Effects of nitrogen and phosphorus addition on forage and soil nutrients in alpine grasslands around Qinghai Lake

This study explored the suitable amount of nitrogen (N) and phosphorus (P) in the alpine grassland in the surrounding area around Qinghai Lake. Using a randomized block design, three levels of N (35, 70, 140 kg·ha^–1) and three levels of P (26, 52, 65 kg·ha^–1) were set, and all combinations of N and P levels were used: N₁P₁(35, 26 kg·ha^–1), N₁P₂ (35, 52 kg·ha^–1), N₁P₃ (35, 65 kg·ha^–1), N₂P₁ (70, 26 kg·ha^–1), N₂P₂ (70, 52 kg·ha^–1), N₂P₃ (70, 65 kg·ha^–1), N₃P₁ (140, 26 kg·ha^–1), N₃P₂ (140,52 kg·ha^–1), N₃P₃ (140, 65 kg·ha^–1). These nine fertilization gradients, as well as no fertilization as a control, were used to study the effects of combined N and P application on forage nutrients and soil nutrients. The results showed that: 1) combined N and P significantly increased the total phosphorus content of forage grasses and forage (P < 0.05). With the increase of fertilization amount, the content of crude fiber of forage decreased, while the crude fat and crude of forage. The increase in ash content plays a catalytic role. 2) Combined N and P significantly increased the total nutrient of the soil (P < 0.05). The soil available phosphorus, ammonia nitrogen and nitrate nitrogen increased by 7.41%～150.00%, 3.92%～108.28% and 3.33%～169.73%, respectively. 3) The fertilization range for the best economic benefit of N and P application was 70 ～ 140 kg·ha^–1 and 26 ～ 52 kg·ha^–1, respectively; these ranges resulted in higher dry weight of pasture, which increased by 1 646.74～5 797.68 CNY·ha^–1 compared to the control.