• My name is Ashley Gorman and my research is looking at the significance of soil erosion to redistribute arable weed seeds, it’s a 4 year project funded by JHI and UoD as part of the CECHR initiative. • It involves 3 scales, lab field and catchment and today I am going to talk about the potentially beneficial roles of seedbanks in erosive agroecosystems. • Firstly, I’ll explain why arable weeds matter, present my conceptual framework for understanding seed fate, take a brief look at the results from a field experiment and then the exciting experimental work looking at seed mucilage and soil properties.
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• Weeds provide important ecosystem benefits within monoculture landscapes, e.g. crop pollination, nutrient cycling, resource provision for invertebrates, birds and small mammals • 35% of crops require pollination, and recently valued by researchers at university of reading as £690 million just in the UK • Accelerated soil erosion degrades the physical and biogeochemical functioning of soils, and weeds may become a vital buffer against short‐term disturbance events • However, they are influenced by cultivation practices, and over the last 50 years, intensification of agricultural management has caused vast biodiversity losses, impairing ecosystem functioning • Which in turn makes arable weeds effective indicators of farmland biodiversity • “On average, the frequency of pollinator visits was 25% higher for crops with adjacent flower strips compared to those without, with bumblebees (Bombus spp.) accounting for 62% of all pollinators observed.” Feltham, H, Park, K, Minderman, J and Goulson, D (2015) Experimental evidence of the benefit of wild flower strips to crop pollination. Ecology and Evolution.
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• I’ve included my conceptual framework today to simply overwhelm you about the number of processes involved in seed fate. • To a certain extent, many of these areas are well described in the literature, such as mortality germination and some stages of secondary dispersal. • However, the impacts of soil erosion on the state and vitality of the seedbank is overlooked • And that gives rise to my research aims surrounding myxospermy which is the secretion of mucilage from the seed coat, creating a critical abiotic process called antitelechory which is the prevention of dispersal • This whole system is of course over‐ridden by management….which leads nicely to my field experiment from 2014
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• This was an incredible opportunity to work on a long term field trial at the mid‐pilmore experimental site in invergowrie, which has previously examined the effects of tillage practices on winter and spring barley crop yield and carbon dynamics – and I was able to use this resource to look at tillage effects on seedbank distribution • There are 5 treatments established from which I sampled from 3 to see how tillage depth influences “viable” seedbank composition • There are 2 subplots, winter and spring barley, which allowed me to look at seasonal germination and potential for crop type associations e.g. promotion of annuals over perennials • And lastly, how tillage affected seed viability with depth e.g. if there has been no‐till for 12 years, will we see fewer viable seeds at lower depths? • I also utilised this experiment to compare seedbank assessment methods of seed extraction and seed emergence which I can about later if anyone is interested
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• 38 species were observed, which were devised into 16 functional groups based on physiological and ecological characteristics • There are lots of data I could go into here but I’m just going to highlight the key message with this canonical variates plot which is that management significantly impacted the abundance, distribution and diversity • No till diversity increased with depth, with surface diversity undoubtedly driven by grasses • I re‐sampled again last year after the conversion of the no‐till plots with a conventional plough and results will be completed this summer • So management does influence seedbank distribution, but why do we care?
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• We care because of the potentially beneficial roles of seed characteristics influencing soil behaviour • PICTURE, after storm desmond in these small tillage plots at pilmore, we see the tramlines becoming a superhighway for transport of seed, nutrients and biota and when we upscale,
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• PICTURE it’s a cause for concern. • If we can say that specific species or specific communities of weeds are important because they are beneficial to soil structure, and under a known quantity can bind soil together, we can start to apply these natural solutions to reduce or even prevent runoff erosion. • Differential mobility capacity and consequently mortality risk, depends on seed morphology • Causing selective erosion of seed, depending on seed size, shape, presence of appendages and myxospermy ability
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• what is it? It’s the secretion of mucilage, a concoction of sticky sugars, which burst out of the cell wall gluing the seed to the ground
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• DIAGRAM When a dry “mature” seed becomes hydrated, volcano‐shaped cytoplasmic column forms, which is gradually displaced by the deposition of a secondary wall known as the columella (C), mucilage ruptures primary cell wall forming a two‐layered capsule and ray‐ like structure (R). • And when it dries, it prevents movement – antitelechory, the abiotic process I mentioned during the conceptual framework at the beginning. • Quantitative studies are rare and limited to a single or a few model species • But they have suggested that due to the adhesive nature of mucilage, it can cause physical alterations to the soil, with the potential to stabilise the structure and enhance water retention • But we don’t know what the capability of specific species, or more importantly community assemblages is to prevent removal or if there is a threshold beyond which soil properties are altered
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• Not all species have mucilage and so using the 38 species I found in pilmore, I found 7 to have mucilage after staining with ruthenium red • But what I didn’t expect to find was how they varied in structure
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• Capsella bursa pastoris, has what is known as a classic pink “halo” shape, a gelatinous capsule, with two layers • The outer later is comprised of pectin and is soluble in water • The inner layer is cellulose, and is insoluble
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• Although the smallest seed tested, it has large halo, irregular outer layer starting to disperse into the solution
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• Plantago, very large thick halo
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• Densely packed inner layer which wasn’t present on my other species, can see the rays clearly
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• This is what the sample looked like after incubation, its glued together forming the shape of the tube, it had even welded to the filter paper after drying like super glue.
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• Veronica, much less intact, whispy structure
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• Very irregular and dense
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• The only elongated spindle rod‐shaped seed, thin layer that was centralised around…
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• The hairs. Doesn’t appear to have a large volume
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• After drying, they created a beautiful (but delicate) lattice
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• Long strings of mucilage coming out from a network of ridges, rather than the typical halo
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• fibres released from seed coat on the ridges
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• Network of these hexagonal mucilage regions, light pink outer and the dark collumella inside
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• When you arrange them in order of relative seed size, the variability is even more exciting • Plantago effectively doubling its size, whereas scenecio is very thin • If the structure could be different across species, what did that mean for the actual sugar composition • When immersing in water, pH differed with plantago, capsella and Arabidopsis around 4.8‐ 5.2 but euphorbia, urtica and senecio were 5.8‐6.4 • What does that imply about the mucilage composition? • Can the adhesive power differ? • How often and at what volume can the mucilage be released? Short periods of rainfall daily for example • The experimental plan will be 3‐fold • I will look at the interaction between mucilage and soil properties • Then, quantify the mass of soil particles that a seed is capable of adhering, relationships between seed surface area and how “sticky” the mucilage is • And finally, examine the adhesive power of different species over varying slopes using a rainfall simulator
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• The initial step was to quantify the seed weight to mucilage ratio • The inner cellulose layer is removable by enzyme digestion and the outer pectin layer can be removed easily in water • The pectin solution can then be freeze dried which ends up looking like THIS and even at this stage, they are all so different • I have ran this process 3 times, altering the incubation temperature and pH and sample size, 2 of which obtained some rough prelim results • Ranges are large due to the seed samples containing non‐seed material and non‐ mature seed. I have since developed a sieving protocol to remove this which will be applied to the next trial.
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• Plantago – over a 1/3 of weight just from the soluble layer, the fraction capable of altering soil properties • Whereas veronica – was under 5% not surprisingly • However, scenecio was surprising at just under a 1/3 despite how thin it, which is most likely due to the fact scenecio is very thin and flat, so relative seed mass is lower
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• Inner layer has been very difficult to remove, and prelim results suggest that there are non‐pectin and non‐cellulose sugars remaining after incubation • This is already documented for Arabidopsis with one author finding dozens of other sugars which then required multiple enzyme digestions. In PhD afterlife, this might be something to come back to
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• I have one final extraction to complete and then I can begin testing the influence of mucilage on soil properties in April • I will be able to alter the concentrations of mucilage that I can apply • And initially I want to focus on a community of species rather than single species effects
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• And hopefully the outcome will help us to better understand seed‐soil interactions occurring in the field
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