混播草地中豆科/禾本科牧草氮转移机理及其影响因素

混播草地中豆科牧草与禾本科牧草(简称豆/禾牧草)之间的氮转移在草地农业系统氮循环中具有重要作用。在豆科/禾本科牧草混播系统和豆科/禾谷类作物间作系统存在一种氮素共享的通道,即在间(混)作中,豆科植物固定大气中的氮在满足自身生长需求前提下,还通过各种途径为伴生的禾本科植物提供氮源。在混播草地中氮素转移途径主要分地上和地下两种。地上途径主要是豆科牧草的地上部分经放牧家畜采食后粪便归还土壤,后又被禾本科牧草吸收利用或者地上凋落物在土壤中经微生物分解矿化释放出有效氮被另一种植物吸收利用(反之亦成立)。地下途径相对复杂,可能有以下3种:1)通过植物根际沉积氮转移。2)通过菌根真菌的菌丝传递。3)通过植物根系分泌物中含氮化合物来转移。目前的研究虽然明确了有可能转移的途径,但哪一种途径是主要的方式?在氮素转移的过程中,某一途径会部分的发生,还是好几种途径同时发生,每一个途径的贡献为多少?这仍缺少关键的证据。本研究针对国内外关于豆/禾混播草地中豆科牧草生物固氮、豆/禾牧草间氮转移的研究现状,重点对混播草地中豆/禾牧草之间的氮素转移数量、转移途径及影响因素等方面进行了分析与总结,并对可能存在的氮素转移机理进行了综述,对今后的研究方向进行了展望,以期为下一步通过将豆科植物引入我国农牧业种植结构来实现农牧业可持续发展模式的研究提供理论资料。

Mechanisms and factors affecting nitrogen transfer in mixed legume/grass swards: A review

Nitrogen transfer between legumes and grasses in mixed grass-legume swards plays an important role in the nitrogen cycle of grassland agricultural systems. There is a nitrogen-sharing pathway in legume-cereal intercropping systems and in grass-legume mixed swards whereby the legume plant not only meets its own nitrogen requirements for growth by way of fixed nitrogen from the atmosphere, but also provides nitrogen for the associated grasses through various pathways. The pathways of nitrogen transfer in mixed grass-legume swards can be categorized as above-ground or below-ground. The above-ground pathway occurs when herbage of legume plants is eaten by grazing animals and returned to the soil via excreta, then absorbed by other plants (including grasses), or when herbage litter falls to the ground and is decomposed and mineralized by microorganisms in soil to release plant-available nitrogen. The below-ground pathway is relatively complex and involves three routes: 1) Decomposition: decomposition of legume root tissues followed by uptake of released N by neighboring plants. 2) Mycorrhizae: transfer of N mediated by plant-associated mycorrhizae. 3) Root exudation: the exudation of soluble N compounds by donors and uptake by receivers. Although the current study identifies the possible routes of nitrogen transfer, many questions remain. Which is the main transfer route? What is the contribution of each route? To what extent can particular transfer routes operate concurrently? Such questions still lack key evidence. This paper briefly describes the current research status of biological nitrogen fixation of legume forages and of legume-grass nitrogen transfer in mixed grass-legume swards. A number of possible nitrogen transfer routes are reviewed, and directions for future research are identified. Better understanding of these questions will pave the way to achieve sustainable development of agriculture and animal husbandry in China, through realization of the potential benefits offered by legumes.