Nutrient cycling in ecosystems: Lecture Content

Introduction to nutrient cycles Driving forces for nutrient cycles in ecosystems Water (hydrological) cycle as a physical model of

nutrient cycling Case study of N, Ca limitation: Hubbard Brook

Experimental Forest, NH Major nutrient cycles & their pool sizes, transfer rates,

control mechanisms, human impacts Nitrogen Phosphorus

What dets comm structure, comp, distribution? Big scale, big picture things….. E in system, nutrients, solar radiation, rain…….=productivity.

Thermodynamics review:

Means that Sun+nutrients + water = plants

Herbivores eat them, preds eat them…….

E always lost w. each transformation from one trophic level to next…..

(What is missing? (Omnis).

•Raymond Lindeman: ecosystems are systems that transform energy.

• This transformation and transfer of energy from one trophic level to the next (feeding level) is inefficient so some energy is lost at each level.

How it is lost,

Primary production –Unabsorbed energy given off as heat. Photosynthesis, Respiration

Secondary production – wastage (bones, stems, uneaten material, ie. Production and Consuption efficiencies), heat

Trophic level transfer efficiency is around 10%.

What OTHER very important trophic level receives lots of available energy due to inefficiency of primary producers and secondary producers (consumers)?

Pyramids of Energy tend to reflect pyramids of numbers

What are the limits, determinants of primary production (see biome lecture!)

Secondary productivity- well…. Primary productivity

Nutrients, unlike energy, are not constantly renewed and used up

Introduction to nutrient cycling They are cycled, between organic (living) and

inorganic pools (and among organic and inorganic ones)

Movement, or cycling, of nutrients requires (ultimately) energy input into ecosystems, e.g., to initiate chemical reactions

We will focus on particular nutrients in this lecture, to try and understand those that are most critical to ecosystem function

One way to understand nutrient dynamics is to use compartmental model to identify both the pools (organic and inorganic) and the fluxes between pools

Generalized compartmental model of nutrient cycles Sedimentary cycles

(e.g., P)

Atmospheric cycles (e.g., N)

To see the coupling of nutrient cycling and energy, consider a simple redox chemical reaction:

Energy releasing reaction is paired with energy requiring one; oxidation side must release more energy than reduction side requires; rest lost as heat

Assimilatory reactions (e.g., photosynthesis) incorporate inorganic forms of nutrients (e.g., carbon) into organic forms (e.g., carbohydrates); dissimilatory rxns. the reverse

Global hydrological cycle drives other cycles (units g18 = teratons (tt) = 1012 metric tons for pools (dark blue). Fluxes in light blue, units of tt/yr.

97% of global H2O pool in oceans

25% of total solar radiation on Earth used to drive hydrological cycle!

Represents difference between precipitation, & evaporation, i.e., 111 - 71

Represents difference between evaporation and precipitation over sea, i.e., 425 - 385

Which nutrient cycles to study? Those that are most limiting to plants (& thus ecosystems), i.e., N, P, S, sometimes Ca because demands high relative to supply (soils, lakes, oceans)

Elemental uptake by plants and soil nutrient reserves for macronutrients (from Stiling text, Table 22.2)

Element Annual Plant Uptake Soil Nutrient Reserves (yr)(kg/ha/yr) (= soil content/annual plant uptake)

Nitrogen (N) 30 40Phosphorus (P) 7 150Sulfur (S) 2 320Potassium (K) 30 430Calcium (Ca) 50 260Magnesium (Mg) 4 2,000Iron (Fe) 1 52,000Sodium (Na) 2 4,600Chlorine( Cl) 0.06 220

Case study: N, Ca limitation in Hubbard Brook Experimental Forest, NH Simplified nitrogen budget for Hubbard Brook

(temperate deciduous forest--northern hardwoods) Inputs via bulk precipitation (hydrological cycle) & net

nitrogen fixation by soil bacteria Outputs via stream water, & by denitrification Internal transfers of N are small relative to pool sizes,

which is typical of limiting nutrients Mineralization (chemical, dissimilatory reactions that

convert nutrient from organic to inorganic form) is slow in Hubbard Brook soils

Low movement of N (low turnover time=pool size/flux) is due to how tightly N is held & cycled by organisms there

Nitrogen budget for forested watershed, Hubbard Brook Experimental Forest (values in boxes are pool sizes, kg/ha; arrows give fluxes in kg/ha/yr)

The hypothesis that nutrients like N tightly held, tightly cycled was tested experimentally at Hubbard Brook

Methods: Entire watershed (water-catchment basin, defined by topography) was cut, harvested (to disrupt biological activity Tree re-growth suppressed; Nutrient inputs, outputs

measured (precipitation gauges, stream gauges) Bedrock “tight” at Hubbard Brook (no groundwater

losses), allowing all outputs to be measured in streams Results?

N, P, Ca increased dramatically in streamwater because of inhibited biological uptake

Why? Increased streamflow (40%), less plant uptake Implications? Forest clearcutting destructive

Clear-cut watershed used to test hypotheses about nutrient cycling by vegetation uptake

Weir, or stream gauge for quantifying water flow, stream chemistry in watershed experiments such as Hubbard Brook

Nutrient increases after clear-cutting in Hubbard Brook streams

Hubbard Brook Study also important to understand effects of acid precipitation on forest dynamics, health Acid precipitation (low pH of rain, snow) caused by

human activities Combustion of fossil fuels, other industrial processes

put nitrous oxides, sulfur oxides in atmosphere, which react with water to form nitric, sulfuric acids

Acidity could affect plants, animals both directly (acid burns) or indirectly (altered soil nutrient availability)

Which was important at Hubbard Brook? Long-term studies show importance of indirect effects

Long-term recovery from acid precipitation, Hubbard Brook, slow

Clean-air Act, 1970

Factors preventing recovery of ecosystem after Clean-air Act?

•Sulfur emissions remained high (fossil fuels not controlled enough)

•Particulate emissions dropped, but this reduced Ca inputs in rain!

•Long-term leaching of Ca from soils via hydronium ions (attaching to clay particles in soil)

•Ca in tree tissues has dropped, causing widespread forest die-back (spruce, sugar maple)

Lessons from Hubbard Brook studies Nutrient limitation, dynamics illustrated by descriptive

(compartmental models) and experimental methods Trees died because of indirect effects, which are

difficult to quantify and demonstrate Natural recovery of acid-damaged ecosystem does

take place, but estimated to be slow (centuries) for nutrient restoration (depends on flux rates)

Nutrient dynamics, regeneration processes important to understand ecosystem processes, effects of human impacts

Regeneration in terrestrial ecosystem via soil processes: microbial activity in detritus food chains (e.g., N), bedrock weathering (Ca, P)

Nutrient cycles & their controls

Things to notice: What are major inorganic sources? How many chemical forms of nutrients? What aspects of physical, biological environment

determine the transformations (fluxes)? What limits the availability of these nutrients in

terrestrial and aquatic ecosystems?

Summary of the nitrogen cycle Ultimate source is atmosphere (huge gas pool) Proximate sources are nitrogen-fixation and lightning Nitrogen fixation is important in variety of ecosystems,

but barely offsets N-losses due to denitrification Oxygen (oxidation potential) determines which

reactions in cycle are important (via microbes) N occurs in many forms because of many oxidation

states (it can act as oxidizing agent or reducing agent) Regeneration in soils via decomposition organic

matter; in H2O via mixing of nutrient-rich sediments Humans add as much N to global ecosystem

(fertilizer) as combined natural causes, leading to eutrophication (increased 1º production)

Chemical transformations in the nitrogen cycle: Note control by microbes, & soil oxygen level

P cycle also of great biological importance

Phosphorus cycle relatively simple chemically, due to fewer oxidation states (plants uptake primarily PO4

3-) Large inorganic pools in soils, bedrock, ocean sediments Control of availability to organisms complex

At low pH, P unavailable by binding to clay, Fe, Al in soil Also unavailable at high pH Mycorrhizae important scavenging P from soils In high-O2 systems, P precipitates out of water,

constituting constant rate of loss from ecosystems Rock weathering, soil decomposition make P available

Humans contribute some P to global ecosystems via fertilizers (& runoff)-->eutrophication aquatic systems

Major pools & fluxes of phosphorus globally (units in billions of metric tons = g15)

Carbon cycle is of great importance to humans Three classes of processes cause C cycling:

Assimilatory, dissimilatory reactions involve living things Exchange of CO2 between atmosphere, oceans Sedimentation, precipitation of carbonates in water

(limestone, dolomite) CaCO3 (insoluble) + H2O + CO2 Ca2

+ + 2HCO3-

(insoluble) Uptake of CO2 by plants, corals pushes reaction to left,

causes Calcium Carbonate sedimentation (or in case of corals, deposition into reef-building structures

Human impacts on carbon cycle (see next lecture): Consumption of fossil fuels increases atmospheric CO2 Global warming causes increased plant uptake, but

even greater release of C (decomposition) from tundras

Conclusions: Energy (sunlight) ultimately required for chemical

circulation (e.g., water movement), transformations Hubbard Brook Experimental Forest studies show

some factors controlling cycling, availability of N, Ca Different nutrient cycles are very different in terms of

the pools, fluxes, interaction with biological organisms, and impacts of humans

Humans are causing global changes in N, C, P cycles, among others, that are altering the biosphere

Acknowledgements: Some illustrations for this lecture from R.E. Ricklefs. 2001. The Economy of Nature, 5th Edition. W.H. Freeman and Company, New York.