Michelsen Anders, Graglia Enrico, Schmidt Inger K, Jonasson Sven, Sleep Darren, Quarmby Chris
1 Department of Plant Ecology, University of Copenhagen, Øster Farimagsgade 2 D, DK-1353 Copenhagen K, Denmark.
New Phytol. 1999 Sep;143(3):523-538. doi: 10.1046/j.1469-8137.1999.00479.x.
Microbial immobilization may decrease the inorganic nutrient concentrations of the soil to the extent of affecting plant nutrient uptake and growth. We have hypothesized that graminoids with opportunistic nutrient-acquisition strategies are strongly influenced by nutrient limitation imposed by microbes, whereas growth forms such as dwarf shrubs are less affected by the mobilization-immobilization cycles in microbes. By adding NPK fertilizer, labile C (sugar) and fungicide (benomyl) over a 5 yr period in a fully factorial design, we aimed to manipulate the sink-source potential for nutrients in a non-acidic heath tundra soil. After 2 yr, N and P accumulated in the microbial biomass after fertilization with no change in microbial C, which suggests that nutrients did not limit microbial biomass growth. After 5 yr, microbial C was enhanced by 60% in plots with addition of labile C, which points to C-limitation of the microbial biomass. Microbial biomass N and P tended to increase following addition of labile C, by 10 and 25%, respectively. This caused decreased availability of NH and P, showing close microbial control of nutrient availability. The most common graminoid, Festuca ovina, responded to fertilizer addition with a strong increase, and to labile C addition with a strong decrease in cover, providing the first direct field evidence that nutrient limitation imposed by immobilizing microbes can affect the growth of tundra plants. Also in support of our hypothesis, following addition of labile C the concentrations of N and K in leaves and that of N in roots of F. ovina decreased, whilst the demand of roots for P increased. In contrast, the most common dwarf shrub, Vaccinium uliginosum, was only slightly sensitive to changes in resource availability, showing no cover change after 4 yr addition of labile C and fertilizer, and little change in leaf nutrient concentrations. We suggest that the differential responses of the two growth forms are due to differences in storage and nutrient uptake pathways, with the dwarf shrub having large nutrient storage capacity and access to organic forms of N through its mycorrhizal association. While the fungicide had no effect on ericoid mycorrhizal colonization of roots or symbiotic function inferred from plant N natural abundance, it decreased microbial biomass C and N after 2 yr. Throughout the fifth season, the availability of soil NO and inorganic P was decreased with no change in microbial biomass C, N or P, suggesting a negative impact of benomyl on N and P mineralization.
微生物固定作用可能会降低土壤中的无机养分浓度，进而影响植物对养分的吸收和生长。我们推测，具有机会主义养分获取策略的禾本科植物受微生物施加的养分限制影响较大，而矮灌木等生长型则较少受微生物的固定 - 矿化循环影响。通过在5年时间内以完全析因设计添加氮磷钾肥料、易分解碳（糖）和杀菌剂（苯菌灵），我们旨在调控非酸性石南灌丛冻原土壤中养分的源 - 汇潜力。2年后，施肥后微生物生物量中氮和磷积累，而微生物碳没有变化，这表明养分并未限制微生物生物量的增长。5年后，添加易分解碳的地块中微生物碳增加了60%，这表明微生物生物量受碳限制。添加易分解碳后，微生物生物量氮和磷分别增加了10%和25%。这导致铵态氮和磷的有效性降低，表明微生物对养分有效性有严格控制。最常见的禾本科植物羊茅（Festuca ovina）对施肥反应强烈，覆盖率大幅增加，而对添加易分解碳反应强烈，覆盖率大幅下降，这首次提供了直接的田间证据，证明微生物固定造成的养分限制会影响冻原植物的生长。同样支持我们的假设的是，添加易分解碳后，羊茅叶片中的氮和钾浓度以及根系中的氮浓度下降，而根系对磷的需求增加。相比之下，最常见的矮灌木笃斯越桔（Vaccinium uliginosum）对资源可用性变化仅略微敏感，添加易分解碳和肥料4年后覆盖率没有变化，叶片养分浓度变化也很小。我们认为，这两种生长型的不同反应是由于储存和养分吸收途径的差异，矮灌木具有较大的养分储存能力，并通过其菌根共生关系获取有机形态的氮。虽然杀菌剂对根系的石楠型菌根定殖或从植物氮自然丰度推断的共生功能没有影响，但2年后它降低了微生物生物量碳和氮。在整个第五个生长季，土壤硝态氮和无机磷的有效性降低，而微生物生物量碳、氮或磷没有变化，这表明苯菌灵对氮和磷矿化有负面影响。
