Contrasting effects of ammonium and nitrate addition on soil organic carbon in Zoige Plateau peatland

Peatland soils, typically nitrogen (N)-limited, are important carbon (C) reservoirs and may be significantly affected by N deposition. Nitrate (NO 3 − -N) and ammonium (NH 4 + -N) are the primary forms of N deposition, with substantial spatial variability. However, how these N forms affect soil organic carbon (SOC) in Plateau peatlands remains unclear. To address this gap, this study stimulated various addition rate of NO 3 − -N and NH 4 + -N (ranging from 0 to 400 kg N ha −1 yr −1 ) to soils sampled from Zoige peatland, China, and then investigated SOC changes and the possible mechanisms driving these responses after a 45-day incubation. Compared to soil without N addition, soils with NO 3 − -N addition had lower SOC, while those with NH 4 + -N addition had higher SOC. NO 3 − -N addition increased enzyme activities and the abundance of soil organic matter (SOM)-decomposing microbes (e.g., Rhodoferax and Nocardioides ), leading to greater dissolved organic C leaching and SOC loss. In contrast, NH 4 + -N addition decreased soil pH, which inhibited the growth of SOM-decomposing microbes, thereby protecting SOC degradation. In summary, in N-limited peatland soil, NO 3 − -N deposition may accelerate SOC decomposition and promote C loss, whereas NH 4 + -N deposition may help mitigate SOC decomposition and prevent C loss.