Storing Nitrogen at standard temperatures

Nitrogen gas (N2) is fairly inert. While it's contribution to the greenhouse effect is minor, it will contribute significantly to atmospheric pressure. Venus, for example, has 2.7 times as much N2, by mass, in it's atmosphere than Earth. So if all the CO2 were removed from the Venusian atmosphere there would still be 2.4 times the atmospheric pressure at the radius. This would force Venusian colonists to live on mountain tops and high plateaus, away from the large bodies of water that would form at lower altitudes. It's therefore necessary to consider Nitrogen storage when terraforming.

Locking away N2 as metal nitrides is difficult and expensive. Storage as compressed, liquid N2 is the easiest option until free atmospheric O2 is available, and Nitrogen Fixation can take over. Nitrogen Fixation is a biological process that converts N2 into nitrogenous organic compounds. Nitrification then converts these organic compounds into an inorganic form, usually nitrate (NO3).

Clover on a terraformed Venus
A mutualistic relationship exists between clover and nitrogen-fixing bacteria. The nitrogen-fixing bacteria on the roots of clover receive protection and nutrients while providing the clover with nitrates. This relationship will allow both species to thrive in the N2/O2 rich atmosphere which will exist during the mid-terraforming stages of Venus. Decomposing clover will release nitrates to the soil where metal nitrates will form. Metal nitrates are stable but water soluable. Dissolved nitrates promote the growth of photosynthetic plants and algae.

Metal nitratesDecomposition °C
Strontium nitrate1100
Cesium nitrate849
Lithium nitrate600
Barium nitrate>592
Thorium nitrate500
Lead nitrate470
Silver nitrate>440
Thallium nitrate>430gas
Potassium nitrate400
Sodium nitrate380
Magnesium nitrate330
Rubidium nitrate>310
Ammonium nitrate210
Copper nitrate170
Aluminium nitrate150
Cerium nitrate150
Nickel nitrate>137gas
Cadmium nitrate>132gas
Calcium nitrate132
Lanthanum nitrate126
Beryllium nitrate125
Zinc nitrate105
Chromium nitrate100
Cobalt nitrate100
Iron nitrate100
Yttrium nitrate100
Uranium nitrate100