N cycling 1999: tropics and temperate on its head

In 1999, two papers were published that began to frame global patterns of soil 15N. One was Handley's. Although they highlighted precipitation, their major significant pattern was latitude.

Martinelli et al also published a paper on global patterns of soil 15N in 1999. Like Handley, they, too, highlighted the importance of latitude. But for one detail, they were exactly the same. The best descriptor for the two results?

Opposite.

Martinelli compared ecosystems latitudinally by comparing temperate and tropical forests.

The authors initial statements were rooted in the idea that tropical forests are less N limited than temperate forests:

--"A number of lines of evidence suggest that N in most tropical forests is relatively more available than is N in most temperate forests. "

--"On average, more N circulates annually through lowland tropical forests, and does so at higher concentrations, than through temperate forests."

--"Emissions of N-containing trace gases are also higher, both absolutely and as a fraction of
N circulating through forests"

--"Comparable data on rates of N mineralization and leaching losses are sparser, but they generally show greater rates of N cycling and loss in many lowland tropical forests."

--"Overall, these observations suggest that N functions as an excess nutrient in most tropical forests, but not in the majority of temperate forests."

There were exceptions to these patterns such as tropical white-sand soils and montane forests and temperate forests dominated by N2-fixing trees or with high anthropogenic N deposition. But the initial premise was that N was more limiting in temperate than tropical soils, causing less N to be lost through fractionating pathways.

Like Handley, Martinelli et al.'s explanations were based on work that Vitousek had been associated with. And again, the authors talk about the "openness" of the N cycle, with open N cycles losing more N through gaseous pathways.

But why the difference?

One point is that Martinelli's tropical systems were primarily from Brazil. Handley's were from  Australia and Africa.

Could the nitrogen cycle just work differently in tropical South America than the other two continents?

Which was the right pattern? Remembering Shearer's earlier work from the US that showed an average soil 15N of +9‰, did the tropics actually have higher soil 15N than temperate ecosystems? Or were they the same?

And why did the different patterns exist?

A paper a few years later would seem to provide the definitive summary of soil 15N patterns, though the why question would linger.

Understanding global N cycling patterns circa 1999: precipitation is (not) important

As soil 15N measurements accumulated, interpreting the characteristics of the N cycle that generated variation in del 15N at broad scales began to develop.

In 1999, two papers were published that began to interpret 15N signatures of soil in terms of N cycle dynamics.

The first was by Linda Handley and others, "The 15N natural abundance (d¹⁵N) of ecosystem samples reflects measures of water availability".

With a title like that, you'd think the main conclusion would be simple.

Not so.

For soils at least, the authors showed strong relationships with latitude (shown above). Relationships with precipitation existed for foliar del15N, but not soils. Although their abstract reads "“The delta15N of whole soil ...when regressed on latitude and rainfall, provided the best model of these data, accounting for 49% of the variation in whole soil delta15N.” Yet, in the text they state that their model that predicted soil 15N with latitude and precipitation "was improved by deletion of rainfall as a model variable." 

Precipitation was not a significant predictor of soil 15N, despite the title and the abstract.

Still, the authors showed that tropical soils were depleted in 15N relative to high-latitude soils. 

The authors explained these patterns based on reasoning from Austin and Vitousek 1998: openness of the N cycle.

The more important inputs and outputs relative to internal cycling, the greater the del15N.

Whether precipitation was or wasn't important would have to wait to further syntheses.

The other paper published in 1999 also drew heavily from the work from Austin and Vitousek and also showed latitudinal gradients of 15N.

Yet, their interpretations were just the opposite.

Steps to understanding global patterns in N cycling

In the progression of our understanding of global patterns of N cycling, 1999 was a big year. Two papers were published that year that began to shape our interpretations about how geographic variations in climate impacted N cycling. They did so by examining broad patterns of nitrogen isotopes in soils and leaves.

Prior to 1999, Georgia Shearer and Danny Kohl had laid out the broadest patterns of nitrogen isotopes in soils, or lack thereof. Georgia had passed away recently, and her story is worth reading. Even though she never had received a PhD, was one of the world's experts in understanding nitrogen isotopes. Danny recently explained to me:


"As I recall, we undertook this study in part in response to a paper from Bremner's lab. They reported that half of the soils they measured had a Delta 15N value below zero and half above. We had reason to be skeptical of the accuracy of the reported data, so we undertook this much larger study...However, we had a hard time thinking of a mechanism that would result in negative values of 15N in soil N) since loss mechanism tend to favor the lighter isotope, resulting in the remaining N having a delta 15N value greater than zero."

The paper that they were looking at was published in Science in 1964. When they looked at the theory, they knew that N comes in to ecosystems with a signature of 0‰ relative to the atmosphere. All the processes known leave behind N enriched in 15N, not depleted. So how could any soils be depleted?


In response, they had collected over 100 soils from 20 US states and looked at the relationships between soil 15N and climate, depth, soil pH, and land use.

The major patterns?

There weren't any.

Soil depth patterns "were not consistent".
Differences among "cropland, pasture,  uncultivated land, and forest...were not striking".
N fertilization had "no systemic effect".

In short, there was a mean (9.2‰) and a range (90% fell between 5 and 12‰)

The take-home-message was that there was little pattern to soil 15N, at least in the US.

Looking for understanding of N cycling? Look elsewhere.

Curiously, their summary stated "less than half of the variation among soils in del15N of the total N could be accounted for by differences in environmental variables or soil characteristics."

Sometimes, though, less than half is more than we had before.

What patterns did they see?

According to their correlations, soils with high del15N came from wet, hot places with high pH, low sand, high clay. And soils with high N content.

Their approach to determining the correlations was not multivariate, but their were hints that soil 15N could be used to understand broad scale patterns in N cycling.

Unfortunately, that paper led to a 20-year drought in advance. Researchers afterwards focussed on using plant 15N as an index of recently fixed N. And it took about 20 years to realize that it didn't work that well.

The shift in approach to understanding 15N came about 21 years after with the publication of work by Martinelli et al. and Handley et al. These two papers offered different views about the major  controls on N cycling that are still reverberating. More on this soon.

Quote from Shearer et al. 1978.

How much progress has their been in 35 years?

Shearer, G., D. H. Kohl, and S. H. Chien. 1978. N-15 Abundance In A Wide Variety Of Soils. Soil Science Society of America Journal 42:899-902.

More on effective ecological monitoring: how to do it right

Long-term monitoring is one of the major pillars of ecological science. Yet, conducting long-term ecological research well is not obvious nor simple. It isn't as simple as "Just repeat."

I mentioned Dave Lindenmayer and Gene Likens' book on ecological monitoring. I don't often pick up books again after reading them, but did. I thought it deserved to be distilled down a bit more in my head.

When I think of the keys to long-term research, much of what they wrote resonated with ideas I've had.

There subsections were:


Good questions and evolving questions
The use of a conceptual model
Selection of appropriate entities to measure
Good design
Well-developed partnerships
Strong and dedicated leadership
Ongoing funding
Frequent use of data
Scientific productivity
Maintenance of data integrity and calibration of field techniques

Plus a section entitled "Little things matter a lot! Some tricks of the trade":  field transport, field staff, access to field sites, time in the field, 

Out of all of those, I think there are three points where things most often go wrong/are ignored:

First, test hypotheses. Monitoring can generate luck, but it's better to have a mental model of how ecosystems work. The long-term data should be used to test that. Do not just monitor a phenomenon, but also the potential underlying determinants. Stream NO3- might be your grand response, but have competing hypotheses about the factors that could be driving stream NO3-.

Second, be outside. Field stations are amazing places. Mostly because people are together looking at complex systems. Up time observing together and down time discussing observations are essential. No great program became great with people working in isolation from one another. Automated data collection is great only if it frees us up to spend the remainder of the time outside observing.

Third, analyze data annually. Don't let it accumulate. When I was doing weekly soil CO2 flux data at Cedar Creek, I analyzed the data that night. Just to see what the pattern was and make sure nothing went bonkers that day. It takes practice to generate (new) hypotheses and be ready for surprises. Groups need to come together to compare trends frequently. If you are not analyzing and discussing your data annually, you're not doing it right. Long-term data analysis is a process, not an event.

Some advances are data driven. Others wisdom-driven. To improve long-term research, go no further than these three points.

Summer Reading: Elixir--a history of humankind and water

Last summer reading for the year.

To start, the title is horrible. This is a book about how mankind, over the past 5000 years has harnessed water for civilization's purpose. It's not about sweetened, alcoholic medicines. Just call it Water. You don't write a book about water and title it "cough syrup".

After cover, the book got better. It's a global survey of human waterworks. How people have harnessed  water for irrigation, sanitation, flood control, even war. Africa, Australia, Peru, China, Greece, Britain, Mexico. It's an amazing cross-section of history. It draws on Fagan's knowledge of archaeology and climate well.

Why read it?

The reviewers were impressed, but you could tell they weren't sure. It's not prescriptive for future society. But it's a rich toolbox from which to draw. To learn about qanats is to appreciate the constraints on Persian society and the great lengths people went to irrigate. It's also clear how much work it was to maintain these systems and why they would have been a key to the organization of civilization. And how incredibly complex and well-organized societies were thousands of years ago.

You also learn a bit better how droughts and floods would have crushed societies, but also how societies buffered themselves against them, too.

From Fenton: On Jealousy

Fenton's essay, "A lesson from Michelangelo" came out in the New York Review of books in 1995. I must have read it my first year of graduate school. The essay was an amazingly condensed survey of the "humanity of ambition" in the artistic world. Every scientist should read it.


Reading Fenton means knowing what it is to "pull a Giambologna", or the pain of "pasquinades" that apply to papers and grants as much as they did critiques of art.

The personalities and psychology of Michelangelo and Leonardo, Wordsworth and Keats...are the best roadmap for understanding the myriad psychologies of how scientists interact and how they express their ambition.

We all have ambition, but it can be expressed in many ways.

Michelangelo set fire to most of his works as his death drew close. Weaver pulled up most of his plot markers.

Leonardo could barely be bothered to sign his works. The great ecologists might sign their papers, but make little effort to control the fate of their ideas, no less require attribution.

Fenton's discussion of Auden is one of his most important lessons:

"Auden wrote a wonderful thing to Stephen Spender in 1942--it is quoted in Auden's Juvenilia --when he said: 'You (at least I fancy so) can be jealous of someone else writing a good poem because it seems a rival strength. I'm not, because every good poem, of yours say, is a strength, which is put at my disposal.' And he said that this arose because Spender was strong and he, Auden, was weak, but this was a fertile weakness."

To view your rival's strength as being at your disposal is one of the greatest propellants in science.