Title: Plant Nitrogen Speaker: Bill Pan. online.wsu.edu

Transcription

1 Title: Plant Nitrogen Speaker: Bill Pan online.wsu.edu

2 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

3 Figure 4-1. The N cycle. Test your command of N terminology in Assnmt 2.1

4 Table 4-1. Approximate Distribution of N Throughout the Soil-Plant/Animal-Atmosphere System N pool Metric Tons % of Total Atmosphere 3.9 x Sea (various) 2.4 x Soil (nonliving) 1.5 x Plants 1.5 x Microbes in soil 6 x Animals (land) 2 x People 1 x

5 Nitrogen Oxidation States NH 4+,NH 3 N 2 N 2 O NO NO 2 - NO 3 - N Oxidation States: Energy requiring reactions Energy yielding reactions

6 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.

7 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

8 Nitrogen Forms the Peptide Bond in Proteins

9 Nitrogen Forms a Coordination Complex with Mg in Chlorophyll

10 Nitrogen Provides Functional Groups at Reaction Sites of Enzymes Such as ATPase

11 Plant N Plant N Accumulation 1 to 5% N Seasonal uptake patterns often follow a sigmoidal pattern: lb N/ac typical ranges for crop plants over season (see texts) Time

12 Nutrient uptake over time Corn Dry Matter Time grain stalk leaves Corn P uptake N P Time

13 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

14 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

15 N Deficiency in Greenhouse Corn

16 N Deficiency in Field-grown Corn (Maize) chlorosis of lower leaves necrosis of leaf tips yellowing of midrib region

17 Field-view of N Deficient Corn General yellowing of canopy May be detectable by on-the-go sensors or by remote sensing

18 Sugar beet Potato Kale Rutabaga Some General N Deficiencies

19 N Toxicities Cucumber N toxicity necrosis of newer growth regions

20 Biological Components of the Nitrogen Cycle Processes and Organisms of: N fixation Mineralization Immobilization Nitrification

21 Biological N fixation Bacteria and algae Symbiotic with legumes some trees & bushes water ferns Non-symbiotic free-living or in loose associations

22 N fixation Industrial Fixation reduces N 2 to NH 4+. Requires C; absence of air and water, and 500 atm pressure. Biological Nitrogen Fixation: Nitrogenase N 2 + 8e- + 16ATP + 10 H + 2NH ADP + 16P i + H 2 Very energetically expensive. Most N fixation by symbioses where plants provide the energy.

23 a) Example of nodules of Bradyrhizobia on soybean b) showing non-inoculated alfalfa (left) and inoculated (right) with proper Rhizobia bacteria

24 Figure 4-5. Conversion of N 2 to NH 4 + by rhizobia inside a legume root nodule.

25 Table 4-7. N 2 Fixed by Legumes in Temperate Climates N fixed (lb/a/yr) Legume Range Typical Alfalfa Beans Chickpeas Clovers (general) Cowpeas Crimson clover Fava beans Hairy vetch Kudzu Ladino clover Lentils Peas Red clover Soybeans

26 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

27 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.

28 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.

29 N fixation is ph-dependent Soil ph Non-legume Low-pH sensitive Low-pH tolerant

30 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.