Testing trends in global N cycling

Some of the best evidence that N availability has been declining globally.
Standardized patterns of wood d15N for forests across the US. From McLauchlan et al. 2017.

Because we monitor, weather all around the world, we can detect changes in global temperatures and precipitation patterns.

Because we monitor gas concentrations throughout the world, we can quantify the increases in CO2 concentrations as well as other gases like methane and N2O.

But what about the N cycle? Has N availability to plants been increasing or decreasing throughout the world?

We really have no idea. There is no global N monitoring network. We cannot tell if plants have been experiencing increased or decreased N availability over the past, say, 100 years.

Considering how crucial the N cycle is to plant productivity worldwide, it seems important to have some index of whether N availability has been going up or down.

Through different projects over the last decade or so, I've been involved in trying to reconstruct N availability over time. Most of these have involved N isotopes in one way or the other, examining patterns in herbarium samples, tree rings, and sediments.

Most of these papers have shown declines in N availability over time.

There's one study left that I never got to, though. And it's been bugging me for about 5 years.

That's seeing if we can see a trend over time in foliar N isotopes at the global scale.

Most simply, if N availability has been increasing globally, all other things equal, d15N should be increasing. If N availability has been decreasing globally, all other things equal, d15N should be declining.

The last time we synthesized global foliar 15N patterns, we stopped data collection in 2006 and had about 12,000 data points. In order to update the database, I've read through about 500 papers that were flagged as potentially having appropriate data. About 250 did. So, Andrew Elmore and I started sending out emails to assemble the new global database.

With the new synthesis, we're now over 40,000 data points assembled so far with a couple dozen more datasets left to incorporate.



Given all the variability that exists at the global scale, we'll likely need all 40,000 data points to detect any trend that might be there (or to be sure there isn't a trend).

As Andrew and I talked about how we wanted to conduct this study, we wanted to make sure that the analyses were trusted. We wanted to preclude any criticisms that, for example, we left out data that didn't fit a given desired outcome**.

**Although most of the other patterns we've uncovered support declining to stable N availability, any of the three outcomes are equally interesting and important, as long as we have strong enough analyses to be certain about the patterns.

To make this project as trusted as possible, we have pre-registered our data analysis plan with the open science framework. See https://osf.io/thnyf/#. This means all of our hypotheses and analysis plans have been declared ahead of data collection. The R code that we will use to analyze the data has also been written and posted. This pre-registration is important in that it locks in our approach ahead of time and eliminates many forms of scientific bias that can misrepresent results. This transparency will be important to creating trust in whatever results we find.

We should be finishing up data acquisition by the end of the year, given the current pace of people getting back to us (which has been pretty good--over 75% of the requests have been answered).

One neat thing about writing all the R code ahead of time is that once we have all the data, we just push the big red button, and we'll see our answer.

Something to look forward to in 2018.

McLauchlan, K. K. et al. 2017. Centennial-scale reductions in nitrogen availability in temperate forests of the United States. Scientific Reports 7: 7856.

Declining protein for cattle in the US

By last count, I had coauthored 98 scientific articles. Number 99 has just been published on-line at Environmental Research Letters. I'll wager that this one is the most important I've written**.

**It doesn't mean it is important on the absolute scale, just the relative scale...

I've said before that scientists have two jobs. The first is to create intellectual tension. The second is to resolve intellectual tension.

For global ecologists, some of the most important intellectual tension right now resides in two opposing ideas regarding nitrogen availability. Some theories suggest that global N availability is rising. Other theories suggest that it should be declining.

Given the central role of N availability in the functioning of the ecosphere, this is some of the most important intellectual tension we have to resolve. There is no doubt that excessive nutrient availability is damaging some aquatic ecosystems. Yet, in terrestrial ecosystems, whether N availability is increasing or decreasing is uncertain. The trajectory of terrestrial N availability determines how the ecosphere will respond to elevated CO2 and what types of policies we must impose. The difference couldn't be starker. If N availability is increasing, then we need to begin to limit anthropogenic N fixation. If the opposite is true, policies that limit N fixation might actually have deleterious effects.

The question about whether N availability to terrestrial plants is increasing or decreasing has suffered from a lack of data. Quite simply, there are no long-term measurements of terrestrial N availability across broad spatial scales that can be used to assess this question. We do not know the trajectory of N availability in grasslands. We do not know the trajectory of N availability in forests.

Paper #99 does not directly assess any of these, but it does one of the next best things for grasslands. It utilizes a unique long-term dataset on forage quality for cattle across the US.

I'll leave the details to the paper, but data suggest that N availability is declining in grasslands across the US. Not one or two experimental plots, but the whole of the Great Plains. Across this broad region, cattle are becoming more protein stressed as their forage is showing declines in protein. It appears that something is causing N availability to decline to plants and plants are responding by reducing their N (and protein) concentrations.

How much has protein declined over 20 years? The equivalent of a decline that causes plant N concentrations to decline about 10 mg protein g-1, or 0.3% N.

How much protein is that for US cattle? It would take about $2B in soybeans to replace all of that protein. Or about half of the soybeans produced in all of Iowa in a year.

By no means is paper #99 the final word on the topic. A fair amount of data support the thesis that N is declining in terrestrial ecosystems. Paper #100, which is still being reviewed, is going to be a major line of evidence in favor of declining terrestrial N availability.

As for this:

Let's just say that there are some intellectual tensions that aren't resolved.

I wouldn't believe it yet.