nitrogen dynamics and greenhouse gas production in yaqui valley ...

NITROGEN DYNAMICS AND GREENHOUSE GAS PRODUCTION IN YAQUI VALLEY SURFACE DRAINAGE WATERS

A DISSERTATION SUBMITTED TO THE DEPARTMENT OF GEOLOGICAL AND ENVIRONMENTAL SCIENCES AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

John Arthur Harrison January 2003

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 Copyright by John Arthur Harrison 2003 All Rights Reserved

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I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and quality, as dissertation for the degree of Doctor of Philosophy. __________________________________ Pamela Matson, Principal Advisor

I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and quality, as dissertation for the degree of Doctor of Philosophy. __________________________________ Robert Dunbar

I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and quality, as dissertation for the degree of Doctor of Philosophy. __________________________________ Scott Fendorf I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and quality, as dissertation for the degree of Doctor of Philosophy. __________________________________ Peter Vitousek Approved for the University Committee on Graduate Studies __________________________________

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ABSTRACT

Agricultural runoff is thought to constitute a globally important source of the greenhouse gas nitrous oxide (N2O), and may also be a significant source of the greenhouse gases methane (CH4) and carbon dioxide (CO2). However, production of N2O, CH4, and CO2 in polluted aquatic systems is poorly understood and scarcely reported,

especially in low-latitude (0-30º) regions where rapid agricultural

intensification is occurring. We measured N2O, CH4, and CO2 emissions, dissolved N2O concentrations, and factors likely to control rates of greenhouse gas production in Yaqui Valley drainage canals receiving agricultural and mixed agricultural/urban inputs. Average per-area N2O flux in both purely agricultural and mixed urban/agricultural drainage systems (16.5 ng N2O-N cm-2 hr-1) was high compared to other fresh water fluxes, and extreme values ranged up to 244.6 ng N2O-N cm-2 hr-1. These extremely high N2O fluxes occurred during green algae blooms, when organic carbon, nitrogen, and oxygen concentrations were high, and only in canals receiving pig-farm and urban inputs, suggesting an important link between land-use and N2O emissions. N2O concentrations and fluxes correlated significantly with water column concentrations of nitrate, particulate organic carbon and nitrogen, ammonium, and chlorophyll a. A multiple linear regression model including ammonium, dissolved organic carbon, and particulate organic carbon was the best predictor of [N2O] (r2 = 52%). Despite high per-area N2O fluxes, our estimate of regional N2O emission from surface drainage (20,869 kg N2O-N yr-1; 0.046% of N-fertilizer inputs) was low compared to values predicted by algorithms used in global budgets. All canals where we measured CO2 were net-heterotrophic. CH4 fluxes, though variable (range: -1372 – 3990 mg CH4-C m-2 d-1), were generally quite high (mean: 3208 ng-C cm-2 hr-1) compared with fluxes from similar systems in other locations. C gas evasion from streams was several times greater than stream export of organic C via lateral transport.

During algae bloom conditions we observed rapid and complete

oxidation and reduction of an entire drainage canal. v

This has not previously been

reported and may have important implications for in-stream transformations and downstream transfer of N, iron, manganese, and sulfur as well as the production of N2O, CH4, and CO2.

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RESUMEN Se ha creído que las aguas residuales agrícolas son importantes fuentes del gas de invernadero, oxido nitroso (N2O), y también puede ser importante por la producción de dióxido de carbón (CO2), y metano (CH4) (también gases invernaderos). Sin embargo, la producción de N2O, CH4, y CO2 en sistemas acuáticos contaminados no esta bien comprendido, ni bien reportado, especialmente en zonas de bajas latitudes tropicales y subtropicales (0-30º), y en zonas de rápido crecimiento agrícola. Medimos emisiones de N2O, CH4, y CO2, concentraciones de N2O disuelto, y factores que puede controlar la taza de producción de gases de invernaderos en los drenes agrícolas y agricolas/urbanas en el Valle del Yaqui. El flujo promedio (por-área) de N2O en ambos drenes con aportación sólo agrícolas y drenes con influencia de una mezcla de fuentes agrícolas y urbanas (16.5 ng N2O-N cm-2 hr-1) fue alto en comparición con los flujos de otros sitios de agua dulce, y los valores extremos fueron hasta 244.6 ng N2O-N cm-2 hr-1. Estos valores extremos de flujo de N2O pasaron durante floraciones de algas verdes, mientras concentraciones de carbón orgánico, nitrógeno reactivo, y oxigeno fueron altas. También sucedieron solamente en drenes, lagunas de oxidación de desechos de granjas porcicolas y de zonas urbanas. Esto sugiere una conexión importante entre el uso del suelo y las emisiones de N2O. Concentraciones y flujos de N2O correlacionaron en una manera significativa con concentraciones (en la columna de agua) de nitrato, carbón y nitrógeno orgánico particular, amonio, y clorofila. Un modelo de múltiple retroceso lineal incluyendo amonio, carbón orgánico disuelto, y carbón orgánico particular como variables independientes fue el mejor pronosticador de [N2O] (r2 = 52%). Aunque flujos de N2O por área fueron altos, nuestro estimado de emisión regional de N2O de drenaje superficial (20,869 kg N2O-N yr-1; 0.046% of N-fertilizer inputs) fue bajo comparado a valores pronosticados por métodos de algoritmos utilizados en estadares globales. Todos los drenes donde medimos CO2 fueron heterotróficos. Flujos de CH4, aunque variables (rango: -1372 a 3990 mg CH4-C m-2 d-1), fueron generalmente altos (promedio: 3208 ngC cm-2 hr-1) comparado con los flujos de sistemas parecidos en otras localidades. El gas vii

de C se desprende a la atmósfera desde los drenes y fue un sendero significativo para proporcionar C del sistema, en varias ocasiones mas grande que la exportación de C orgánico por transporta lateral. En mediciones que duraron 24 horas de un dren, observamos cambios químicos muy rápidos que correspondieron a cambios en la concentración de oxigeno (O2). Durante este periodo, la nitrificación y la producción de N2O pararon por lo menos en 8 horas, y la desnitrificación se detuvo al menos en 6 horas. Concentraciones de CH4 y CO2 también mostraron un ciclo diel fuerte. Los cambios químicos observados afectaran profundamente las transformaciones y la transferencia de N, fierro, manganeso, azufre, y también la producción de los gases de invernaderos N2O, CH4, y CO2.

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ACKNOWLEDGMENTS

Thanks to my dissertation committee for support and guidance: Pamela Matson, Peter Vitousek, Robert Dunbar, and Scott Fendorf. Thanks to members of the Matson and Vitousek Labs and to the Yaqui Valley Research Group for comradery, support, and guidance throughout my Ph.D.: Kathleen “Kitty” Lohse, Ted Schuur, Sharon Hall, Michael Beman, Carrie Nielsen, Karen Carney, Barbara Cade-Menun, Peter Jewett, Jeanne Panek, Bill Riley, Amy Luers, Eve Hinkley, Toby Ahrens, Lee Addams, Roz Naylor, Wally Falcon, Greg Asner, Carlos Valdes, and others. Special thanks to Ivan Ortiz-Monasterio for his invaluable advice and logistical support. Thanks to Gustavo Vasques, Dennis Rogers, Luis Mendez, Manuel Muñoz, Lindley Zerbe, Juan Bustamante, Don Chayo, Igenio, Jesus Nieblas, and “Los Chicos” (Cuervo, Gallo, Oso, y El Viejo) for assistance in the field and laboratory as well as for inspiration. Thanks also to Lori McVay, Elaine Andersen, and Lorraine Araujo for logistical support on the home front. I am also grateful to the following agencies and organizations for their financial support of my thesis research: The National Science Foundation for Dissertation Enhancement and Predoctoral Fellowship Awards, NASA for an Earth Systems Science Fellowship, The Packard Foundation, The Bechtel Foundation, NASA’s Land-Use/Land-Cover Change program, and Stanford University’s McGee Fund. Finally, many heartfelt thanks to my dear family and friends, whose love and support have made my thesis possible: Penny, Marvin, and Rebecca Harrison, Katie Prager, Michael Kiparsky, Jeremy Eddy, Sarah Jane Lapp, Darcy Karakelian, Joern Hoffman, Ana Porter, Jeff Dukes, Julia Verville, High Speed Buck, the Fine Tuners, and many others. ix

DEDICATION

To the residents of the Yaqui Valley, and to all those who supported me during this work…

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TABLE OF CONTENTS

Abstract and Abstract in Spanish ....................................................................................... iv Acknowledgments............................................................................................................viii Dedication .......................................................................................................................... ix List of tables .................................................................................................................... xivi List of figures .................................................................................................................. xvv INTRODUCTION............................................................................................................... 1 Chapter 1: Patterns and Controls of Nitrous Oxide Emissions from Waters Draining a Subtropical Agricultural Valley .................................................................................... 6 Introduction ................................................................................................................... 6 Site Description ............................................................................................................. 8 Watershed................................................................................................................ 8 Drainage canals ..................................................................................................... 11 3. Methods......................................................................................................................... 12 Measurement Program .......................................................................................... 12 Gas Collection and Analysis ................................................................................. 13 Determination of Gas Transfer Coefficient........................................................... 14 Other Chemical Analyses...................................................................................... 15 Denitrification and Nitrification Potential Assays ................................................ 17 Intact Core Experiments........................................................................................ 18 Sectioned Core Experiment................................................................................... 19 Statistical Analyses ............................................................................................... 20 Regional Flux Estimates........................................................................................ 21 Results and Discussion................................................................................................ 21 N2O fluxes ............................................................................................................. 21 Potential Controls on N2O Emission..................................................................... 22 xi

Nitrate.................................................................................................................... 28 Organic Carbon ..................................................................................................... 32 Ammonium............................................................................................................ 33 pH ......................................................................................................................... 36 Temperature .......................................................................................................... 37 Discharge and Flow Rate ...................................................................................... 37 Salinity .................................................................................................................. 38 Algae Blooms (Chlorophyll a) and Related Variables.......................................... 39 Multiple Regression Models ................................................................................. 39 Regional Significance of Aquatic N2O Flux and Implications for Global Estimates ......................................................................................................... 40 N Transfer to Estuaries.................................................................................... 42 Denitrification ................................................................................................. 44 Chapter 2: Rapid-onset Anoxia Affects Nitrogen Transfer and Greenhouse Gas Production in Mexican Stream.................................................................................. 466 Methods....................................................................................................................... 59 Gas Concentrations ............................................................................................... 59 Other Chemical Analyses...................................................................................... 60 Chapter 3: Patterns and Controls of Methane (CH4) and Carbon Dioxide (CO2) Evasion from Urban and Agricultural Drainage waters.............................................. 62 Introduction ............................................................................................................... 622 Methods....................................................................................................................... 62 Study Site .............................................................................................................. 62 Drainage canals ..................................................................................................... 63 Experimental Design ............................................................................................. 63 Gas Collection and Analyses................................................................................. 63 Other Analyses ...................................................................................................... 64 Statistical Analyses ............................................................................................... 65 Results and Discussion................................................................................................ 65 xii

Methane................................................................................................................. 65 Patterns and Magnitudes of Flux..................................................................... 65 Importance of CH4 Evasion at the Landscape Scale ....................................... 67 Carbon Dioxide ..................................................................................................... 68 Simple and Multiple Regressions.................................................................. 689 Patterns and magnitudes of flux ...................................................................... 69 Importance of CO2 evasion at landscape scale................................................ 70 Total Carbon.......................................................................................................... 71 Appendix A: Abbreviations .............................................................................................. 73 Appendix B: Estmation of Gas Transfer Coefficient ........................................................ 74 Appendix C: Poisoned Versus Unpoisoned Septum Vials ............................................... 79 Appendix D: N2:Ar and N2:O2 Measurements Via Membrane Inlet Mass Sectrometer, Dssolved Gs Aalysis (MIMS-DGA) ........................................................................... 81 Appendix E: Some Important Unanswered Questions...................................................... 82 Appendix F: Some Tips For Future Yaqui Researchers ................................................... 84

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LIST OF TABLES

Number

Page

Table 1:

Current estimates of global N2O sources .......................................................... 7

Table 2:

Canal watershed areas and mean annual discharges ....................................... 12

Table 3:

Correlations between measured N2O concentration and independent variables such as temperature, DO, Chl a, dissolved inorganic nitrogen [DIN], dissolved organic carbon [DOC], particulate organic carbon [POC], particulate organic nitrogen [PON], [NO3-], [NH4+], [PO43-], [NO3-], [NH4+], pH, salinity, and turbidity...................................................... 35

Table 4:

CH4 fluxes from tropical and subtropical freshwater systems, including the ones in this study............................................................................................. 66

Table 5:

Correlations between measured CO2 flux (ug-CO2-C cm-2 hr2) and independent variables such as temperature, DO, Chl a, dissolved inorganic nitrogen [DIN] defined as [NO3- + NO2- + NH4+], dissolved organic carbon [DOC], particulate organic carbon [POC], particulate organic nitrogen [PON], [NO3-], [NH4+], [PO43-], [NO3-], [NH4+], pH, salinity, and turbidity.. 70

Table 6:

Abbreviations used in text............................................................................... 73

Table 7:

ANOVA table for ppb N2O in septum vial headspaces. See Figure 24 for data used in this ANOVA................................................................................ 79

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LIST OF FIGURES

Number 1

Page Recent increases in anthropogenic N fixation in relation to “natural” N fixation .............................................................................................................. 2

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Model-predicted global distribution of dissolved inorganic nitrogen transport............................................................................................................. 3

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Study site ........................................................................................................... 9

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N2O flux in 8 Yaqui Valley drainage canals over two winter wheat cycles (October 1999- September 2001) determined from supersaturation data ....... 24

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Per-area rates of N2O flux in study drainage canals (ng N cm-2 hr-1) ............. 25

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[N2O], [NO3-], and [NH4+] in cores collected from PA1 by depth.................. 26

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Time series plots of A) [NO3-] (mg L-1), B) [DOC] (mg L-1), C) [NH4+](mg L-1), D) pH, E) [Chl a] (mg L-1), F) [PON](mg L-1), G) Temperature (ºC), H) discharge (m3 s-1), and I) salinity (g L-1) .................... 27

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Mean annual [NO3-] vs. mean N2O flux in several large rivers and the drainage canals in this study............................................................................ 30

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A & B) Potenital denitrification and N2O production in sediment slurries as a function of NO3- and Organic C availability............................................ 31

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Potential for nitrification and denitrification to produce N2O. Inset: NH4+ enrichment versus N2O production ................................................................. 34

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Annual N2O-N flux from study drainage canals ............................................. 41

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Study site for diel experiment ......................................................................... 47

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Diel measurements from July 2-3, 2001 ......................................................... 51

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Diel measurements from July 2-3, 2001 continued......................................... 53

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N2O flux and Chl a values over 23 months of bi-weekly sampling................ 58

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Chamber-estimated CH4 flux (ug-C cm-2 h-1) from canals A1 and PA1......... 67 xv

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Chamber-estimated CO2 flux (ug-C cm-2 h-1) approximately bi-weekly from canals A1 and PA1 ................................................................................. 69

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Schematic of triple tracer addition experiment ............................................... 74

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Photo of tracer addition ................................................................................... 74

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Concentrations of i) SF6, ii) rhodamine WT, and iii) bromine over time at two different sites in Drainage Canal PA1...................................................... 75

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Typical sampling setup for floating chamber flux measurements .............76-77

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Typical N2O chamber flux data....................................................................... 77

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Schematic of septum vial used for headspace analysis ................................... 78

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N2O concentrations in poisoned and unpoisoned septum vial headspaces from a 24-hour period from noon 12/7/00 to noon on 12/8/00. ...................... 79

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A comparison between YSI-measured O2 and O2 measured via O2:Ar method using Membrane Inlet Mass Spectrometer......................................... 81

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Measured and predicted N2:Ar over 24 hours................................................. 81

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INTRODUCTION

Globally, industrial nitrogen (N) fixation for agricultural use has increased from less than 10 Tg y-1 (Tg = 1012g) in the 1950's to over 80 Tg y-1 in the 1990’s (FAO 1995), and approximately half the N fertilizer ever produced has been applied during the last fifteen years (Galloway and Cowling 2002). This use of N fertilizer, together with approximately 40 Tg y-1 fixed by leguminous crops and 21 Tg y-1 N inadvertently fixed during fossil fuel combustion, has more than doubled the rate of N input to terrestrial systems via natural N fixation (Vitousek and Matson 1993; Figure 1). Increased N fertilizer use has led to massive increases in agricultural yield (food grown per unit area) and has allowed humans to largely avoid the food shortages historically predicted to accompany the recent population boom. In this sense, N fertilizer has been an enormous boon to humans. However, the recent increase in N use has had serious environmental drawbacks as well. Along with increased anthropogenic N fixation, levels of riverborne N have increased dramatically (Turner and Rabalais 1991, Howarth et al. 1996), leading to coastal oxygen depletion, increases in toxic and nuisance algae blooms, sedimentation, and loss of biodiversity (Turner and Rabalais 1994, Steidinger 1981, Jickells 1998). In addition to these local and regional effects of river N loading, this loading may affect the global climate system by altering the balance of greenhouse gases produced and consumed in freshwater systems (Paerl 1997, Seitzinger and Kroeze 1998).

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Figure 1. Recent increases in anthropogenic N fixation in relation to “natural” N fixation. Modified from (Vitousek and Matson 1993).

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\ A)

B)

C)

Figure 2. A) Synthetic N fertilizer use in 1990. B) Model predicted N fertilizer use in 2050, and C) Model-predicted dissolved inorganic nitrogen export to coastal systems in 1990. All figures modified from (Seitzinger et al. 2002a). Anthropogenic N loading is increasing particularly rapidly in the developing tropics and subtropics (Figure 2). For example, consumption of fertilizer by developing 3

nations has grown from 12% of the global total in 1960 to 65% in 2000 (Fertilizer Institute 2002). Yet, consequences of anthropogenic N loading for aquatic systems remain almost completely unstudied outside of the developed world. In my thesis I have investigated the link between agricultural intensification (the largest human source of reactive nitrogen globally), N dynamics, and greenhouse gas production in the surface drainage waters of the Yaqui Valley, an intensively farmed, rapidly developing region in Sonora, Mexico. In Chapter 1, I characterize and quantify fluxes of nitrous oxide from the drainage waters of the Yaqui Valley and investigate the mechanisms controlling the production and emission of N2O. In this chapter, I report some of the highest per-area fluxes measured in surface fresh-water systems. I also explore relationships between N2O concentration and several factors thought to control rates of N2O production. Finally, I present calculations suggesting that despite high per-area fluxes, regional N2O fluxes from surface drainage waters in the Yaqui Valley are low compared to values predicted by algorithms used in global estimates. In Chapter 2, I focus on diel chemical dynamics in one particular drainage canal, and show for the first time that a flowing, freshwater system can undergo a complete cycle of chemical reduction and oxidation within 24 hours. I also explore some of the implications of this rapid change in chemical conditions for downstream N transfer and greenhouse gas production.

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In Chapter 3, I characterize and quantify CO2 and CH4 fluxes from Yaqui Valley surface drainage waters. Yaqui drainage canals appeared to be net producers of both CO2 and CH4. Per-area CH4-C fluxes, though variable, were generally quite high compared to other aquatic systems, but about an order of magnitude less than CO2-C fluxes. Finally, C gas efflux was several times greater than lateral export of organic C, suggesting that it may be an important pathway for C loss from the Yaqui Valley drainage system. Together, by quantifying rates of key processes and the links between them, this work adds significantly to our understanding of nutrient dynamics and trace gas emissions, enhancing our capacity to manage agricultural and coastal resources and ecosystems sustainably.

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CHAPTER 1: PATTERNS AND CONTROLS OF NITROUS OXIDE EMISSIONS FROM WATERS DRAINING A SUBTROPICAL AGRICULTURAL VALLEY

INTRODUCTION Because it is an important greenhouse gas that also plays a role in the destruction of stratospheric ozone, nitrous oxide (N2O) has received considerable attention (Khalil and Rasmussen 1983, Matson and Vitousek 1990). Currently, global atmospheric N2O concentration is increasing at the rate of 0.2-0.3% per year, and fertilization of agricultural fields is thought to be the single most important source of the observed increase ((IPCC 2001), Table 1). After a decade or more of research, we now have a reasonably good understanding of the relationship between nitrogen (N) fertilization and N2O flux from soils (de Klein et al. 2001), (Harrison and Webb 2001). However, much less is known about the gaseous loss of fertilizer N once it has left agricultural fields in solution or particulate forms. Some researchers have estimated that N2O emissions from indirect emissions (emissions from surface water and groundwater not within agricultural fields) are currently as large as direct emissions from fields (Table 1, (Mosier et al. 1998), and others have projected that increases in N loading to rivers will triple river N2O production by 2050 (Kroeze and Seitzinger 1998). However, uncertainty surrounding estimates of N2O emissions from indirect sources is large, ranging over one and a half orders of magnitude and accounting for over 60% of the uncertainty in current estimates of total anthropogenic N2O emissions (Table 1). This uncertainty is due to a 6

combination of poor understanding regarding the mechanisms controlling N2O production and a lack of studies of off-site emissions of N2O from agriculture (de Klein et al. 2001), (Brown et al. 2001). Tg N yr-1

Range (Tg N yr-1)

2.1

(0.23-11.9)

2.1 2.1 0.5 1.3

(0.4-3.8) (0.6-3.1) (0.2-1.0) (0.7-1.8)

Subtotal

8.1

(2.13-21.5)

Natural: Ocean tropical soils temperate soils

3.0 4.0 2.0

(1.0-5.0) (2.7-5.7) (0.6-4.0)

Subtotal

9.0

(4.3-14.7)

Total sources

17.1

(11.93-36.3)

Anthropogenic: Agriculture indirect emissions direct emissions cattle and feedlots biomass burning industrial sources

Table 1. Current estimates of global N2O sources compiled from Intergovernmental Panel on Climate Change (IPCC) (2001), and Mosier et al. (1998).

Previous work with sediments and soils has indicated that N2O is formed principally as a byproduct of two microbially mediated N transformations: denitrification and nitrification. Denitrification, the microbial reduction of nitrate (NO3-) to N2O and dinitrogen (N2) under anaerobic conditions, is thought to be controlled by inorganic N availability, organic carbon (C) availability, oxygen (O2) concentration, and temperature (Nishio et al. 1983), (Seitzinger 1988) (Firestone and Davidson 1989, Robertson 1989). Nitrification, the oxidation of ammonium (NH4+) to NO3- under aerobic conditions (with N2O as a byproduct), is controlled by NH4+ availability, temperature, and redox 7

conditions (Firestone and Davidson 1989).

Previous work has suggested that the

proportion of total denitrification or nitrification emitted as N2O depends on the relative availability of resources for microbes, as well as ambient redox conditions and temperature (Seitzinger et al. 1984, Seitzinger 1988) (Joergensen et al. 1984). Consequently, environmental variables such as N availability, organic C availability, temperature, and oxidation-reduction conditions are likely to influence the rate at which N2O is produced. In this study, we used a combination of field and laboratory approaches to estimate rates of N2O production in Yaqui Valley drainage canals and to improve our understanding of the controls on those rates. These approaches included: 1) regression analysis examining relationships between measured N2O fluxes and factors likely to influence N2O production, 2) in situ sampling of sediment cores, 3) potential nitrification and denitrification assays, and 4) intact core experiments.

SITE DESCRIPTION WATERSHED The Yaqui Valley is located between 26° 45' and 27° 33' latitude N and 109° 30' and 110° 37' longitude W (Figure 3). Containing 226,000 ha of intensively managed, irrigated, wheat-based agriculture, the Yaqui Valley is the birthplace of the Green Revolution for wheat and one of Mexico’s most productive breadbaskets. This valley is

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considered to be agro-climatically representative of 43% of wheat production in the developing world (Meisner et al. 1992).

Figure 3. The Yaqui Valley- Study drainages are outlined in black and drainage canals are portrayed as dark rectilinear networks. Water in the Valley generally flows southwest toward the Gulf of California, shown in black. Dark circles (●) represent the locations of pig farms, and the city of Obregón (Pop. 300,000+) is shown in the upperright.

Mean annual temperature in the valley is 22.5 ºC, and mean annual precipitation is 28.7 cm, with 82% of that precipitation occurring during a “wet” season (JulyOctober). Over the course of this 23-month study, total rainfall in the region was just 37.5 cm, and this included only 3 rain events with greater than 2 cm rainfall in 24 hours 9

(Arizona Meteorological Network / PIAES (Patronato para la Investigación y Experimentación Agrícola del Estado de Sonora 2002). Occasional heavy rain events associated with major tropical storms may play an important role in this system on the several year time-scale, but this topic is beyond the scope of this study. In the Yaqui Valley, the use of fertilizer N has increased markedly in the past three decades of development; between 1968 and 1995, fertilizer application rates for wheat production increased from 80 to 250 kg-N ha-1 per 6-month wheat crop, and survey results indicate substantial increases in fertilizer inputs in just the past decade (Naylor et al. 2000). Today the most common farming practice for wheat production in the valley is a pre-planting broadcast application of urea or injection of anhydrous ammonium (at the rate of 150-200 kg-N ha-1), immediately followed by irrigation. Smaller allocations of fertilizer are commonly added later in the crop cycle along with additional irrigation water. These valley-wide fertilization/irrigation events occur principally in November, but continue intermittently throughout the winter and early spring. These events lead to large N losses to the atmosphere, ground water, and surface waters (Matson et al. 1998, Panek et al. 2000, Riley et al. 2001)). Summer crops, primarily maize, are grown when reservoir water storage is high and irrigation water is available; an additional 250-300 kgN ha-1 is typically applied to summer maize. However, during our study, summer crops were not planted due to water shortages.

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DRAINAGE CANALS In the Yaqui Valley, surface irrigation runoff, livestock waste, and largely untreated urban sewage enter the coastal zone directly via a system of 5 principal, and 14 smaller, open waterways (hereafter referred to as "drainage canals" or “canals”) (Figure 3). These perennial constructed and natural waterways have muddy to sandy bottom sediments and are lined by shrubby vegetation (mainly Tamarix sp. and Cercidium microphyllum (palo verde)). Sampling sites were free of macrobenthic flora and fauna. Approximate drainage areas of study canals ranged from 800 to 43,000 ha, and mean annual discharges ranged from 1,600 m3 (A3) to 59,000 m3 (A1), averaging 17,017 m3 (Table 2). Over the period of this study, flow in these drainage canals was regulated almost entirely by upland water use (taken from a reservoir) because rain events were small and infrequent.

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Canal PA1 PA2 A1 A2 A3 A4 A5 A6

Area Served (Ha) 22400 3200 43600 6400 800 2800 6400 8800

Mean Annual Discharge 1987-1996 (1000 m3) 52000 NA 59000 14500 1600 5500 13000 15500

Canal Surface Area (Ha) 68-134 10-19 133-262 19-38 2-5 9-17 19-38 27-53

Table 2. Canal watershed areas and mean annual discharges.

3. METHODS

MEASUREMENT PROGRAM We sampled 8 drainage canals biweekly for 23 months (November 1999September 2001) (Figure 3). In these canals, we measured fluxes of N2O as well as several environmental variables likely to influence and co-vary with N2O emission. These variables included DO, NO3-, NH4+, temperature, DOC, POC, PON, salinity, Chl a, turbidity (NTUs), wind-speed and direction, water velocity, and pH. Drainages were chosen to vary widely with respect to the factors likely to influence N-cycling and N2O flux.

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GAS COLLECTION AND ANALYSIS We estimated gas fluxes using two techniques: floating chambers (as in Livingston and Hutchinson (1995)), and headspace equilibration (modified from Robinson et al. (1988). We applied the floating chamber technique in sites A1 and PA1, the two largest drainage canals. In this technique, we fitted 10 cm high, 25-cm-diameter acrylonitrile-butadiene-styrene (ABS) plastic chambers with floating collars and tied them to stakes driven into canal bottoms. We sampled four chambers simultaneously at 10-minute intervals, and flux measurements lasted 30 minutes.

During each flux

measurement, we measured water flow rates and depth just behind each chamber. We calculated concentrations of chamber gases using least squares linear regression, and calculated N2O fluxes by regressing gas concentration within a chamber against sampling time, correcting for temperature and chamber volume as in Matson et al. (1998). Minimum detectable flux of N2O was approximately 0.3 ng N2O –N cm-2 h-1. In addition to the floating chambers, we employed a headspace equilibration technique at all 8 sites. For this we pre-sealed 60 ml glass Wheaton bottles with gray butyl stoppers, then evacuated and flushed them with helium. 15 ml aliquots of canal water were injected into bottles, brought back to the laboratory, and gently shaken for 4 hours at 25 ºC. Headspace gas was then extracted and analyzed for N2O, and original [N2O] was calculated using the appropriate solubility tables (Weiss and Price 1980). Ambient air samples were also collected at each site for use in flux calculations.

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On several occasions, we compared samples poisoned with saturated mercuric chloride (HgCl) solution to samples lacking HgCl. There was no detectable difference between poisoned and unpoisoned vials in 24 hours with respect to N2O (Appendix C). We made all headspace measurements between 4 and 12 hours after sample collection, and most samples were not poisoned. We measured N2O using a Shimadzu gas chromatograph configured with electron capture detector

(ECD).

The ECD contained

63

Ni as the isotope source and an

argon/methane mixture was used as the carrier gas (Matson et al. 1998). Standards ranged from 500 ppbv to 500 ppmv N2O, and 500 and 900 ppbv standards bracketed every 15 samples. Coefficients of variation for standards never exceeded 2%.

DETERMINATION OF GAS TRANSFER COEFFICIENT A gas transfer coefficient was estimated via a triple tracer experiment in PA1 and was validated by comparing chamber fluxes with simultaneously measured N2O concentration. In the tracer experiment, a pulse of dissolved SF6 gas (volatile tracer), rhodamine (visual tracer), and bromine (conservative tracer) was added to one of the larger canals (PA1) during a period with minimal wind in a manner similar to (Wanninkhof et al. 1990). The slug of tracer was sampled over time, and the difference in rate of loss between the volatile tracer (SF6) and conservative tracer (bromine) was used to calculate a gas transfer coefficient according to Kilpatrick et al. (1989) (Also, see Appendix A). In the case of PA1, this transfer coefficient was 4.67 cm hr-1 for N2O at a Schmidt number of 600, assuming a flat water surface according to Wanninkhof (1992). 14

This estimate is in rough agreement with an estimate based on comparisons between N2O concentration and chamber fluxes (mean = 7.8 cm hr-1 ± 3.5 cm hr-1 (1 S.D.). Fluxes were calculated according to: F = k*(Cw – Ceq)

(1)

(Liss 1974) where F is gas flux across the air-water boundary, k is the gas-transfer velocity, Cw is the dissolved gas concentration in the water column, and Ceq is the dissolved gas concentration in equilibrium with the atmosphere. Because wind-speed can play an important role in regulating gas transfer velocity over the range observed in our study (0.1-7.5 m s-1), we accounted for variation in wind-speed using the relationship: y = 1.91e0.35k

(2)

where y is wind speed in m s-1, and k is the gas transfer velocity in cm hr-1 at a constant Schmidt number (in this case 600) (Raymond and Cole 2001). We used on-site wind data after June 26, 2001. Prior to this date, we used wind data from a nearby weather station (