Nitrogen Cycling in Soils dissolved NH 4 (aq) N 2 O + NOx (gas) NO x (gas)  15 N=? HNO 3 (gas  liquid) aerosol excess NO 3 - > 0.5  m NH 4 + < 0.5  m aerosol NH 4 + /NO 3 - (aq/s) transport deposition upwelling sea-air flux dissolved NO 3 (aq)  15 N=? NH 3 (gas) deposition with transport N 2 (gas) NO x (gas) N 2 (gas) N 2 O (gas)  15 N=1-3 upwelled NO 3  15 N=10-15 NH 3, NO x soil production N2 fixation by plants

Simplified View of Soil N Cycle Nitrate, N20, N2 Nitrate, ammonium, org N

N cycle within soil:

Model of Soil N At steady state (inputs=outputs:

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

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 15 N) –Dissolved organic N or erosion of soil organic N do not affect N isotopes of soil N Globally, the 15 N 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=10 12 g) 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

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 NO 3 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