Nitrification And Denitrification In Subalpine Coniferous Forest Soil Of Western Sichuan, China

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 soil nitrogen cycle that are involved in the control of long-term nitrogen losses in ecosystems through nitrate leaching and gaseous N losses. To better understand how nitrification and denitrification work in subalpine coniferous forest soil and how they're affected by the 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. Furthermore, gross nitrification rates were measured during growing season of plants in the natural coniferous forest, the coniferous broadleaved forest and the 40 year-spruce plantation. The factors that may control nitrification rates were investigated. In order to study the direct effects of soil temperature and water content on nitrification, we also designed a controlled laboratory experiment and developed the linear and quadratic equations to describe these effects. The main results are as follows. 1. Forest restoration stage had no significant effect on gross nitrification rates (One-way ANOVA, P<0.05). There was no significant difference in the temperature coefficient (Q10) for gross nitrification rate among all the forest types (One-way ANOVA, P<0.05). 2. There was obvious seasonal variation for gross nitrification rates in the natural coniferous forest, the coniferous broadleaved forest and the 40 year-spruce plantation soil. The highest value in the natural coniferous forest was found in July, and September in the coniferous broadleaved forest, August in the 40 year-spruce plantation. 3. The controlled laboratory experiment suggests that soil temperature and water content were positively related to gross nitrification rates (P<0.05) and had a profound interactive effect. Water content might play a more important role in the variations of nitrification rate compared with soil temperature. 4. Gross nitrification rates were not related to the pH value, the organic matter, total nitrogen and C/N. Litterfall was negatively related to gross nitrification rates in the coniferous broadleaved forest and the 40 year-spruce plantation, but there were strong positive correlations between nitrification rates and litterfall from June to September in the natural coniferous forest (P<0.01). 5. Denitrification rates in all the forest soils were low and forest restoration stage had no significant effect on denitrification rates (One-way ANOVA, P<0.05). Denitrification rates were not closely related to water content, the pH value, organic matter and total nitrogen. Nevertheless, we found that C/N obviously affected denitrification rates (P<0.05). 6. Gross nitrification is more responsible for the nitrogen loss from the soils, compared with denitrification.