1 / 43

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

overton
Download Presentation

Toby Ahrens 27 Sept 2004

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

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 Coastal eutrophication Declassified Keyhole satellite image March 8, 1978 (Beman, unpublished data)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

More Related