Incorporating biochar into urban agroecology

Final report for GNC19-282

Project Type: Graduate Student
Funds awarded in 2019: $14,140.00
Projected End Date: 11/30/2022
Grant Recipient: University of Michigan
Region: North Central
State: Michigan
Graduate Student:
Faculty Advisor:
John Vandermeer
University of Michigan
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Project Information


Given that 81% of the US population lives in urban areas (2010 census), urban agriculture represents a key sector for increasing food security and improving quality of life for urban communities. However, urban soils often pose issues that limit agricultural productivity: including compaction (~50%), high pH (~8), and contamination with heavy metals (Lead, Arsenic) caused by hotspots of industrial activity and unregulated waste, often at concentrations definitively toxic to humans. This is especially true in Detroit, where, for example, 4 neighboring oil refineries have created the most polluted zip code in Michigan. As a result of uniquely severe soil issues like these, urban farmers, such as those in Detroit, often need to import topsoils and/or compost, which can be prohibitively costly, but is often the sole option for soil remediation due to city-level regulations on maximum urban compost pile sizes. Ultimately, importing organic soils does not represent an ideal nor feasible strategy for the sustainability of crop production by urban farmers. This project addresses urban soil compaction issues by evaluating the effects of biochar application, alongside legume mixtures, on soil structure, nutrient retention, soil biological activity, and plant growth. We found that biochar significantly lowers bulk density of fine-textured soils, improves organic matter and nutrient concentrations, holds more soil water, and affects soil invertebrate communities.

Project Objectives:

To complete this project, we engaged collaborative participation with several farmers, and analyzed soil responses related to soil structure, nutrient retention, and invertebrate community structure, and from this project produced scientific articles and seminar presentations about biochar's effects on (1) soil aggregation, structure, and theoretical background, (2) nutrient concentrations in an urban Technosol, (3) microbial community composition variation by biochar amendment treatment, and (4) invertebrate community composition and diversity in bulk soils.


Click linked name(s) to expand/collapse or show everyone's info
  • Naim Edwards (Educator and Researcher)
  • Seneca Lee (Educator and Researcher)
  • Gordon Fitch (Educator and Researcher)


Materials and methods:
Cover crop selection

Cowpea was chosen as a legume with bacterial-root symbioses that provide biological nitrogen fixation, adding soil nitrogen that biochar lacks, being a decay-resistant form of carbon, making a combination that studies show improves the effectiveness of biochar (urea is also an alternative nitrogen source). Cowpea also has African origins, making it of cultural importance to local Detroit urban farmers. Buckwheat was chosen as a companion crop for its ability to scavenge soil phosphorus, grow quickly, and attract pollinators via high floral density (SARE cover crop guide).

The following season an early cover crop mix of red clover and hairy vetch was planted to continue supplying nitrogen and root carbon to soils.

Research results and discussion:
Soil bulk density

Overall, biochar amendments showed significant effects on and explained substantial variance in soil bulk density within the top 10 cm (~30-60%, Table, Figure). These effects were mediated by the amount applied, its particle size, and whether it was paired with compost. Here, the amount applied seemed to show the largest effects, with application at 5% of topsoil volume being better than none, but no added benefit at 10% until reaching 20% by volume topsoil, and no differing effect by topsoil depth. Biochar also showed an effect in addition to that of compost amendment; an effect that seemed to persist even when biochar was applied alone without compost in the top 5 cm. Particle size showed a noticeable effect as well, with small particles more strongly lowering bulk density, especially at 10 cm, and larger particles possibly increasing it.

Soil bulk density statistical results

These effects compared variably to our hypotheses. Increasing application amount lowering bulk density agreed with our hypothesis, while the effects of particle size and comparison with compost were surprising. Previous studies suggest that biochar primarily helps when co-applied with compost; however, it may be that this is not true specifically for the function of lowering bulk density, but is still true for other functions, like increasing nutrient fertility or potential yield. In this fine-textured soil, we thought large particles help counteract the consolidating effects of many fine soil particles. However, it may be that 1 mm biochar particles are too large to help work at the micron scales of sand and silt (0.02 mm), and that instead the particles here named Small are actually the more appropriate size to counteract collective soil particle behavior at these small scales.

These bulk density results suggest that small amounts of biochar can help transition post-construction urban soils for cultivation, in low-till systems. Biochar use was comparable to compost with respect to soil bulk density, so depending on costs, biochar might be a useful option, such as a more stable amendment that composes slower than compost does throughout the growing season.

For wet aggregate stability, biochar did not significantly explain its variation among analyzed samples (n=16). Changes in aggregate stability may occur over longer periods of time, or when overall soil structure has improved after sustainable cultivation over five years or more. Aggregate size fraction mass data may respond differently and is under analysis.

Soil chemistry

Biochar also had significant effects on and explained substantial variance across various soil nutrients within the top 10 cm of soil (~20-55%). In the top 5 cm, biochar amendment rates significantly increased organic matter, phosphorus, potassium, and percent magnesium and calcium saturation – with all nutrients showing similar response patterns as in bulk density, where 20% application is the highest and either 5% and/or 10% being intermediate. Particle size also appeared to have some effect on nutrients, with Mixed (unsieved) or Small sizes possibly increasing organic matter and available magnesium and its percent saturation. At 5-10 cm depth, however, intermediate application rates of 5 and 10% appeared to show slightly higher magnesium. On the other hand, particle size seemed to have more of an effect on calcium than magnesium.

Soil chemistry statistical results

These chemistry results suggest that biochar can work to increase nutrient fertility at the very top of soil in low-till systems, such as by accumulating microbial biomass and necromass along biochar surface area, but that nutrient dynamics slightly at depth become more nuanced, such as according to potentially different nutrient susceptibilities to leaching by dissolution and runoff. Accordingly, biochar may be more useful in improving basic soil physical and chemical properties, but not as much for micro-nutrient availability management.

Soil chemistry graph 1 Soil chemistry graph 2 Soil chemistry graph 3Bulk soil microbiome sequencing data are under analysis for diversity and taxon trends with findings in prep for a public pre-print article.

Soil invertebrates and moisture

Overall, biochar had a significant positive effect on average soil moisture and significant yet slight negative effects on invertebrate biomass and diversity, but no detectable response of other measured variables after the first 3 months.

Soil moisture was 2% higher in soils with unsieved biochar particles (Mixed, including those > 2 mm), and most of this effect appeared to come from the smallest (< 1 mm) particles (S). Larger (> 1 mm) biochar particles (L) appeared to perhaps reduce soil moisture on average, as hypothesized, but this was not detectable as statistically significantly different from control (None, N) plots, given natural variations among plots of the same treatment. Soil moisture in the subsequent season tended to appear different based on biochar treatment, with possible functional delineations at 1-2” depth. However, amended topsoils showed similar moisture levels across biochar application rate, particle size, and comparative compost treatments.

Soil invertebrate biomass was significantly lower in soils amended with small particles, and invertebrate diversity (alpha / richness) was significantly lower in the soils with unsieved (Mixed, standard) biochar applied. Though effects came from different treatments here, they make sense given that most of the effects, e.g. on moisture, appear to come from smaller particles (S, < 1 mm), which presumably have more surface area (biochar already has a lot) and uniform shape compared to larger particles (L, > 1 mm). The loss in invertebrate biomass and richness could be due specifically to fewer ground beetles, which have thicker exoskeletons, or using a bit more inference based on the results of a few other studies, due to negative effects of biochar on the reproductive rates of springtails, which are one of the most abundant groups of soil invertebrates, and a weaker effect of some relatively positive (i.e. less negative) biochar effects on younger invertebrate life forms due to more moist soil environments.

Early cultivation  soil measurements

Average responses ± 1 std error to biochar treatments: None (N), Mixed/unsieved (M), Small (S, < 1 mm), and Large (L, > 1 mm). Meso and macro fauna were distinguished by taxonomic Order averaging below or above a 2 mm cutoff.

The effects of biochar found here may be slightly negative more directly on soil fauna, but the short-term positive effects on soil moisture may help save water for irrigation. The later benefits to crop growth and yield may be more variable as they depend on the the quantity and frequency of rainfall in the season, especially for soils with low organic matter. However, biochar application, which was of comparable cost to compost in this study and could be cheaper if sourced locally, could be an option to withstand variable rain and save money on irrigation, which can be expensive for home-owning urban growers in cities like Detroit.

Plant biomass
Cowpea and buckwheat cover crops grew well, however pods nor pollination appeared to show noticeable responses to biochar amendments after the first growing season. In the second trial beets and field peas established in part but did not reach substantial maturity. By this time in field growth, encroaching weed competition under no-till may have been a source of belowground competition. For initial soil management, strong root activity from native plants may be very healthy, so tradeoffs between faster soil regeneration for the long-term and annual short-term crop growth might be worth considering. Our recent study on the site (Edwards et al 2022) suggests that minimal-till by roto-till may be an optimal balance for the multi-functionality that urban growers need to manage, especially on engineered soils transitioning to cultivation use.
Callahan, Benjamin J, Paul J McMurdie, Michael J Rosen, Andrew W Han, Amy Jo A Johnson, and Susan P Holmes. 2016. “DADA2: High-resolution Sample Inference from Illumina Amplicon Data.” Nature Methods 13 (7): 581–83.
Caporaso, J. G., C. L. Lauber, W. A. Walters, D. Berg-Lyons, C. A. Lozupone, P. J. Turnbaugh, N. Fierer, and R. Knight. 2011. “Global Patterns of 16S rRNA Diversity at a Depth of Millions of Sequences Per Sample.” Proceedings of the National Academy of Sciences
Edwards, Naim, Nicholas Medina, and Elizabeth Asker. 2022. “Mixing Cover Crops Suppresses Weeds and Roto-till Improves Urban Soil Compaction and Infiltration.” agriRxiv 2022 (January): 20220304308.
Schloss, Patrick D., Sarah L. Westcott, Thomas Ryabin, Justine R. Hall, Martin Hartmann, Emily B. Hollister, Ryan A. Lesniewski, et al. 2009. “Introducing Mothur: Open-Source, Platform-Independent, Community-Supported Software for Describing and Comparing Microbial Communities.” Applied and Environmental Microbiology 75 (23): 7537–41.
Taylor, D. Lee, William A. Walters, Niall J. Lennon, James Bochicchio, Andrew Krohn, J. Gregory Caporaso, and Taina Pennanen. 2016. “Accurate Estimation of Fungal Diversity and Abundance Through Improved Lineage-Specific Primers Optimized for Illumina Amplicon Sequencing.” Edited by D. Cullen. Applied and Environmental Microbiology 82 (24): 7217–26.
Future analyses expected to also be posted publicly, i.e. via Google Scholar (
Participation Summary
2 Farmers participating in research

Educational & Outreach Activities

2 Consultations
1 Curricula, factsheets or educational tools
2 Journal articles
1 On-farm demonstrations
1 Published press articles, newsletters
5 Tours
3 Webinars / talks / presentations
1 Workshop field days

Participation Summary:

20 Farmers participated
3 Ag professionals participated
Education/outreach description:

Overall, activity at this site was relatively high as new and burgeoning hub for urban agriculture. The experiences gained alongside this project owe much thanks to the following contributors to various activities at the site, in no particular order -- L'Oreal, Pamela, George, Jacki, Olivia, Kat, Devin, Detra, Stathis, Lauren, Lizzy, Mary, Detroit Hives, Kido, Keep Growing Detroit, initial consultants at MSU, and many other folks who've engaged to some degree with discussion or labor related to this project, including many others not on this list, plus the many visitors touring the site throughout the peak growing season.

Engagement was limited the initial year due to Michigan universities’ pandemic policies, but we participated in a promotional video for the site and connected with several interested neighbors and informally with some local growers on several days while gathering data, since the site is relatively new, and is aiming for acceptance by the most immediate neighbors of practice of the broader mission, which needs building after leasing of the site.

We also produced a short internal report about the project, connected with local agricultural non-profits, and made slides as part of a resulting thesis presentation. (see description for credits)

Masters Practicum Presentation1 P1. Masters Practicum Presentation1 P2

Following this, a public webinar about the origins and science of biochar was offered and recorded in collaboration with Keep Growing Detroit, which is the largest farmer support non-profit organization in Detroit. Content covered included indigenous origins of biochar as a soil amendment in the Amazon (Woods et al. 2009) as well as across other continents, scientific evidence for hypotheses about how it works, and local project results. Some of ~20 participants asked follow-up questions about how biochar could be made locally and which crops would benefit the most from its application.
Woods, William, Teixeira Wenceslau, Johannes Lehmann, Christoph Steiner, Antoinette WinklerPrins, and Lilian Rebellato, eds. 2009. Amazonian Dark Earths: Wim Sombroek’s Vision. Berlin: Springer.
In addition, a scientific article was prepared for publication, submitted and pre-printed for public access in the agriRxiv host server on a previous study conducted at the site, as the product of a collaboration between growing field trials and scientific data analysis practices. The analysis code is also publicly visible, and both article and code are citable.
A short online media survey of a Detroit urban farmer group on the most popular sustainable farmer growing habits, including the use of biochar, with 47 resulting survey respondents. Compost addition was the most popular practice used by 36% of respondents, followed by cover crops and mowing at about 20%, fertilizer at 12%, and biochar and tillage at 6%. This shows that urban farmers do use a variety of practices, and that research projects like these and demonstration projects would best serve local growers here by addressing specific issues related to each one.
Farming practice survey
An additional talk was given recently at the DPFLI site's open house, where we discussed ongoing results from this research and possible future research directions with input from the audience. This talk was one of several given by other researchers and local growers on various topics in urban agroecology, such as air pollution, heat island effects, and urban forestry and pollination.
Future analyses also expected to be publicly available as scientific pre-prints and/or journal articles, e.g. via Google Scholar (

Project Outcomes

2 Grants received that built upon this project
1 New working collaboration
Project outcomes:

Improving urban soil health is still likely to benefit urban growers. While biochar may still likely be an external input, it should be most sustainable when made or purchased from wood as local as possible, and mixed with compost and a legume crop so as not to bind and make nutrients less accessible to plants. It may be a long-term practice for improving soil quality over initial years rather than immediately increasing short-term yields. Future analyses for additional objectives will reveal how different dimensions of soil health respond specifically to biochar. Overall, the current site used has certainly been a place of exchange of ideas among nearby farmers and sustainable agriculture practitioners, and is of much benefit to re-orient and innovate perspectives, paradigms, and norms of academic research.

Knowledge Gained:

Working with the Detroit Partnership for Food, Learning, and Innovation has put into perspective the kind of research that is most useful to farmers, and that academic research often asks related but separate and more conceptual questions. Areas of land, as the most fundamentally valuable assets in society (e.g. the source of the majority of inter-generational wealth) might be ideally used for setting up plots that mimic what small-scale, rather than industrial or entrepreneurial scale, urban farmers do in practice (including vested interests in specific crops with more local vs. regional or national markets). Additionally, conceptual studies can be paired and done in smaller and more controlled settings. Furthermore, data collection could be more targeted to mimic what farmers would typically collect data on when doing an intensive survey of some fraction of a production system each year, as is recommended. Then, much of the conceptual aspects of how nature is functioning in the agricultural system can be teased apart analytically, such as with qualitative comparison to simple (parsimonious) model representation and simulation, rather than relying on strict independent manipulation. The latter may usually weigh the most as quality of evidence for constructing or deducing knowledge, but this is worth discussing different ways of knowing, and validating how different strategies of approaching research questions match the breadth of societal problem or specific research question or concept being investigated. Ultimately, we explored more deeply and concretely (vs. typical formal education credits) the ways that academic researchers should best serve in service to farmers and the public.

Biochar initially changes moisture levels of transitioning soils mainly due to small sized pieces <1 mm. As of now, compost may be just as effective in maintaining field soil moisture levels as biochar, which may save grower time and money by allowing flexibility in practice according to local pricing and availability. Cover crops can do well for initial cultivation of post-demolition urban soils, as they can grow solidly with little maintenance. Vegetables on the other hand may require at least occasional tillage when starting from low-vegetation post-demolition urban soils. Depending on recent vegetation history, however, many urban lots may harbor relatively rich soils if starting from several previous years of fallow root activity from grass, herbs, and weeds. Overall, growers already do many field trials and share information among local social networks, but the trials are often not shared in written form (pers. comms.). Future engagement should analyze the contextual value of experimental plot replication, especially under limited field space, to allow for natural variation in yield responses and hypothesis falsifiability to reduce confirmation bias, as well as detailed record-keeping in written form, to diversify and potentially improve on existing local knowledge generation.
Success stories:

We have had several fruitful discussions about how urban agriculture ought to look with growers and students of nearby universities, further revealing the nuances and philosophical diversity of doing ecological and agricultural research depending on the community of assumed beneficiaries, be them other researchers, practitioners, suburban entrepreneurial growers, or younger urban growers with unique experiences. For example, one volunteer generated an idea for resourceful drip-line systems using recycled unused hoses that are still usable with little repair as a way of innovating urban growing with minimal input cost, and a communal outlook. Similar ideas are already implemented, such as a shared tool shed. Finally, individuals have shown consistent interest in the specifics of how biochar works while visiting, highlighting a role for future organized group discussions.


By significantly lowering bulk density of fine-textured soils, improving organic matter and nutrient concentrations, and holding more water, biochar can be useful for mitigating effects of drought on soils and plants, and reducing irrigation, which can be expensive, and may have separate issues with systemic contamination such as in Flint, MI. There also may be a trade-off with invertebrate diversity (as predators), even with positive effects on microbial diversity (as prey). There remain only a handful of studies considering the invertebrate diversity of agricultural soils, and to our knowledge none to date in response to biochar use in urban systems, so future efforts should continue to monitor invertebrate activity to better understand the full complex ecology of soil food webs and how they respond to agricultural management. Specifically, biochar may have positive effects on some aspects of soil health, but the specific and net effects on food webs requires more research by engaged practitioners.

Together with other studies, biochar is best used alongside a source of nitrogen, which can be a legume plant (e.g. bean cover crop), compost, urea, or otherwise synthetic fertilizer (extra cost). Compost is relatively accessible and/or preferred by urban growers, which serves as both a solid nitrogen and phosphorus source. Before mixing, if possible, separating larger pieces of biochar to concentrate smaller ones, especially if from a home-made source, may make biochar more effective in retaining moisture for water-dependent crops like leafy greens. Additionally, if newly cultivating a vacant lot that has had little vegetation and has compacted clay soils, occasional tillage may be important for growing sensitive crops like vegetables especially from seed. Overall, keeping as detailed written records as is feasible can facilitate better information sharing across local grower networks.
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.