Effect of soil copper on the response of soil fungal communities to the addition of plant residues

Organic residues provide the fundamental energy supply supporting soil fungal communities. Provision of adequate energy is required for soil microbial communities to adapt and function in the presence of ecological stress, such as copper (Cu) contamination. However, contamination can also lead to decreased ecological fitness of microorganisms, limiting their ability to access substrates. Thus, complex interactions exist between substrates, metals, energy supply/accessibility, fungal communities and their processes, and these have implications for ecosystem processes. We investigated the interaction between energy resources and Cu tolerance on soil fungal communities, including Fusarium and Trichoderma (model disease causing and beneficial genera). Using quantitative PCR and DGGE fingerprinting, the effects of increasing soil Cu levels (0 to >3000 mg Cu kg(-1) soil) on size and structure of soil fungal communities were tested under basal and plant-residue (medic; Medicago trunculata) added conditions. The interaction between increasing soil Cu levels and the addition of plant resources on fungal community structure was tested using multivariate analysis. The relative size (DNA copies per unit of soil DNA) of soil fungal communities, including Trichoderma and Fusarium, significantly (P<0.05) increased (94% and 32% respectively) with addition of medic to soil. In medic-applied samples, the bacterial to fungal ratio decreased, demonstrating the selective influence of the cellulose-rich substrate on the fungal community. Under the high nutrient conditions fungal DNA increased as a fraction of the total soil DNA, demonstrating the tolerance of fungi to Cu (relative to other microbiota) given adequate energy resources. Copper had no impact on the abundance of Fusarium or Trichoderma, but significantly affected community structure (PERMANOVA; P<0.05). With increasing Cu, species selection and replacement could be observed, particularly in soils where medic had been included. Plant residue addition itself was a highly selective factor affecting the structure of communities of Trichoderma and Fusarium (P<0.05). The effects of increasing Cu could be seen in both medic and basal soils for Trichoderma, but only in the basal treatments for soil Fusarium. This was due to very low dispersion in Fusarium community structure in the medic-added treatment (PERMDISP; P<0.05). The results show the interactive influence of organic matter inputs and heavy metal contamination on size and structure of soil fungal communities. The data show that species selection and replacement is an important mechanism for community adaptation to increasing levels of soil Cu, and this mechanism can be influenced by addition of resources to the soil. (C) 2010 Elsevier GmbH. All rights reserved.