Biochar and Soil Mites: Behavioural Responses Vary with Dosage and Feedstock

Robert Godfrey1,2, Sean Thomas2, Kyle Gaynor2

1. Department of Biology, McMaster University 2. Faculty of Forestry, University of Toronto

Background• Biochar, charcoal used as a soil amendment, is produced from the thermal decomposition

of biomass under low oxygen conditions (pyrolysis)1

• Pyrolysis converts C in biomass to aromatic forms, stable in soil for hundreds of years2

• Therefore, biochar has great potential as a means to sequester C, reducing losses of CO2 to the atmosphere from decay and incineration of forestry and agricultural wastes1

• High variability in chemico-physical properties of chars (eg. pH, porosity) depending on feedstock and pyrolysis conditions (peak temp, time)2

• Oribatid mites are the most numerous & diverse soil arthropods (up to 300,000/m2 of forest soil)3

• Oribatids are important in decomposition and mineralization3, yet there is a complete lack of data on biochar responses, though some research conducted on earthworms4

Research Questions• Does the oribatid mite Oppia nitens respond behaviourally to biochar exposure (ie. attraction/avoidance response)?

• Does the response vary according to dosage and type of char? • Does char avoidance correlate significantly with a specific char property (eg. pH)?

• Oppia nitens, common temperate soil mite, raised in culture from individuals taken from Haliburton Forest, ON

• Soil: at 70% water-holding capacity, ~7% OM, from Haliburton Forest, ON• Four biochars: maple-birch sawdust mix (87%-13%), beech sawdust, crushed spruce chip,

barn waste• Plastic preference trial containers, each divided in half with mesh Table 1. The four biochars used in the experiment. Biochar properties vary depending feedstock and pyrolysis conditions.

Peak pyrolysis temp (oC) pH % Moisture % AshMaple-birch 550 7.39 2.70 2.74Barn waste 525 10.18 ? ?Spruce chip 250 7.43 1.37 1.25Beech 378 6.18 ? 1.60

Figure 2. Preference trial container, show-ing char separated from soil by mesh.

Figure 3. Oppia nitens adult, viewed with light microscope.

Figure 4. Containers housed within humid box (2 treatments, 8 replicates per)

Results• Trial data analyzed using Excel and R statistical software• Each replicate subjected to a binomial test to generate individual P-values• Fisher’s test for combined probabilities used to combine binomial test results to generate a single P-value for each treatment • HA: true probability of ending up on one side is not equal to 0.5• Statistically significant response observed for all treatments except control & 51.2% maple-birch• Average proportion of mites on 100% dosage side = 0.52, proportion =0.17 for 51.2% dosage across all chars• No significant correlation between mite response and pH (r=0.56), % ash (r=- 0.98, P=0.15)

Results cont.

Conclusions• Oppia nitens responds behaviourally to biochar exposure (eg. avoids 100% char)• Biochar response consistently greater for 100% char vs. 51.2%, though magnitude of re sponse varies with type of char• Oppia nitens showed slight preference for 51.2% spruce char mix vs. soil• Insufficient data to determine which char property is the main driver of the response

Discussion• Some property of chars is repellent to mites at very high char concentrations• Biochar application might therefore have negative effects on oribatid populations and thus decomposition and soil formation, depending on char properties/application rate• Slight preference for spruce mix might be explained by spruce dominance across much of Oppia nitens’ range (in boreal)• Mites could be stressed by volatile organics (eg. phenols), likely more in barn wasteFurther research:• Conduct fecundity and mortality trials • Test other oribatid species and mites at different life-stages• Test other biochars and dosages, continue char characterization• Investigate mite aggregation and “herding” to better understand behavioural response

Figure 6. Average proportion of mites on biochar substrate side of container for each treatment. Error bars extend one SD on ei-ther side of the mean. Line at 0.5 to indicate whether attraction or avoidance response observed (avoidance below line).*=P<0.05, **=P<0.01, ***=P<0.001

References1. Thomas, S. 2013. Biochar and its potential in Canadian forestry. Silviculture Magazine, Jan. 2013, 4-6.2. Seastedt, T. R. 1984. The role of microarthropods in decomposition and mineralization processes. Ann. Rev. Ent. 29:25-46.3. Rostad, C., and D. Rutherford. 2011. Biochar for soil fertility and natural carbon sequestration.in U. S. G. Sur vey, editor. USGS, Reston, VA.4. Weyers, S., and Spokas, K. 2011 “Impact of biochar on earthworm populations: A review,” App. Env. So. Sci. 2011:12.

AcknowledgementsSincerest thanks to Sean Thomas for his guidance and patience. My gratitude goes also to Kyle Gaynor, Tara Sackett, Nigel Gale, Henry Hong, CGCS, and the more than 1280 mites who stoically tolerated moderate-to-profound discomfort in the name of science.

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Maple-birch Barn waste Spruce chip Beech

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Biochar

51.2% 100%

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Figure 5. Scanning electron micrograph of Oppia nitens (125x). Image by Henry Hong.

100 µm

Materials and Methods

• Each char (except beech) prepared in two dosages, 100% and 51.2% by volume, ~70% wa-ter-holding capacity

• ~15mL of soil added to one side, 15mL of char substrate to the other in each of eight con-tainers used per treatment (null treatment conducted with soil on both sides as a control, test for aggregation behaviour)

• Substrate left ~18hrs in humid box to equilibrate, then 10 adult mites added to each side• Containers left for another 24hrs, then mites on each side counted

Figure 1. Wood waste with biochar (Image from Farm Management Canada)