Nutrient Cycling in Soils

Transcription

1 Nutrient Cycling in Soils Kristin A. Fisher, Ph.D. Nutrient Management Specialist Agricultural Nutrient Management Program University of Maryland, College Park

2 Nutrient Cycling in Soils N, P & K cycles sources, losses, availability Choice of timing and placement on N fertilizer efficiency Calculating legume credits C cycling and C:N ratios

3 Why N, P & K? Macronutrient Roles N P K Component of amino acids, proteins, DNA, chloroplasts Cell membranes, DNA, ATP Regulates opening and closing of stomates; important for photosynthesis, water and nutrient uptake

4 Nitrogen Most complicated many reactions mediated my soil organisms N has gaseous forms oxidation and reduction

5 additions losses translocations transformations (J.J. Meisinger, USDA)

6 Additions: crop & animal residues biosolids compost fertilizers N fixation (J.J. Meisinger, USDA)

7 N Fixation Mechanisms Natural fixation Atmospheric additions electrical discharge of lightning Biological processes symbiotic for example between Rhizobium bacteria and leguminous plants non-symbiotic carried out by free living bacteria and blue-green algae

8 Photo credits: Harold Evans, OSU

9 Biological N Fixation

10 Calculating Legume Credits Crop lbs N/Ac Perennial Crops Alfalfa 100 to 150 Red Clover 40 Winter Annual Crops Hairy Vetch 75 to 150 Crimson Clover 50 to 100 Summer Annual Crops Lespedeza 20 Soybeans 15 to 40 (Photo credit: Ray Weil)

11 N Fixation Mechanisms Synthetic or industrial processes Industrial fixation for fertilizer manufacturing Haber-Bosch process N 2 + 3H 2 2NH 3 (g) High temperature fixation internal combustion engines

12 Volatilization losses: hot, dry weather Losses: Nitrate leaching Decades Days/wks Weeks/months Years Eroded N: slopes; heavy rain Denitrification losses: waterlogged soils Crop uptake Leaching losses: soil texture; rain amount (J.J. Meisinger, USDA)

13 Leaching (a liquid loss) Primarily as NO 3 - moves freely downward transported by drainage water especially important in sandy soils can lead to pollution of groundwater An economic loss with environmental consequences

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15 Nitrate leaching Minimal under natural vegetation (forests) Greater under modern row-crop productions A risk if N management is sub-optimal

16 Reasons for Excessive NO 3- Leaching Inefficient N management heavy one-time applications improper timing over-application of manure/sludge Enhanced by periods of heavy rainfall

17 Enhancing Synchrony: Minimize Vulnerable Nitrogen If the N is not yet applied, it can not be lost Importance of split applications (Penn State Agronomy 12)

18 Effects of Over-Application of Nitrogen in Maryland 200 Grain Yield, bu/ac or Soil NO3-N lbs N/ac Fert. N, lbs N/ac Corn Grain Yield Fall Soil NO3 0-3 ft Source: Frank Coale

19 Ammonia Volatilization (a gaseous loss) Loss of ammonia-n to the atmosphere Ammonium in the presence of hydroxyl (OH - ) can produce ammonia gas NH 4+ + OH - H 2 O + NH 3 Affects all surface-applied N sources urea, ammonium nitrate, manure Enhanced by warm, dry atmospheric conditions

20 Diurnal variations (Pote & Meisinger, J. Soil & Water Cons.)

21 Treatment Total losses of applied NH 3 N Subsurfer 12% 8% Light Disking 22% 32% Surface- Applied (Control) 74% 95% Starting amount of NH 3 -N: 0.65% FW or lbs NH 3 -N/ac

22 Managing Ammonia Losses Know where and when ammonia loss occurs First day of application Sunny, warm, low humidity, breezy conditions Encourage injection or light disking for incorporation of manures Spread and incorporate manures in the early morning or evening (when dew still present)

23 Denitrification (a gaseous loss) NO 3 - N 2 O N 2 nitrate ions nitrous oxide gas dinitrogen gas biological reduction of nitrate NO 3- transformed to gaseous compounds favored by saturation of the soil (anaerobic conditions)

24 Denitrification Photo credit: www. Agleader.com

25 Transformations Gaseous forms: N 2 NH 3 NH 3 volatilization Fixed N Natural Atmospheric Redox N 2 NH 3 NH 3 NO 2 - NO 2- NO 3 - NO 3- N 2 Magdoff, 1992 Building Soils for Better Crops

26 Nitrification (a transformation) Biological transformation of ammonium (NH ) to nitrate (NO ) Aerobic conditions and moderate ph suppressed below ph 5.5 Soil bacteria Nitrosomonas and Nitrobacter

27 Nitrification: A Two-Step Process Nitrosomonas 2 NH O 2 2 NO H 2 O + 4 H + + e Nitrobacter 2 NO 2- + O 2 2 NO 3- + e

28 N Cycle: A Biological Phenomenon Influenced by: ph temperature moisture oxygen status (aeration) most bacteria suppressed at low ph

29 Temperature & N Cycling Water freezes Optimum for nitrification Optimum for mineralization Most microbes killed Biological zero Optimum for growth of corn, cotton, potatoes

30 Moisture & Oxygen Status

31 Plant-available N in soil solution Ammonium (NH 4+ ) Nitrate (NO 3- ) Small organic molecules Small organic molecules

32 Transformations of Organic N Proteins Amino sugars Amino acids 2-5% of total soil N NH 4 + NO 3 - NO 2 - Graphic from N.C. Brady, 1974

33 Mineralization (a transformation) Conversion of organic N to inorganic N, ammonium (NH 4+ ) Mediated by bacteria and fungi

34 Immobilization (a transformation) Conversion of inorganic N (NH 4 + & NO 3- ) to organic N soil organisms assimilate nutrients into biomass Soil organisms are extremely numerous and very well distributed in the soil outcompete plants for available nutrients under many circumstances

35 The Fate of N in Organic Additions C:N ratio is the amount of C relative to the amount of N in a given material A high C:N ratio (>30:1) N immobilized A low C:N ratio (<20:1) N mineralized

36 a Cornell On-Farm Composting Handbook, Rynk et al, 1992 b The Nature and Properties of Soils, Brady and Weil, 1999

37 Changes in NO 3- levels when organic additions have wide C/N Adapted from F.J. Stevenson, 1986

38 Managing C:N Ratio of Inputs Manage cover crops incorporate cover crops while in vegetative state leave mature cover crops on surface Monitor soil N and plant growth when incorporating straw, sawdust and other high C:N materials

39 Principles of N Management Maintain soil ph appropriate for crop Reduce runoff/erosion Apply N fertilizers and manure when plants need it, where they need it Timely incorporation of manures and sewage sludge when practical Use cover crops to scavenge residual N

40 Practice Question #1 In which form is nitrogen most likely to be lost from soils in Maryland? a) Dinitrogen gas (N 2 ) b) Nitrate (NO 3- ) c) Ammonium (NH 4+ ) d) Organic N

41 The Phosphorus Cycle Simpler cycle than N - no oxidation and reduction - no gaseous forms Soluble P can be fixed to less available forms

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43 Transformation of Organic P INORGANIC PHOSPHORUS POOL ORGANIC PHOSPHORUS POOL HPO 4-2 H 2 PO 4 - P mineralization and immobilization occur simultaneously in the soil

44 Plant-available P Orthophosphate (water-soluble phosphate) - H 2 PO 4 - at ph 5 to HPO 4 2- at ph > 7.2 Phosphate moves slowly in soil by diffusion

45 P Fixation (a transformation) A set of processes through which P is converted to less available forms Precipitation Adsorption

46 P fixation Adsorption In acid soils, P is adsorbed to surfaces of Al/Fe oxides and clay minerals In neutral and calcareous soils P is adsorbed to surfaces of CaCO 3 and clay minerals

47 P Fixation Precipitation as secondary P compounds In acid soils, P combines with iron (Fe) and aluminum (Al) to form insoluble compounds In neutral and calcareous soils, P combines with (Ca) to form insoluble compounds

48 N.C. Brady, 1974

49 Availability of Fixed P P relatively available when first fixed dissolution from newly formed minerals desorption from sorption sites Availability decreases as time passes

50 Occluded P Sorbed P tends to become occluded P as time passes sorbed P covered by coatings of iron, aluminum, or manganese oxides unavailable

51 Principles of P Management Maintain soil ph for desired crop Apply P fertilizers when needed, where most efficiently utilized band starter fertilizer When practical, incorporate manure Utilize practices that reduce soil erosion and runoff

52 Practice Question #2 The optimum ph range for maximizing plant-available P is: a) b) c) d) >7.5

53 Potassium Cycle Simpler cycle than N - no oxidation and reduction - no gaseous forms Soluble K can be fixed to less available forms in some soils - different mechanism than P - minimal water quality issues

54 PLANT RESIDUES Sources Losses Maryland Cooperative Extension, 2000

55 Plant-available K K + dissolved in soil solution Exchangeable K + Moves primarily by diffusion along moist soil pores

56 K in soil solution Only % of total K in soils Readily available for plant uptake

57 Exchangeable K K + at exchange sites on soil colloids in dynamic equilibrium with K + in soil solution replenishes the soil solution Readily available for plant uptake Usually less than 1% of total K in soils

58 Fixed K Fixed as K + trapped in interlayers of clay minerals Non-exchangeable Minimally available by weathering About 1-10% of total K in soil

59 N.C. Brady, 1974

60 Mineral (structural) K K is a structural component of primary minerals such as feldspars and micas 90 98% of total K in soil Relatively unavailable

61 K Loss Pathways Crop removal luxury consumption Leaching coarse soils/high rainfall Erosion

62 Luxury consumption of potassium by plants. If excess amounts of potash fertilizers are applied to soil, plants will absorb potassium in quantities exceeding that required for optimum yields. This may be wasteful if crops are completely removed from the soil. N.C. Brady, 1974

63 Practice Question #3 Potassium is lost from soils by a) Microbial transformation to gaseous K + b) Leaching through clayey, finetextured soils c) Luxury consumption by plants

64 Principles of K Management Maintain soil ph for desired crop Utilize practices that reduce soil erosion Split application reduce losses to luxury consumption

65 Nutrient Cycling in Soils N, P & K cycles sources, losses, availability Choice of timing and placement on N fertilizer efficiency Calculating legume credits C cycling and C:N ratios

66 Questions?