SOC Mineralization And CO₂, CH₄ Production Under Long-Term Different Fertilizations From Two Typical Paddy Soils Of South China

The output of soil organic carbon from soil to atmosphere is an important component of terrestrial ecosystem C cycling. Mitigation for greenhouse becomes a focus in paddy soil that is a vital source of greenhouse gases as well as a main organic carbon pool and material for production food among farmland soil in our country. With increase of population and demand for food, a great deal application of chemical fertilizer, a kind of essential measure for increasing yield, is applied. However, little information is known for C cycling characteristic under long-term application of fertilization. The research based on long-term trial field for the stability of C pool and carbon emission can provide useful data to understand profoundly relationship of carbon source and sink.The author selected two typical paddy soils under long-term different fertilization: one(18 years) from Tai Lake region was divided into four fertilization treatments: no fertilization(NF), NPK(CF), chemical fertilizers plus manure(CFM), chemical fertilizers plus straw(CFS). The other(24 years) from Jinxian, Jiangxi province was divided into five treatments: no fertilzation(NF), N, NP, NPK, NPK plus manture(NPKM).Field measurement and laboratory incubation was performed to investigate soil respiration, soil organic mineralizaiton under submerged condition, soil capacity of oxidation methane, and the response of SOC mineralization to elevated temperature. The main results were as follows:Soil basal respiration CO₂ flux ranged from 12.2-25.2mg.m^-2·h^-1, being smaller than the reported values for forest and grassland soils, and those from dry croplands. Fertilization can significantly increase soil CO₂ emission. The daily flux under long-term chemical fertilization alone increased by 55%～85% compared to that under combined organic and chemical fertilization. Furthermore, a more prompt response of CO₂ flux to soil temperature at 5cm depth under long-term chemical fertilization alone was observed than that under combined fertilization. A close relationship was observed between CO₂ flux and the levels of microbial N, and C_mic/N_mic, P availability, respectively, which indicated that nutrient levels were important factors influencing soil respiration and CO₂ emission.The SOC mineralizaiton was different with moisture and soil type. The mineralizaiton rate(0.20-0.52 mg·gOC^-1d^-1) of Huangnitu was significantly higher than that of red soil, which indicated C bio-availability was lower and stability was higer in red soil than Huangnitu. However, the relative value of SOC mineralizaiton rate under aerobic and that under anaerobic condition was different for different soil type. The mineralization rate of SOC was higher for red soil under aerobic condition than that under anaerobic condition. Whereas, for Huangnitu it was necessary to study further.A significant difference of C bioavailability was observed among different soil particle size fraction. The content and C bioavailability of SOC contained the large particle size fraction(2000-200μm) that was the major contributor to greenhouse gases production from soil, was highest. Althouth content of SOC existing in the clay was also higher than silt size fractions, CH4 was hardly produced for the SOC higher stability and the lower C bioavailability. Furthermore, the organic carbon also was affected hardly by management measure.The production and emission of greenhouse gases has a close relationship to microbial diversity and activity. PCR-DGGE method was introduced to study the effect of long-term fertilization on the soil ability to oxidize CH₄ and methanotrophs community structure. The results showed that application of long-term organic fertilizer plus chemical fertilizer was helpful to increase the soil ability to oxidize endogenously methane and microbial diversity. The results further indicated that the variation of methanotrophs diversity might cause the change of soil ability to oxidize CH₄.SOC stability was involved in protection mechanism in soil. The development of aggregates and higher content of DCB-Fe was the major mechanism of SOC for paddy soil. SOC for itself stability also was affected by fertilization measurement. The results showed that stability coefficient(Kos), relative quantity of active component in TOC, was reduced by fertilization. Correspondingly, the portion of active component increased. A closely negative correlation existed between Kos and CO₂ emission, so we can study bioavailability of SOC by Kos.The mineralization rate under anaerobic was significantly lower than that under aerobic, and the factors which affected the response of SOC mineralization to temperature were different under two kinds of incubation condition, respectively. One a significant correlation was observed between Q₁₀ and DCB-Fe under aerobic condition. Another was done between Q₁₀ and micribial quotient under anaerobic condition. Therefore, we can infer that different mechanism play the dominant role for SOC mineralization under different moisture condition.Sumarrily, fertilization affected significantly nutrient level, soil CO₂ emission, SOC stability and microbial community structure. Fouthermore, the relative intension of SOC sequestration and greenhouse gases emission was changed by different fertilizations. These results provided some theoretic basises for us to reach the three"win" aim that is not only increase of crop yield and SOC content but also mitigation for greenhouse gases by agriculture management and nutrient regulation. Especially, it is very important for mitigating global warming.