The trajectory of nitrogen in grasslands

N concentrations for grasses from Konza's 1D watershed from 1983-2010.

Plant N concentrations might seem like another esoteric ratio, but they are the key to a number of ecosystem services. In grasslands, they determine the nutritional quality of grass for grazers, how much C plants take up, as well as how fast dead grass material decomposes. And grass that doesn't decompose fast is more likely to fuel burns later.

Whether plant N concentrations have been increasing or decreasing in grasslands is one of the greatest unknowns for modern ecosystem ecologists.

For example, Kendra's previous work had shown that N concentrations had declined by 25% in Kansas grasslands over the past 75 years. That study relied on plants collected for herbaria in Kansas over the past century.

Based on the timing of declines and what we know for other supporting evidence, the most likely explanation for the declines in plant N concentrations was that increasing CO2 concentrations have been driving down plant N concentrations.

Despite this single line of evidence, could grass N concentrations have actually been increasing?

N deposition rates have been increasing, for example. When N deposition is high enough, it's enough to eutrophy the grasslands with a cascade of effects.

To answer this question, we examined the N concentrations of grasses collected over 25 years at Konza Prairie. The grasses come from a single watershed under the same burn regime (annual burning) with no grazing during this time.

Now what trajectory the plants would take was uncertain. When grasses were first measured CO2 concentrations were 343 ppm. By 2010, they were 390 ppm. 14% higher.

Was that enough added CO2 to pick up a signal?

The quick answer was no. There were no significant declines in N concentrations (or 15N:14N ratios for that matter).

Could there be another driving factor that was offsetting the decline? We checked a lot of things. No trends in climate. No trends in species composition. No trends in productivity. No trends in water availability.

The grasslands was really similar to what it was like in 1982**.

**The Nature Conservancy's Joe Fargione's response to this was "Conservation works!" Essentially, we could hold a grassland pretty similar to what it was before.

Given the differences in results, now the rectification begins. The herbarium data suggest declines in N availability and plant N concentrations. The Konza data suggests no significant declines in either.

Is the difference time scale? Local conditions? Collection protocols?

Unknown at this point.

One thing is clear, though. Neither study supports eutrophication of the Kansas grasslands. Despite elevated N deposition, there is no indication of greater N availability.

Looking forward, the future of the grasslands is still uncertain for so many reasons.

For those grasslands that are preserved, whether N availability, plant N concentrations, and forage quality will decline is a question that only further monitoring and testing will be able to answer.

McLauchlan, K. K., J. M. Craine, J. B. Nippert, and T. W. Ocheltree. 2014. Lack of eutrophication in a tallgrass prairie ecosystem over 27 years. Ecology 95:1225-1235.