Effects of phosphorus application rates on winter wheat yield and phosphorus use efficiency in drylands of South Shanxi Province

Although rational application of phosphate could increase winter wheat grain yield, phosphate fertilizer can easily be transformed into slowly available or insoluble phosphates in calcareous soils. This could limit phosphorus use efficiency with recovery efficiency of only 5%~25%. It is therefore critical to determine optimal phosphate application rate to enhance high production efficiency and yield of winter wheat. Phosphorus use efficiency is generally affected by several factors including wheat variety, soil fertility and ecological conditions. This implies that results of optimal phosphate application rate could be different in regions with different hydro-climatic and agronomic conditions. In this study, a field experiment was conducted to analyze the effects of phosphate application rates by evaluating selected stress-resistance indexes, including contents of proline (Pro), malondialdehyde (MDA), and nitrate reductase (NR) activity of flag leaf of winter wheat. Winter wheat cultivar "Linhan 6" yield and phosphorus use efficiency were also analyzed along with the distribution of soil phosphorus fractions under four phosphate levels (P₂O₅ rates of 0 kg·hm^-2, 60 kg·hm^-2, 120 kg·hm^-2, and 180 kg·hm^-2) in Xiangfen County. The county is located in the rain-fed cultivation area of Shanxi Province. The study explored suitable phosphate application rate and laid the scientific basis for pro-environmental production of wheat under rain-fed conditions. The results showed that winter wheat stress-resistance strengthened which subsequently increased spike number and yield of winter wheat with increasing phosphate rate. However, no significant difference was observed in kernel number and 1000-kernel weight under different phosphate rates. In the 0~120 kg(P₂O₅)·hm^-2 range, NR activity of flag-leaf, spike number and yield of winter wheat increased significantly. However, MDA and Pro contents of flag-leaf decreased significantly with increasing phosphate rate. When phosphate application rate reached 180 kg(P₂O₅)·hm^-2, MDA and Pro contents slightly decreased, or even increased with increasing phosphate application rate. With the exception of the heading stage, no significant difference was also noted in spike number, yield and NR activity of winter wheat. The phosphate fertilizer use efficiency was relatively low (9%~13%) because of P-fixation. The highest phosphate fertilizer use efficiency was in the application range of 60~120 kg(P₂O₅)·hm^-2. The sequence of phosphate fractions in the 1.0 m soil profile was HCl-Pi > Residual-P > HCl-Po > NaOH-Pi > NaHCO₃-Pi > NaOH-Po > H₂O-Pi > NaHCO₃-Po > H₂O-Po. Most of phosphate fractions were HCl-P and Residual-P, accounting for 75% and 20% (in terms of proportion) of soil total phosphate. Also H₂O-P, NaHCO₃-P and NaOH-P together accounted for 5% of soil total phosphate. The applied phosphorus mainly occurred in the 0~20 cm soil layer during the growth period. The amount of increase in different Hedley phosphorus was 0~39.11 mg·kg^-1 and P-fixation increased with increasing phosphate fertilizer rate. Based on a comprehensive stress-resistance index, yield and phosphorus use efficiency, the recommended optimal phosphate application rate was 120 kg(P₂O₅)·hm^-2 in the rain-fed winter wheat conditions in southern Shanxi Province or in drylands with similar ecological conditions.