Thinking through the sagebrush nitrogen cycle

What happens when you add urea fertilizer to a sagebrush ecosystem? This is something that I’ve been puzzling on for some time, as it pertains to some land management strategies being employed for the purpose of wildlife conservation here in Wyoming.  Here in this post will be my first attempt at describing the pathways that fertilizer might follow once added to the ecosystem.

Nitrogen entering the system as urea [CO(NH2)2] or ammonium (NH4+) can be lost as ammonia (NH3) gas. In the presence of water and the microbial enzyme urease, urea hydrolyzes to ammonia and CO2 (both gases).

This ammonia gas can be lost to the atmosphere unless it reacts with another water molecule to form ammonium and a hydroxide (OH), a source of soil alkalinity. However, in alkaline (high pH) soils where hydroxide is abundant, ammonium can combine with hydroxide, reversing the reaction back to ammonia gas. Nitrogen lost as ammonia gas is usually re-deposited to the soil surface as NH4+ within a few miles.

Ammonium can be directly taken up by plants and microbes as a nutrient. In the presence of oxygen, (and oxygen is abundant in any non-water-saturated soil, ammonium can go through the microbial process of nitrification, in which it is converted via multiple poorly-understood intermediate steps, to another plant-available nutrient, nitrate (NO3). Intermediate steps in this pathway can result in N losses from the ecosystem via emissions of NO and N2O gases.

Nitrate can be taken up by plants or microbes, can leach with groundwater in moist soils, or when soils are saturated with water and oxygen is absent (such as during snowmelt), can be converted to N2 gas by the process of denitrification. This step-wise process also releases nitrous oxide (N2O) and nitric oxide (NO) gases as side products. Escape of any of the three gases from the soil represents a loss of nitrogen from the ecosystem.

In the atmosphere, NO reacts with oxygen to form nitrogen dioxide (NO2). Both NO and NO2 (together referred to as NOx) react with volatile organic compounds (VOCs; produced naturally by plants, but also emitted in vehicle exhaust, and from paints and solvents) in sunlight to form the air pollutant ozone (O3; see image above). N2O is a greenhouse gas that is 298 times more effective in trapping heat than is CO2 (100 year global warming potential).

Wyoming big sagebrush tends to occur on soils that are slightly alkaline (have high pH), which favors the loss of urea and/or NH4+ as ammonia gas. This high pH also favors the nitrification (NH4+ → NO3) pathway. The question is which is more favored. I don’t know.

All of these pathways require at least some water, so when soils are very dry, reactants will accumulate until water becomes available, at which point there will be a large pulse of activity and emissions. Only when soils are very wet will denitrification (NO3 → N2) or coupled nitrification-denitrification (NH4+ → NO2 → N2; see image below) proceed to completetion. If soils are incompletely or patchily saturated, the denitrification process may be incomplete. Denitrification also requires a carbon source, which may be a limiting factor  in these carbon-poor semiarid soils, and along with incomplete saturation may cause the denitrification process, when it occurs at all, to be incomplete and thus promote formation and gaseous loss of troublesome compounds NOx and N2O instead of the inert N2.

Plants and soil microorganisms (particularly fungi and bacteria) compete for available soil N (NO3 and NH4+). Generally microbes are thought to be superior competitors, and plants are only able to take up what is left over after microbes meet their requirements (although there is abundant literature suggesting the story is much more complicated). However, ecological studies overwhelmingly show that when added nitrogen is tracked through an ecosystem, much more of it can be found in soil than in plants.

Plants also compete for the N amongst themselves. Introduced perennial grasses (i.e. crested wheatgrass) tend to outcompete native perennials (thickspike wheatgrass, blue grama), but invasive annual grasses (like cheatgrass) are often the best competitors of all for available N.  Sagebrush are likely similar in competitive ability to perennials. Sagebrush seedlings are not competitive and highly vulnerable to competition from both grasses and from mature sagebrush.

Perennials tend to respond by increasing the concentration of nitrogen in leaves and tissues (and likely forage quality), but not biomass or cover, while invasive annuals tend to respond to added N by increasing their biomass (and thus percent cover), which is why annuals tend to “win” by growing bigger and shading out the others. Sagebrush have been observed respond either way, or even to do both. Based on this, it seems like sagebrush should be competitive enough to use any plant-available N so long as cheatgrass isn’t present.

Putting it all together looks something like this:

Messy, huh? Luckily when we write the paper, someone more artistic than I will do the illustrations.