The carbon storage capacity of grassland soils is greater than that of cropland, but grassland management changes the amounts of methane (CH_4) and nitrous oxide (N_2O) emitted from soils. To investigate the effects of forage species, renovation, and farmyard manure (FYM) application on CH_4 and N_2O emissions from grassland soils, substantiative experiments were performed on grassland plots in Nasu, Japan. The experiments are described in this thesis as follows. Chapter 3 describes the effects of forage species composition on CH_4 and N_2O emissions. Orchardgrass (OG), white clover (WC), and OG/WC grasslands emitted -1.8, -2.4, and -1.8 kg CH_4 -C ha^-1 year^-1 and 0.39, 1.59, and 0.67 kg N_2O -N ha^-1 year^-1, respectively. WC increased the permeability of the soil to air but did not significantly increase annual CH_4 consumption by grassland soil, although it did significantly increase annual N_2O emissions from the soil. Chapter 4 investigates the effect of grassland renovation on N_2O emissions. Renovated plots emitted 2.1 to 5.3 kg N ha^-1 and unrenovated control plots emitted 0.96 to 2.8 kg N ha^-1. Grassland renovation significantly increased N_2O emission from the soil. In the renovated plots, N was mineralized from incorporated roots and stubble, and the soil moisture was distributed within the range favorable for N_2O production and emission. Greater N_2O emission was observed with increasing soil moisture just before renovation and with precipitation just after renovation. Chapter 5 describes the effects of FYM application on CH_4 and N_2O emissions from grassland soils. Grassland plots that received FYM plus chemical fertilizer (manure plot) or chemical fertilizer only (fertilizer plot) were compared. In both plots, the sum of N applied as fertilizer and N estimated to be mineralized from FYM was adjusted to 210 kg ha^-1 year^-1. The manure plot emitted 7.0 to 11.0 kg N_2O -N ha^-1 year^-1 and the fertilizer plot emitted 4.7 to 9.1 kg N_2O -N ha^-1 year^-1. FYM application significantly increased annual N_2O emissions from the grassland soil. The manure plot emitted -0.74 to -0.16 kg CH_4 -C ha^-1 year^-1 and the fertilizer plot emitted -0.84 to -0.52 kg CH_4 -C ha^-1 year^-1. FYM application did not significantly increase annual CH_4 emissions. These results show that, to mitigate N_2O emissions from grassland soils, it is important to reduce the amount of N fertilizer applied in accordance with the amount of N fixed by WC and N mineralized from FYM. It is also important not to renovate grassland at a time of year when soil temperature and soil moisture are relatively high.

草地は畑地より炭素貯留機能が大きいが, 草地管理は温暖化係数の高いメタン (CH_4) や亜酸化窒素 (N_2O) の排出量を変化させる。本研究は, 草種構成, 草地更新, 堆肥散布が CH_4 と N_2O の排出量に及ぼす影響を明らかにするため, 那須の火山灰土壌の草地において実証試験を行った。第3章では, 草地の草種構成が CH_4 と N_2O の排出量に及ぼす影響を述べた。オーチャードグラス (OG) 単播草地, シロクローバ (WC) 単播草地, OG/WC 混播草地の CH_4 と N_2O の排出量は, それぞれ -1.8, -2.4, -1.8kg C ha^-1 y^-1, 0.39, 1.59, 0.67kg N ha^-1 y^-1 であった。WC は土壌通気性を高めたが CH_4 年間吸収量を有意に増加させなかった。WC が栽培されると N_2O 年間排出量が有意に増加した。第4章では, 草地更新時の N_2O 排出量を述べた。草地更新を行った草地 (更新草地) と行わない草地 (対照草地) の N_2O 排出量は, それぞれ 2.1～5.3, 0.96～2.8kg N ha^-1 で草地更新は N_2O 排出量を有意に増加させた。更新草地では, すき込まれた根と刈株から窒素無機化が起こり, 土壌水分は N_2O が生成され易く, また排出され易い領域に分布した。更新時期の土壌水分や更新直後の降雨量が多いと N_2O 排出量が増加した。第5章では, 堆肥散布が草地からの N_2O と CH_4 の排出量に及ぼす影響を述べた。堆肥と化学肥料を散布した草地 (堆肥区) と化学肥料のみを散布した草地 (化学肥料区) の N_2O と CH_4 の排出量を比較した。なお, 両処理区の窒素供給量は 210kg N ha^-1 y^-1 に調整した。堆肥区と化学肥料区の N_2O 年間排出量は, それぞれ 7.0～11.0, 4.7～9.1kg N ha^-1 y^-1で堆肥散布が N_2O 年間排出量を増加させた。堆肥区と化学肥料区の CH_4 年間排出量は, それぞれ -0.74～-0.16, -0.84～-0.52kg C ha^-1 y^-1 で堆肥散布は CH_4 年間排出量を増加させなかった。以上の結果から, N_2O 排出量を抑制するため, WC の窒素固定や堆肥の無機化を考慮して施肥量を削減し, 地温や土壌水分の高い時期を避けて更新を行うことが重要と指摘される。