| Climate warming is a global environment issue today, the main reason for this issue is the increasing concentration of greenhouse gases in the atmosphere, and CO2, CH4 and N2O has been paid much attention due to their substantial contribution to global warming. The recent report of IPCC (2007) pointed out that, the GWP value of the unit molecule CH4 in 100 years scale was 25 times than that of CO2, the concentration of CH4 in 2005 was about 1774 ppb, twice as much as the concentration of the preindustrial. The natural emissions source of CH4 in atmosphere include the natural wetland, paddy field, gas leak, the sea, the forests, fossil fuels, ruminant animals, termite, biomass burning, plants and litter, etc. In addition to natural wetlands, the CH4 emission from the agricultural soil produced by human activity release is an important source of greenhouse gases.Based on the observation of CH4 emission, the aim of this paper is exploring the influence of different cultivation systems and environmental factors of CH4 emissions from croplands, in order to find the best cultivation system to reduce CH4 emissions, providing the theoretical basis of CH4 mitigation. The CH4 emission was measured by static chamber-gas chromatographic techniques. The field plots experiment was established in the Key Field Station for Monitoring of Eco-Environment of Purple Soil of the Ministry of Agriculture of China located in the farm of Southwest University (30°26'N,106°26'E), Chongqing. Four tillage treatments including conventional tillage with rice only system (DP), conventional tillage with rotation of rice and rape system (SH), no-till and plain culture with rotation of rice and rape system (XM), no-till and ridge culture with rotation of rice and rape system (LM) were selected as research objection. Results were showed as following:1. The seasonal changes of CH4 emissions under different tillages(1) In the rice growing season, the seasonal variations of CH4 fluxes under different tillages were almost identical all with a double-peak. During the rice-growing stage the average fluxes of CH4 varied considerably, the CH4 emission flux under DP, LM, XM and SH treatments ranged between 0.163～17.110,0.538～20.218,-0.094～13.360 and -0.311～9.376 mg-m-2·h-1, respectively. The CH4 emission from DP treatment was significantly higher than other treatments (p<0.01). There was no significant difference between LM and XM treatments (p=0.052), the CH4 emissions from LM and XM treatments were significantly higher than SH treatment (p<0.01).The CH4 emission in each rice-growing stage was obviously different, but identical under different tillages, the order was jointing> heating and flowering> mature stage> tillering> turning green stage. The results showed that the duration of the jointing and heating and flowering stage (28 days) was less than 1/3 of the rice-growing season (102 days). The amount percent of CH4 under DP, LM, XM and SH treatments in this stage reached to 48.4%,61.3%,54.8% and 61.4%, respectively.(2) In the rape growing season, the CH4 emission rate under all of the treatments was considerably lower than that of in the rice growing season, with little variation amplitude. The CH4 emission rates in LM, XM and SH treatments during the rape growth period were -0.446～0.465,-0.371～0.451 and -0.489～0.630 mg·m-2-h-1, respectively, but the CH4 emission from DP treatment with no rape planting raged between -0.584-3.767 mg-m-2·h-1. The average fluxes of CH4 from DP, LM, XM and SH treatments were 0.417,-0.003,-0.031 and 0.016 mg-m-2·h-1,respectively. DP treatment presented CH4 emissions process, LM, XM and SH treatments were behaved for a weak CH4 emissions even absorption (a CH4 sink).The CH4 absorption occurred in moss bud, presenting CH4 emission process in other stages under DP treatment; LM, XM and SH treatments absorbed CH4 in seeding but fluxed in pod maturity.(3) The CH4 emission in non-rice-growing season was lower than rice-growing season and raged between -0.854～5.514 mg-m-2·h-1. In addition to DP treatment had larger CH4 emissions, LM and XM treatments processed CH4 emissions rarely, and SH treatment appeared as a CH4 sink.2. The effects of tillage on CH4 emission from croplands(1) During the rice growth period, the proportion of CH4 emission under different tillage was up to 77.22%-103.84%, and the emission of CH4 decreased in following order:DP (196.216±22.943)> LM (153.418±21.203)>XM (119.058±8.372)>SH (79.550±3.994 kg-hm-2).Throughout rice growth time, similar tendency occurred in the four treatments, which the emission of CH4 decreased as the order:DP>LM>XM>SH.(2) During rape growth, the proportion of CH4 emissions account for year-round farmland emissions was small under DP, LM, XM and SH treatments. Especially, the proportion was only 1% under LM, XM and SH treatments, as for DP treatment, which was 7.67%. The emission of CH4 decreased under four treatments in following order: DP (19.495±5.517)> LM (0.805±1.693)> XM (0.536±2.251)> SH (-1.046±1.335 kg-hm-2).Throughout rice growth, the CH4 emissions were different in each period. DP treatment showed a CH4 emission except tai bolls. As for LM, XM and SH treatments, the emission occurred in Pod maturity period, and seeding stage showed an absorption for CH4.(3) During fallow period, the emission of CH4 was highest for DP treatment compared with other treatments, and the proportion account for year-round was up to 15.1%. And the emissions under four treatments ranked in the order: DP (38.400±8.945)> LM (0.620±0.696)> XM (0.161±0.516)> SH (-3.746±0.719 kg-hm-2).(4) As our results shown, the CH4 emissions arranged from 76.609±6.406 kg-hm-2 to 254.112±37.405 kg·hm-2 in experimental period. Compared with LM, XM and SH treatments, the CH4 emission was significant high under DP treatment. Similarly, a higher emission was observed under LM and XM than SH (p<0.01); however, seldom difference was found between LM and XM (p>0.05). The rank is in the order: DP (254.112±37.405 kg-hm-2)> LM (154.574±24.150 kg-hm-2)> XM (118.173±10.223 kg·hm-2)> SH (76.609±6.406 kg-hm-2). On the whole, compared with DP, the treatments of LM, XM and SH decreasing the emission of CH4, which proportion were 39.2%,53.5% and 69.9%, respectively.3. The effects of environmental factors on CH4 emission from croplandsOur findings revealed that a significant positive relation was observed between three temperatures (the soil temperature underground 5 cm depth, earth s surface and in the cabinet) and CH4 emissions (p<0.05) during the rice growth period. Especially for the temperature of underground 5 cm depth, the correlation coefficients was 0.697, 0.560,0.598 and 0.665 for DP, LM, XM and SH treatments, respectively. Nevertheless, the relation was not obvious between soil temperature and CH4 absorption during other period (p>0.05). No significant correlation was showed between CH4 emissions and WFPS under various tillage systems. In addition, the correlation of logarithm of CH4 emissions and water depth was not obvious. Also, the correlation of between CH4 flux and DOC in soil was not found. |
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