Effects of Water and Nitrogen Control on the Growth Physiology, Yields, and Economic Benefits of Lycium barbarum Plants in a Lycium barbarum + Alfalfa System

In the production of economic forests, there are common issues such as excessive application of water and fertilizer, redundant plant growth, and low economic benefits. Reasonable water and fertilizer management can not only help address these problems but also improve the absorption and use efficiency of water and fertilizer resources by plants, promoting the green and efficient development of the fruit and forestry industry. In order to explore a suitable water and nitrogen management mode for Lycium barbarum, field experiments were conducted in this study from 2021 to 2022. Specifically, four irrigation modes (according to the proportion ratio of soil moisture content to field moisture capacity θf, 45-55% θf (W1, severe water deficiency), 55-65% θf (W2, moderate water deficiency), 65-75% θf (W3, mild water deficiency), and 75-85% θf (W4, sufficient irrigation)) and four nitrogen application levels (0 kg·ha-1 (N0, no nitrogen application), 150 kg·ha-1 (N1, low nitrogen application level), 300 kg·ha-1 (N2, medium nitrogen application level), and 450 kg·ha-1 (N3, high nitrogen application level)) were set up to analyze the influences of water and nitrogen control on the plant height, stem diameter, chlorophyll content, photosynthetic characteristics and yield, and economic benefits of Lycium barbarum in the Lycium barbarum + Alfalfa system. The study results show that the plant height and stem diameter increment of Lycium barbarum increase with the irrigation amount, increasing first and then decreasing with the increase in the nitrogen application level. Meanwhile, the chlorophyll contents in Lycium barbarum continuously increase throughout their growth periods, with Lycium barbarum treated with W4N2 during all growth periods presenting the highest contents of chlorophyll. In a Lycium barbarum + Alfalfa system, the daily variation curve of the Lycium barbarum net photosynthetic rate presents a unimodal pattern, with maximum values of the daily average net photosynthetic rate and daily carboxylation rate appearing among W4N2-treated plants (19.56 μmol·m-2·s-1 and 157.06 mmol·m-2·s-1). Meanwhile, the transpiration rates of Lycium barbarum plants continuously decrease with the increased degree of water deficiency and decreased nitrogen application level. W1N2-treated plants exhibit the highest leaf daily average water use efficiency (3.31 μmol·s-1), presenting an increase of 0.50-10.47% in efficiency compared with plants under other treatments. The coupling of water and nitrogen has significantly improved the yields and economic benefits of Lycium barbarum plants, with W4N2-treated and W3N2-treated plants presenting the highest dried fruit yield (2623.07 kg·ha-1) and net income (50,700 CNY·ha-1), respectively. Furthermore, compared with other treatment methods, these two treatment methods (W4N2 and W3N2) exhibit increases of 4.04-84.08% and 3.89-123.35% in dried fruit yield and net income indexes, respectively. Regression analysis shows that, in a Lycium barbarum + Alfalfa system, both high yields and economic benefits of Lycium barbarum plants can be achieved using an irrigation amount of 4367.33-4415.07 m3·ha-1 and a nitrogen application level of 339.80-367.35 kg·ha-1. This study can provide a reference for improving the productivity of Lycium barbarum plants and achieving a rational supply of water and nitrogen in Lyciun barbarum + Alfalfa systems in the Yellow River Irrigation Area of Gansu, China, and other similar ecological areas.