Abstract:Fertilization plays a vital role in maintaining the productivity of short-rotation plantations. Eucalyptus plantations are one of the fast-growing and high-yield plantations around the world and are numerous in south China. In order to improve nitrogen use efficiency, slow-release nitrogen fertilizers are being widely adopted. However, few studies have been done to assess the effect of slow-release fertilizer on the soil-atmosphere exchange of greenhouse gases (GHGs), such as carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4). To clarify temporal changes of soil GHGs fluxes following slow-release N fertilizer application, field control trials with four levels of N application (Control: 0 kg/hm2; Low N: 84.2 kg/hm2; Medium N: 166.8 kg/hm2; High N: 333.7 kg/hm2) were initiated in a Eucalyptus plantation in Guangxi, southern China. At the beginning of growing season, the nitrogen fertilizer, urea formaldehyde (a kind of slow-release fertilizer), was applied according to the local fertilization practice (once a year). Static chamber and gas chromatography techniques were used to quantify soil GHGs exchange monthly during the study period from May to November 2013. Environmental factors, such as soil temperature at 5 cm depth and soil water content at 10 cm depth, were synchronously monitored while the GHGs were collected. Before N application, no significant differences were observed for soil GHGs fluxes in all N application treatments. The results showed that (1) CO2 emission fluxes, N2O emission fluxes, and CH4 absorption fluxes under four levels of nitrogen application were 276.84-342.84 mg m-2 h-1, 17.64-375.34 μg m-2 h-1 and 29.65-39.70 μg m-2 h-1, respectively. Fertilization resulted in a remarkable but short increase in soil respiration over the first 2 to 3 months during the observation period, and the differences in soil respiration between the High N treatment and the control treatment were significant. Nitrogen application significantly increased the N2O emission and persisted for 5 to 6 months after fertilization. Each N application treatment had a significant effect on N2O emission. Moreover, High N treatment had a significantly negative effect on CH4 oxidation. (2) During the growing season, CO2 emission had a significantly positive correlation with N2O emission (P < 0.01), and CH4 uptake had a significantly negative correlation with both CO2 emission and N2O emission (P < 0.05 and P < 0.01, respectively). With the increase of the amount of fertilizer, the CO2 emission fluxes increased and CH4 oxidation fluxes decreased,respectively. (3) Soil temperature and soil water content were the main factors influencing soil respiration, N2O emission, and CH4 oxidation. Soil temperature and soil water content had significantly positive effects on CO2 and N2O emission fluxes, and soil temperature had significantly negative effects on CH4 absorption fluxes. In conclusion, during the growing season in a Eucalyptus plantation, slow-release nitrogen application not only significantly in creases soil N2O emission, but also had significant effects on CO2 emission and CH4 oxidation after High N treatment. Our results can provide parameters for accurately assessing the effects of slow-release nitrogen application on GHGs fluxes in a Eucalyptus plantation.
