Nitrification and denitrification in subalpine coniferous forests of different restoration stages in western Sichuan, China

Abstract

Nitrification is the biological conversion of organic or inorganic nitrogen compounds from a reduced to a more oxidized state. Denitrification is generally referred to as the microbial reduction of nitrate to nitrite and further gaseous forms of nitric oxide, nitrous oxide and molecular nitrogen. They are functionally interconnected processes in the soil nitrogen cycle that are involved in the control of longterm nitrogen losses in ecosystems through nitrate leaching and gaseous N losses. In order to better understand how nitrification and denitrification change during the process of ecosystem restoration and how they are affected by various controlling factors, gross nitrification rates and denitrification rates were determined using the barometric process separation (BaPS) technique in subalpine coniferous forests of different restoration stages. The results showed that forest restoration stage had no significant effects on gross nitrification rates or denitrification rates (One-way ANOVA (analysis of variance), ρ⟨0.05). There was no significant difference in the temperature coefficient (Q₁₀) for gross nitrification rate among all the forest sites (One-way ANOVA, ρ⟨<0.05). Gross nitrification rates were positively correlated with water content ( ρ⟨0.05), but not with soil pH, organic matter, total nitrogen, or C/N ratios. Denitrification rates in all the forest soils were low and not closely correlated with water content, soil pH, organic matter, or total nitrogen. Nevertheless, we found that C/N ratios obviously affected denitrification rates ( ρ⟨0.05). Results from this research suggest that gross nitrification is more responsible for the nitrogen loss from soils compared with denitrification.