Effects of Green Manure and Wheat Straw Combined Returning Application on Photosynthetic Characteristics and Yield of Spring Wheat Under Reduced Nitrogen Levels

The aim of this study was to investigate the effects of green manure combined with wheat straw returning application on grain yield and leaf photosynthetic characteristics of spring wheat (Triticum aestivum L.) under different nitrogen reduction levels, which could provide theoretical basis and technical support for establishing spring wheat multiple cropping with green manure saving and efficient production pattern.

The experiment was conducted at the Wuwei Oasis Agricultural Experimental Station of Gansu Agricultural University in 2022 and 2023. The split-plot designed experiment was adopted. The main plot was designed with different returning materials at three levels, including green manure and wheat straw combined returning application (W-GS), green manure returning application alone (W-G), and fallowing after wheat harvesting (W-F). The split plot was designed with four nitrogen application levels, including conventional nitrogen application rate (N1, 225 kg N·hm^-2), nitrogen application rate reduced about 15% (N2,190 kg N·hm^-2), about 30% (N3,155 kg N·hm^-2), and about 45% (N4, 120 kg N·hm^-2). The chlorophyll value (SPAD value), photosynthetic characteristics and grain yield of the top expending leaf of spring wheat was measured, and the leaf water use efficiency and light energy utilization efficiency was analyzed.

Negative effect of nitrogen reduced application was observed on the grain yield of spring wheat, however, this effect was compensated even the grain yield of spring wheat was increased by green manure combined with wheat straw returning application. With the same nitrogen application rate, compared with that of W-F, the grain yield of spring wheat under W-GS and W-G increased by 15.3% and 9.4%, respectively, and the grain yield of spring wheat with W-GS was increased by 5.4% than that of W-G. The grain yield of spring wheat under W-F was decreased by the decreasing of nitrogen application rate, while that of W-GS had no significant difference between N3 and N1. The grain yield under W-GSN3 was not reduced compared with that under W-GSN1 and W-FN1. The photosynthetic characteristics of spring wheat leaf with nitrogen reduced application rate was improved by green manure combined with wheat straw returning application. The SPAD value, net photosynthetic rate (P_n), stomatal conductance (G_s), transpiration rate (T_r) and leaf water use efficiency (LWUE) of spring wheat leaf under W-GS were significantly increased, respectively, compared with that of W-G and W-F under the same nitrogen application rate. With the decreasing of nitrogen application rate, the SPAD value, P_n, G_s, T_r and LWUE of spring wheat leaf under W-F was decreased significantly, however, there was no significant difference between N3 and N1 in W-GS. Comparison between combined treatments, there was no significant difference in photosynthetic characteristics of spring wheat under W-GSN3 compared with W-GSN1 and W-FN1. The light use efficiency (LUE) of spring wheat was significantly increased by 5.2% W-GS than that of W-F. The LUE of spring wheat with W-F decreased by the decreasing of nitrogen application rate, however, the LUE of spring wheat with W-GS was not significant difference between N3 and N1. There was no significant difference was observed in the LUE of spring wheat under W-GSN3 compared with that under W-GSN1 and W-FN1. The results of the path analysis indicated that, the grain yield of spring wheat was mainly determined by thousand kernel weight, which was affected by the regulation on net photosynthetic rate and transpiration rate via stomatal conductance of spring wheat leaf.

The green manure and wheat straw combined returning application mainly increased the thousand kernel weight of spring wheat by improving the photosynthetic characteristics of spring wheat leaf, and finally did not reduce the grain yield of spring wheat under 30% nitrogen reduced application rate. Therefore, the green manure and wheat straw combined returning application under nitrogen application rate of 155 kg·hm^-2 was recommended as a nitrogen saving and efficient production pattern of spring wheat multiple cropping with green manure in arid oasis irrigation region.