Soil Science Journal Club : climate

Next journal club meeting - carbon in China

Andrew.Rate — Wed, 01 Jul 2009 05:41:00 GMT

The next meeting (already notified by Talitha) is on Tuesday 7 July, 1pm, second-floor lunch area, Soil Science building, UWA.

The article to be mused upon is :
Piao S, Fang J, Ciais P, Peylin P, Huang Y, Sitch S, Wang T. 2009. Carbon balance of terrestrial ecosystems in China. Nature, 458:1009-1014.

Astonishing reasoning on Greenland from U.S. scientists

Andrew.Rate — Mon, 22 Jun 2009 05:47:00 GMT

The good news (for some of the approximately 57,600 Greenlanders, at least): Greenland now has much more control over its destiny, following the decision from Denmark to allow self-rule for Greenlanders (see the article at the Sydney Morning Herald).

The not-so-good news: the following excerpt from this, and other, articles on this event is a real jaw-dropper:

"US scientists believe Greenland's northern tip is especially rich in oil and gas and they say global warming could help unlock the untapped wealth under the ice-cap and provide a solid foundation for an independent economy."

Let's just hope that these are the same ubiquitous and apocryphal "U.S. scientists" who seem to crop up occasionally in news articles to justify not-to-be-disputed points of view. Let's also hope they have sufficient integrity to be greenhouse sceptics.

It's hard to imagine a more cynical and blithely pragmatic approach to the issue of global warming, whether or not one has doubts about the anthropogenic hypothesis (which would be too large a can of worms ever to be opened on this blog).

Image from cartophilia.com

Too much carbon... in soils, now?

Andrew.Rate — Thu, 01 Nov 2007 03:21:00 GMT

Musings on:
Stewart CE, Paustian K, Conant RT, Plant AF, Six A. 2007. Soil carbon saturation: concept, evidence and evaluation. Biogeochemistry 86:19-31.

On first glance I thought that this was too obvious to be significant - if carbon input fluxes (e.g. litter fall) are increased (in a single step), then of course soil carbon will increase, but asymptotically to a new maximum, caused by establishment of a new steady state with increased losses due to mineralisation, etc.
I was wrong; this simplistic understanding assumes, as Stewart et al. point out, that every increase in input flux will eventually increase soil C content. In fact, with this assumption the relationship between input flux and new equilibrium soil carbon concentration turns out to be linear; this is what most models of soil organic carbon dynamics (e.g. CENTURY or RothC) do.

Stewart et al.'s hypothesis is that with increasing input fluxes to soil, the soil C content does not keep increasing but reaches a maximum value. Using sites having a range of annual carbon inputs (0-6 Mg C/ha/y), with treatments ranging in duration from 12-51 years, they find that the data as a whole are described best by a model that assumes that soil carbon content, or at least one pool of soil carbon, reaches a limiting value.

Clearly (if correct) Stewart et al.'s findings have implications for carbon sequestration in soils for controlling CO₂ concentrations in Earth's atmosphere (again, the authors make a point of emphasising this). Soils, then, behave the same way as reforestation/revegetation in terms of carbon sequestration (ecosystems reach steady states eventually in terms of productivity vs. loss). Soil is not an infinite sink for carbon; neither is the biomass growing on it.

It's early days before this hypothesis is fully evaluated across a range of biomes, but it needs to be taken notice of. There may be some issues with the treatment of data; it wasn't clear, for example, whether clustering of data for different sites affected the fitting procedure for the carbon saturation model. But we'd better understand stuff like this if we're to be serious about exploiting the role of soils in global carbon dynamics.

Soil image from Australian Society of Soil Science Inc.

Globally consistent nitrogen release (backlog No. 2)

Andrew.Rate — Tue, 18 Sep 2007 01:15:00 GMT

Article for 2 July 2007:
Parton W, Silver WL, Burke IC, Grassens L, Harmon ME, Currie WS, King JY, Adair EC, Brandt LA, Hart SC, Fasth B, 2007. Global-scale similarities in nitrogen release patterns during long-term decomposition. Science 315:361-364.

Finding a global pattern in nature would be a worthy achievement for any scientist, and this is what this large team seems to have done. The research itself was truly of a global scale - a ten-year study of leaf litter decomposition with sites in most of the earth's biomes.

The similarity that the authors found was that the amount of nitrogen release and immobilization (as a fraction of total litter N) was independent of climate zone. The rate of net nitrogen release was not, however, independent of climate. N release also depended on initial N content, with consistent remaining-N vs. remaining mass of litter plots for different categories of initial N content. In fact, the initial N content was found by the authors to be "the dominant driver of net N immobilization and release ... regardless of climate" (p.364).

An interesting result was obtained when comparing data for humid and arid grasslands, where a clear difference between biomes was observed. The authors suggested that this represented the importance of photodegradation as a mechanism in arid climates (seen previously, for example, by Austin & Vivanco, 2006, who we've cited before).

In a way, the existence of a global similarity or pattern owes its existence less to the comprehensive dataset, than to the way in which the authors looked at their data. That's probably a lesson for us all...

Soil carbon review (backlog No. 1)

Andrew.Rate — Mon, 17 Sep 2007 08:21:00 GMT

This one's from 21 May 2007:
Davidson EA, Janssens IA (2006) Temperature sensitivity of soil carbon decomposition and feedbacks to climate change. Nature, 440:165-173.

A useful article on the face of it, especially in light of Article 3.4 of Kyoto which allows for carbon credits on the basis of increased soil carbon storage. It provides a good summary of many of the concepts and issues relating to carbon cycling in soils...

...but we were frustrated by a few things. Not the least some omissions and errors: for example, activation energies are not, as the authors claim (p. 165), "related to the ambient temperature..." (they create temperature dependence for chemical reactions, but do not themselves depend on temperature - Wiki). The process of carbonate weathering and formation in soils was not considered, a simplification of the idea of "soil" carbon that does not necessarily make much sense given that carbonate dissolution depends on the partial pressure of CO₂. We also felt the list of factors affecting temperature sensitivities of soil organic carbon decomposition (p. 167) was perhaps incomplete. No mention was made, either, of the observations from arid ecosystems that photodecomposition of soil organic matter may be important (see Austin AT, Vivanco L 2006 Nature 442:555-558).

The temperature-sensitivity of soil organic carbon decomposition has been contentious recently, but the authors make no firm conclusion either way, nor makes any great deductive leaps in conclusion. This may be fair enough, given the complexity of the issue. And that's perhaps what the article lacks - sufficient complexity (especially in light of the omissions). It certainly makes for good undergraduate reading, and has well-drawn and informative diagrams that will no doubt find their way into numerous lectures. But cutting-edge review? - maybe not.

Soils: carbon sink or source?

Andrew.Rate — Tue, 03 Jul 2007 06:17:00 GMT

Article for 26 March 2007:

Meir P, Cox P, Grace J. 2006.The influence of terrestrial ecosystems on climate. Trends in Ecology and Evolution 21:254-260.

This is a review article that promised a stimulating discussion of the global function of soils and their response to one of the most significant (or at least newsworthy) issue of the 21^st century - Climate Change.

The past few decades have seen considerable effort from the scientific community into understanding the interactions between global carbon cycles and global climate. Much of this effort has gone into understanding the ocean-atmosphere systems (perhaps understandably, given the proportion of Earth's surface occupied by oceans) with the uncertainties about oceanic responses to projected increases in atmospheric CO₂ now estimated to be lower than for other Earth sub-systems.

The authors review short- to medium-term mechanisms by which terrestrial ecosystems influence global climate. There is no discussion for climatic effects over "geological" time scales, such as biotic-atmosphere-silicate weathering effects and feedbacks, or the influence of the clay mineral factory in soils on organic carbon stabilisation. It's a frustrating review though, identifying many areas where we have insufficient knowledge of the influence of terrestrial carbon cycling on climate, but not really pointing the way forward except to identify, in general terms, the type of information needed.

We wondered whether the scope of the problem is just too large, requiring a "theory of everything" to generate any definitive answers. Nevertheless, it's a useful review, and is well-suited as reading material for our first-year unit in Earth Systems as an introduction (or revision material) to short- to medium-term carbon cycling.

Photograph by blog author