Monday, September 17, 2012

Global soil 15N: the most recent synthesis was 10 years ago

Soil nitrogen isotopes have the potential to elucidate long-term patterns of N cycling. In short, the ratio of 15N/14N in soil organic matter should differentiate soils that lose more N through fractionating pathways like gaseous N loss vs. those that primarily lose N through non-fractionating pathways such as organic N loss or NO3- leaching.

In 1999, there were two major syntheses that were at odds. Handley's synthesis stated that high precipitation sites had low soil 15N--except it wasn't significant. They did show good relationships with latitude. Martinelli showed that tropical forests differed in soil 15N from temperate forests, which again highlights the importance of latitude.

The only problem is that the two papers showed different results. Handley had low-latitude soils with lower 15N, Martinelli higher.

Taken at face value, relative importance of denitrification should be higher in the temperate systems if one follows the Handley data, tropical systems for Martinelli.

A few years later, in 2003, Amundson et al. conducted a new synthesis. Like Handley, the data were global and across all ecosystem types. They examined soil 15N to 10 cm and 50 cm. They also calculated "regional" averages, which diminished the potential "pseudo replication".

The synthesis regressions, shown above seemed to have solved the confusion. Latitude wasn't the driver of soil 15N, it was climate. Hot sites had high soil 15N. So did dry sites.

From this, they concluded,

"Because most undisturbed soils are near N steady state, the observations suggest that an increasing fraction of ecosystem N losses are 15N-depleted forms (NO3, N2O, etc.) with decreasing MAP and increasing MAT. Wetter and colder ecosystems appear to be more efficient in conserving and recycling
mineral N."

The paper seemed to calm the waters on patterns and interpretations. Handley's earlier results on precipitation were supported. The latitude question was resolved. Interpretations were based on careful modeling of isotopic dynamics.

Or not. 

A few problems.

First, there were still no data that supported their interpretations that low soil 15N was associated with "efficient" conservation and recycling--the "openness" argument from earlier. They should have said that cold, wet systems lose a smaller fraction of N through fractionating pathways like gaseous N loss. The N cycle could still be "open" and lose a lot of N relative to rates of cycling, just lose it through DON or NO3- for example. 

Second, they still hadn't shown that precipitation had significantly impacted soil 15N. The P value for the 10-cm soil 15N samples was 0.14. I reanalyzed their data here:

For 50 cm, the P value was 0.09. Still hardly definitive.

For global syntheses, the number of soils examined was still incredibly small. For 0-10 cm, just 85 sites. For 0-50 cm, just 47. Remember, in 1978, Shearer et al. had done over 100 soils from the US alone. 

The lack of replication isn't a personal fault of the authors--although they could have asked Georgia and Danny for their raw data**. As a discipline, although there had been major questions about global patterns of N cycling, there was never a global effort to nail them down.

**Apparently no one ever asked them for their data. Danny recently wrote me "Although I was not certain, it turns out that I disposed of all of the raw data when my office was moved recently.  You, of course, have access to the paper. I'm afraid that is all there is. I never thought for one instance that anyone would ever be interested in those original data. I am sorry that I cannot be more helpful and sorry that our tediously acquired data will not be part of your data base."

The reason that the low replication is important is that the results are still highly sensitive to a few points. If I exclude 3 points from the regression, MAT is not significant.

So, where does that leave us? 

1) No one has yet to show that high precipitation sites would differ inherently in long-term N cycling characteristics than low-precipitation sites.

2) Hot sites have been shown to have significantly higher soil 15N than cold sites, but it's tenuous.

3) The interpretations of soil 15N are still shaky. Even accepting the conventional wisdom on how to interpret soil 15N, what these patterns implicate about the N cycle have not yet been resolved.

Lastly, there is one more thing that is conspicuously absent from all of these syntheses:


The C and N cycles are tightly linked, but the syntheses have been done in absence of understanding carbon.

In a bit, I'll show that also looking at carbon has the potential to fundamentally change our interpretations of global N cycling.

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