Species Interactions Shape Nitrogen Utilization Characteristics and Influence Soil Quality in Jujube-Alfalfa...

Intercropping legumes offers a sustainable approach to enhance resource efficiency and yields, yet the effects of different legume densities and nitrogen addition levels on soil quality within such systems remain unclear. We conducted a comparative analysis of crop yield, nitrogen use efficiency, and soil quality between intercropping and monoculture systems, and further examined the effects of four planting densities (D1: 210 kg ha^-1, six rows; D2: 280 kg ha^-1, eight rows; D3: 350 kg ha^-1, ten rows) and four nitrogen application levels (N0: 0 kg ha^-1; N1: 80 kg ha^-1; N2: 160 kg ha^-1; N3: 240 kg ha^-1) within a jujube-alfalfa (Ziziphus jujuba Mill. and Medicago sativa L. respectively) intercropping system. The results showed that intercropping significantly enhanced land productivity within the agricultural system, with the highest yields (alfalfa: 13790 kg ha^-1; jujube: 3825 kg ha^-1) achieved at an alfalfa planting density of 280 kg ha^-1. While the intercropping systems generally improved productivity, an alfalfa planting density of 350 kg ha^-1 resulted in an actual yield loss due to excessive nutrient competition at higher densities. As the planting density of alfalfa increased, its competitive ratio declined, whereas the competitive ratio of jujube trees increased. Compared to monocropping systems, intercropping systems demonstrated a clear trend of enhanced nitrogen utilization efficiency and improved soil quality, particularly at an alfalfa planting density of 280 kg ha^-1. At an alfalfa density of 280 kg ha^-1, the intercropping system exhibited increases of 15.13% in nitrogen use efficiency (NUE), 46.60% in nitrogen partial factor productivity (NPFP), and 32.74% in nitrogen nutrition index (NNI), as well as improvements in soil quality of 19.53% at a depth of 0-20 cm and 15.59% at a depth of 20-40 cm, compared to the monoculture system. Further analysis revealed that nitrogen utilization efficiency initially increased and then decreased with a rising competitive ratio of alfalfa. Accordingly, soil quality was improved along with the enhanced nitrogen utilization efficiency. Thus, at an alfalfa planting density of 280 kg ha^-1, resource use efficiency and soil quality were maximized as a result of optimal interspecific competitiveness and the highest nitrogen use efficiency, with minimal influence from the application of nitrogen fertilizer.