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Toby Ahrens 27 Sept 2004

Linking Spatial Variability of Soil N Retention Mechanisms to Landscape-level Fates in Yaqui Valley, Mexico. Toby Ahrens 27 Sept 2004. The hook…. Where do I want to be? Quantify degrees of N contamination under different management regimes

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Toby Ahrens 27 Sept 2004

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  1. Linking Spatial Variability of Soil N Retention Mechanisms to Landscape-level Fates in Yaqui Valley, Mexico Toby Ahrens 27 Sept 2004

  2. The hook… • Where do I want to be? • Quantify degrees of N contamination under different management regimes • Investigate the value of spatial data sets varying in resolution • Probability of “dangerous” N application • What do I need to get there? • Link process and transport models • Improve process models • Include abiotic retention mechanisms

  3. Yaqui Valley, Sonora, MX

  4. Coastal eutrophication Declassified Keyhole satellite image March 8, 1978 (Beman, unpublished data)

  5. Nitrate-contaminated groundwater <10 ppm 10-40 ppm 116 ppm

  6. Two Q’s guiding my efforts… • What soil characteristics control N availability, retention and loss in OM-deficient ag soils? • Can the spatial variability of retention-dominating characteristics be linked to aqueous N fates throughout the Valley?

  7. Watershed-level aquifer vulnerability studies: not mechanistic… • Watershed-level aquifer vulnerability studies use empirically-derived equations to predict leaching… • Makes management extension hard! • Process- and index-based models not spatially explicit • Makes fate + transport link hard! • RHESSys used similar reasoning for natural systems

  8. Model schematic… soil properties soil mineralogy crop yield (+N use) management unit crop type groundwater depth NLOSS Solute transport un/saturated boundary conditions • Output maps: • Leaching vulnerability • Aquifer contamination • Coastal N sources

  9. NLOSS • Nitrification: hole-in-the-pipe sequence • Denitrifier kinetics: double Monod • Decomp: 3+ pools, not important to denit. • Outputs: Leached N, trace gases • Soil moisture: • Solve Richard’s equation • Accounts for macropore plow (using Bypass model of Eckersten and Jansson 1991) • Mechanistic treatment of anaerobic fraction

  10. NLOSS, cont’d • No sorption or fixation • NH4+ either taken up by plants, immobilized, or nitrified. • Resolution in unsaturated zone needs to be 10 cm or less

  11. Lee’s model…

  12. Major modeling efforts: • Soil submodel including sorption isotherms and mineral fixation • Spatial data referencing • Solute transport component to saturated hydrology model • Saturated/unsaturated boundary layer conditions

  13. Two Q’s guiding my efforts… • What soil characteristics control N availability, retention and loss in OM-deficient ag soils? • Can the spatial variability of retention-dominating characteristics be linked to aqueous N fates throughout the Valley?

  14. Applied N: 250 kg/ha Gaseous losses: NO+N2O: 2-5% NH3: __% N2: __% Plant uptake: 31% Drainage canals: NO3- + NH4+: 2-5% NO+N2O: <0.1% ? Leached: 2-5% (14-26%) ? ? References: Riley et al. 2001 Harrison 2003 Matson et al. 1998 Ortiz-Monasterio, pers. comm. ?

  15. Q1: A series of field and lab experiments to identify the major pools, turnover rates and bioavailability of N… Reactive N Abiotic Biotic Fixation Adsorption Clay minerals OM AEC CEC

  16. Q1: How much N is immobilized by abiotic vs. biotic processes during an irrigation event? H1: Abiotic mechanisms are significant sinks of ammonium due to moderate CECs, mineralogy, timing of N addition and low availability of labile C for heterotrophs… Hypotheses… Reactive N • Study 1: • Sterilize with HgCl2 • Add 15N-(NH4)2SO4 • Extract with KCl • Dry and grind soils for 15N • analysis Heterotrophs Nitrifiers Abiotic Biotic Plant uptake Fungi/myc. Denitrifiers Fixation Adsorption DNRA Methods: Johnson et al. 2000 Wolf et al. 1999 Preliminary data: Panek et al. 2000 Riley et al. 2001 Clay minerals OM AEC CEC

  17. Q2: How much adsorbed N is associated is exchangeable? H2: The high pHs and presence of 2:1 Si-O clays results in moderate CEC and low AEC… CEC kinetics follow a Langmuir isotherm model… Hypotheses… Reactive N Study 2: CEC: expose soils to range of [NH4+], centrifuge, filter and ion chromotography AEC: same, but use NO3- Kinetics: establish isotherms (also extract initial ions…) Abiotic Biotic Fixation Adsorption Methods: Philips 1999 Methods of Soil Analysis 1996 Preliminary data: Limon-Ortega et al. 2001 Riley et al. 2001 Clay minerals OM AEC CEC organic mineral

  18. Q3: What is the potential for soils to fix NH4+, and is there field evidence of fixation? H3: Soils are rich in montmorillinitic clays, and significant amounts of NH4+-N is associated with those minerals… Low OM concentrations leads to low OM-associated fixation Hypotheses… Reactive N • Study 3: • Mineralogy • XRD: particle size sep, then • dry slurry on glass slide… • Spectroscopy in lab • Fixed N • Sequential dissolution: size • sep then KCl, H2O2, HF • OK for labeled/sterilesoil… Abiotic Biotic Fixation Adsorption Clay minerals OM AEC CEC Methods: USDA 1996 Paramaasivam and Breitenbeck 2000 Preliminary data: Osher, pers. comm. Enriquez et al. 1998 Cade-Menun, unpub. data

  19. NH4+ fixation in clay minerals Fixed and exchangeable NH4+ in an illite (Stevenson et al., 1982)

  20. Q4: Is there a potential for abiotic reactions between NO2- or NHx and SOM? H4: Chemodenitrification and NO2- rxns with OM not important due to pH. Don’t yet have an educated guess as to the OM fate of NH4+, but I don’t expect much to end up in limited OM… Hypotheses… Reactive N • Study 4: • OM N pools: steam • distillation with acids and • bases, recover NH3 • OK for labeled/sterilesoil… Abiotic Biotic Fixation Adsorption Clay minerals OM Methods: Davidson et al. 2003 Stevenson 1982,1996 Preliminary data: Riley et al. 2001 AEC CEC OM fraction

  21. Aforementioned studies not exclusive… • Largest pathways will be followed with more detailed studies… • Clay mineral fixation: bioavailability or remineralization studies • Biotic immobilization: heterotrophic vs nitrifiers, etc. • OM fixation: NMR or mass spec to look at actual compounds involved in fixation

  22. Future directions and broader implications… • Does better spatial resolution increase our ability to predict SW/GW vulnerability? • And the degree of how mechanistic submodels are? • What is the probability of “dangerous” N application? • Couple to economic optimization model following the Mastrandrea and Schneider (2004) method…

  23. more weathered less weathered

  24. Resolution of input data:Field based…

  25. Resolution of input data:or NPP driven by remote sensing…

  26. Timeline ’04 ’05 ’06 ‘07 Fall Win Spr Sum Fall Win Spr Sum Fall Win Spr Sum Fall Planning, cont’d XXX Field sampling XXX N pools + rates XXX Total C,N, etc. XXX Abiotic vs. biotic XXX CEC, AEC XX NH4 fixation XXX OM fixation XXX Modeling – submodel dev’t XXXXXXXXXX Multiple constraints – 15N? XXXXXXX 15N analyses XXXX Model completion XXXXXXXXXXXXXXX Writing Plant vs. leached N XXXXX Spatial leaching vulnerability XXXXX Value of spatial process models XXXXX Dangerous N application XXXXX Graduate X

  27. Ivan’s 1st question: Triticales Pasta Durum

  28. Riley et al. 2001…

  29. Tractor-mounted hollow stem augers for subsoil sampling http://www.geology.sdsu.edu/classes/geol552/hollowstem.htm

  30. Aquifer depth Addams, 2004

  31. Clay content in shallow subsurface Addams, 2004 Figure ‎4‑28 Estimated fraction of clay in Layer 1 (“Shallow Horizon”). Estimated values for the cells of the model grid range between 0-1 and were interpolated from measurements at well locations (blue).

  32. Anticipated publications • Controls on N availability to plants and N loss to surface and groundwater differ in an agricultural soil in Northwestern Mexico. • Spatial variability of montmorillonite and management history explain contaminated groundwater distribution in an agricultural landscape in Northwestern Mexico. • How mechanistic do process-based spatial models need to be to predict N fates at the landscape scale? • What is the probability of “dangerous” N application in an irrigated wheat system in Northern Mexico?

  33. Applied N: δ15N Plant uptake: Drainage canals: Leached: References: Panek et al. 2001

  34. N oxidation states Davidson, 1991

  35. Chapin et al, 2002

  36. Major subsurface flowpaths Haag and Kaupenjohann, 2001

  37. Corridors and retention zones Haag and Kaupenjohann, 2001

  38. Lytropic series • Al3+>H+>Ca2+>Mg2+>NH4+>K+>Na+ • PO43->SO42->Cl->NO3-

  39. 26% of area not in wheat production

  40. % of area not in wheat • 26% in winter growing season (120 day growing season) • ~80+% in non-winter growing season • > 60% of the time, a given area is not is use for wheat production

  41. Can what I’ve proposed get me there? • N fates in soil + groundwater: Yes, with flexible approach… • N to estuaries: will need collaboration with Adina’s lab • N modeling Q’s: Yes, keep uncertainties explicit…

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