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Bruce W. Wood

Guiding Principals for Managing Pecan Orchard Nitrogen. B.W. Wood. Bruce W. Wood. Point #1- -From a Tree’s Perspective: “ Not all Forms of Nitrogen (N) are Equal” . N 3- (i.e., surplus of 3 electrons): Ammonium N 2+ (i.e., deficient 2 electrons): Hydrazine (rocket fuel)

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Bruce W. Wood

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  1. Guiding Principals for Managing Pecan Orchard Nitrogen B.W. Wood Bruce W. Wood

  2. Point #1--From a Tree’s Perspective: “Not all Forms of Nitrogen (N) are Equal” • N3- (i.e., surplus of 3 electrons): • Ammonium • N2+ (i.e., deficient 2 electrons): • Hydrazine (rocket fuel) • N3+ (i.e., deficient 3 electrons): • Amine (urea, peptides, proteins); ammonia • N4+ (i.e., deficient 4 electrons): • Nitrite • N5+ (i.e., deficient 5 electrons): • Nitrate N-atoms have different oxidation states Increasing availability of chemical energy (gain or acceptance of electrons; reduction; gain of energy) Decreasing availability of chemical energy (loss or donation of electrons; oxidation; release of energy) Different N forms possess different chemical energy potentials for plant metabolism and are therefore differentially valued by the tree.

  3. From a Tree’s Perspective: “Not all Forms of Nitrogen (N) are Equal”* Under normal conditions, most nitrate within the plant is eventually converted to ammonium, but this is energetically expensive. • N3- (i.e., surplus of 3 electrons): • Ammonium • N2+ (i.e., deficient 2 electrons): • Hydrazine (rocket fuel) • N3+ (i.e., deficient 3 electrons): • Amine (urea, peptides, proteins); ammonia • N4+ (i.e., deficient 4 electrons): • Nitrite • N5+ (i.e., deficient 5 electrons): • Nitrate Increasing availability of chemical energy (gain or acceptance of electrons; reduction; gain of energy) Decreasing availability of chemical energy (loss or donation of electrons; oxidation; release of energy) *Different N-forms possess different chemical energy potentials for plant metabolism.

  4. Point #2—Pecan Is an Ammonium-loving Species-Consider Pecan Ecosystems and Their Ecophysiology- “Ammonium-loving” vs. Ammonium intolerant (“nitrate-loving”) vs. N-Intermediate species. (Ammonium-loving= higher tolerance and greater physiological preference for ammonium relative to that of nitrate-loving species.)

  5. Pecan Alluvial Lands: ●A Co-dominate “Climax” species in certain forest types, and a “Subclimax” in other forest types. Water Table http://nrs.fs.fed.us/fmg/nfmg/bl_hardwood/eco/spechar/index.html ►Pecan is also native to deep well drained pockets on “upland soils”; and near small rivers/creeks on ridges of well drained “first bottom”, or where the “second bottom” nearly intersects the watercourse.

  6. Top soil zone is rich in decaying organic matter Organic-N and Ammonium-N are common (“ammonia-loving” species) Nitrate-N dominates (ammonia intolerant; “nitrate-loving” species) Lower N:A ratio [N(3-)H4]+1 N(5+)O3 Higher N:A ratio [N(5+)O3]-1

  7. Pecan is exposed to considerable reduced-N (organic and ammonium) in its natural habitats So, pecan exhibits strong evidence of being an “ammonium-loving” (reduced-N) species!!! (typically shade tolerant mid- late-successional species are ammonium-loving) Lower N:A ratio “Nitrate-loving”: continuous shoot growth species (Pioneer species) [N(3-)H4]+1 Higher N:A ratio [N(5+)O3]-1 Water Table

  8. Z There is biological variation in species response to the two N-loving categories

  9. Point #3—Trees Can Respond Differently to Different Soil Solution Nitrate:Ammonium Ratios • “Vegetative growth” is favored by nitrate-N* usage [Nit./Amm. = 5-10:1 (?)]: • Tree nitrate can act as a quasi-hormone to trigger downstream hormonal changes that result in enhanced vegetative growth • Nitrate-N triggers “switching” of tree resource partitioning to favor vegetative growth structures • Less fruiting • More likely to trigger deficiencies of essential and beneficial nutrient elements • Nitrate-N is best if trying to get trees to grow fast; potential issues with low P and S • Note: Urea-N is converted to ammonium and then nitrate very rapidly in most soils, with little or none absorbed as urea by roots; however, foliar applied urea-N is rapidly absorbed and used by the plant as a preferred high energy N-form. • “Reproductive growth”is favored by ammonium-N* usage [N:A = 2- 3:1 (?)]: • Tree ammonium-N enhances synthesis of carbohydrates, amino acids, peptides, proteins (and enzymes), and nucleic acids, giving enhanced reproduction. • Ammonium-N triggers “switching” of tree resource partitioning to favor reproduction • More fruiting and reduced incidence and severity of nutrient element deficiencies • Because ammonium competes with K in uptake by roots, be careful to ensure good K nutrition *Root uptake of nitrate and ammonium-N is regulated by tree demand and sugar supply in roots

  10. Synthetic Nitrogen Sources: Ammonical (Ammonium) • Ammonium: • ~100-1000:1 preference by roots over nitrate • A high energy N-form (highly reduced N atoms) • Not as easily lost in soils as nitrate due to leaching or denitrification, as ammonia binds to soil particles • Uptake is best at pH 7, declines as soil pH drops • Trees tend to have higher carbohydrate and protein levels than when fertilized with nitrate-N • Easier to get phytotoxicity using ammonium, due to rapid and great uptake preference by roots, so have to be careful to not over fertilize (very toxic to plant cells). Better micronutrient nutrition. • Acidifies soils as it is converted to nitrate in the soil, and therefore affects availability of other elements (1 molecule releases 3H+ ions into soil solution; low CEC soils (e.g., sands) are more susceptible than higher buffered high CEC soils (e.g., clays and loams) • Rapidly converts to nitrate form in soils unless “Nitrification Inhibitors” are use to retard oxidation; conversion is more rapid in high pH soils. • Subject to loss when soils are waterlogged and also due to denitrification and mineralization • Can suppress uptake of Alkali and Alkali-earth metals; e.g., reduces K uptake, so can trigger K deficiency if tree is low in K (e.g., increased June-drop of fruit). Can also suppress Ca, Mg, and transition metals (Fe, Mn, Cu, Zn, Ni, Mo) uptake. • Increases tree uptake of “non metals” (e.g., S, P, Cl, and Si) [N(3-)H4]+

  11. Synthetic Nitrogen Sources: Nitrate • Nitrate: • The dominate N-form in most fertilized orchard soils and their soil solutions • Most N absorbed by roots is nitrate, even in ammonium-loving species • A low energy N-form (highly oxidized N atoms) • More easily lost in soils than ammonium due to leaching , as not as much is bound to soil particles • Trees will hyperaccumulate (luxury consumption) nitrate, but the accumulated N is not necessarily assimilated and used for desired end products‼! • Trees are far more tolerant to high nitrate than to high ammonium, so low chance of phytotoxicity • All synthetic N-forms will convert to nitrate in soil within 2-3 weeks unless they are bound to soil particles or organic matter • Can suppress uptake of non-metals; so trigger P or S deficiency (also Cl and Si) if tree is low in P or S and add a lot of nitrate. • Enhances uptake of K, Ca, Mg, and transition metals [N(5+)O3]-

  12. Commercial Orchards Are Artificial Pecan Habitats Do we want orchards to possess the positive traits of their native habitats? ►If: -Relatively little “reduced-N” -Lots of nitrate-N (“oxidized-N”) -Lots of water via irrigation ►Then: -More nutrient deficiencies -Lots of vegetative growth -Less reproductive growth

  13. Point #4--N Usage Efficiency by the Tree Depends on Status of Other Elements, Especially Sulfur (S) and Micronutrients (Fe, Cu, Mo, Ni…) • The N and S assimilation (integration into needed N or S containing biochemicals) processes within pecan trees are tightly linked, with a deficiency of one “repressing” assimilation (conversion into needed biochemicals) of the other. • Can have high leaf tissue N; yet, have a physiological deficiency of N because of insufficient sulfur (also applies to Mn, Cu, Zn, Fe, Ni, and Mo) • Mo, Fe, and S are key components of nitrate reductase enzyme; and Cu, Fe and S are key components of nitrite reductase (to make ammonium in the plant) • Cysteine and Methionine are S containing amino acids, and are also key components of almost all plant enzymes. • Ni is key component of urease and probably certain other N-associated enzymes • N @ ~2.3-2.8%? dw; S @ 0.20-0.50% dw • Failure to consider tree S status (same is true for micronutrients) when applying N can be a costly mistake. • Spring foliar sprays of a balanced micronutrient mix is potentially very important for best usage of spring allied N (especially to alkaline soils)

  14. Common Synthetic Nitrogen Sources* * Best source of N is usually decomposing organic matter

  15. Other Considerations • Most pecan orchards are excessively fertilized with N, especially nitrate-N; needing only ≈70-100 lbs of N/acre/year if done right. • In most cases, we get by with applying less N/acre if ammonium-N is properly applied. • Excessively shady trees do not require as much N as well sun-exposed trees, but will hyperaccumulate nitrate-N and vegetative growth will be triggered, creating more shade in the orchard • Use primarily ammonium-N or urea-N on “mechanical hedge-pruned” trees (us as little nitrate-N as feasible). • Use predominately nitrate-N in young orchards (<5-6 years old), but predominately ammonium-N or urea-N in bearing orchards (> 7 years old)

  16. Thanks!!! • Image by Dr. Ted Cottrell, USDA-ARS

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