Soil Moisture & Nitrogen: How Water Drives Nutrient Availability

When it comes to the availability of nitrogen, whether its source is organic or added as chemical and inorganic, water and temperature are the biggest influencers in the soil.  Oftentimes, water and temperature influence biology, namely bacteria, that affect the availability and status of nitrogen.  However, water also physically affects the status of nitrogen as well.  Let’s go through some of the mechanisms where nitrogen and water interact.

Nitrification – This process is the conversion of ammonium nitrogen cations (NH4+) into nitrate nitrogen anions (-NO3).  Bacteria (Nitrosomonas and Nitrobacter) convert the ammonium ions first into nitrite (-NO2), then into nitrate.  While soil temperature primarily drives the bacteria’s activity, soil moisture also plays a role.

This graph shows the relative activity of nitrification based on the relative soil moisture content.  The values on the X axis represent key soil moisture levels: WP = Wilting Point, the soil moisture content at which the plant typically cannot extract any more water.  FC = Field Capacity, the level at which the soil is moist but still has pores of airspace.  SAT = Saturated, the level of soil moisture where the gaps in the soil have been filled with water, leaving no room for air.

This relationship is modeled in the DSSAT crop growth model structure but is based on observations of actual dynamics.  As you can see, nitrification is maximized when the soil is at Field Capacity; it makes sense, since the Nitrosomonas and Nitrobacter bacteria are aerobic (operate in air).  At Wilting Point the nitrification is minimized somewhat, but at Saturated levels it is completely shut down; again, due to the aerobic nature of the bacteria.

Mineralization – This is the conversion of organic nitrogen into inorganic (and therefor plant-available) as ammonium.  Mineralization follows similar patterns as Nitrification; therefore, conditions that cause organic nitrogen to convert to Ammonium will also likely convert that Ammonium to Nitrate.

Denitrification – When soils become saturated for a few days, anaerobic bacteria increase and work on converting nitrate into first nitrous oxide (N2O), then as atmospheric nitrogen (N2).  Even if soils are above Field Capacity but are not completely saturated, some denitrification will take place, losing nitrogen into the air.

Leaching – Depending on the porosity and drainage of the soil, free nitrate may also be lost from the rooting zone via leaching, or downward movement of nitrate with water flow.  Water movement will take free nitrate with it where it goes.

Plant Uptake – While these are all ways in which water is involved in the loss of nitrogen, it is also necessary for the delivery into the plant.  Most nitrogen is taken up by corn plants in the form of nitrate, even though most chemical fertilizers are in the ammonium form.  However, because ammonium nitrogen is fixed to the soil colloid and doesn’t move, unless roots physically intercept the ammonium, it won’t be taken up by the plant.  Being converted into nitrate allows the nitrogen to move with the water (for good or bad), giving it a greater chance of being intercepted and taken up through mass diffusion.

Water and nitrogen go hand in hand.  Plants need water for nutrient transport and cooling, and oftentimes nitrogen (as nitrate) will be right there and go along for the ride.  Identifying loss mechanisms is important for proper timing of nitrogen applications to reduce losses.  When using irrigation, the farmer has the added benefit of adding water when they need it; it too needs proper management and timing.  Together, water and nitrogen should be managed as players on the same team, not as two different games.