Cycling and Biogeochemical Transformations of N, P and S OCN 401 - Biogeochemical Systems Reading: Schlesinger, Chapter 6 1. Nitrogen cycle Soil nitrogen cycle Nitrification Emissions of N gases from soils Global N 2 O emissions Atmospheric N deposition 2. Phosphorus cycle Importance of P transformations Phosphorus cycling Soil P transformations Phosphorus pools 3. Sulfur cycle The importance of sulfur cycling Sulfur cycling Atmospheric sulfate deposition 4. Potassium cycle Outline 1
ALL OF THESE PROCESSES ARE MICROBIALLY MEDIATED Soil Nitrogen Cycle soil organic matter (low C:N; slowly decomposable) Litter input Litter input ASSIMILATION OF FIXED N MICROBIAL DEATH plant residues (high C:N; readily decomposable) microbial biomass N ASSIMILATORY IMMOBILIATION REDUCTION N 2 Denitrification N 2 Nitrogen Fixation AMMONIFICATION MINERALIZATION Exchangeable NH 4 + NH 4 + DIFFUSION NITRIFICATION NO 3 - DIFFUSION Fixed NH 4 + NO 3 - How to Measure the Soil N Cycle Changes in plastic litter bags to measure N concentrations over time Changes in trenched plots with plants removed Changes in 15 NH 4 and 15 NO 3 (with and without inhibitors for specific processes) Other techniques: Acetylene reduction for nitrogen fixation Acetylene block for denitrification - specific inhibitor of N 2 0 N 2 (discussed later) 2
Nitrification Lower rates under these conditions: low O 2 low ph low soil moisture (can also be inhibited by too much water) high litterfall C:N (slow mineralization of N due to increased immobilization of NH 4 + by microbes) Higher rates with high soil NH 4, but generally not responsive to other nutrients Usually higher after disturbances like fires Emissions of N Gases From Soils Emitted gases include: Ammonia (NH 3 ) Nitric oxide (NO) Nitrous oxide (N 2 O) Dinitrogen (N 2 ) Emissions are important because they remove nitrogen available for uptake by plants N 2 O flux is also important because N 2 O is a greenhouse gas 3
Processes of Nitrogen Gas Emissions Rapid conversion of NH 4 + to NH 3 occurs at high ph and low soil moisture, and results in gas loss to the atmosphere: NH 4 + + OH - NH 3 + H 2 O High organic waste loads (e.g., feedlots) promote NH 3 loss NO and N 2 O are byproducts (intermediates) of nitrification and denitrification Global N 2 O Emissions www.fao.org/docrep/004/y2780e/y2780e07.htm 4
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Atmospheric N Deposition An important source of nitrogen cycling in many systems Mostly in the form of acidic wet and dry deposition (HNO 3 ), originating from fossil fuel and plant combustion (NO x ) Some areas also have significant deposition of ammonium, largely originating from livestock organic waste 6
Inorganic Nitrogen Wet Deposition 1985 http://nadp.sws.uiuc.edu/amaps2/ 2005 7
The Nitrogen Cycle Component Input to soil Loss from soil Atmospheric fixation & deposition Animal manures & biosolids Crop harvest Atmospheric Nitrogen Volatilization Industrial fixation (commercial fertilizers) Biological fixation by legume plants Organic Nitrogen residues Immobilization Mineralization uptake Ammonium (NH 4 + ) Nitrate (NO 3 - ) Denitrification Runoff and erosion Leaching www.ipni.net Importance of P Transformations The level of available P during soil development may be the primary determinant in terrestrial NPP: P is present in low concentrations in rocks N is abundant in the atmosphere Bacteria that fix N 2 gas to biologically available N require P Other essential plant nutrients (e.g., S, K, Ca, Mg) are more abundant than P 8
Phosphorus Cycling Unlike N, P cycling includes significant inorganic (mineral) reactions that make it much more difficult to study These reactions tend to interfere with the availability of P They also complicate the measurement of various forms of P Use sequential extractions to determine P-mineral binding Unlike N, gaseous P (phosphine, PH 3 ) is negligible in biogeochemical cycles and can be ignored Unlike N, P does not have multiple oxidation states (no redox reactions) 1) Organic matter P Phosphorus Pools P in live plants and animals P in microbes P in dead organic matter 2) Soluble P (P in dissolved form) 3) Labile P (P readily released into solution) 4) P in minerals and occluded P (tightly adsorbed or absorbed) Igneous apatite (Ca 5 FP 3 O 12 ) Biological forms of apatite P co-precipitated with CaCO 3 Fe- and Al-bound P, etc. 9
The Phosphorus Cycle Component Input to soil Loss from soil Animal manures & biosolids residues Crop harvest Atmospheric deposition Mineral fertilizers Organic phosphorus Microbial residue Humus Leaching (usually minor) www.ipni.net Mineralization Immobilization uptake Soil solution Phosphorus HPO 4-2 H 2 PO 4-1 Weathering Primary Minerals (apatite) Desorption Adsorption Dissolution Precipitation Runoff and erosion Mineral surfaces (clays, Fe and Al oxides, carbonates) Secondary compounds (CaP, FeP, MnP, AlP) 10
The Importance of Sulfur Cycling S, along with C, H, O, N, and P, is one of the major constituents of living tissue While essential to life, S is also relatively abundant It is therefore an essential plant nutrient, but not ordinarily a limiting plant nutrient s take up SO 4 2- by reduction and incorporation into amino acids S is released in many forms during decomposition Sulfur Cycling The S cycle is analogous to the P cycle, with some important differences Like P, sulfur undergoes important geochemical, as well as biological, cycling Like N, and unlike P, sulfur occurs in multiple oxidation states SO 4 2- [sulfate] is abundant in seawater (28 mm) Like N and unlike P, sulfur has significant gas phases: H 2 S [hydrogen sulfide] Organic gases COS [carbonyl sulfide] (CH 3 ) 2 S [dimethylsulfide] A locally important S source is H 2 SO 4 [sulfuric acid] in acid rain 11
Sulfate Ion Wet Deposition 1985 http://nadp.sws.uiuc.edu/amaps2/ 2005 12
The Sulfur Cycle Atmospheric deposition Atmospheric Sulfur SO 2 gas Crop harvest Component Input to soil Loss from soil Volatilization Animal manures & biosolids Organic Sulfur www.ipni.net residues Absorbed or mineral sulfur Immobilization Mineralization uptake Sulfate Sulfur (SO 4- ) Oxidation Bacterial oxidation Mineral fertilizers Elemental Sulfur Bacterial reduction Leaching Runoff and erosion Reduced Sulfur Potassium Cycling The potassium (K) cycle is almost entirely inorganic The major role of K in living organisms is osmotic control K is taken up, retained and excreted in ionic form (K + ) The amount of K in soil solution is relatively small but is in near equilibrium with the much larger amount of exchangeable K from which it is replenished Soils also contain K in more slowly exchangeable forms (fixed K) which act as sources for crops K present in clay minerals becomes available as these minerals weather forum.fwag.org/data/public/727_4.doc 13
The Potassium Cycle Component Input to soil Loss from soil Crop harvest residues Animal manures & biosolids Mineral fertilizers uptake Exchangeable potassium Runoff and erosion www.ipni.net Leaching Soil solution potassium (K + ) Mineral potassium Fixed potassium Lecture Summary Nitrogen cycling is biologically mediated among soil pools (organic N, NH 4, NO 3 ), with important shunts to gaseous forms Nitrogen fixation, the process that returns gaseous nitrogen back to being biologically available, is comparatively rare Unlike N, phosphorus is also involved in geochemical (mineral) reactions that may make P less available for biotic cycling Sulfur has important analogies with both N and P, including both biological and geochemical reactions, and gas-phase reactions 14