The “Nitrogen Throttle” hypothesis of primary productivity

Imagine a global change scenario. Models predict precipitation will decline for a region. Other things are likely to happen, too. For example, N deposition is likely to increase. But, with soil water determining productivity, the model returns the prediction that productivity will decline, too. As a result, ranchers will fail. Forestry will be diminished. But what if the model was structured wrong? What if soil moisture only indirectly determined productivity and N was actually more limiting? The decline in precipitation might be less important that the increase in N. The original prediction might be 100% wrong.

Globally, nitrogen is the nutrient that limits primary productivity the most. Also, secondary productivity, since protein is often more limiting to herbivores than energy. Yet, whether nitrogen or water limits productivity is still open. Places with little precipitation have little plant biomass. Yet, even vegetation in dry places still responds to N addition.

Separating between water and nitrogen limitation is complex enough that for all intents and purposes, ecologists have ceased to try. Not that it doesn’t matter any more—those that were interested in the topic have likely just stopped hitting intellectual walls. And some simple scenarios show how important it is to get the fundamentals right.

There are two major hypotheses regarding water and N limitation. First, the “Water Stress” hypothesis states that water directly limits production. As soil moisture declines, water supply to plants declines, water stress increases, and plants reduce stomatal conductance to match supply and demand and/or limit cavitation risk.

The other hypothesis is the “Nitrogen Throttle” hypothesis. As soil moisture declines, water stress increases for plants, but this is not what limits photosynthesis. Instead, microbial N mineralization declines causing N supply to plants to decline. Here, low moisture ‘throttles’ N mineralization. As a result of the lowered N supply, plants decrease their stomatal conductance in order to match C and N supplies. Under more extreme soil moisture stress, plants begin to senesce their leaves to maintain minimum growth requirements if not store N for when favorable growing conditions return.

How important is each hypothesis in explaining patterns of productivity? Hard to know, but as I’ve described before there is evidence that nitrogen throttling happens and is important. For example, why would there be soil moisture left deeper in the soil profile if plants were limited by water? Why is it that plants that can photosynthesize at -9 MPa water pressure begin to senesce at -3 MPa?

Like other competing hypotheses, proving one over the other is all but impossible. Factorial resource additions can’t solve the problem when adding water also increases N supply. In the end, it’ll be parsimony that will be tested as lines of evidence are compared.