Ecosystem ecology and sustainability in the Chinampa raised-field agriculture of Mexico City A Master of Science thesis in Environmental Risk by Jorge Federico Miranda Vélez Supervisor: Niels H. Jensen Institute for Nature and Environment Roskilde University September 2018 Foreword I feel genuine pity for the poor souls who will read this thesis. They’ll drag themselves through page after page of dark references and impossibly convoluted approximations about the ecosystem ecology of carbon in the 15th-century Chinampas of central Mexico, an environment they likely just heard about for the first time, and then, after what seemed an eternity of squinting and drinking strong coffee, they’ll make it to the end, teary-eyed and gasping for air, only to turn the page and read the heading: "Nitrogen" Acknowledgements I suppose it’s only fair to name all the people that are to blame for this particular piece of work. After all, I can’t be responsible for all of it: My supervisor, Niels, for trusting me with this whole thing, but mostly for showing me how much of our world is contained within the soil we grow on. My loving wife, Nika, for putting up with my endless rants about nitrogen fluxes and even proof- reading the whole manuscript, and in general for refusing to let me starve myself to death in the living room armchair, typing this thesis on nothing but coffee. Katrine, Rikke, Torben and Anne, for not shooing me out of their laboratories with a broom, but instead helping me with my poorly-timed questions and requests. And finally, Diego, Maya, Leo and Armando from Humedalia A.C. for sharing their time, their space, their soil and their love for the wetlands of Xochimilco. This thesis would’ve never been written were it not for them. i Abstract This thesis presents an analysis of the prehispanic and modern chinampas of central Mexico through the lens of ecosystem ecology and soil science, aimed at gauging the actual richness and sustainability of this agricultural practice. To do so, it makes use of historical accounts and modern archaeological findings to reconstruct a cohesive conceptual model of what the Chinampa agricultural system, as well as the chinampa raised fields themselves, likely looked like in the late 15th century A.D. Onto this conceptual model, the Chinampa’s original ecosystem ecology is overlaid by calculating pools and fluxes of carbon, nitrogen and phosphorus from known aspects of this agricultural system; and where detailed ecological knowledge is lacking, historical accounts and parallel ecosystems are brought in to fill in the gaps through extrapolation and inference. These conceptual models and theoretical nutrient cycles are brought closer to the ground by the combination of field work performed in the late 2017 and early 2018 in the extant chinampas of Xochimilco, Mexico, and subsequent laboratory analyses of soil and sediment samples collected in the field. The analyses, performed at Roskilde University in Denmark, focus around the quantification of carbon, nitrogen and phosphorus in the soil of the extant chinampas and sediment from the modern canals of Xochimilco. Firstly, total carbon, nitrogen and phosphorus, as well as extracted nitrate and ammonium, were quantified in samples from a series of soil cores 1 m deep. Secondly, an experiment was carried out to estimate the biogeochemical changes in the canal sediment often used as soil amendment, a practice known as mucking. This experiment was run twice, once for 21 days and once for 64 days, and sediment samples were later analysed for total carbon and nitrogen, as well as nitrate and ammonium content and total, inorganic and Olsen phosphorus. Finally, making use of the experimental results obtained and literature centred on the study of the present-day chinampa landscape, a new conceptual model and ecosystem ecology are discussed in a manner that considers present socio-environmental conditions and current agricultural practices that in the modern Chinampa landscape. This thesis concludes with recount of the most important ecosystem processes and agricultural practices that can have important roles in the sustainability, and ultimately the survival, of Chinampa agriculture in Mexico. ii Contents Foreword ............................................................................................................................................................. i Acknowledgements ............................................................................................................................................. i Abstract .............................................................................................................................................................. ii 1. Introduction ................................................................................................................................................... 1 2. Problem Formulation ..................................................................................................................................... 5 2.1 Sustainability in this thesis ...................................................................................................................... 5 3. The Premodern Chinampa from Myth and Theory ....................................................................................... 7 3.1 Environment, construction and morphology .......................................................................................... 8 3.2 Carbon pools and fluxes ........................................................................................................................ 14 3.3 Nitrogen pools and fluxes ...................................................................................................................... 25 3.4 Phosphorus pools and cycles ................................................................................................................. 32 4. The Chinampa Environment Today ............................................................................................................. 40 5. Methodology ............................................................................................................................................... 45 5.1 Soil Core and Water Sampling ............................................................................................................... 45 5.1.1 Site selection................................................................................................................................... 45 5.1.2 Soil core extraction ......................................................................................................................... 48 5.1.3 Bulk density sample extraction ...................................................................................................... 49 5.1.4 Canal water and groundwater sampling ........................................................................................ 49 5.2 Muck decomposition experiment ......................................................................................................... 49 5.2.1 Muck extraction .............................................................................................................................. 50 5.2.2 Experiment setup ........................................................................................................................... 50 5.2.3 Sediment sample extraction and storage in Mexico ...................................................................... 52 5.3 Soil Properties Analysis .......................................................................................................................... 52 5.3.1 Bulk Density .................................................................................................................................... 52 5.3.2 Organic Matter Content ................................................................................................................. 52 5.3.3 CaCO3 Qualitative Test ................................................................................................................... 53 5.4 Nutrient Analyses .................................................................................................................................. 53 5.4.1 Total Carbon and Total Nitrogen by Organic Elemental Analysis ................................................... 53 5.4.2 Total phosphorus in Core samples by ICP-MS ................................................................................ 53 5.4.3 Canal Water Sample Preparation ................................................................................................... 55 5.4.4 Phosphorus Analysis of Groundwater and Canal Water by ICP-MS ............................................... 55 iii 5.4.5 Muck decomposition experiment (EXP) sample water content determination and preparation . 55 5.4.6 Total Carbon and Nitrogen by Organic Elemental Analysis ............................................................ 55 5.4.7 Total and Inorganic Phosphorus Acid Extraction ............................................................................ 56 5.4.8 Olsen (Labile) Phosphorus Extraction ............................................................................................. 56 5.4.9 Phosphorus Analysis in EXP subsamples by Colorimetry ............................................................... 56 5.4.10 KCl extraction from wet EXP samples and top Core samples ....................................................... 57