Title: Plant Nitrogen Speaker: Bill Pan online.wsu.edu
Lesson 2.3 Plant Nitrogen Nitrogen distribution in the soil-plantatmosphere Chemical N forms and oxidation states Biological roles of N in plants Seasonal N accumulation, N deficiencies Biological N fixation
Figure 4-1. The N cycle. Test your command of N terminology in Assnmt 2.1
Table 4-1. Approximate Distribution of N Throughout the Soil-Plant/Animal-Atmosphere System N pool Metric Tons % of Total Atmosphere 3.9 x 10 15 99.3840 Sea (various) 2.4 x 10 13 0.6116 Soil (nonliving) 1.5 x 10 11 0.0038 Plants 1.5 x 10 10 0.00038 Microbes in soil 6 x 10 9 0.00015 Animals (land) 2 x 10 8 0.000005 People 1 x 10 7 0.00000025
Nitrogen Oxidation States NH 4+,NH 3 N 2 N 2 O NO NO 2 - NO 3 - N Oxidation States: -3 0 + 1 + 2 + 3 + 5 Energy requiring reactions Energy yielding reactions
Plant Nitrogen Uptake Most plants are capable of absorbing ammonium and nitrate. Relatively little organic N is directly absorbed by crop roots, it must first be mineralized into these inorganic forms before plant uptake can occur. Since nitrate is the predominant inorganic N form in soil solutions, it is typically the primary form absorbed by crop roots.
Biological Role of N in Plants Forms peptide bond linking amino acids in proteins Constituent of other important biochemicals such as chlorophyll Provides functional groups in enzymes for reaction site or attachment of substrates, cofactors
Nitrogen Forms the Peptide Bond in Proteins
Nitrogen Forms a Coordination Complex with Mg in Chlorophyll
Nitrogen Provides Functional Groups at Reaction Sites of Enzymes Such as ATPase
Plant N Plant N Accumulation 1 to 5% N Seasonal uptake patterns often follow a sigmoidal pattern: 100 450 lb N/ac typical ranges for crop plants over season (see texts) Time
Nutrient uptake over time Corn Dry Matter Time grain stalk leaves Corn P uptake N P Time http://maize.agron.iastate.edu/corngrows.html#nutrient
N deficiency symptoms: General patterns 1. Chlorosis (loss of green color, usually seen as yellowing) of leaves Lack of chlorophyll Seen esp. in OLDER leaves, since N is very mobile in plant 2. General stunting of plant Decreased carbohydrate production Decreased protein production
N Deficiency Symptoms: Examples Corn (maize): chlorosis of lower leaf, progresses up the midrib first Potatoes: Stunted plants, chlorosis of lower leaves, upward cupping Wheat: smaller leaves, less tillering, chlorosis of lower leaves Peach: reddish leaves with shothole appearance Remember, by the time you see symptoms, plant health / quality / yield have ALREADY been diminished
N Deficiency in Greenhouse Corn
N Deficiency in Field-grown Corn (Maize) chlorosis of lower leaves necrosis of leaf tips yellowing of midrib region
Field-view of N Deficient Corn General yellowing of canopy May be detectable by on-the-go sensors or by remote sensing
Sugar beet Potato Kale Rutabaga Some General N Deficiencies
N Toxicities Cucumber N toxicity necrosis of newer growth regions
Biological Components of the Nitrogen Cycle Processes and Organisms of: N fixation Mineralization Immobilization Nitrification
Biological N fixation Bacteria and algae Symbiotic with legumes some trees & bushes water ferns Non-symbiotic free-living or in loose associations
N fixation Industrial Fixation reduces N 2 to NH 4+. Requires 300-400 C; absence of air and water, and 500 atm pressure. Biological Nitrogen Fixation: Nitrogenase N 2 + 8e- + 16ATP + 10 H + 2NH 4 + 16ADP + 16P i + H 2 Very energetically expensive. Most N fixation by symbioses where plants provide the energy.
a) Example of nodules of Bradyrhizobia on soybean b) showing non-inoculated alfalfa (left) and inoculated (right) with proper Rhizobia bacteria
Figure 4-5. Conversion of N 2 to NH 4 + by rhizobia inside a legume root nodule.
Table 4-7. N 2 Fixed by Legumes in Temperate Climates N fixed (lb/a/yr) Legume Range Typical Alfalfa 50 300 200 Beans 20 80 40 Chickpeas 20 100 50 Clovers (general) 50 300 150 Cowpeas 60 120 90 Crimson clover 30 180 125 Fava beans 50 200 130 Hairy vetch 50 200 100 Kudzu 20 150 110 Ladino clover 60 240 180 Lentils 40 130 60 Peas 30 180 70 Red clover 70 160 115 Soybeans 40 260 100
Table 4-5. Economically Important Microorganisms Involved in Biological N Fixation Organisms General Properties Agricultural Importance Azotobacter Aerobic; free fixers; live in soil, water, rhizosphere (area surrounding the roots), leaf surfaces Minor benefit to agriculture; found in vascular tissue of sugarcane, with abundant sucrose as a possible energy source for N 2 fixation Azospirillum Microaerobic; free fixers; or found in association with roots of grasses Inoculation benefits some nonlegume crops, shown to increase root hair development Rhizobium Fix N in legume-rhizobium symbiosis Legume crops are benefited by inoculation with proper strains Actinomycetes, Frankia Fix N in symbiosis with nonlegume wood trees alder, Myrica, Casuarina Potentially important in reforestation, wood production Blue-green algae, Anabaena Contain chlorophyll, as in higher plants; aquatic and terrestrial Enhance rice in paddy soils; Azolla (a water fern) Anabaena-Azolla symbiosis; used as green manure
Inoculation Inoculation may be required. Rhizobium only survive a few years in the soil without the proper host. Commercial strains selected for most N fixed on specific crops. Inoculate seeds themselves. Mix bacteria with peat and gum (sugar) to coat seeds. Best survival if done just before planting. Or inoculate within or below the seed row.
Figure 4-7. Soybean yield as influenced by P availability and inoculation. (Singleton et al., 1990, Applied BNF Technology: A Practical Guide for Extension Specialists, NifTAL, Paia, Hl.) Leibig s Law of the Minimum in action.
N fixation is ph-dependent 18 16 14 12 10 8 6 4 2 0 5 5.5 6 6.5 7 Soil ph Non-legume Low-pH sensitive Low-pH tolerant
Multiple N Pathways in Legumes Crop legumes typically assimilate soil N like non-legumes, while biologically fixing atmospheric N. Higher soil N availability inhibits N fixation.