硫代葡萄糖苷调控肉牛排泄物氧化亚氮生成的规律与分子机制

Nitrous oxide (N2O) is a potent greenhouse gas as well as the key component depleting the ozonesphere of the earth. The excreta of beef cattle produces a large amount of N2O during storage or when used as fertilizer. It is of great importance to mitigate N2O emission from excreta of beef cattle. Glucosinolates (GLs) are specific components of Brassica plants. GLs can be hydrolyzed to isothiocyanates (ITCs) and other products by myrosinase. Present results show that ITCs significantly inhibit a key process of nitrification for N2O formation, and the N2O emission from the excreta of beef cattle fed with brassica grass is significantly lower than other grass. It is speculated that ITCs are excreted from the urine of cattle and consequently inhibit the N2O formation. However, it is unclear if ITCs are actually excreted in the urine of beef cattle. The objetcives of the project are to study the metabolism of rapeseed meal GLs by beef cattle, to clarify the excretion pattern of GLs metabolites from beef cattle and the inhibition effect on N2O from excreta, and estabolish the theretical basis of inhibiting N2O formation through feeding rapeseed meal to beef cattle.

氧化亚氮（N2O）是高强度温室气体及破坏地球大气臭氧层的关键成分。肉牛粪尿在贮放或作为肥料施用过程中产生大量N2O。减少肉牛N2O排放对保护地球环境意义重大。硫代葡萄糖苷(glucosinolates, GLs)是十字花科植物(Brassica spp.)的特有成分。该成分在硫代葡萄糖苷水解酶(myrosinase)作用下可水解成异硫氰酸酯 (isothiocyanates, ITCs)等成分。现有结果表明，ITCs能够显著抑制N2O生成的关键步骤，而采食十字花科牧草的肉牛粪尿的N2O产量显著低于其他牧草。因此推测，采食十字花科牧草的肉牛可能从尿中排泄ITCs等代谢产物，抑制了N2O生成。而ITCs能否通过肉牛尿液排泄，尚不清楚。本项目拟研究肉牛对菜籽饼GLs的代谢规律，阐明肉牛对菜籽饼GLs代谢产物ITCs的排泄规律及抑制N2O生成的效果，为通过饲喂肉牛菜籽饼抑制N2O生成奠定理论基础。

大气中CO2、CH4和N2O快速积累造成地球气候变暖。N2O是高强度温室气体，也是破坏大气臭氧层的关键成分。家畜粪尿的含氮化合物可被土壤微生物转化为N2O。减少家畜的氮排泄和调控尿液成分是降低N2O生成的关键。肉牛的日粮氮沉积率仅为20%左右，肉牛的粪尿可产生大量N2O。硫代葡萄糖苷（Glucosinolates, GLS）是十字花科植物的特有成分。据报道，采食十字花科植物油菜绵羊尿液的N2O排放量少于采食黑麦草的绵羊。这可能是油菜GLS的代谢产物从绵羊尿中排出，抑制了N2O的生成。GLS可被水解为硫氰酸盐（Thiocyanate，SCN）和异硫氰酸酯（Isothiocyanate，ITC）等成分。肉牛采食菜籽饼GLS时，是否从尿中排泄SCN、ITC等成分而影响尿液N2O生成，并不清楚。本项目研究了： (1) 肉牛对菜籽饼GLS的代谢规律；GLS代谢产物在肉牛尿中的排泄规律及其对N2O生成的作用; (2)温度和CuSO4对菜籽饼GLS脱毒及其饲喂肉牛的效果；（3）十字花科植物紫甘蓝提取物（red cabbage extract, RCE）的花色苷在肉牛体内的代谢规律及其对尿液N2O生成的作用。结果显示，菜籽饼GLS在肉牛瘤胃内主要被分解为SCN，SCN可从肉牛尿中排出。日粮中包含8%菜籽饼不影响肉牛的氮沉积。尿液N2O-N排放量与尿液SCN浓度之间存在线性正相关关系。SCN对尿液N2O生成没有抑制作用，反而具有促进作用。SCN通过提高土壤Nitrosomonas和细菌amoA的丰度，促进了N2O-N生成。土壤培养试验结果表明，人工尿N2O-N排放量和SCN浓度也存在线性正相关关系。日粮中添加8%DM菜籽饼（GLS 18.1μmol/g DM）, 尿液SCN排泄量由0提高到2.56 mmol/d，尿液N2O-N排放量提高42.2%。因此，对菜籽饼进行脱毒处理，可减少采食菜籽饼的肉牛尿液的SCN排泄量，进而降低尿液N2O排放量。菜籽饼GLS脱毒试验显示，25℃和1.6 g CuSO4·5H2O/L与60℃和3.2 g CuSO4·5H2O/L处理的GLS总去除率均在62.7-68.5%范围内。日粮中添加120 g RCE/d不影响肉牛氮沉积，提高了牛尿的马尿酸N/尿N比值，减少尿液N2O-N排放量33.1%。研究结果对于减少反刍家畜排泄物N2O排放具有应用价值。