1. Plant-Microbe Interactions  Plant-microbe interactions diverse – from the plant perspective: Negative – e.g. parasitic/pathogenic Neutral Positive – symbiotic  This lecture  important positive interactions with respect to plant abundance and distribution – related to plant nutrient and water supply: Decomposition Mycorrhizae N 2 fixation Rhizosphere  the role of this interaction in the N cycle

2. I. Decomposition Primary supplier of plant nutrients – particularly N & P A. Raw material Soil organic matter derived primarily from plants – Mainly leaves and fine roots Wood can be important component in old growth forests Input rates – Generally follow rates of production Deciduous = evergreen

3. B. Processes 1. Fragmentation – Breakdown of organic matter (OM) into smaller bits = humus By soil ‘critters’ – including nematodes, earthworms, springtails, termites consume and excrete OM  incomplete digestion nematode springtail (Isotoma viridis) termites

4. 2. Mineralization Breakdown OM  inorganic compounds Microbial process: accomplished by enzymes excreted into the soil Microbial uptake Immobilization Plant uptake Nitrite NO 2 - Nitrate NO 3 - energy for nitrifying bacteria* Nitrification For Nitrogen proteins (insoluble) amino acids energy for heterotrophic bacteria proteases Ammonium NH 4 + Mineralization * In 2 steps by 2 different kinds of bacteria – (1) Nitrosomonas oxidize NH3 to nitrites + (2) Nitrobacter oxidize nitrites to nitrates

5. NH 4 + proteins mineralization NO 3 - plant uptake 1) Nitrate (NO 3 - ) Preferred by most plants, easier to take up Even though requires conversion to NH 4 + before be used  lots of energy vs. taking up & storing NH 4 + problematic More strongly bound to soil particles Acidifies the soil Not easily stored C. N uptake by plants – Chemical form taken up can vary 2) Ammonium (NH 4 + ) – Used directly by plants in soils with low nitrification rates (e.g. wet soils)

6. proteins NH 4 + mineralization microbial uptake immobilization NO 3 - nitrification plant uptake amino acids 3) Some plants can take up small amino acids (e.g. glycine) Circumvents the need for N mineralization Facilitated by mycorrhizae Direct uptake

7. D. Controls on rates of decomposition 1) Temperature – Warmer is better <45°C 2) Moisture – intermediate is best Too little  desiccation Too much  limits O 2 diffusion T Soil Moisture % Soil Microbial Respiration

8. 3) Plant factors – Litter quality a) Litter C:N ratio (= N concentration) If C relative to N high  N limits microbial growth Immobilization favored N to plants  Decomposition rate as fn(lignin, N) Deciduous forest spp b) Plant structural material Lignin – complex polymer, cell walls Confers strength with flexibility – e.g. oak leaves Relatively recalcitrant High conc.  lowers decomposition

9. Consequence of controlling soil OM chemistry and microclimate … Plants important factor controlling spatial variation in nutrient cycling c) Plant secondary compounds Control decomposition by: Bind to enzymes, blocking active sites  lower mineralization N compounds bind to phenolics  greater immobilization by soil Phenolics C source for microbes  greater immobilization by microbes Anti-herbivore/microbial Common are phenolics – e.g. tannins – Aromatic ring + hydroxyl group, other compounds OH R

10. A.Symbiotic relationship between plants (roots) & soil fungi Plant provides fungus with energy (C) Fungus enhances soil resource uptake Widespread – Occurs ~80% angiosperm spp All gymnosperms Sometimes an obligate relationship II. Mycorrhizae

11. B. Major groups of mycorrhizae: 1) Ectomycorrhizae – Fungus forms “sheath” around the root (mantle) Grows in between cortical cells = Hartig net – apoplastic connection Occur most often in woody spp

12. 2) Endomycorrhizae – Fungus penetrates cells of root Common example is arbuscular mycorrhizae (AM) Found in both herbaceous & woody plants Arbuscule = exchange site Arbuscule in plant cell

13. C. Function of mycorrhizae: 1)Roles in plant-soil interface – a)Increase surface area & reach for absorption of soil water & nutrients b)Increase mobility and uptake of soil P c)Provides plant with access to organic N d)Protect roots from toxic heavy metals e)Protect roots from pathogens 2) Effect of soil nutrient levels on mycorrhizae Intermediate soil P concentrations favorable Extremely low P – poor fungal infection Hi P – plants suppress fungal growth – taking up P directly N saturation

14. III. N 2 Fixation N 2 abundant – chemically inert N 2 must be fixed = converted into chemically usable form Lightning High temperature or pressure (humans) Biologically fixed  Nitrogenase – enzyme catalyzes N 2  NH 3  Expensive process – ATP, Molybdenum  Anaerobic – requires special structures

15. Symbiosis with plants – Mutualism Prokaryote receives carbohydrates Plant may allocate up to 30% of its C to the symbiont Plant provides anaerobic site – nodules Plant receives N A. Occurs only in prokaryotes: Bacteria (e.g. Rhizobium, Frankia) Cyanobacteria (e.g. Nostoc, Anabaena)  Free-living in soil/water – heterocysts  Symbiotic with plants – root nodules  Loose association with plants Anabaena with heterocysts

16. Those with N 2 -fixing symbionts form root “nodules” – anaerobic sites that “house” bacteria soybean root alpine clover Examples of plant–N 2 -fixing symbiotic systems – 1) Legumes (Fabaceae) Widespread bacteria = e.g., Rhizobium spp.

17. Problem of O 2 toxicity – Symbionts regulate O 2 in the nodule with leghemoglobin Different part synthesized by the bacteria and legume Cross-section of nodules of soybean nodules

18. Buffaloberry (Shepherdia argentea) - actinorhizal shrub (Arizona) 2) Non-legume symbiotic plants – “Actinorhizal”= associated with actinomycetes (N 2 -fixing bacteria) genus Frankia Usually woody species – e.g. Alders, Ceanothus Ceanothus velutinus - snowbrush Ceanothus roots, with Frankia vesicles Bacteria in root or small vesicles

19. B. Ecological importance of N 2 fixation 1) Important in “young” ecosystems – Young soils low in organic matter, N

20. 2) Plant-level responses to increased soil N conc: Some plants (facultative N-fixers) respond to soil N concentration  Plant shifts to direct N uptake N fixation  Number of nodules decreases

21. 3) Competition: N fixers-plant community interactions N 2 -fixing plants higher P, light, Mo, and Fe requirements  Poor competitors Competitive exclusion less earlier in succession Though - N 2 fixers in “mature” ecosystems Example N-fixing plants important in early stages of succession: Lupines, alders, clovers, Dryas

22. IV. N losses from ecosystem Leaching  to aquatic systems Fire  Volatization Denitrification  N 2, N 2 O to atmosphere – Closes the N cycle! Bacteria mediated Anaerobic Natural N cycle PLANT REMAINS N2ON2O

23. From - Peter M. Vitousek et al., "Human Alteration of the Global Nitrogen Cycle - Causes and Consequences," Issues in Ecology, No. 1 (1997), pp. 4-6. ANTHROPOGENIC SOURCES Annual release (10 12 g N/yr) Fertilizer80 Legumes, other plants40 Fossil fuels20 Biomass burning40 Wetland draining10 Land clearing20 Total from human sources210 Altered N cycle NATURAL SOURCES Soil bacteria, algae, lightning, etc.140 Annual release (10 12 g N/yr)

24. V. Rhizosphere interactions – the belowground foodweb Zone within 2 mm of roots – hotspot of biological activity Roots exude C & cells slough off = lots of goodies for soil microbes  lots of microbes for their consumers (protozoans, arthropods) “Free living” N 2 -fixers thrive in the rhizosphere of some grass species Fine root

25. Summary Plant–microbial interactions play key roles in plant nutrient dynamics  Decomposition –  mineralization, nitrification …  immobilization, denitrification …  Rhizosphere – soil foodweb  Mycorrhizae – plant-fungi symbiosis  N fixation – plant-bacteria symbiosis Highly adapted root morphology and physiology to accommodate these interactions N cycle, for example, significantly altered by human activities