Nitrogen allocation and utilization in alpine meadows of the Sanjiangyuan region under exogenous additions of different nitrogen forms

Objective We studied the absorption and utilization of different N forms by alpine meadow plants, and the fate of residual N in the soil to provide a theoretical basis for efficient and environmentally friendly fertilization of alpine meadows in the Sanjiangyuan area.

Methods A field microplot experiment was carried out using the ¹⁵N tracer method at the Alpine Meadow Experimental Station in Chengduo County, Qinghai Province, from June 2020 to September 2021. The three N sources tested were (¹⁵NH₄)₂SO₄, Ca(¹⁵NO₃)₂, and CO(¹⁵NH₂)₂, applied at N 300 kg/hm² in early 2020. The N content and uptake in the above- and under-ground parts of forages were measured, and the N content in the 0–15 cm and 15–40 cm soil layers was analyzed in 2020 and 2021.

Results 1) In the same year of N application, compared with Ca(¹⁵NO₃)₂ and (¹⁵NH₄)₂SO₄, CO(¹⁵NH₂)₂ increased N absorption by 276.4% and 48.1% in the aboveground part, 360.3% and 129.2% in the underground part of forages, 275.6% and 78.4% for the N residual amount in the 0–15 cm soil layer, and 240.1% and 115.6% in the 15–30 cm soil layer (P<0.05) . 2) In the following year of N application, compared with Ca(¹⁵NO₃)₂ and (¹⁵NH₄)₂SO₄, CO(¹⁵NH₂)₂ increased the N absorption in the aboveground part by 206.9% and 66.1%, 473.2% and 234.4% in the underground part of forages, 635.6% and 129.5% for residual N amount in 0–15 cm soil layer, and 22.5% and 188.2% in 15–30 cm soil layer (P<0.05). 3) In the same year of N application, the fate of (¹⁵NH₄)₂SO₄ and Ca(¹⁵NO₃)₂ were in the order ¹⁵N loss>¹⁵N soil residue>¹⁵N plant uptake and ¹⁵N soil residue>¹⁵N plant uptake>¹⁵N loss for CO(¹⁵NH₂)₂. In the following year of N application, the fate of (¹⁵NH₄)₂SO₄ and Ca(¹⁵NO₃)₂ were: ¹⁵N loss>¹⁵N plant absorption>¹⁵N soil residue and ¹⁵N plant absorption>¹⁵N loss>¹⁵N soil residue for CO(¹⁵NH₂)₂.

Conclusion CO(¹⁵NH₂)₂ promotes a high nitrogen uptake rate by forages, lower loss rate, and higher soil residual nitrogen. Therefore, it can be used as the preferred nitrogen form in the alpine meadow pasture in the Sanjiangyuan region.