Urban Biogeochemistry

David Lewis opened by reviewing biogeochemistry definitions and background before describing his research in Florida.
  • Urban biogeochemistry: "the movement and processing of matter and energy through and within urban systems."
    • Two broad principles relevant to biogeochemistry were identified:
    • (1) Coupled Material Cycles through
      • (a) organisms' simultaneous need for multiple material species
      • (b) bimolecular stoichiometry
    • (2) Ecosystem Fluxes
  • Lewis suggested connecting these two principles as a framework for urban biogeochemistry.

Lewis' research explores the overarching question: How do the massive transfers of matter and energy across urban ecosystem boundaries affect coupled biogeochemical cycles? Specifically, the transfer of one element or compound may influence the transfers of others, and depletion of an element of compound form resource acquisition zones may affect material cycles in those locations.

Lewis’ research focused on wetland extraction sites outside of Tampa, Florida, from which water is channeled to urban areas.
  • Wetlands sequester large amounts of carbon as soil organic matter (SOM).
    • 14% of biosphere soil carbon is in wetland soil, but wetlands are only 1% of the planet.
    • The amount of carbon stored in soil is depleted by microbial respiration, which depends on (1) properties of the soil organic carbon and (2) the environment.
    • Properties of soil organic matter determine the partitioning of carbon between labile and stable fractions.
    • Properties of the environment determine whether labile carbon is used.
  • SOM provides nitrogen immobilization through microbes that transfer a large fraction of nitrogen from mineral nitrogen to soil organic molecules

Urban water demand generates a massive water flux from the hinterlands, reducing the hydroperiod of rural wetlands as a result of consumption driven water table drawdowns. How do these water table drawdowns affect the coupled carbon and nitrogen cycles of soils in the Tampa area?
  • Soil carbon responds to hydroperiod, with the labile carbon pool increasing approximately linearly with the percent of time a wetland is inundated. The reduced labile carbon pool of rural watersheds experiencing high extraction rates has feedbacks to nitrogen retention and storage in stable pools.
  • Nitrogen uptake and storage was measured by placing soil cores with isotopically labelled nitrogen and retention was measured in soil organic matter. The retained amount of labelled nitrogen was correlated with the amount of labile carbon, and the amount of nitrogen stabilized was similarly correlated with the percent of time inundated.
  • Result: Water extraction from rural wetlands to support urban areas reduces rural wetland hydroperiod, which reduces the labile carbon pool as well as the rate or nitrogen retention and stabilization.
Image borrowed from: http://lewislab.org/research/wetlands/
Image borrowed from:
In addition to direct impacts on rural wetlands, extraction can also perturb downstream hinterlands. Reclaimed water has been explored as an alternative to water from wetland extractions; however, some current work in Lewis' lab explores whether the high salinity of reclaimed water has negative impacts on soil aggregate formation and microbial survival.

Summarized Conclusions:
Atmospheric CO2 concentrations are buffered by carbon storage in soils, which are the largest C pool in the terrestrial and surface ocean biosphere. Most soil carbon is stored in wetlands. Urban water management decisions generate a massive H2O flux across the rural to metro boundary, resulting in water depletion in rural wetlands, reduced C storage, and reduced N storage.

Post Lecture Joint Discussion:
  • Lucy asked whether the chemistry of reclaimed water in Tampa differs from other parts of the country? David was unsure, which led to a question about how much reclaimed water is needed in the area because of high precipitation and ground water recharge rates. David explained that many of the karst depressions that form lakes and ponds are highly valued by residents who build homes adjacent to water features. Municipal governments can't utilize these water sources due to residents concerns, so despite high precipitation rates there is still not enough water being fed into the urban system.

  • There seem to be conflicting goals and values in the way that managers are siphoning water from wetlands, while simultaneously mandating that they be protected from development. Water management entities are desirous of ecological information that can help them understand the consequences of their decisions and plans. A major driver of the work done in the Lewis lab revolves around responding to the information needs of the water management community. Managers want to know what the consequences of their past activities have been and how to make it better in the future. While there could be many strategies to solve multiple problems at once,most are politically unfeasible. There are many residents, including retirees, transplants, and transients, that believe they know how to best manage the local environment, and often decisions are influenced by a small minority of vocal people who undermine what seem like good environmental decision making. (the word "nutters" was used several times)

  • The good news about the water table drawdowns occurring in the area are that many of the shallow aquifers recharge very quickly. While some of the deep aquifers are essentially depleted, decisions to cut back on the use of water might quickly translate into ecosystem [[#|recovery]]. That said, they do experience saltwater intrusions that could render a lot of the freshwater resources unusable. these intrusions are driven by ground water pumping and extraction near the coast. David is unsure of the percentage of impacted ground water resources, but does know that it happens.

  • Drainage of water from wetlands likely effects wildlife, but has extensive effects on the plant community -- so much so that many wetlands have been flatly covered into pine forests.There are people investigating the fauna question, but mainly from a comparative standpoint between preserved wetlands in urban and non-urban areas.

  • There are also interesting associations between landuse legacies and nutrient cycling. In Phoenix, a framework was developed that centered around the idea that land management decisions can be very strong [[#|movers]] of nutrients out of soils. Given that land use changes can affect the storage of nitrogen and carbon, researchers wanted to know about the different land uses and how they effect this storage. They found that different land uses have different storage levels -- especially in areas with know legacies (houses developed on agricultural lands versus those developed on former desert land). This finding begged the question, 'does agricultural legacy influence current residential N and C pools'? In short, yes it does. They found significant differences in the biogeochemical signals of residential areas with agricultural legacies versus those with desert legacies. Also, older development has higher C and N pools, more so than recently urbanized land.

  • Regarding urban ecosystem convergence, biogeochemical cycles are likely not that different among cities with similar climates. In terms of development shape and pattern -- fluxes are probably more similar or different depending on climatic system. The real homogenization is likely in the realm of shared culture in big cities -- things that supersede climate. However, biogeochemical legacies - like those observed in Phoenix - would not be present in a midwestern city because agricultural practices have depleted C and N pools over the last few centuries. These types of climatic and geographic differences likely escape the homogenization processes observed in cities.

At BU, following Lewis' lecture, we continued our discussion by exploring whether urban areas have a unique biogeochemistry.
First, we established two definitions: Nutrients - chemical elements that organisms require for life. Biogeochemistry - study of the cycling of nutrients through the biosphere, hydrosphere, lithosphere and atmosphere.Six elements (C,H,N,O,P,S) comprise 95% of the biosphere, and the cycling of C,N, and P has been heavily modified by humans. Using the carbon cycle as a model, we noted that the cycling of carbon in urban areas is largely governed by the conventional carbon cycle model, but that urban areas do have unique inputs and outputs. We then focused on perturbations to the nitrogen cycle. Since 1970, global population has increased by 80%, with a corresponding 120% increase in the creation of reactive nitrogen, made possible largely by the Haber-Bosch process. Only 14% of the fixed nitrogen in fertilizer is consumed by humans, and much of the excess and diverted nitrogen is transported to the atmosphere, followed by deposition on land or in water. These excess inputs in urban areas have implications for urban soils and the eutrophication of urban waterways.

At NCSU, our pre-lecture discussion began with a presentation by Rene Valdez about the impacts of invasive species in urban environments. Focusing on the management aspects urban invasive species, Rene presented two case studies: (1) The Monk Parakeet, and (2) the Asian Longhorned Beetle. Management refers to the prevention, eradication and control of invasives. However, in both cases there are conflicting ideals between residents and managers about how (or if) these species warrant stringent control efforts. The concern with the Monk Parakeet is that they next on utility poles and pose a fire risk. Simultaneously, they are associated with human population density due to backyard feeding in winter months. The conflict between managers and residents arises within this juxtaposition between fire risk and aesthetics. Residents like them because they are pretty. Managers are concerned about the risks posed by their nesting habits in densely populated areas. Control efforts in Chicago are ongoing, while NYC has introduced a bill to protect the birds despite the risks. As is typical with most insect species, they receive little affection from managers or residents. As a corollary, Chicago has allocated 70 million dollars over 10 years to implement an eradication plan for the Asian Longhorned Beetle. This plan received support from the USDA, Illinois State Natural Resources Agency, and a strong community outreach campaign. However, many residents oppose the plan and are upset about the removal of suburban trees when the damaging effects of the beetle are unapparent.

These two cases bring up interesting questions about the nature of urban invasive species and our attempts to manage their spread. For instance, how do we bridge the gaps between management and control efforts, and public perception of invasives? Also, much of our discussion centered on whether these species are actually invasive, which is a question we have touched on several times this semester – the idea that urban invasive species are classified based on human perception (its invasive only if its doing something we don’t like).

After Rene’s presentation we moved on to a short discussion about this weeks readings. The class generally agreed that the Kaye paper was an important introduction to urban biogeochemistry, and the science suggesting that urban environments have unique biogeochemical signatures is solid. Melissa then shifted the discussion to human alterations to chemical pools, and the compounding effects on fluxes. Most of us (none of us?) are biogeochemists, so this was our introduction to the topic. The conversation then shifted to the usefulness of modeling (predictive & analytical). Global predictive modeling is useful for identifying carbon/emissions/climate trends, but the prevalence of these types of models makes it difficult to separate them into good vs bad categories. Should we be focusing on mitigating the impacts of climate change rather than continuing to measure emissions and model climate change scenarios?


Kelly Suttles - In the book, articles, and David's lecture I found great information for the carbon and nitrogen cycles and how urban systems change those cycles. Not having taken any other ecology class, it is great for me to see these cycles re-iterated, so I can really learn them. Before reading "Beyond Urban Legends" and David's lecture, I did not realize soil is a huge carbon reservoir. Now it makes perfect sense. It will be important for me to understand these cycles and the urban effects on them for my thesis work. For my research I am particularly interested in downstream effects on a national forest from projected upstream forest conversion to urban. Sediment, stream flow, and nitrogen will be important water quality indicators.

I appreciated our class discussion that placed large pools of nutrients in the appropriate contexts. Mitigation strategies like forest carbon sequestration may have their place, but the real issue is limiting fossil fuel conversion to carbon dioxide in the first place. This is an enormous flux that cannot be offset by growing enough trees or some other vegetation.

After what I learned in class, I look forward to discussing urban riparian areas with Alan Yeakley. I would like to know more about the functioning of preserved urban wetlands depending on the level of surrounding impervious surface. Since the urban hydrological network is completely managed by pipes designed for the sanitary city, riparian areas cannot provide the ecosystem services they are designed for. From my limited understanding the minimum buffer areas that we place around riparian areas likely do nothing to restore function; it is more likely they just prevent further degradation. How big should riparian buffer areas be? Are there certain places/parts of a river where larger pervious buffers could have a greater impact?

Mustafa Saifuddin - I came across a TED talk that is only peripherally related to this week's discussion, but it is related to some of the broader conversations we've been having all semester, and it's just super cool, so I thought I'd share it here! In the talk, designer/researcher Jae Rhim Lee, describes the Infinity Burial Project, which is a project to develop burial bodysuits housing fungal strains that promote human decomposition while sequestering toxins. It reminded me of our discussion with Steve Raciti on the relationship between design and science, and also the lecture by Kristen Schwarz on the distribution of hazards in soil. In this instance design is super important because it deals with the very sensitive subject of death, but then the design also addresses serious urban ecological problems by attempting to reduce cycling of toxins consumed by humans, which can be exacerbated by, for example, cremation. (If you're interested in this nutrient-cycling related artsy stuff, you should also check out another project of hers in which she modified her diet to produce urine optimized for cabbage fertilization and then made kimchi. Sorry, I know this is getting way off topic, but I think it's neat stuff!)

Rene Valdez

I found the presentation by David Lewis to be fascinating and fairly easy to follow. I was definitely intimidated by the idea of a lecture on biogeochemistry but the presentation allowed for people (like me) with less experience in nutrient cycling to follow along. In this course we have spent a fair amount of time thinking about cycles and changes of ecological and social systems related to urban environments. Viewing the Earth as a system of moving, changing chemicals was a little different but makes perfect sense and compliments previous system approaches and diagrams.

I've heard from multiple sources that wetlands are ecologically important and are disappearing. I have heard the arguments that wetlands are good for biodiversity, for storm and flooding mitigation, and for pollution dilution but wetlands as areas for carbon sequestration and nitrogen capture was new to me. The research on the coupling of carbon and nitrogen in wetland systems is definitely important from an environmental standpoint and should be taken into consideration when developing and implementing environmental policy in areas with wetlands. I am also curious to see how and if research with reclaimed water will yield new solutions to our growing water scarcity issues.

I also happen to think that my presentation on urban invasive species was interesting. Invasive species management as a whole does not have a unified theme, goal, or policy so even trying to present a short of presentation was more challenging than it should have. A 'set in stone' framework does not and may never exist. Probably because invasive species have such variable impacts, a species by species approach may be more apt.

Steve Decina

Mustafa, I found the videos intriguing. Natural burials seem, well, natural. I have always been fascinated by Tibetan Sky Burials, but have never really thought about the consequences of things like BPA or heavy metals on the health of the scavenging vultures. Then again, when I think about the lifestyle of people buried this way in Tibet, I have to imagine that there are not a lot of heavy metals or BPA in them to begin with. I think that Jae Rhim Lee's decompiculture project is a provocative way of thinking about the problem at hand; our bodies are full of toxins from the water, food, and air we ingest. I have seen people buying "organic clothing" and insisting that their food never touches plastic (both mostly at Whole Foods) and have been instantly judgmental of such effete and delicate attitudes, but when you consider the bioaccumulation of toxins in our bodies, it makes sense. I knew a girl who died at 26 years old from breast cancer, and did not have the BRCA gene. Doctors said that her cancer was "environmental in nature." She worked for a chemical company and had two EPA Superfund sites within three miles of her home. It makes me wonder who should be held responsible for her death and the deaths of countless others who die of cancer every year with little to no genetic basis for the cancer.

These questions are biogeochemical in nature. I think that David Lewis nicely addressed the ways that urban resource use can have far-ranging effects on the upstream and downstream "hinterlands" (we need a new name for this; it evokes far-away, bucolic places and not our own backyards, which is what they really are). The draining of the baldcypress swamps due to inappropriate water management and use is frustrating. If people made a conscious effort to use less water, this would be a non-issue. How do you get people to stop watering their lawns, to fix leaky faucets, to cut down their showers to ten minutes. Criminalize lawn-watering? I would vote for that, but the public would be up-in-arms. There needs to be a shift in public perception about resource use and waste disposal, something that drives the point home. This will either happen through effective science and policy (what we're here for) or natural disasters so great that people are forced to change (happening already).

Sravya: Thanks Mustafa for sharing those very cool videos! I really enjoyed last weeks discussion. In particular, I enjoyed Professor Hutyra's slides about The Fate of Haber-Bosch Nitrogen. It was really interesting track the nitrogen being transferred from each stage, and to see how much nitrogen content is lost. Additionally, I also liked contemplating if different equations must be used for biogeochemical cycles in an urban setting. I personally feel that the equations used are fundamentally similar, but that perhaps different assumptions and constants must be added to better represent the overall dynamics in urban areas. Specifically, I really liked Professor's Hutyra's example of limiting nutrients: if we supply the limiting reagent in biological systems, eventually another nutrient will end up limiting the overall reaction. Additionally, I am sure everyone has seen this article about Mayor Menino joining BU, and leading a new initiative on urban life. It sounds fascinating! Also they mentioned a few comments from Professor Hutyra!

Ryan Foley

Mustafa, how in the world did you find this video? But seriously, all jokes aside, this was pretty interesting. It seems simple; a great idea and painless (pun intended) way to help our ecosystem. Upon watching the video, I immediately began to wonder what it would take for something like this to actually work- to get people to actually do this. Personally, I could care less what happens to my body when I die, but I sure as Hell know my neo-Christian grandparents won't be gung-ho about this idea.

We talk about social pressures with regard to helping our planet. After all, it's the tendencies that we, as people, have during our day to day lives that affect our planet. But what do you think it would take for our species to come to terms with this idea? The world is religious place. And something that is so sacred, and religiously practiced seems almost impossible to actually make happen. It's one thing to talk about it and make a TED video to show it's possible, but an entire different ball-game to see it happen. I personally don't see people ever turning to this method. But one thing's for sure; I'm never going to forget this video, so I'll be the first person to say I was wrong, in 60 years!

Lital: This presentation about wetlands reminded me of something a professor told one of my classes last year; Louisiana is sinking. More specifically, the wetlands on the coast are sinking- at a substantial rate, I might add. I find it interesting that societies across the world are worrying about rising oceans; cities and states are constructing climate action plans, investing billions of dollars to ensure their coastlines remain intact, and yet this is already occurring in our own backyards AND NO ONE IS NOTICING?! According to an LA Times article, Louisiana has already lost the size of Rhode Island. People are focused on major urban areas because of the alarmingly high population that would be affected, but losing wetlands could be just as detrimental due to the enormity of probable repercussions to the air we breathe, the food we eat, and the water we drink. Why doesn't the public care about wetlands? I'd like to also mention that this article is from 2002... The first news article about this topic on google (of 6) is from 2002.
Steve was discussing how we could realize water conservation efforts to reduce the draining of baldcypress swamps. I really think it comes down to a lack of incentives. People don't consider what they can get out of fixing the leaky pipe or halving the time of lawn-care (apart from the saved money- but this doesn't amount to enough as a true incentive). I blame this on the lack of knowledge on wetlands. The majority of people don't understand what wetlands are good for. They think that because only ~250 people live on the sinking land, it wouldn't have a such a great effect on the country. I don't have a true solution to this problem either. I imagine the first step is to make people more aware of the issue. Help them understand what David Lewis was so clearly explaining to us on Friday. Who knows, maybe one of us will write the seventh article on Louisiana's sinking wetlands?

Katy Lawless:
I enjoyed that David Lewis’s presentation was about the effects urban areas have on non-urban areas. Most of the topics we’ve discussed this semester have focused on issues in urban areas. It was interesting to learn about impacts on non-urban areas since urban areas obtain natural resources from outside of the city. I also enjoyed learning about the ecosystem services that wetlands provide. I have learned about wetland ecosystems before, but I was not aware of the important role this type ecosystem has in biogeochemistry. It is unfortunate that there has been a loss in wetland areas. Like Lital mentioned, a lack of wetland conservation is most likely due to a lack of knowledge about wetlands. Since residences cannot be built on top of wetland without altering the land, most people do not see the value of wetlands. There definitely needs to be more education about the importance of wetlands, especially as a water resource, so there can be more effort in conserving these ecosystems.

Wyatt Sanders:

David Lewis: What a great and engaging lecturer! I wish that I had his video lecture series (yet to be published) on nutrient cycling for my putative future graduate oral exams. It is immensely helpful to get these kind of clear, concise and engaging framework explanatory lectures. It gives one such a three-dimensional framework upon which to hang future bits of deeper understanding of the subject. This is so crucial, for me, to have a solid macroscopic view of difficult concepts and processes, and its often the most difficult part of the pedagogical mission.

David's work is interesting to me as a native Californian with a preoccupation with the ecology and politics of water. Florida and California have a lot of similarities in my mind (Phoenix too...). Its interesting to think of the stakeholder component who see's water mostly from a recreational or viewshed analysis of value. In my own hometown, there has been outcry in recent years for the Army Corps of Engineers to permit releases of water from Isabella Reservoir, 50 miles east of the city in the Southern Sierra's collision with the Transverse Ranges. That's not really where the flow stops, of course. It's allowed to run riot down the Kern Canyon until it spills out into the fertile, be-citrused foothills-but it does make it the last ten miles or so into the city proper, being diverted far and wide to 10,000 kinds of crops in Kern County. But the citizens would like to see some water in their river in the summertime, rather than the broad, dry sand canal that they normally have. And so they have forced the Corps to trickle out a flood into urban portion of the riverbed, wastefully (some say) spilling crop-useful water needlessly to the sand.

Our pre-lecture discussion today included three papers. one: a proof of concept of urban biogeochemistry. check. I think we're all on board with the foundational concepts of the genre, now. Two: another bedrock paper, a Pickett et al number that we heretofore hadn't read, and I liked this one. A sort of Socratic method in which preconceptions one (might) have about the urban environment are taken down, backed up by data from the Baltimore LTER. A good paper for you, Mom, and your neighbor. Three: a convulsively excited missive regarding the possibilities of a global megacity C emission monitoring network. Even writing the phrase gets me thinking of those cotton trailers along the side of I-5 in the valley, with their somewhat inscrutable proclamations regarding the UN and the US of A. Things got dark here, maybe the darkest I've yet seen in the class, but then again it could've just been me. This is where we began to talk about models. The efficacy of models, the ascendancy of models, the madness of models.