Effects of optimal irrigation and nitrogen coupling on Bromus inermis yield, nitrogen, phosphorus, and potassium stoichiometric characteristics

Objectives Reasonable regulation of water and nitrogen effectively ensures artificial grassland's efficient and sustainable development. This study aimed to explore the appropriate combination of irrigation and nitrogen supply for the high yield and nutrient content of Bromus inermis and understand the response of the grassland ecosystem to the combined irrigation and nitrogen effect.

Methods A field experiment was conducted in the artificial grassland in the Hexi Corridor region, taking Bromus inermis as the test material. We used sprinkler irrigation technology to compensate for the soil moisture content at the joining stage, corresponding to 75%–85% (W₀), 45%–85% (W₁), 55%–85% (W₂), and 65%–85% (W₃) of field water capacity. Four N application rates of 0, 60, 120, and 180 kg/hm² (N₀, N₁, N₂ and N₃) were set up under each soil moisture level, giving 16 irrigation and nitrogen combinations. The grasses were harvested three times to determine dry matter yield. The stoichiometric characteristics of nitrogen (N), phosphorus (P) and potassium (K) were recorded in each cut.

Results Irrigation and N addition increased the total yield of Bromus inermis by 22.0%–58.1% and 31.1%–88.9%, respectively. The interaction of the two factors (P<0.01) affected Bromus inermis yield. Compared with N₀W₁, N₁W₂, N₂W₃ and N₃W₀ increased Bromus inermis yield by 66.6%, 158%, and 144.7%, respectively, with the maximum yield recorded under N₂W₃ treatment (12260 kg/hm²). For a peak production of 13308 kg/hm² Bromus inermis, regression analysis revealed that the N application rate and irrigation were 152.3 kg/hm² and 552.1 mm, respectively. Water compensation increased the N, P, and K content of Bromus inermis. N addition (P<0.05) increased that of N and K but decreased P (P<0.01) content of Bromus inermis. The interaction of the factors imposed did not affect the N, P, and K content of Bromus inermis. The N/P and K/P ratios of Bromus inermis increased at first, then reduced as irrigation levels increased; N application rate enhanced the plant N/P and K/P ratios, but no interaction effect was found. Regression analysis showed that N and K contents reached the maximum (23.6 mg/g and 23.1 mg/g) at 136.8 kg/hm², 546.3 mm and 139.3 kg/hm², 552.5 mm, respectively. For P content, the threshold could not be determined within the test range adopted in this study.

Conclusions Irrigation and N application could increase the yield and NPK contents of Bromus inermis. However, the N application rate had a greater yield increase effect than irrigation. Irrigation and N application had a coupling effect on the yield of Bromus inermis within a short period, but not N, P, and K stoichiometry. The coupling effect of irrigation and N may be related to soil drought degree, N application rate and grass growth period. According to the regression prediction, the suitable range of irrigation and N input for Bromus inermis in the Hexi region was 546.3–552.5 mm and 136.8–152.3 kg/hm².