Effects of Different Phosphorus Fertilizer Application Rates on Photosynthetic Characteristics, Yield and Water Use Efficiency of Broad Bean Mulched in Alpine Region

To investigate the effects of different mulching methods and phosphorus (P) fertilizer application levels on the photosynthetic characteristics, yield, and water use efficiency of broad bean, so as to provide data support for the selection of broad bean planting modes and soil nutrient management practices in alpine regions.

Used spring broad bean 'Qinghai No.13' as experimental material, a field experiment was conducted to investigate the effects of different mulching methods and P fertilizer application levels on soil hydrothermal conditions, the photosynthetic characteristics of broad bean, yield, and water use efficiency in the eastern of Qinghai Province from 2020 to 2023. For plot setup, the random block design was used, and three mulching methods were set up, i.e. double ridges and furrows mulch (DRM), and three ridges and furrows mulch (TRM), and no mulch (NMF), with three P fertilizer application levels (P0, no fertilizer; P1, 9.10 kg P·hm^-2; P2, 18.2 kg P·hm^-2).

(1) Compared with NMF, DRM and TRM treatments increased the daily mean soil temperature within the 10 cm soil layer by 16.1% to 20.5% and 16.7% to 23.0%, respectively, and also increased soil water content and storage in the 0-2 m soil layer of broad bean growing season. (2) Compared with NMF, DRM and TRM treatments decreased the leaf transpiration rate, stomatal conductance, intercellular CO₂ concentration, net photosynthesis rate and leaf area index during the flowering and podding periods, and deceased grain yield, aboveground biomass, and water use efficiency of broad bean too. The highest mean annual grain yield was 2 273 kg·hm^-2 under NMF treatment, followed by 1 030 kg·hm^-2 under TRM and 943 kg·hm^-2 under DRM. (3) P fertilizer application enhanced the net photosynthetic rate, transpiration rate, intercellular CO₂ concentration, stomatal conductance, and leaf area index in the flowering and podding periods of broad bean under NMF and DRM treatments, but the trends under TRM treatment were reversed. (4) The correlation analysis indicated that the grain yield, aboveground biomass, and water use efficiency were significantly positive correlated with the net photosynthetic rate, transpiration rate, intercellular CO₂ concentration, stomatal conductance, and leaf area index, and were significantly negative correlated with the daily mean soil temperature of the growing seasons.

Ridge-furrow plastic film mulching treatments (DRM and TRM) inhibited the growth of broad bean in later growth stages, slowed down the growth rate of leaf area index, reduced photosynthesis, which resulted in the decreased grain yield, aboveground biomass, and water use efficiency. Under the conditions of this experiment, no mulch with flat planting combined with 18.2 kg P·hm^-2 phosphorus fertilizer significantly enhanced photosynthetic area and photosynthesis, thus increase the grain yield and water use efficiency of broad bean, which could be used as an effective practice to increase broad bean productivity in alpine region.