Agricultural Research Service, Rangeland Resources Research Unit, USDA, 1701 Centre Ave, Fort Collins, CO 80526, USA.
Oecologia. 2011 Mar;165(3):755-70. doi: 10.1007/s00442-010-1845-4. Epub 2010 Nov 27.
Carbon allocation and N acquisition by plants following defoliation may be linked through plant-microbe interactions in the rhizosphere. Plant C allocation patterns and rhizosphere interactions can also be affected by rising atmospheric CO(2) concentrations, which in turn could influence plant and microbial responses to defoliation. We studied two widespread perennial grasses native to rangelands of western North America to test whether (1) defoliation-induced enhancement of rhizodeposition would stimulate rhizosphere N availability and plant N uptake, and (2) defoliation-induced enhancement of rhizodeposition, and associated effects on soil N availability, would increase under elevated CO(2). Both species were grown at ambient (400 μL L(-1)) and elevated (780 μL L(-1)) atmospheric [CO(2)] under water-limiting conditions. Plant, soil and microbial responses were measured 1 and 8 days after a defoliation treatment. Contrary to our hypotheses, we found that defoliation and elevated CO(2) both reduced carbon inputs to the rhizosphere of Bouteloua gracilis (C(4)) and Pascopyrum smithii (C(3)). However, both species also increased N allocation to shoots of defoliated versus non-defoliated plants 8 days after treatment. This response was greatest for P. smithii, and was associated with negative defoliation effects on root biomass and N content and reduced allocation of post-defoliation assimilate to roots. In contrast, B. gracilis increased allocation of post-defoliation assimilate to roots, and did not exhibit defoliation-induced reductions in root biomass or N content. Our findings highlight key differences between these species in how post-defoliation C allocation to roots versus shoots is linked to shoot N yield, but indicate that defoliation-induced enhancement of shoot N concentration and N yield is not mediated by increased C allocation to the rhizosphere.
植物遭受刈割后通过分配碳和吸收氮可能与根际中的植物-微生物相互作用有关。植物的碳分配模式和根际相互作用也可能受到大气 CO2 浓度升高的影响，而这反过来又可能影响植物和微生物对刈割的响应。我们研究了两种广泛分布于北美西部草原的多年生草本植物，以检验以下两个假设：（1）刈割诱导的根分泌物增加是否会刺激根际氮的有效性和植物氮的吸收；（2）在高 CO2 条件下，刈割诱导的根分泌物增加及其对土壤氮有效性的相关影响是否会增加。这两个物种在水分限制条件下分别在大气 CO2 浓度为 400 μL L(-1)（环境水平）和 780 μL L(-1)（升高水平）下生长。刈割处理后 1 天和 8 天，测量植物、土壤和微生物的响应。与我们的假设相反，我们发现，刈割和升高 CO2 都减少了柳枝稷（C4）和冰草（C3）向根际的碳输入。然而，两种植物在处理 8 天后，与未刈割植物相比，刈割植物的地上部分氮分配量都有所增加。这种响应在冰草中最大，与刈割对根生物量和氮含量的负面影响以及减少刈割后同化产物向根的分配有关。相比之下，柳枝稷增加了刈割后同化产物向根的分配，并且没有表现出刈割引起的根生物量或氮含量减少。我们的研究结果突出了这两个物种在刈割后根系与地上部分的碳分配与地上部分氮产量之间的关系方面的关键差异，但表明刈割诱导的地上部分氮浓度和氮产量的增加不是通过向根际增加碳分配来介导的。
