What Impact Does Nitrogen Have on Alpine Grasslands of the Tibetan Plateau?

Scientists reviewed 57 studies to see how nitrogen affects the alpine grasslands of the Qinghai‑Tibet Plateau. Adding nitrogen makes plants grow more: above‑ground biomass rises by 43% and below‑ground by 16%. Dissolved organic carbon in the soil also climbs by 11%, and total soil organic carbon (SOC) increases about 4%. Yet the picture isn’t all positive. Soil respiration – CO₂ released from the ground – drops by 5%, while microbial respiration jumps by 22%. Nitrogen makes the soil more acidic (pH falls by 0.2 units) and cuts the C/N ratio by nearly 2 points. Soil nitrate and ammonium drop by 52% and 20% respectively. Microbial communities shift: their nitrogen content rises by 15%, carbon falls by almost 3%, and the fungal‑to‑bacterial ratio falls by 31%. In short, short‑term additions (≤5 years, ≤30 kg N ha⁻¹ yr⁻¹) boost carbon storage thanks to more plant growth and microbial sequestration. Over the long term, acidification and microbial changes promote carbon loss. Experts advise keeping nitrogen inputs below 10 kg N ha⁻¹ yr⁻¹ to protect these high‑altitude ecosystems.

The meta‑analysis pooled 794 observations, allowing separation of three key variables: nitrogen dose, application duration, and chemical form (NH₄⁺ vs NO₃⁻). Lower doses (≤30 kg N ha⁻¹ yr⁻¹) showed a linear increase in plant biomass, likely because nitrogen is a limiting nutrient in these typically nutrient‑poor soils. The biomass boost supplies more plant residues to the soil, fueling SOC formation through necromass processes. Simultaneously, microbial respiration rose by 21.9%, indicating heightened decomposer activity, yet total soil respiration fell, suggesting that carbon fixation by plants outweighs CO₂ loss. The pH reduction (‑0.20 units) is significant because acidification can inhibit enzymes involved in lignocellulose decomposition, favoring organic carbon preservation. However, prolonged acidification reshapes the microbial community: the fungal‑to‑bacterial ratio (F/B) drops by 31%, indicating a shift from efficient fungal decomposers to faster but less carbon‑stabilizing bacteria. This shift correlates with a decline in stable microbial carbon (MBC) and an increase in microbial nitrogen (MBN), reflecting microbial adaptation to a more nitrogen‑rich yet acidic environment. Long‑term implications include a potential reduction in soil carbon sequestration capacity, especially if nitrogen additions exceed 10 kg N ha⁻¹ yr⁻¹, a threshold beyond which acidification and microbial community changes outweigh initial benefits. Sustainable management must monitor not only nitrogen quantity but also pH, C/N, and F/B ratios, integrating mitigation practices such as liming or rotating with leguminous species to balance the nitrogen cycle.