Alpine Meadow Could Be Better Restored Before Moderate Degradation Using Inorganic Nitrogen on the Tibetan Plateau

ABSTRACT Nitrogen (N) fertilization is considered a powerful alternative approach to restore degraded alpine meadows. However, whether the form of available N matters and its efficiency along a degradation gradient remains largely unexplored. A four‐year N addition experiment with different available N forms (NH 4 + ‐N, NO 3 − ‐N, and Glycine) was carried out on the undegraded, moderately (MD) and severely degraded (SD) alpine meadows on the Tibetan Plateau. Plant aboveground productivity was greatly increased in undegraded and MD alpine meadows but belowground productivity was enhanced in SD alpine meadow, especially in the two inorganic N treatments. When inorganic N was added, plants accumulated more N compared to the control in undegraded alpine meadow. Plants accumulated more N in the MD alpine meadow than in the undegraded alpine meadow in all forms of N addition treatments as the plant N uptake rate was almost doubled in the MD alpine meadow when N was added. In the SD alpine meadow, most of the added N moved to the microbial N pool with the largest increase observed in the NO 3 − ‐N treatment. Leaching remained almost unchanged in NH 4 + ‐N and Glycine in undegraded and MD alpine meadows but increased in the SD alpine meadow, especially in the NO 3 − ‐N treatment. However, leaching was low, accounting for less than 5% of the added N even in the SD alpine meadow. Our results indicate (1) the added N was mostly up taken and retained in plants or microorganisms regardless of N forms, (2) inorganic N is more efficient in increasing plant productivity for MD but not for SD alpine meadow, (3) fertilization using inorganic N to restore the degraded alpine meadow is efficient for MD but not for SD alpine meadows.