Nitrogen Cycling in Soils

dissolvedNH4 (aq)

N2O + NOx(gas)

NOx

(gas)15N=?

HNO3

(gasliquid)

aerosolexcess

NO3-

> 0.5 m

NH4+

< 0.5 m

aerosol NH4

+/NO3- (aq/s)transport

deposition

up

wel

ling

sea-air flux

dissolvedNO3 (aq)15N=?

NH3

(gas)

deposition with transport

N2

(gas)NOx

(gas)

N2

(gas)

N2O(gas)

15N=1-3

upwelled NO3

15N=10-15

NH3, NOx

soil production

N2 fixation by plants

Simplified View of Soil N Cycle

Atmospheric NDeposition(Iex; IexRex)

N2 Fixation

(Ifix; IfixRfix)

N Losses toenvironment(Nskex;

15Nskexαex)

Plan t Nuptakefro m soil (Nskp; 15Nskpαp) Plan t Nreturn to

soil(Npks; 15Npks)

SOIL PLANTSSS

Nitrate, N20, N2

Nitrate, ammonium, org N

N cycle within soil:

€

orgN→ NH4 → NO3

plantavailable1 2 4 4 3 4 4 →

anaerobic{ N2O→ N2

backtoatmosphere1 2 4 3 4

Model of Soil N

€

dNdt

=I atm+kplant−soilNplant−ksoil−plantNsoil −kenvironmentNsoil

At steady state (inputs=outputs:

€

Nsoil =Itotal

kenvironment

Geographical Distribution of Soil N

•Soil N linked to C (maybe other way round)

• they are still independent of each other

•N more effectively conserved during plant decomposition (C/N ratios decline with time)

• Soil N patterns follow global soil C patterns

•Inputs increase with precipitation (temp?)

•Losses increase with temperature, deficiency of other nutrients

Soil N vs. Soil Age: Input and decomposition for San Joaquin Valley annual grasslands

0

10

20

30 0

2000

4000

0

1

2

MAP (mm.)

Total Soil N (kg m-2)

MAT (oC)

• illustrates trend that with increasing C content (due to both decreasing temp and increasing precip), the C/N ratio of the SOM increases.

•Reflects C/N values closer to plant

•Reflects lower degrees of decomposition

Climate Controls Total N Amounts and C/N Ratios: How does it affect form of N losses

•N isotope composition of soil N reflects the form of N lost from soils–Nitrate, N2O, N2 (forms of N lost from plant available forms) enrich remaining soil N in 15N)–Dissolved organic N or erosion of soil organic N do not affect N isotopes of soil N

• Globally, the 15N increases with increasing temperature and decreasing moisture, which implies that plant avaible forms of N are increasingly lost as climate becomes becomes hot and/or hot and dry.

–Hot dry climates are limited by water rather than N, so plant available N can leak out–Hot/wet environments (Brazil, etc.) are commonly limited by other elements (such as P) so plant available N forms can also leave…..

How have humans altered the global N cycle?

Natural N Cycle:• lightning: <10 Tg N/yr (Tg=1012g)• biological N fixation: 90 to 140 Tg N/yr

Altered N Cycle (INPUTS):• N fertilizer: 80 Tg N/yr•Fossil fuel burning: > 20 Tg N/yr• N fixing crops: 40 Tg N/yr

Altered N Cycle (OUTPUTS)•Land clearing/cultivation new lands: 20 Tg N/yr•Drainage wetlands and oxidation: 10 Tg N/yr• Total oxidation of N from all ag soils in world (sum)= ~4000 to 5000 Tg N

Atacama Nitrates: Major World Source Until 20th Century and Geological Enigma

Desert PavementGypsum/anhydrite prisms

Na2SO4 and nitrates

Humans have doubled the N inputs to Earth

Human impact varies with N form:

Ecosystem Response to Increased N Inputs

• Increased ecosystem productivity (areas with N limitation)

•Increased C sequestration (up to 1.3 Gt C estimated)

•N saturation

• increased NO3 leaching from soils/rivers

•Changes in species composition (loss of biodiversity)

•Decline in productivity

•Loss of Ca and Mg

•Increase in Al

•NE US, Europe

N in Rivers

N Effects on Biodiversity: Minnesota

Characterists of N Saturated Ecosystems