Effect Of Long-Term Fertilization On CO₂ Emission And Decomposition Of SOM From A Rice-Wheat Rotated Purple Soil

The production and release of greenhouse gases that have caused global warming,have become one of the most serious environmental problems in the world. Among them, CO₂ is one of the most important greenhouse gases, and the farmland ecosystem is an important source of CO₂ emissions. There are many factors that affecting soil respiration, including soil properties, environmental factors, land use, fertilization measures, etc. in witch fertilization is one of the significant factors. In order to understand the effect of long-term fertilization on the decomposition of SOM and the CO₂ emission, this study, based on six different fertilization treatments at the National Monitoring Base for Purple Soil Fertility and Fertilizer Efficiency in Beibei, Chongqing,investigated the CO₂ flux among different treatments by close chamber method for 1year, and collected soils for the aerobic incubation experiment. The treatments are CK（no fertilizer）; NP（NP fertilizer）; NK（NK fertilizer）; PK（PK fertilizer）; NPK（NPK fertilizer）. The main purpose of the study is to: 1) to understand the effect of long-term fertilization on soil physical-chemical properties in every season; 2) to reveal the effect of long-term fertilization on CO₂ emission in every season; 3) to investigate the effect of long-term fertilization on SOM decomposition and mineralization kinetics. The results are as follows:I. The seasonal dynamic change of CO₂ emission.CO₂ flux of different fertilization appeared similar trends with soil temperature in seasons. From May to August in 2014, the CO₂ flux increased with the increasing soil temperature. Then the soil temperature decreased gradually, and the CO₂ flux decreased and reached to the bottom in December, 2014. Then after January in 2015, the CO₂ flux increased with the increasing soil temperature again.The average of CO₂ emission flux was 7.05 mg C·m-2·hr-1⁵4.89 mg C·m-2·hr-1,39.80 mg C·m-2·hr-1⁶9.46 mg C·m-2·hr-1 and 30.55 mg C·m-2·hr-1⁴5.31 mg C·m-2·hr-1during rice, fallow and wheat season for each treatments. The average of CO₂ flux was lower in wheat than that in rice and fallow season for all treatments, in which the average CO₂ fluxes of CK and PK were a little greater in rice season than in fallow season, and the average CO₂ fluxes of other 4 treatments were greater in fallow than in rice season.II. The correlations between CO₂ emission and soil environmental factors.During the rice season, the soil CO₂ fluxes had a significant negative correlation with soil moister content（R=-0.277^-0.802, P < 0.05 or P < 0.01） and a positive correlation with soil temperature; During wheat season, the soil CO₂ fluxes had a significant positive correlation with soil temperature（R= 0.336 0.845, P<0.05 or P<0.01）, but the correlation with soil moister content was not obvious. The correlations between CO₂ fluxes and soil temperature and mositer content were not obvious during fallow season.CO₂ average flux had significant positive correlations with soil Olsen-P during rice（R2=0.743,n= 6,P<0.05） and wheat season（R2=0.574,n= 6,P<0.05）.III. Effect of long-term fertilization on soil CO₂ emission.The cumulative emission of CO₂ from all sites in the whole year were between2820 to 4111 kg C·hm-2, in which CK was the least and NPK was the greatest. The percentages of the cumulative CO₂ in rice, fallow and wheat season were 28%³2%,20%²7% and 43%⁴8%, respectively. As for the same treatment, the cumulative CO₂ showed a trend of wheat season > rice season > fallow season.As for a same season, P-amended treatments（i.e., PK, NP, NPK） increased CO₂ emission by 23.9%²5.7% compared to the treatments without receiving P fertilizer.That showed, the application of P fertilizer significantly stimulated the CO₂ emission.N-amended treatments（i.e., N, NP, NK, NPK） increased CO₂ emission by 2.2%⁶.5%compared to the treatments without receiving N fertilizer.That showed, the application of N fertilizer stimulated the CO₂ emission, but the effects were not obvious.IV. Effect of long-term fertilization on characteristics of soil organic carbon mineralizationThe CO₂-C release data fitted the two-pool first-order decay model well in this study. The active-C pool was 3.13⁵.81 mg·（g C）-1, 1.79⁴.72 mg·（g C）-1and 3.21⁴.02mg·（g C）-1in profile 0²0cm, 20⁴0cm and 40⁶0cm. And the slow-C pool was7.76¹2.00 mg·（g C）-1, 3.98⁸.81 mg·（g C）-1 and 4.99 7.46 mg·（g C）-1 in profile0²0cm, 20⁴0cm and 40⁶0cm. The active-C pool and slow-C were greater in profile0²0cm than in profile 20⁶0cm for most treatments. As for the same treatment in the same profile, the slow-C was 0.29⁵.23 times greater than active-C, but the mineralization rate for the former was about 2.8%⁷.3% than the latter. And themineralization half life for the former is 10.5³5.0 thimes longer than the latter.The N application would increase the active-C in profile 0²0m and decrease active-C in profile 20⁴0cm and slow-C in profile 0⁴0cm. The P application would increase the active-C in profile 0²0m and decrease active-C in profile 20⁴0cm and slow-C in profile 0²0cm.The N application would quicken the velocity of turnover in active-C pool in profile 0⁴0cm, but slower the velocity of turnover in slow-C pool in profile 0²0cm.The P application would slower the velocity of turnover in active-C pool in profile0⁴0cm, but quickenr the velocity of turnover in slow-C pool in profile 0²0cm.