Effect Of Long-term Differentiated Fertilization On N₂O Emission From A Rice-Wheat Rotated Purple Soil During Wheat Growing Seasons

Agricultural soils is an important anthropogenic sources of atmospheric N2O. The emission of N2O mainly comes from soil microbial denitrification and nitrification process, and affected by comprehensive effect of some factors such as soil, climate, fertilization and cultivation, et al. Therefore, the emission of N2O presents an complex space-time variability. Purple Soil is the most important agricultural soil type of Sichuan and Chongqing area, but its N2O emission intensity and rule are still not clear. This study based on seven different fertilization levels treatments in National Purple Soil Fertility Monitoring Station, at Beibei of Chongqing, China.Changes of N2O flux in two wheat seasonsfrom November 2008 to May 2009 and November 2009 to May 2010) among different treatments were monitoring in situ by close chamer method. The seven treatment are respectively:no fertilization+no crop residue(R-), no fertilization+crop residue(R+); PK fertilization+no crop residue(PKR-); N fertilization+no crop residue(NR_); NPK normal fertilization+no crop residue(FnR_); NPK normal fertilization+crop residue(FnR+); NPK high fertilization+crop residue(FhR+)。The amount of normal nitrogen application was 135 kg N·hm-2, straw returning amount to 7.5 t·hm-2. The main results are as follows:Ⅰ. The dynamic change of N2O emissionThe N2O transient emissions fluxes of no nitrogen treatments, in the two wheat seasons, are less than 55 and 30μgN·m-2·hr-1 respectively, and the fluctuation over time is small. For the average emissions, there was no significant difference between treatments and between the inter-annual., the range is 13.46±6.71～21.11±13.03μgN·m-2·hr-1.Unlike no nitrogen treatments (R., R+ and PKR.), soil N2O emissions of nitrogen treatments showed an obvious seasonal variation, its peak emissions (120.39～289.25μgN·m-2·hr-1) occurred mainly after application of base fertilizer and topdressing(in the first two months of wheat growth period), soil moisture increased by rainfall also results in some smaller emission peaks. The average of N2O emission fluxs of the nitrogen treatments in range of 35.9±31.77～47.27±62.91μgN·m-2-hr-1 and 31.3±14.27-44.28±22.58μg·N·m-2·hr-1 respectively in the two seasons. The FnR+ and FhR+ were the highest in the two seasons respectively. In addition to FhR+treatment, the average N2O emission flux of other nitrogen treatments in the second wheat season were lower than that in the first wheat season, this may be because the lower soil moisture in second wheat season weakened the denitrification.II. Effect of long-term different fertilization and straw management on soil N2O emissionsThe N2O cumulative emission of R_ (control) being not applied nitrogen fertilizer and straw which did not return to field was 0.63 kg N hm-2 on average, respectively 0.66 and 0.60kg in the two wheat seasons. There is no significant difference of cumulative emissions between PKR- and R- in the two wheat season, but the mean N2O cumulative emission of R+ is 42.5% higher than R., indicating that the straw returning in favor of N2O generation and emission. For N2O emission promoting effect, chemical nitrogen fertilizer is more obvious than straw. Compared with R+ treatment, the NR- and FnR-respectively 118% and 84% higher than it. The N2O cumulative emissions of the treatmens, which were applied NPK fertilizers and straw returned to field, are 97%-99% higher than that which straw returned to field only. Indicating that chemical fertilizer and straw applied together has an synergistic effect on N2O emissions.Under the condition of straw not returning, the difference of cumulative emissions between partial nitrogen treatment NR. (1.97 kg N-hm-2) and NPK fertilizers treatment FnR. is not obvious in the first wheat season, but in the second season, the former is 44% higher than the latter (two wheat season average higher by 21%). Similarly, The N2O cumulative emission of NR. were 11～12% higher than the treatments applied chemical fertilizer and straw together, Therefore, considering the soil N2O emission reduction, balanced fertilization should be used as far as possible and avoid partial nitrogen, in agricultural production activities.III. N2O Emission factorsThe establishment of national or regional emission inventory must on the basis of soil N2O emission factor.The emission factors of the exogenous input of nitrogen (including straw and nitrogen)in the two wheat seasons of each treatment in the range of 0.50%-0.98%, an average of 0.69%±0.20%, which is less than the IPCC(2006) recommended value (1%). Consider chemical fertilizers or straw input of nitrogen, the range were 0.54%-0.98% and 0.56%-0.66% respectively, mean value were 0.85%±0.21% and 0.61%±0.04% respectively. N2O emission factor of chemical nitrogen was significantly higher than straw, indicating that the organic materials and fertilizers should adopt different emission factors for N2O emissions estimates.