桤木(Alnus nepalensis)是一种重要的非豆科固氮植物，广泛分布于喜马拉雅山脉东部地区。在东南亚地区的传统农业生态系统中，多用桤木作为休耕树种，或将其与农作物间作。桤木根瘤固氮量随季节和年龄而变化，在桤木_小豆蔻(Elettaria cardamomum)系统中15年达到高峰（155 kg•hm－2）；桤木通过共生固氮对系统产生的氮增加量在纯桤木林中7年达到高峰（117 kg•hm－2）。桤木与农作物间作可显著提高农作物的产量，桤木小豆蔻立地上小豆蔻的经济产量是在森林小豆蔻立地上的2.2倍。桤木休闲地的休闲效果明显好于自然休闲地，其地上部分生物量在休闲6年后是自然休闲地的4倍，N蓄积量是自然休闲地的3倍，P、K蓄积量是自然休闲地的2倍。桤木根系特征似乎最适合混农林系统，其细根生物量（FRB）集中于土壤剖面上层10 cm范围内，在此范围内，FRB在“树+农作物”间作条件下比在 “只有树”条件下高5%；在两种立地条件下，60%以上的细根都分布于树干周围0.5 m内，大部分木质根(直径>0.5 mm)都分布于土壤上层0～10 cm处，长度都不超过1 m。桤木可加速系统的养分循环。桤木凋落物降解速率比非固氮植物快，并且与其它植物凋落物混合后的降解速率与自身凋落物降解速率一样快。在传统刀耕火种系统中，用桤木替代自然林休闲在3～6年内即可恢复土壤肥力，改善土壤理化性质，显著缩短休闲周期。该文综述了近30年来桤木在传统农业生态系统中改良土壤效应的研究成果，以提高人们对桤木的生态作用的重视程度，使人们更好地将桤木利用到农业生态系统中，达到发展山区农业和保护生态环境双赢的目的 。
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 follow s: 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 t he 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.