| Nitrous oxide (N2O) and nitric oxide (NO), two important greenhouses gases, have strong direct and indirect greenhouse effects. Agricultural soils are important sources of N2O and NO, and the emissions of N2O and NO are intimately linked to local climate (e.g., temperature and precipitation), soil properties and management measures such as fertilization, irrigation and plowing. As one of the major crops in our country, the planting areas of cotton are approximately 4% of total farmland. High nitrogen (N) and sufficient water are needed during the growing period for cotton productions, which could promote more emissions of N2O and NO. So far, however, studies on N2O and NO emissions from cotton fields are rare.The objective of this study is to know the emissions of N2O and NO from an irrigated cotton field in Shanxi Province. Using a manual static opaque chamber/gas chromatograph and chemiluminescence measuring systems, the fluxes of N2O and NO were measured year-round from usual-nitrogen and no-nitrogen treatments. The results showed as follows:Cotton fields were the sources of N2O and NO during the measuring period. In the usual-nitrogen treatment (7.5 kg/mu Type urea-based compound [N-P2O5-K2O (24 %-10%-6%)],27.0 kg N ha-1), the mean fluxes of N2O and NO were 8.3-124.0 and-6.7-77.0μg N m-2 h-1, with the average (±tandard error) 31.4±2.0 and 8.2±0.9μg N m-2 h-1, respectively. For the no-nitrogen treatment, the mean fluxes of N2O and NO were 5.9-65.5 and-6.7-26.6μg N m-2 h-1, and averaged (±tandard error) 22.7±1.1 and 6.5±0.6μg N m-2 h-1, respectively. The cumulative emissions from the treatments of fertilization and unfertilization were 2.36±0.18 and 1.97±0.11 kg N2O-N ha-1yr-1 for N2O, and were 0.72±0.02 and 0.66±0.01 kg NO-N ha-1yr-1 for NO, respectively. The direct emission factors were calculated to 1.44%±0.33% for N2O and 0.22%±0.10 % for NO.The emissions of N2O and NO from cotton field showed seasonal fluctuations, with the intensive emissions appearing in spring/summer and low emission occurring in autumn/winter. The peak of N2O emission (124.0μg N2O-N m-2 h-1) was observed in twelve days after fertilization with the emission pulses lasting 21 days. The cumulative N2O emissions in usual-nitrogen treatment during this period (from 10th July to 1st August) (0.54 kg N ha-1) accounted for 23% of the annual emission, and it was significantly higher than no-nitrogen treatment during the same period (p<0.01, n=22). The peak of and NO appeared at seventh day after fertilization with, and the high emissions lasted only two days.The fluxes of N2O and NO were significantly related to soil temperature (5cm), water-filled pore space (WFPS,0-6 cm) and mineral nitrogen content (p<0.01). Excluding the high fluxes after fertilization, N2O and NO emissions exponentially increased with increasing soil temperature. But in no-nitrogen fertilization, both N2O and NO fluxes linear increase with soil temperature. Both emissions of N2O and NO showed the tendency with first increase and subsequent deincrease. The combination of soil temperature and WFPS could explain 38% and 25% changes of N2O and NO fluxes, respectively. This study have investigated the characteristic of N2O and NO emissions, the magnitude of emissions, emissions factors, and the relationships between the fluxes and environmental factors and management in an irrigated cotton field in Shanxi. Our results contribute to reduce the uncertainty of N2O and NO emissions for agricultural inventories. However, N2O and NO emissions from cotton fields are affected strongly by great spatial variations and complexity of agricultural managements in China. The multi-point observations are necessary to quantity the regional emissions of N2O and NO. |
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