Effects of reduced chemical nitrogen input combined with organic fertilizer application on the productivity of winter wheat and summer maize rotation and soil properties in central Henan Province

To explore the potential of nitrogen (N) reduction in winter wheat and summer maize rotation system located at central Henan Province and pursue the best crop nutrient management strategy, the effects of reduced chemical N fertilization (RCN) combined with organic fertilizer application (OFA) on rotation system productivity and soil properties were assessed through a field experiment over three years. Ten treatments were employed in the present study: without fertilization (CK), farmers’ chemical N application rate (100%CNF), gradually reducing the chemical N application rate by 20% (80%CNF, 60%CNF, and 40%CNF), applying organic fertilizer alone (OF), and combined application of RCN and OFA (100%CNF+OF, 80%CNF+OF, 60%CNF+OF, and 40%CNF+OF). The differences in grain yields and aboveground biomass of wheat and maize, soil physicochemical properties, enzymes activities, and bacterial communities among treatments were analyzed. In the absence of OFA, the highest grain yield and aboveground biomass for wheat and maize were achieved with 80%CNF. Compared with 100%CNF, grain yield and aboveground biomass of wheat in 80%CNF significantly increased by 9.67%−10.55% and 30.53%−35.76%, respectively, and those of maize in 80%CNF significantly increased by 28.06%−51.42% and 29.62%−41.27%, respectively, suggesting that farmers’ conventional fertilization constituted excessive N supply. Furthermore, reducing 20% of farmers’ chemical N fertilization significantly increased rotation system productivity. The OFA could further release the space for N reduction, which was confirmed by the lack of statistical difference in wheat grain yield among 60%CNF+OF, 40%CNF+OF, and 80%CNF; and in maize grain yield between 60%CNF+OF and 80%CNF. The RCN alone or in combination with OFA did not alter the contents of soil organic matter, easily oxidized organic carbon, and total nitrogen, or pH, but substantially reduced the NO₃⁻-N content. The effects of RCN and OFA on the contents of NH₄⁺-N, available phosphorus, and available potassium varied among treatments. Compared to 100%CNF, RCN combined with OFA significantly increased the activities of urease and arylsulfatase and reduced the activity of β-glucosidase but did not affect the activities of sucrase, alkaline phosphatase, and dehydrogenase. The combination of RCN with OFA improved the α diversity of the bacterial community. This was indicated by the Simpson and evenness indices in 60%CNF+OF and 40%CNF+OF being significantly higher than those of 100%CNF. The RCN combined with OFA also significantly changed the β diversity of the bacterial community, which was more dependent on RCN rather than OFA. At the phylum level, Proteobacteria, Actinobacteria, and Acidobacteria dominated the bacterial community. The combinations of RCN and OFA significantly reduced the average relative abundance of Actinobacteria by 10.92%–14.39%. At the genus level, RCN combined with OFA significantly increased the average relative abundances of unclassified GP6 and Sphingomonas, whereas reduced the average relative abundances of Nocardioides, Kribbella, Lechevalieria, Promicromonospora, Massilia, Glycomyces, and Dongia. Redundancy analysis demonstrated that available potassium and NO₃⁻-N contents were the two most important soil environmental variables driving the bacterial community structure. In addition, co-occurrence network analysis also indicated that RCN strengthened the interactions of species in the bacterial community and yielded highly complex and connected associations among bacterial taxa. In the current study, reducing 60% and 40% of farmers’ chemical N fertilization in wheat and maize production, respectively, and in combination with 3000 kg∙hm⁻² of OFA in each season could maintain relatively high rotation productivity. This fertilization regime not only maximized the potential of N reduction in the rotation system but also improved soil microbial diversity and community structure. Notably, the optimal fertilization regime obtained here must be re-verified across crop varieties, organic fertilizers, and fields. Moreover, the long-term effects of RCN and OFA on yield and soil properties in winter wheat and summer maize rotation systems in central Henan Province need to be further assessed.