关键词: 桤木, 共生固氮, 氮增加量, 生物量, 养分循环, 农业生态系统, 混农林业

Abstract:

Alder (Alnus nepalensis), an important non-legume nitrogen-fixing plant, occurs widely in the eastern Himalayas. In traditional agroecosystems of Asia, alder is commonly used as a fallow tree or is intercropped. There has been growing concern about ecological and economical values of alder, and this has resulted in greater interest in the utilization of this species. Soil-improvement functions of alder in traditional agroecosystems were reviewed based on research results for the last 30 years, in order to bring attention to alder's role in traditional agroecosystems, promote the development of agriculture in mountain areas and protect the environment.

Soil improvement functions of alder in traditional agroecosystems were as follows: 1) Seasonal nitrogen accretion in alder stands was high during the growing season between June and November (95%-98% of total annual accretion) and very low from December to April (2%-5%). Annual accretion of nitrogen was the highest (117 kg·hm^-2) in a 7-year stand. Annual symbiotic nitrogen fixation in alder-cardamom (Elettaria cardamomum) plantations was the highest (155 kg·hm^-2) in a 15-year stand and decreased with increasing plantation age. 2) Alder performed well with intercropping or as a fallow tree. When intercropped, it could improve the agronomic yield of crops. The agronomic yield of cardamom was 2.2 times greater under the canopy of alder than under natural forest. As a fallow tree after a 6-year fallow period, the above-ground biomass in alder fallows was 4 times greater than in grass fallows, N accumulation was 3 times greater than in grass fallows and P and K were both about double the level found in grass fallows. 3) Alder seemed to have the most desirable rooting characteristics for agroforestry systems. Most fine roots were concentrated in the upper 10 cm of the soil, but coarse roots were concentrated at 10-20 cm depth. The fine root biomass (FRB) in intercropping was 5% greater than when grown as a fallow tree. Sixty percent of the fine roots were distributed 0.5 m from the tree trunk. Most of the woody roots (>5 mm diameter) were present within 0-10 cm depth near the tree trunk in both intercropping and fallow-tree situations. Total root length of woody roots was <1 m. 4) Nutrient cycling rates were higher in alder plantations than under other plantations. Rates of litter production and decomposition were 1.59 and 1.79 times higher, respectively, in alder-cardamom stands than in forest-cardamom stands. The litter from N₂-fixing species may accelerate the decomposition of other types of litter. When mingling with alder litter, other types of litter decomposed as rapidly as the alder litter. 5) In the traditional swidden agricultural system, fertility of the soil under alder fallow could be rejuvenated in 3-6 years, which reduced the fallow period remarkably, and physical and chemical properties of the soil could be improved greatly.

Although the functions of the alder for improving soil have been researched much, the problem of how to simultaneously achieve the best economical and ecological values of co-planting crops and the alder in mountain areas needs further study.

Key words: Alder (Alnus nepalensis), Symbiotic nitrogen fixation, Nitrogen accretion, Biomass, Nutrient cycling, Agroecosystems, Agroforestry