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I recently found out that alder trees have root nodules which contain nitrogen-fixing bacteria and that alders are primary colonizers in primary succession. That leads me to this question: since there are really no fungi in the soil and since the alder is a colonizing species, where does the fungi come from? Or is my assumption that there are no fungi in the soil incorrect?

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    $\begingroup$ Welcome to Biology S.E.! If you need any assistance, please visit The Help Center. $\endgroup$ – L.B. Apr 20 '15 at 3:15
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    $\begingroup$ Where did you hear that alder trees are primary colonizers? Primary colonizers tend to be organisms such as mosses and bryophytes. I don't know for sure, but I would be willing to bet good money that the fungi make it there long before the alder trees do $\endgroup$ – C_Z_ Apr 20 '15 at 17:00
  • $\begingroup$ Cyanobacteria can form nodules in plants, and actinobacteria also called a sort of association which is called "actinorhyza" similar to mycorrhiza. But how could you conclude there is no fungi? fungi are everywhere, and they play a key-role in degradation of lignin etc biopolymers from plants $\endgroup$ – Always Confused Sep 7 '16 at 5:40
  • $\begingroup$ How you conclude that nodules contain nitrogen fixing bacteria? did you found that from experimental analysis? $\endgroup$ – Always Confused Sep 7 '16 at 5:41
  • $\begingroup$ Many types of bacteria can form nodules or such. In case of your alder tree root nodules; it might be some sort of Actinorhiza. See en.wikipedia.org/wiki/Frankia_alni , en.wikipedia.org/wiki/Root_nodule , en.wikipedia.org/wiki/Actinorhizal_plant. $\endgroup$ – Always Confused Nov 6 '16 at 6:14
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Any soil microbiology textbook will tell you that there are a great number of fungi in most soils. I believe there are 12 or 13 species of non-legumes capable of fixing atmospheric nitrogen. Podocarpus is one of them. Look at the roots of podocarpus and you will see nodules all along the roots.

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