Effects of Reducing Nitrogen Application on Leaf Stay-Green, Photosynthetic Characteristics and System Yield in Maize-Soybean Relay Strip Intercropping

The aim of this study was to explore the characteristics of leaf green retention, photosynthesis and system yield of maize and soybean under different planting modes and nitrogen (N) application levels.

The effects of planting methods (maize monoculture (MM), soybean monoculture (SS), maize intercropping (IM), soybean intercropping (IS)) and N application levels (0 N application (NN), reduced N application (RN: 180 kg N·hm^-2) and constant N application (CN: 240 kg N·hm^-2)) on leaf stay-green, photosynthetic characteristics, dry matter accumulation and system yield of maize and soybean leaves were studied by field positioning experiment.

The maize yield increased with the increase of N application, and the soybean yield increased first and then decreased with the increase of N application; Under RN, the seed dry matter accumulation of IM was the largest, the total yield of maize-soybean intercropping system was the highest, and the system productivity index (SPI) was the largest too. Under intercropping, the leaf green period of each crop was longer, the photosynthetic characteristics were more stable than that of monoculture, and better than that of monoculture at seed formation stage; Under all N application levels, the percentage of green leaves under intercropping treatment was significantly higher than that under monoculture. The maximum green leaf attenuation rate of IM appeared 7 d, 5 d and 1d later than that of MM, respectively, while IS was 7 d, 0 d and 11 d later than SS, respectively. Compared with monoculture, the intercropping could significantly reduce the average attenuation rate of maize leaves, prolong the days of maximum attenuation rate and reduce the attenuation degree of green leaves. The photosynthetic rate of each crop was higher under intercropping than monoculture, and the reduced N application was higher than the constant N application. At R2 stage, the photochemical quenching coefficient (QP) under IM was 12.78% higher than that under MM, and the non-photochemical quenching coefficient (NPQ) was 21.30% lower; NPQ decreased with the increase of N application level, while the ratio of RN to NN decreased by 17.11%. The fluctuation range of SPAD value of intercropping was weaker than that of monoculture, and showed a stable upward trend. In maize R2 stage, IM was 34.52% higher than MM; In soybean R2 and R6 stage, IS was 10.39% and 29.48% higher than SS, respectively, and the SPAD value of RN was the highest. At R2 stage, IMRN was 17.46% higher than IMNN, and MMRN was 35.02% higher than MMNN; in soybean R6 stage, ISRN was 7.71% and 6.67% higher than that of ISNN and ISCN, and SSRN was 10.03% higher than that of SSCN.

Under reduced N application condition, the maize-soybean intercropping significantly prolonged the green holding period of leaves; After flowering, the photosynthetic rate of leaves, the function of PS Ⅱ photosynthetic mechanism and chlorophyll remained at a high level were more stable than that of monoculture, and the accumulation of seed dry matter was enhanced, which gave full play to the production potential of maize and increased the yield of soybean, so that the total yield of intercropping system was significantly increased.