Department of Horticulture, Virginia Tech, Blacksburg, Virginia 24061.
Mycologia. 2002 Nov-Dec;94(6):921-32.
Ectomycorrhizal fungi allow their host plants access to organic forms of N through enzymatic mineralization of the substrate and enhanced absorption of amino acids and mineral N. The cost to the plant is carbohydrates that support fungal growth and metabolism. Enrichment of soils with mineral N, as through atmospheric deposition, may affect the growth and function of these fungi by direct effects of increased N availability on fungi and indirect effects through reduced plant C allocation to roots. We tested the potential of N enrichment and altered carbohydrate supply to affect the growth and protein mineralization activity of 10 ectomycorrhizal fungi in sterile liquid media. Nitrogen treatments consisted of organic N only vs organic plus mineral N. Carbon treatments consisted of 5 g per liter glucose vs. no glucose added. Fungi differed widely in their growth and mineralization responses to these variables. Seven of 10 fungi had at least 20% reduced growth with reduced carbohydrates. Only 2 of 10 increased growth by 20% or more with increased mineral N. Carbohydrates affected growth more in a purely organic N environment suggesting an energy limitation to mineralization. Protein mineralization activity tended to be depressed by reductions in carbohydrates and increased by increased mineral N. The high sensitivity of fungal growth to carbohydrates suggests important indirect effects of N enrichment via altered C allocation in host trees. Principal Components analysis separated most fungal species along an axis representing a gradient from high protein mineralization efficiency to high intrinsic growth rate. Those fungi with slow growth and efficient mineralization activity corresponded closely to fungi often cited as late successional species, while fungi with high growth rates and low mineralization efficiency are often categorized as early successional. One fungus, Cenococcum geophillum, separated from others on an axis representing strong N dependence in growth. Nitrogen enrichment has the potential to alter the composition and function of the ectomycorrhizal fungus community. Physiological differences among species provide a starting point for predicting community responses and anticipating ecosystem consequences.
外生菌根真菌通过基质的酶促矿化和增强对氨基酸和矿质氮的吸收，使宿主植物能够获得有机形式的氮。植物的代价是支持真菌生长和代谢的碳水化合物。通过大气沉降等方式增加土壤中的矿质氮，可能会通过增加氮的可用性对真菌产生直接影响，并通过减少植物向根部分配碳而对真菌产生间接影响，从而影响这些真菌的生长和功能。我们在无菌液体培养基中测试了氮富集和改变碳水化合物供应对 10 种外生菌根真菌生长和蛋白质矿化活性的潜在影响。氮处理仅包括有机氮与有机氮加矿质氮。碳处理包括每升 5 克葡萄糖与不添加葡萄糖。真菌的生长和矿化反应差异很大。在减少碳水化合物的情况下，有 7 种真菌的生长至少减少了 20%。只有 10 种中的 2 种真菌的生长增加了 20%或更多，而矿质氮增加了。在纯有机氮环境中，碳水化合物对生长的影响更大，这表明矿化存在能量限制。碳水化合物的减少会抑制蛋白质矿化活性，而矿质氮的增加则会促进蛋白质矿化活性。真菌生长对碳水化合物的高度敏感表明，通过改变宿主树木的碳分配，氮富集会产生重要的间接影响。主成分分析将大多数真菌物种沿一条代表从高蛋白质矿化效率到高固有生长速率的梯度的轴分离。那些生长缓慢但矿化效率高的真菌与常被引用为晚生种的真菌密切相关，而那些生长速度快、矿化效率低的真菌通常被归类为早生种。一种真菌，Cenococcum geophillum，在一条代表生长强烈依赖氮的轴上与其他真菌分离。氮富集有可能改变外生菌根真菌群落的组成和功能。物种之间的生理差异为预测群落反应和预测生态系统后果提供了起点。
