There’s no place like home? An exploration of the mechanisms behind plant litter–decomposer affinity in terrestrial ecosystems

This week’s post is by PSE member and star #psejclub commenter Relena Ribbons. If you’d like to write a guest post, please get in touch!

I was really excited to see a new article synthesizing recent publications on decomposition and litter affinity effects in New Phytologist by Austin et al. One of the most well-known examples of litter affinity effects is the Home Field Advantage (HFA) hypothesis, whereby litter decomposition is accelerated in its home environment. Given the importance of nutrient cycling in terrestrial systems the patterns observed in litter decomposition have attracted a large audience, and expansions and new hypotheses have been postulated including the substrate-matrix interaction (SMI) or the phenology-substrate match (PSM) hypotheses. The authors go on to briefly describe each of these hypotheses, while also providing examples that don’t align with HFA, and conclude the article by raising additional research questions and future directions to elucidate the mechanisms behind observed litter affinity affects. Following the yellow brick road on home field advantage leaves us asking more questions, which is a good thing.

I really liked that the authors provided the background on HFA, expanded on to SMI and PSM, as it serves as a logical trajectory of how these hypotheses have been developed, and provides a good introduction for readers who may not be familiar with litter affinity effects. The authors state that the most cited explanations for litter affinity effects result from a specialization of microbial community, which is due to a constant and chronic input of similar litter quality over time which results in a microbial community that is optimized to degrade those particular litter inputs.  However, the authors go on to assert that standard indices of lignin, C : N ratios, and nutrient content alone, may not be sufficient to explain an observed HFA. For example, recalcitrant litter decomposes more quickly in a less favourable home environment and simply as a function of its litter quality or site fertility (see  Vivanco and Austin 2008 for further insights to this particular point and figure 1- copied below). Furthermore, high-quality litter may decompose faster in a fertile microsite as a result of higher resource availability for decomposers independent of its origin, which is known as a priming effect.

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What I found especially insightful was the section of this article discussing green leaf hitchhikers, and discussion incorporating the legacy of the phyllosphere community. I see the term rhizosphere used quite frequently in journal articles, but I have not had the same exposure to the term phyllosphere (which is used to refer to the above-ground parts of plants). The microbiology of the rhizosphere (refers to the belowground habits of plants, e.g. plant roots) has received more attention than the microbiology of the phyllosphere (see Vorholt’s 2012 Nature Review Microbiology publication for further discussion), which I found interesting as historically scientists have first explored above-ground components.

Austin et al. went on to discuss how green leaves can serve as a conduit for introducing not only the litter nutrients, but also serve as a vector for carrying fungi and bacteria from green leaves into the decomposer community. In effect, microbial community might be hitching a ride from green leaves, down to the leaf litter (as leaves senesce) and could jump-start decomposition, alter the microbial decomposer community, and ultimately play an important mechanistic role in explaining observed HFA effects. The authors further explored the potential connections between volatile organic compounds used in plant defences, as a means for attracting a specific decomposer community. This got me thinking about all those caterpillars I have seen crawling towards my garden this summer, and what their role might be in decomposition. The authors highlight that further exploration of macrofauna decomposers is also warranted (see figure 2 from their paper below as you contemplate whether there is more to the story of Eric Carle’s Very Hungry caterpillar).

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I found this particularly interesting as I think about my own experimental work, and the potential intersection between pathogens and pests and observed patterns in litter decomposition and nutrient cycling. Has this paper jump-started any new lines of inquiry in your research? Are there any future directions you think the authors should have considered in this paper? Any points where you are confused, or wish the authors explained the concepts in more details? Are any of your looking at caterpillars differently after reading this article? Share your thoughts on Twitter and Facebook using the #psejclub tag.

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2 thoughts on “There’s no place like home? An exploration of the mechanisms behind plant litter–decomposer affinity in terrestrial ecosystems

  1. Alix

    The green leaf hitchhiker ‘hypothesis’ is an interesting one. Has anyone tested this? The hold up for me is: presumably whatever (microbe?) is on the green leaf, it is doing some non-decomposition function (the leaf isn’t dead yet), right? Although perhaps it’s eating any dead leaf cells that are sloughing off (do dead leaf cells slough off, the way I imagine dead human skin cells sloughing off and the skin microbiome feasting?). It’s intriguing and interesting to think that once the leaf falls to the ground and cut off from the plant’s life line, and the microbe goes with it, that the microbe has some sort of ‘switch’ that turns on a decomposer function. Totally reasonable, I guess, considering evolution–if any microbes CAN turn on that switch, they’re MUCH more likely to survive than those hitchhikers that CAN’T perform decomp in any way (and probably just senesce away with the leaf). It’s a neat hypothesis to test, and reminds me of vertical transmission of seed fungal endophytes (see Hodgson et al. 2014, Ecology and Evolution). Definitely opens an interesting direction for linking aboveground/belowground trophic/nutrient cycles!

    Reply
  2. mgenung

    Nice post, Relena, and interesting comment, Alix. I agree that the hitchhiker hypothesis is interesting. In terms of the function of microbes on green leaves, you could look at this review article (Lindow and Brandl Applied Environmental Microbiology 2003, which may be dated, but should still serve as a decent introduction). Some relevant points – green leaf bacterial communities are highly variable over time, as physical and nutrient conditions in the phyllosphere change. Also, the types of bacteria found on green leaves (e.g. pigmented bacteria) vary from those found on roots. So, one holdup might be establishing that a certain community persists after leaf senescence and influences decomposition, despite the fact that bacterial community composition is frequently changing on the green leaf just due to different conditions throughout the growing season (more detail on this from studies such as: IP Thompson et al. Plant Soil 1993). I like Alix’s “switch” idea as a potential mechanism for this.

    Reply

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