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.

Fishes of Ohio

I remember clearly over 20 years ago reading Fishes of Ohio by Milton Trautman. First published in 1957, the book is primarily a key and description to the fishes of the state.

The part I remember most was the introduction. I remember it describing Ohio when settlers first came to settle Ohio. Trautman summarized early records and painted a picture of lands with clear waters and an amazing abundance of fish.

In my mind, the abundance of fish was represented by a story that the boardwalks of Cleveland were built on the backs of fish.

I related that story the other day, but thought I should go back and find if I was remembering the details correctly. So, I purchased a copy, sat down, and started working through the 700+ page tome.

"The state of Ohio, situated in the midlands of the United States, is squarish in outline". It's not the most flattering beginning to a book, but it's a true representation of the state.

After this, Trautman describes historical accounts of Ohio. The waters were so clear that "early pioneers drank as readily from flowing streams as the did from springs." The abundances of fish are characterized, too. Before 1800, in the Maumee River "A spear may be thrown into the water at random, and will rarely miss killing one!"

After 1800, things start to go downhill.

I looked through this introduction and couldn't find the line about the boardwalk.

I went into the sections on individual fish.

In my mind, the boardwalks were built on the backs of sturgeon or maybe blue pike, a subspecies of walleye.

The section on lake sturgeon describes them being so abundant that fisherman sometimes placed them in large piles and set fire to them. Nothing on boardwalks.

Blue pike? 26M pounds caught in the 1950's, but nothing on their use as a base for sidewalks.

Google books has been no help, nor google. Nor bing.

I don't think it was in this book.

I have no idea where I read that.

Still, it was good to read through it again. Books like these just don't get written much anymore. And certainly there are few individuals left that spend more than 25 years making more than 2000 collections of fish--some half million individuals identified-- to help map the distribution of fish of a state.

Map of streams of US

I found this map of the US today. It shows all of the streams and rivers in the lower 48 states. You can find it here.

It seem like everywhere in the US, even dry places, have at least temporary streams. But not everywhere.

What fascinates me about the map are not that there are so many streams, but the regions where there aren't any.

That strip in the Dakotas is the Missouri Coteau. It's the western extent of the eastern glaciers. It looks something like this:

There really are no streams there. Pot hole lakes and well drained soil. But really no streams.

In the middle, you can also see the Sandhills of Nebraska. Again, almost no streams. At least on the surface. Almost all the water drains through the soil and feeds aquifers. 

You can also see the high plains of Texas to the south of there and south of there the coastal sand plain of Texas. Again, no streams there. 

In the northwest, there are the buttes of the Snake River valley:

Again, no streams or rivers up there.

South of the Snake River Valley is the Great Salt Lake Basin. 

Down in Florida, the Everglades are prominent.

I really have no other insight about the map, except it's an interesting way to look at the geography of the US. 

Book review: Statistics Done Wrong: the Woefully Complete Guide

How to Lie with Statistics came out in 1954. It has long been considered a classic with over a half-million copies sold.

The second edition of Statistics Done Wrong might be a true successor to the classic. The first edition, recently released, is still a good read for any scientist.

The author, Alex Reinhart, spends covers some basics about statistics and then empirical cases where statistics have been used incorrectly.

It's a good book. I learned a few things while reading it and was impressed to see that important examples from recent news were included as cautionary tales. I think most scientists should spend the time to read through this. If they don't learn anything, they should at least feel good about that. My guess is that they would.

That said, the difficulty with the book is that it isn't mature yet. The section on p-values is probably the most important, but I finished the section not quite sure what the author wanted to impress upon the reader. The reader is admonished to use ranges instead of p-values, but it just isn't clear why. As the book matures, the author should have better examples to get his points across. In contrast, his examples for base rate fallacies were mature. They were poignant and the reader should have a clear idea how to calculate what percentage of positives are likely false.

Another problem with any statistics book is that statistics is too broad to be covered in any thin volume no more than a single book could cover how to lie with language.

At any rate, Statistics Done Wrong is an admirable effort. At the very least it should set off alarm bells with researchers to ask questions about their statistics a little more deeply.

I'm looking forward to seeing what the second edition holds.

xkcd cartoon on global mammal weights

Just a bit sad....

The trajectory of nitrogen availability

It is a simple fact that N availability is rising throughout the world, likely causing a planetary boundary to be exceeded. Considering that humans have doubled global N2 fixation, it's impossible that it hasn't.

It is also a simple fact that CO2 concentrations have been rising, which likely should be causing N to become progressively more limiting.

It is also a simple fact that no one has taken the time to comprehensively address whether N availability has been increasing or decreasing in the ecosystems of the world. There are almost no time series of direct measurements of N supplies or availability to test whether N availability is going up or down.

As a result, it is unresolved as to whether N availability is increasing or decreasing in ecosystems across the world.

Andrew Elmore and Dave Nelson (with a little help from me) report in the latest issue of Nature Plants new data that looks at whether N availability is increasing or decreasing in US eastern deciduous forests.

Short answer: N availability looks to be decreasing.

Using ratios of N isotopes in wood as a proxy for N availability, Elmore et al. show that N availability has been declining in the forests they examined for some time.

That's a pretty big result.

Not only do they show this, but they also show that the declines are tied to spring phenology. Years with warmer springs have the lowest N availability.

Mechanistically, one link between phenology in N availability is that years with warmer springs have greater increases in plant demand for N than any increases in N supplies, leading to declines in N availability.

One question that arises from this work...if N availability is declining in these forests, how sure are we that we have crossed a planetary boundary for N? Are the world's terrestrial ecosystems really eutrophying?

Elmore, A. J., D. M. Nelson, and J. M. Craine. 2016. Earlier springs are causing reduced nitrogen availability in North American eastern deciduous forests. Nat Plants 2:16133.