Effects of planting density and film mulching on the integrated productivity of soybean in young apple orchard of the Loess Plateau

Young apple tree and soybean intercropping is an important approach to the mutual benefits winnings between ecology and economic for apple production in Loess Plateau of Northwest China. The intercropped soybean is not only the drive force for ecological benefit, but also the dominant component for economical benefit under the intercropping system in young apple orchard. Few studies focused on productivity of intercropped-soybean under the soil condition in orchard, and evaluated its integrated productivity from the perspective of grain yield, protein quality, biological nitrogen fixation (BNF) and nitrogen byproduct, nitrogen in straw and straws included. The present study aimed to determine the integrated practices for the optimally integrated productivity of soybean by planting density and film mulching. Soybean crops with three densities (high density: 24×10⁴ plant·hm^-2; medium density: 16×10⁴ plant·hm^-2; low density: 10×10⁴ plant·hm^-2) and two mulching practices (film mulching, without film mulching) were planted in the young apple orchard of the Loess Plateau in 2018 and 2019. Indicators of grain yield, grain protein content, BNF rate, biomass and nitrogen distribution in organs, etc. were investigated under different treatments. The results showed that planting density was dominant for soybean grain yield and protein yield. The similar superiority over the high density was found both for grain yield and protein yield under the medium density and low density. Relative to high density, soybean grain yield and protein yield was increased by 23.8% and 24.5% on average of medium and low density, respectively. Under medium and low density, film mulching was beneficial to synergistic interaction between root nodules biological nitrogen fixation (BNF) and shoot development, and then greatly boosted accumulation of straw dry matter and nitrogen in straws while without decreasing soybean grain yield significantly. In detail, under film mulching for soybean crops with medium density, straw dry matter, nitrogen accumulation in straws and BNF from straws was increased by 1.76 t·hm^-2, 39.50 kg·hm^-2 and 32.42 kg·hm^-2 respectively, and contributed 98%, 79% and 67% to the improvement of soybean total dry matter, total nitrogen accumulation and total BNF in corresponding. Under film mulching for soybean crops with low density, straw dry matter, nitrogen accumulation in straws and BNF from straws was increased by 1.81 t·hm^-2, 33.70 kg·hm^-2 and 25.41 kg·hm^-2 respectively, and contributed 102%, 58% and 59% to the improvement of soybean total dry matter, total nitrogen accumulation and total BNF in corresponding. In addition, film mulching boosted protein content in grain by 9.6% on average of three densities, mainly due to the coaction from a slight decline in grain yield and increase of nitrogen absorption by soybean plant. Finally, soybean crops with medium density and film mulching achieved the optimally integrated productivity that comprised of grain yield of 3.55 t·hm^-2, protein yield of 1.27 t·hm^-2, BNF of 256.80 kg·hm^-2, nitrogen accumulation in straws of 134.87 kg·hm^-2, straw dry matter of 6.84 t·hm^-2 and the better grain protein quality in relative to its no mulching. This result indicates that the integrated practice by higher (medium) density with plastic film mulching for soybean can not only ensure plentiful grains with better protein quality, but more importantly, it is conducive to harvest more straws and nitrogen accumulation in straws that can be returned to local farmland for sustainable intensive production.