Feedback between nutrient availability, NPP and N release

Transcription

1 Feedback between nutrient availability, NPP and N release 1

2 Redfield ratios A typical plant = 45% C, 1.5% N, 0.2%P or C:N = 30 : 1 and C:P = 225 : 1 or C:N:P = 225 : 7.5 : 1 N:P = 7.5 : 1 Mobility of nutrients In plants N limitation most non-tropical terrestrial ecosystems early 1 succession P limitation highly weathered soils, i.e. tropical soils calcareous soils Leaves affected old whole plant old old whole plant old Cation (3+) Cation (2+) Macronutrients: Ca2+ Mg2+ Micronutrients: Fe3+ Fe2+ Mn2+ Zn2+ Cu2+ Ni2+ Cation Neutral Anion (+) (0) (-1) K+ NH4+ Anion (-2) NO3H2PO4- HPO42SO42B(OH)3 ClMoO42-2

3 What are the sources of nutrients? Sources of nutrients Weathering = (parent material, climate, vegetation, topography, time) Parent material Climate Vegetation Topography Time Atmospheric sources Rain, clouds, fog Sedimentation Largest reservoir of N 3

4 Decomposition rate measurement Carbon is lost to the atmosphere as CO 2 in the process of respiration Based on the data, the decomposition rate can be calculated Recycling of Nutrients decomposition Climate Litter chemistry Soil organisms Climate Temperature influence on decomposition 7.2oC, 621 mm 12.2oC, 720 mm 14.4oC, 806 mm Decomposition of red maple litter at three sites. Warm and wet conditions, decompose faster. Temperate deciduous forest 4

5 Lignin contents influence litter decomposition Variety of organisms involved in decomposition Processes: leaching, fragmentation, changes in physical and chemical structure, ingestion and excretion of waste products. Bacteria are dominant decomposer (animal) Fungi (plant) Aided by detritivores Soil Cation exchange complex Tendency for adsorption varies: cations: Al 3+ > Ca 2+ > Mg 2+ > NH 4 + > K + > Na + anions: PO 4 3- > SO 4 3- > NO 3- > Cl - 5

6 How do ions get to roots? Root growth Diffusion Mass flow Diffusion D = Dl θ ƒ 1/b where Dl = diffusion coefficient in free solution θ = volumetric water content of soil ƒ = an impedance factor, length of the diffusion pathway b = soil buffer capacity, determined by the supply of ions adsorbed on the CEC Mass flow Transpiration stream Wetting stream Lateral flow Pathways of ion uptake Acclimation/adaptation to nutrient availability Uptake kinetics Root morphology Rhizosphere chemistry Growth strategies Symbioses Uptake kinetics Response to low nutrient supply Increase maximum inflow rate Induction of high-affinity transport system 6

7 Pathways of ion uptake Root allocation and morphology Root mass ratio (% roots) Root hairs Cluster roots Cluster roots Hakea prostrata 1 µm P 75 µm P 1 µm P 75 µm P Rhizosphere chemistry Excretion of H+ or organic acids reduces ph Zn, Mn, B, Mn, Fe Excretion of chelating agents Fe and Zn; releases PO 4 2- Excretion of phosphatases cleave organic bonds to release PO 4 2- Excretion of organic acids, carbohydrates, and amino acids stimulates microbial activity Plant Growth Strategies Plants from low-nutrient environments slow growth low nutrient demand long-lived tissues; C-based defenses low tissue nutrient content; slow decomposition Plants from high-nutrient environments rapid growth high nutrient demand short-lived tissues; N-based defenses high tissue nutrient content; rapid decomposition 7

8 Mycorrhizal associations of fungi and plant roots promote nutrient uptake Fungi assist the plant with the uptake of nutrient from the soil (extended water and nutrients absorption) Plant provides the fungi with carbon, a source of energy. Ectomychorrhizae (EcM) Arbuscular Mycorrhizae (AM) Function: promote plant growth, increase a plant s uptake of minerals by penetrating a greater volume of soil than the roots. Endomycorrhizae (a) Ectomycorrhizae (b) Ectomycorrhizae Vesicular/Arbuscular Mycorrhizae Resource transfer via hyphal network Mycorrhizal fungi work well under poor nutrient conditions 8

9 Symbiotic N fixation restricted to more limited group of plants (legumes, actinorhizal species), high host specificity nitrogenase enzyme plant host provides C energy to N-fixer bacteria N-fixer provides available N to host plant Paradox of nitrogen limitation Nitrogen is the element that most frequently limits terrestrial NPP N2 is the most abundant component of the atmosphere Why doesn t nitrogen fixation occur almost everywhere??? Why don t N fixers have competitive advantage until N becomes nonlimiting? Environmental limitations to N fixation Energy availability in closed-canopy ecosystems N fixers seldom light-limited in well-mixed aquatic ecosystems (e.g., lakes) Nutrient limitation (e.g., P, Mo, Fe, S) These elements may be the ultimate controls over N supply and NPP Grazing N fixers often preferred forage 9