Plant Nutrients (1): Nitrogen and Sulfur

Plant Nutrients (1): Nitrogen and Sulfur

Main Objectives Capable of asking a systematic set of questions about all relevant nutrients, and providing a basic answer to each questions. Comprehend the global nitrogen and sulfur cycles in plant-soil systems. Understand forms and transforms of nitrogen. Be able to relate N-transformation to soil N availability to plants, and to relevant environmental issues.

Key terms and Concepts Macro-nutrients, micro-nutrients (trace elements) Human-enhanced global nitrogen cycle Nitrogen mineralization and immobilization Ammonium volatilization Ammonification Nitrification Denitrification N 2 -fixation Rhizobium Frankia Nitrogen deposition

From: Gardiner & Miller 2004. Soils in Our Environment, Page 280

From: Singer & Munns 2006 Soils: An Introduction, pages 191

A system of inquiry for essential nutrient elements: 1. Why do plants need it? 2. Is it mobile once taking up by plants? 3. What is the relative quantity of plant need (macro, secondary, & micro)? 4. Where does the element come from (or what are the sources globally and locally?) 5. How does it cycle through time and space? 6. Does the element go through chemical and/or biological transformations in plantsoil systems? If it does, do you understand all the key processes that regulate such transformations? 7. What factors and processes influence or control the availability of each element to plants? 8. Can you relate the above questions to management decisions? 9. Can you relate the above questions to relevant environmental issues?

Nitrogen (N) as a nutrient element 1. Why do plants need N? Plants need nitrogen to make proteins (enzymes), chlorophyll, etc. 2. Is it mobile in among plant tissues? Yes. Plants do move N around. 3. What is the relative quantity of plant need (macro, secondary, & micro)? The fourth largest amount (under C, O and H) among all essential elements, and the highest among elements from soils. So it is the first MACRO-nutrient.

4. Where does the element come from? (or what are the sources globally and locally?) (see figures) Note: Before plants and animals can make use of nitrogen, the diatomic form of N 2 gas must be "fixed" first, or transformed into usable (reactive) forms. Major fluxes on land: Internal cycling (plant-soil) (~1200 million metric tons [or pg=10 12 g] per year) Input to land: (1) biological N 2 -fixation (~145 million metric tons per year) (2) Human activities (fertilizers[100], Fossil fuel[25], etc.[30]) (~155) (3) N 2 -fixation in lightening (~20) (4) sea spray (~15) Total input: 335 Output from land: (1) denitrification (landmass:120; Rivers & lakes:90) (~210???) (2) river flow to ocean (~80) (3) dust from land to ocean (~10) Total output: 300 Land net balance: 335-300 = 35 Why is nitrogen cycling on land far from zero balance?

5. How does it cycle through time and space? Total natural input: 110 (Bio-N 2 -fix)+20 (N 2 -fix by lightening)=130 pg yr -1 Total human-driven input:100 (N-fert)+25 (fossil fuel)+35 (N 2 -fixing crops)=160 pg yr -1 So, 160 / 130 X 100 = 123%, or human-driven N-input has more than doubled the natural N-cycling rate on land globally. Gruber & Galloway, Nature 451:293-296, 2008

Hypoxia and Eutrophication Increased nutrient input to aquatic ecosystems may cause eutrophication. Eutrophication leads to excessive growth of algae and cyanobacteria. Later after death of these excessive biomass, much increased decomposition by bacteria depletes oxygen in the water, which causes fish kills and other detrimental effects the Dead Zone.

Figures from: http://www.cropscience.org.au/icsc2004/plenary/2/140_hatfield.htm Nitrogen fertilizer consumption for six regions throughout the world for the 1961 to 2002 period. (Data source is http://faostat.fao.org/faostat). Grain production in Australia, China, South America, and United States relative to N fertilizer use for the 1961-2002 period. (Data source is http://faostat.fao.org/faostat).

From: Rothamsted Long-term Experiment: Report of 2006 Short-straw Cultivars

Vitousek et al., 2009. Science 324:1519-1520

6. Does the element go through chemical and/or biological transformations in plant-soil systems? If it does, do you understand all the key processes that regulate such transformations? Yes. It does go through both chemical and biological transformations as follows: Ammonification/decomposition, Nitrogen assimilation/immobilization, Nitrification, Denitrification, and N 2 -fixation, 6-1. Ammonification/Decomposition: Ammonification is the biological conversion from organic nitrogen to inorganic nitrogen ammonium (NH 4+ ) : R-NH 2 (organic-n) NH 4 + Many microorganisms are capable of carrying out ammonification. Most decomposers do. So any conditions that influence SOM decomposition are important controlling factors for ammonification (e.g., microbial substrates, water, O 2, temperature, etc). 6-1.1 Ammonia volatilization: When urea (NH 2 -CO-NH 2 ) or ammonium salts or animal manure is applied to soils of high ph values (or basic), ammonium hydroxide (NH 4 OH) can form, which will further increases the ph and results in N-loss to the atmosphere as ammonia (NH 3 ). This process is called ammonia volatilization. Ammonia (NH 3 ) volatilization often occurs to a large extent: (1) for surface application to high-ph calcareous soils, or soils with low CEC, and/or low water content, higher temperature, or applied on grass/pastures.

6-2. Assimilation & immobilization Nitrogen assimilation is the process in which inorganic nitrogen (NH + 4 or NO 3- ) is converted into organic nitrogen forms. A typical one may like this: NH + 4 or NO - 3 R-NH 2 (represents organic-n). This process happens only within biological systems (or living cells). Soil microbial N assimilation is also called N immobilization. 6-3. Nitrification Nitrification is a series of oxidation processes starting from ammonium and ending with nitrate: NH + 4 (N 2 O; NO; NO 2 ) NO - 2 NO - 3 Both Nitrosomonas and Nitrobacter (nitrifying bacteria) are chemolithotrophic, both can be active under low ph since nitrification lowers the ph of the environment. From NH + 4 to NO - 3 is a very significant change, especially in soils. Most NH + 4 is absorbed on soil particle surface (cation exchange), while NO - 3 is virtually free to move. So NO - 3 leaching can be a problem of losing available N and polluting the environment. Nitrate in ground water is a serious problem: 1. Nitrate + amino compounds nitrosamines (highly carcinogenic) 2. Nitrate can be reduced in gastrointestinal tract of infants into toxic nitrite (NO 2- ), which combines with hemoglobin of the blood, causing respiratory distress or the socalled blue baby syndrome. 3. Nitrate reduction to nitrite may also occur in the rumen of live stock, causing animal disease.

6-4. Denitrification Denitrification is a series of processes starting from nitrate (NO 3- ) and ending with N 2 : NO 3 - NO 2 - (NO 2 ; NO; N 2 O) N 2 Some typical denitrifying bacteria:(paracoccus denitrificans, Thiobacillus denitrificans, Pseudomonans spp.). Key points about Denitirfication: 1. Denitrifying microbes use NO 3 - or NO 2 - as electron acceptors. 2. Denitrification only occurs under anaerobic condition. 3. Denitrification needs reducing substrates as energy source. 4. The enzyme system is totally inhibited by free oxygen, but not ammonia.

Figure 12.6

6-5. N 2 -Fixation Nitrogen (N 2 ) fixation is a process of converting N 2 to ammonia (ammonium), or nitrate. There are three major ways of N 2 -fixation: (1) Natural lightening; (2) Artificial synthesis; and (3) biological N 2 -fixation. (1). N 2 -fixation in Lightening (N 2 NO x NO 3 - ) It is not very clear how exactly this happens. At the global scale, lightening may fix as much as 70 to 100 million metric tons of nitrogen per year. (2). Artificial N 2 -fixation (The famous Haber-Bosch process; 200 ATM, 500 ºC, catalyst): 4N 2 + 12H 2 8NH 3 The above equation represents the major industrial production of nitrogen fertilizers. Now total amount of nitrogen fertilizer use in the world is about 100 million metric tons per year. Producing nitrogen fertilizers is a very energy-intensive business. (3). Biological N 2 -fixation Some procaryotic microorganisms are capable of carrying out N 2 -fixation: N 2 + 8H + + 2e + 16 ATP 2NH 3 + H 2 The enzyme responsible for catalyzing the above reaction is called nitrogenase. All known nitrogenase are super sensitive to free O 2. Any amount of free O 2 will poison the enzyme and stop N 2 fixation. Biological N 2 fixation needs a large amount of energy supply to break the three covalent bonds of N 2. Globally, biological N 2 -fixation may contribute approximately 140 million metric tons of nitrogen per year to land systems, and roughly equal amount to oceans. (3.1) Symbiotic (mutualistic): Rhizobium types, and actinomycete-types (3.2) "Free-living": e.g., Azotobacter, Azospirillum, Beijerinckia, and cyanobacteria (blue green algae). These procaryotes contain the enzyme nitrogenase, and capable of fixing N 2. They are commonly found on plant surface (leaf and sheath, or the phyllosphere) of moist climate or inside plant tissues, and some are found on root surfaces, where available organic substrates supply are high and free O 2 is blocked out.

High-voltage electrical discharges, such as lightning, can oxidize N 2. N 2 ---> NO x ---> NO 3 - Internal combustion engines produce NO and NO 2 because the high internal temperatures and pressures cause atmospheric N 2 and O 2 to react.

Industrial Nitrogen Fixation total N fertilizer use in the world: 90-110 million metric tons per year. 200 ATM + 500 C 4N 2 + 12H 2 -----------------> 8NH 3 Catalyst

Legumes (Fabaceae) Bacteria: Rhizobium and Bradyrhizobium Legume plants: peas, lentils, beans, alfalfa, lupines, peanuts, clover Pea root system

Root nodules of a clover plant (a legume).

Soybean roots and nodules Nodules

Nonlegumes Cyanobacteria Bacteria Actinomycetes (Frankia) associated with angiosperms like red alder

Alder root nodules Red Alder (Alnus rubra) forms a symbiotic association with an actinomycete of the genus Frankia.

"Free-living N-fixers: e.g., Azotobacter, Azospirillum, Beijerinckia, and cyanobacteria A microbial mat at a hot spring in eastern Oregon. Some of the organisms in the mat are cyanobacteria that can fix nitrogen. A transmission electron micrograph of Acetobacter diazotrophicus. These bacteria live and grow inside sugarcane plants and fix N 2 for both the plant and themselves.

Five processes in cycling nitrogen through the biosphere fixation ammonification (decompostion) nitrification Denitrification Assimilation (Immobilization) Assimilation or Immobilization Animal protein Ammonium NH 4 + Industrial fixation N 2 in atmosphere Biotic N- fixation Protein (plants and microbes) Nitrification by bacteria Nitrites NO 2 - Fixation by lightning Nitrates N0 3 - Denitrification (by bacteria) Assimilation or immobilization

7. What factors and processes influence or control the availability of such element to plants? All the transformation processes, and environmental factors that influence them. 8. Can you relate the above questions to management decisions? Chemical fertilizers vs. organic farming 9. Can you relate the above questions to relevant environmental issues? Altering the global nitrogen cycle Eutrophication Acid rain Food production and the world population growth

SULFUR (S) 1. Why do plants need it? Sulfur is an essential part of protein (e.g., Methionine and Cysteine). Some organic sulfur-containing compounds may have defense functions. 2. Is it mobile once assimilated into plant tissues? Yes. It acts more like N in plant tissues. 3. What is the relative quantity that plants need (macro, secondary, & micro)? Approximately 0.1 % of plant dry matter. It is in the "secondary" group in term of quantity. 4. Where does the element come from (or what are the sources globally and locally?) Most sulfur is in sedimentary rocks and other rocks. Oceans have a considerable amount of sulfur in the form of SO 2-4. Very little sulfur is in organic forms either in soils or in biomass. 5. How does it cycle through time and space? Rock cycle, weathering, leaching, and transformation given below. 6. Does the element go through chemical and/or biological transformations in plant-soil systems? If it does, do you understand all the key processes that regulate such transformations? (1) Desulfuration (analogous to ammonification): RSH(mercaptan), or DMS(CH 3 SCH 3 ) + H 2 O ----RH + H 2 S (2) Oxidative transformations: H 2 S (DMS, RSH) + O 2 -- S 0 + O 2 --- H 2 SO 4 Sulfur oxidation produce acid, so Thiobacillus thiooxidans and Thiobacillus ferrooxidans are used in mining. Fossil fuel sulfur: SO 2 + H 2 O ---- H 2 SO 3 (sulfurous acid) or SO 3 + H 2 O ---- H 2 SO 4 (sulfuric acid) (3) Reductive sulfur transformations: SO 2-4 --- SO 2-3 --- S 0 --- H 2 S Sulfate reduction occurs at wide range of ph values, pressure, temperature and salinity. Sulfate reduction is inhibited by oxygen, nitrate, and ferric ions. H 2 S is very toxic to aerobes and plant roots. Assimilatory sulfate reduction happens in many organisms. The valence change and the different forms of sulfur transformations are analogous to nitrogen transformation. 7. What factors and processes influence or control the availability of such element to plants? Microbial activities/ SOM decomposition; Sulfur concentration in the soil parent material; S deposition. 8. Can you relate the above questions to management decisions? 9. Can you relate the above questions to relevant environmental issues? Draining wetlands? Acid deposition?