment, but higher than the other two treatments in both years, and the maize yield of the (ChemF + OrgF) treatment was significantly higher (14.9%) in 2021 than 2020. Yields were significantly positively correlated with aboveground N accumulation and fertilizer use efficiency, measured using the nitrogen partial productivity (NPP), nitrogen agronomic efficiency (NAE) and nitrogen fertilizer recovery rate (NFRR). The protein content tended to increase and the oil content tended to decrease under (ChemF + OrgF) applications in soybeans. The (ChemF + OrgF) treatment had the lowest starch content in maize. There was no significant difference in the nitrogen harvest index among treatments, while the NPP, NAE and NFRR were the highest for the application of chemical N fertilizer and significantly decreased with the addition of organic N fertilizer. We conclude that the mixture (1:1) of chemical and organic N fertilizer increased the seed yield and quality of maize, but only the seed yield of soybean.