The Carbon Balance Of Vegetable Fields Converted From Paddy

Global warming as a major feature of global change has become the focus of environmental issues, and carbon dioxide（CO₂） as the most important greenhouse gas has become a hot research issue. According to the Intergovernmental Panel on Climate Change（IPCC） reports, land use conversion has become the second-largest source of carbon emissions following fossil fuels. However, it remains unclear about the impact of changing land use patterns on the various soil carbon pools. In southern China, conversion from paddy field to vegetable field is a typical change in land use. Current studies on this issue mostly concentrated on vegetable field carbon balance in one year or on vegetable field carbon balance years later the conversion from paddy field to vegetable field. Therefore, it is lack of new researches whose data are time series, or whose vegetables differ from each other. Consequently, lack of these studies exerts a great impact on the global carbon cycle estimates. In this experiment, changes of carbon balance under land use changes were selected as the starting point for study vegetable carbon balance in subtropical areas of paddy fields and paddy fields converted. Then, changes of carbon balance and related controls were determined to explain the different inter-annual disparity among years and estimates the carbon balance in vegetable and paddy field,. Last, guidance on land use patterns were provide.Six paddy fields in Changsha Environmental Agricultural Research Station, Institute of Subtropical Agriculture, The Chinese Academy of Sciences were selected as the study areas, three of which were vegetable field converting from paddy field. Three sub-processing was set up in each vegetable filed and paddy field: crops applied nitrogen（CN）, fertilization without crops（BL） and crops without N fertilizer（CK）. Continuous observation was executed in three years. Plus, variation characteristic of three treatments emissions and soil CO₂ of ammonium nitrogen（NH4+-N）, nitrate（NO3--N）, soluble organic carbon（DOC） were marked. The main findings are as follows:1. Cumulative emissions of heterotrophic respiration in year of 2013, 2014 and 2015 paddy were 4758.82, 3628.61 and 4720.73 kg C hm-2, respectively. There was no significant difference among the three variances. During the study period, paddy heterotrophic respiration ranges from-43.09 to 275.62 mg C m-2 h-1. Low heterotrophic respiration was observed in flooding period, while high heterotrophic respiration was observed during drying period. Besides, heterotrophic respiration in spring leisure time was larger than heterotrophic respiration in winter recreation time. During 2013 to 2015, paddy heterotrophic respiration is not related with T among years, but significantly positively correlation was found at leisure season. Surprisingly, the study found no significant correlation between paddy heterotrophic respiration and WFPS, NH4+-N, NO3--N, and DOC.2. Cumulative emissions of heterotrophic respiration in the year of 2013, 2014 and 2015 were 6174.57, 5942.29 and 4546.49 kg C hm-2, respectively. There was no significant difference among the three variances. Heterotrophic respiration during the study vegetable ranges from 7.51 to 599.12 mg C m-2 h-1. Severe heterotrophic respiration was detected in summer, while the weak one was found in cold winter. The results of this study indicate that T in the observation period were highly significant positive correlated with heterotrophic respiration, and no continuous discovery about WFPS, NH4+-N, NO3--N, DOC and vegetable heterotrophic respiration correlation. In the paddy field converted into a vegetable field, heterotrophic respiration were showed no significant difference in the years 2013, 2014 and 2015（P=0.056）, a significant difference（P = 0.005）, the difference was not significant（P=0.777）.3. Accumulation ecological system respiration of paddy field with nitrogen fertilizer applied in the year of 2013, 2014 and 2015 were 13519.48,10210.56 and 17106.94 kg C hm-2, and variance analysis showed that significantly different was found for each year. Ecosystem respirations of CK accumulation of paddy field were 11818.18, 9220.27 and 9270.56 kg C hm-2, respectively. CN and CK analysis of variance results showed no significant difference in 2013 and 2014, so there are significant differences in 2015. Paddy ecosystem respiration of the study period in range from-3.59 o 3.70 mg C m-2 h-1, while field ecosystem respiration during the rice-growing more intense and stronger than during the late period of early rice; ecosystem respiration in paddy field Leisure of relatively weak, but casual spring be greater emission peaks. In this study, paddy field ecosystem respiration for three consecutive years in between years there and T highly significant positive correlation was observed in 2013 and NH4+-N was significantly negatively correlated, the remaining time is not observed with the WFPS, NH4+-N, NO3--N and DOC correlation4. Cumulative ecological fluxes of CN in the year of 2013, 2014 and 2015 were 10530.72, 14766.24 fluxes and 9825.25 kg C hm-2, respectively. Analysis of variance showed no differ significantly in 2013 and 2015, and 2014. However, both were significantly different; the ecological CK cumulative emissions 7943.38, 13849.80 and 8384.27 kg C hm-2, respectively. Variance analysis was were significantly different in CN and CK, 2013. Vegetable ecosystem respiration was mainly influenced by the impact of vegetable crops, different crops in the growth cycle exhibit different emission intensity, Red Cabbage 48.09±4.02 mg C m-2 h-1, pepper 1（Season 1） 132.42±13.55 mg C m-2 h-1, radish 1（season 1） 143.22±8.67 mg C m-2 h-1, water spinach 215.00±14.09 mg C m-2 h-1, 2 radish（season 2） 127.70±7.97 mg C m-2 h-1, pepper 2（season 2） 120.33±9.17 mg C m-2 h-1. ANOVA showed no season’s peppers and radishes significant differences, and moss and red peppers, radishes and spinach were significantly different. Vegetable ecosystem respiration during the study and T has a very significant positive correlation, in 2014 there was a significant negative correlation with the WFPS, in 2014 and 2015 with the DOC had a significant positive correlation. After the transition paddy vegetable, 2013, 2014 and 2015 during research on ecosystem respiration showed significant differences.5. Paddy field acted as carbon sink during the growing period. Conversely, in the rice fallow period, it is shown as a carbon sink. Generally, rice ecosystems manifested as carbon sinks, and 10.46, 9.98 and 12.08 t C hm-2 were observed in 2013, 2014 and 2015 respectively.6. Vegetable Crops include red moss ecosystem, radish-1,-2, and radish spinach are realized as carbon sinks, which were 5.99, 10.11, 6.47 and 0.11 t C hm-2, respectively. The chili pepper-1 and-2 are expressed as carbon, which are 1.84 and 1.93 t C hm-2, respectively. Estimating carbon changes by using two crops as a year as a vegetable, 2013, 2014 and 2015 respectively, which are 4.15, 10.22 and 4.54 t C hm-2, respectively.In summary, inter-annual variation of carbon circle were observed in paddy fields and vegetable fields due to varied hydrothermal conditions differing from year to year. After the paddy fields ben converted into vegetable plots, significant changes in the sensitivity of vegetation to water heat reduces carbon sinks, but by selecting unit area large biomass crops can reduce this kind of difference.