No abstract

Monthly measurements of carbon dioxide ͑CO 2 ͒, methane ͑CH 4 ͒ and nitrous oxide ͑N 2 O͒ fluxes in peat soils were carried out and compared with groundwater level over a year at four sites ͑drained forest, upland cassava, upland and lowland paddy fields͒ located in Jambi province, Indonesia. Fluxes from swamp forest soils were also measured once per year as the native state of this investigated area. Land-use change from drained forest to lowland paddy field significantly decreased the CO 2 ͑from 266 to 30 mg C m -2 h -1 ͒ and N 2 O fluxes ͑from 25.4 to 3.8 g N m -2 h -1 ͒, but increased the CH 4 flux ͑from 0.1 to 4.2 mg C m -2 h -1 ͒ in the soils. Change from drained forest to cassava field significantly increased N 2 O flux ͑from 25.4 to 62.2 g N m -2 h -1 ͒, but had no significant influence on CO 2 ͑from 266 to 200 mg C m -2 h -1 ͒ and CH 4 fluxes ͑from 0.1 to 0.3 mg C m -2 h -1 ͒ in the soils. Averaged CO 2 fluxes in the swamp forests ͑94 mg C m -2 h -1 ͒ were estimated to be one-third of that in the drained forest. Groundwater levels of drained forest and upland crop fields had been lowered by drainage ditches while swamp forest and lowland paddy field were flooded, although groundwater levels were also affected by precipitation. Groundwater levels were negatively related to CO 2 flux but positively related to CH 4 flux at all investigation sites. The peak of the N 2 O flux was observed at Ϫ 20 cm of groundwater level. Lowering the groundwater level by 10 cm from the soil surface resulted in a 50% increase in CO 2 emission ͑from 109.1 to 162.4 mg C m -2 h -1 ͒ and a 25% decrease in CH 4 emission ͑from 0.440 to 0.325 mg C m -2 h -1 ͒ in this study. These results suggest that lowering of groundwater level by the drainage ditches in the peat lands contributes to global warming and devastation of fields. Swamp forest was probably the best land-use management in peat lands to suppress the carbon loss and greenhouse gas emission. Lowland paddy field was a better agricultural system in the peat lands in terms of C sequestration and greenhouse gas emission. Carbon loss from lowland paddy field was one-eighth of that of the other upland crop systems, although the Global Warming Potential was almost the same level as that of the other upland crop systems because of CH 4 emission through rice plants.

Greenhouse gas emissions were measured from tropical peatlands of Kalimantan, Indonesia. The effect of hydrological zone and land-use on the emission of N 2 O, CH 4 and CO 2 were examined. Temporal and annual N 2 O, CH 4 and CO 2 were then measured. The results showed that the emissions of these gases were strongly affected by landuse and hydrological zone. The emissions exhibited seasonal changes. Annual emission of N 2 O was the highest (nearly 1.4 g N m −2 y −1 ) from site A-1 (secondary forest), while there was no significant difference in annual N 2 O emission from site A-2 (paddy field) and site A-3 (rice-soybean rotation field). Multiplying the areas of forest and non-forest in Kalimantan with the emission of N 2 O from corresponding land-uses, the annual N 2 O emissions from peat forest and peat non-forest of Kalimantan were estimated as 0.046 and 0.004 Tg N y −1 , respectively. The emissions of CH 4 from paddy field and non-paddy field were estimated similarly as 0.14 and 0.21 Tg C y −1 , respectively. Total annual CO 2 emission was estimated to be 182 Tg C y −1 . Peatlands of Kalimantan, Indonesia, contributed less than 0.3% of the total global N 2 O, CO 2 or CH 4 emission, indicating that the gaseous losses of soil N and C from the study area to the atmosphere were small.

The DNDC (DeNitrification-DeComposition)-Rice model, one of the most advanced process-based models for the estimation of greenhouse gas emissions from paddy fields, has been discussed mostly in terms of the reproducibility of observed methane (CH 4 ) emissions from Japanese rice paddies, but the model has not yet been validated for tropical rice paddies under alternate wetting and drying (AWD) irrigation management, a water-saving technique. We validated the model by using CH 4 and nitrous oxide (N 2 O) flux data from rice in pots cultivated under AWD irrigation management in a screen-house at the International Rice Research Institute (Los Bañ os, the Philippines). After minor modification and adjustment of the model to the experimental irrigation conditions, we calculated grain yield and straw production. The observed mean daily CH 4 fluxes from the continuous flooding (CF) and AWD pots were 4.49 and 1.22 kg C ha À1 day À1 , respectively, and the observed mean daily N 2 O fluxes from the pots were 0.105 and 34.1 g N ha À1 day À1, respectively. The root-mean-square errors, indicators of simulation error, of daily CH 4 fluxes from CF and AWD pots were calculated as 1.76 and 1.86 kg C ha À1 day À1, respectively, and those of daily N 2 O fluxes were 2.23 and 124 g N ha À1 day À1, respectively. The simulated gross CH 4 emissions for CF and AWD from the puddling stage (2 days before transplanting) to harvest (97 days after transplanting) were 417 and 126 kg C ha À1 , respectively; these values were 9.8% lower and 0.76% higher, respectively, than the observed values. The simulated gross N 2 O emissions during the same period were 0.0279 and 1.45 kg N ha À1 for CF and AWD, respectively; these values were respectively 87% and 29% lower than the observed values. The observed total global warming potential (GWP) of AWD resulting from the CH 4 and N 2 O emissions was approximately one-third of that in the CF treatment. The simulated GWPs of both CF and AWD were close to the observed values despite the discrepancy in N 2 O emissions, because N 2 O emissions contributed much less than CH 4 emissions to the total GWP. These results suggest that the DNDC-Rice model can be used to estimate CH 4 emission and total GWP from tropical paddy fields under both CF and AWD conditions.

Intensive field experiments were conducted from 1999 to 2001 to examine the effects of farmland improvement on methane (CH4) emission from two rice paddy fields in Niigata, Japan. Rice cultivation and field management were similar in both paddy fields; however, one field had a subsurface drainage system installed 0.6–0.8 m below the soil surface (drained paddy field) and the other had no such system (non‐drained paddy field). Methane emissions from the drained paddy field during each rice‐growing season were approximately 71% lower than those from the non‐drained paddy field. The subsurface drainage system lowered the groundwater level and top of the gley soil layer to the drainage pipe level, enhanced soil permeability, and resulted in more oxidized soil conditions in the fallow season. The lower total and hot water extractable carbon in the plowed layer soil of the drained field versus the non‐drained field strongly suggests that the organic substrate that gives rise to CH4 decomposed more quickly in the drained field. Ferrous iron concentrations in the fresh plowed layer soil, collected from before submergence up to mid‐summer drainage, were also much lower in the drained field. This indicated that ferrous iron produced during the flooding seasons was quickly oxidized to ferric iron in the fallow season, which then acted as an electron accepter and inhibited CH4 production in the subsequent rice‐growing season. In contrast, the continuous reductive conditions in the non‐drained field (even in the fallow season) prevented most of the ferrous iron from being oxidized. Therefore, installing a subsurface drainage system greatly reduced CH4 emissions by improving aerobic conditions and reducing CH4 production potential. Methane emissions with a large inter‐annual variation in the rice‐growing season from the non‐drained field were positively correlated with soil moisture in the plowed layer before submergence, which, in turn, greatly affected CH4 emission in the following rice‐growing season.