1. Contribution of fungi activity for N 2 O emission in no-tillage with cover crop fields Zhaorigetu 1,2, T.Nishizawa 2, Y.Sato 2, M.Komatsuzaki 1, H.Ohta 1 1 Tokyo University of Agriculture and Technology, 2 Ibaraki university. Background & Objectives Winter cover crops would be a effective tool to enhance the potential of soil carbon sequestration in the soil, however, N 2 O emission may offset that positive potential due to decompose of cover crop residue. It is also expected that soil microbial population and activities affect residue decompose process and greenhouse gas emission from soil, however, limited information is available regarding the effects of cover cropping and tillage system on soil microbial population dynamics and N 2 O emission. This research was aimed to evaluate the influence of cover crops and tillage system on bacteria population and N 2 O flux. This study was conducted on a Humic Allophane soil to evaluate the effects of tillage systems (plow, rotary and no-till treatment), cover crop species(hairy vetch, rye and fallow)and soil N level (0 and 100 Kg N ha -1 ) on soil microbial populations.Cereal rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth) were planted in the fall, 2003. Before planting of field rice (Oryza sativa L.), the cover crop residues were incorporated into the soil in the spring, 2004, fallow was used as the control (no cover). Soil samples were taken from three different depths (0-10, 10-20, 20-30 cm) on 14 May 2004, two weeks after the seeding of upland rice and on 4 October 2004, two days before the harvest. Culturable bacteria were enumerated using an agar medium of 100-fold diluted conventional nutrient broth (DNB). Fungi biomass was estimated from ergosterol content of the soil according to the method of microwave-assisted methanol extraction followed by HPLC analysis.The N 2 O flux from the field was measured by the closed chamber method(37×30.5×13.5cm).This chamber was buried under 6cm in depth and the N 2 O concentration in the sample gas was analyzed with GC-ECD. Material & Methods Results & Discussion 1)Cover crop species and tillage systems were strongly affected soil microbial populations in the soil from 0 to 10 cm depth in May. Magnitude difference of soil microbial populations occurred between hairy vetch in no-till plot and fallow in rotary plot. However these differences were moderate in the soil from 10 to 30 cm depth layer. In October, bacterial populations of all the soil depths decreased compare with in May and no significant differences were observed among cover crop and tillage treatments. (Figure 1). 2)No-tillage with cover crop treatment showed significantly high soil fungi biomass, compare with other tillage system, therefore the interaction between cover crop and tillage system were obvious. (Figure 2). 3)High N 2 O emission were found in tillage or cover crop incorporate season in April. Cover crop and no- tillage treatment increased N 2 O emission compare with other treatment(Figure3). 4)For N 2 O emission significant difference was observed between soil N level, N 2 O emission was approximately two times higher in 100 kg N ha -1 than in 0 kg N ha -1. Rye and hairy vetch showed higher N 2 O emission than in fallow. No-tillage showed higher N 2 O emission than in rotary and plow treatment. The impact of cover cropping on N 2 O emission was small compare with N fertilization. Higher correlationship was observed between N 2 O emission and fungi biomass, however, the correlationship between bacteria population and N 2 O emission was low in July,2005(Figure 4 and Figure5). Conclusion These microbial reactions adopting cover crop residue incorporation in different tillage system strongly associate with the depth of cover crop residue incorporation into the soil and cover crop residue characters. These difference of microbial community may influence on N 2 O emission from agricultural soil. Because of an abundance of unculturable microbes in soil, further studies using molecular approaches will be needed to obtain more exact pictures of cover crop/tillage system-soil microbe relationships in the view point of reducing the N 2 O emission. Figure 1.Soil bacteria profile at the different tillage systems and cover crop types in May and October, 2005. Horizontal bar shows standard deviation. Figure 2.Ergosterol content at the different tillage systems and cover crop types in May and October,2005.Horizontal bar shows standard deviation. Figure 3 Effects of tillage system and cover cropping on N 2 O flux in the upland rice field(2006). Rye Hairy vetch Fallow Rotary Plow No-till 100 N kg ha -1 0 N kg ha -1 Figure 5 Correlation between N 2 O emission and bacteria population (A) and fungi biomass which was estimated by ergosterol content (B) （ July,2005). Figure 4 Annual N 2 O emission at the different tillage systems and cover crop types (2005). A B May October RotaryPlowNo-tillage