Soil Science Journal Club : carbon

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.

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.

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.

the elusive humic substances

Andrew.Rate — Wed, 25 Jul 2007 08:10:00 GMT

Article for 23 April 2007:
Kelleher BP, Simpson AJ, 2006. Humic substances in soils: are they really chemically distinct? Environ. Sci. Technol. 40, 4605-4611.

For a long time it has been assumed that the humic substances are a distinct class of organic materials in soils and other natural systems, many workers believing them to be poorly ordered macromolecules formed by polycondensation reactions of a diverse range of biological molecules. Kelleher and Simpson's article suggests that this may not be the case, potentially changing our understanding of the largest pool of carbon in terrestrial environments. Their findings are based on advanced applications of NMR spectroscopy (¹H and ¹³C 2-D techniques, mostly; see Figure at right).

The main conclusion of the article seems to be that humic substances are mainly very complex mixtures of known compounds, rather than a distinct category in their own right. There are some disclaimers, of course; the authors allow for the possibility that distinctly "humic" macromolecules exist in amount below the fairly coarse detection limits of the NMR techniques, and also admit that older organic matter may contain chemically distinct substances.

One issue (at least) was unresolved, and confusing to us: if discrete biological compounds make up most of the humic mixture, than why do humic substances seem to be resistant to decomposition in soils?

Kelleher and Simpson's conclusion may be very important, though. As they point out, the dynamics of terrestrial carbon are critical to understanding global carbon cycles, a key to anthropogenic climate change scenarios. The chemical nature of the carbon compounds involved is a major determinant of carbon dynamics, so we do need to know what humic substances actually are.

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

Kudos for the humble clay

Andrew.Rate — Tue, 03 Jul 2007 05:42:00 GMT

Article for 7 May 2007:

Kennedy M, Droser M, Mayer LM, Pevear D, Mrofka D. 2006. Late Precambrian oxygenation: inception of the clay mineral factory. Science 311, 1446-1449.

Quote: "...the advent of soils sufficiently biotic for clay formation likely predated complex terrestrial ecosystems."

Many articles emphasise the importance of soils, but perhaps not in quite as spectacular terms as this one, which suggests that the intimately linked co-evolution of early life and soils on Earth allowed the subsequent development of complex organisms and their ecosystems.

The mechanism for this begins with the intense weathering of rock materials, facilitated by [micro]organisms, to form phyllosilicates - the "clay" minerals. Clays have been well-known for some time to stabilise organic compounds in soils and sediments against decomposition, due to adsorption of organic compounds, creation of microporosity, and so on. If organic matter does not decompose aerobically, then oxygen is not consumed and the net global effect of these processes was to allow oxygen concentrations in the Earth's atmosphere to increase to values allowing the development and survival of higher organisms.

The novelty of the ideas in this paper are those of scale - taking phenomena which have been well-established in disciplines like soil science and considering their implication for the evolution of the Earth system, over the last billion years or so. Their ideas are supported by their data - phyllosilicate contents of sedimentary rocks (normalized to quartz) certainly increase from near-zero at 800 Ma towards the 500 Ma mark, in three stratigraphic sequences from Australia, Baltica and China. Marine sulfate concentrations and strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) also increase over this time frame, consistent with the assumed increase in atmospheric O₂.

Further novelty, and perhaps controversy, is in the authors' own admission that this represents a non-uniformitarian approach to Earth system development. Our group is mainly composed of non-geologists, and presumably such approaches exist elsewhere in contemporary geology (not surprisingly, we also had some trouble understanding the significance of strontium isotope ratios).

Some unanswered questions:
- What was consuming O₂ prior to the stabilisation of organic matter by clays, if significant life had not yet developed on earth (and presumably, organic carbon stocks were low) ?
- Are three stratigraphic sequences enough?

Transmission electron microscope image by Dr Jian Li, CSIRO Minerals

Coffee and charcoal

Andrew.Rate — Thu, 30 Nov 2006 04:12:00 GMT

The UWA club barista was happy to sell coffee to non-members on the promise that a card-carrying one would soon arrive, and so our first meeting began.

After the inevitable housekeeping issues were covered, we were discussing:
Marris, E. 2006. Putting the carbon back: Black is the new green. Nature 442:624-626 (with supporting information from Glaser et al. (2001) below). The main thrust being the terra preta soils of Amazonia, and charcoal additions to soils in general.

It turned out to be a surprisingly small world. Natalie, who has worked in Amazonia, has a colleague who had worked with some of the terra preta enthusiasts mentioned in Ms Marris' article. She mentioned that this sort of issue was "huge" in South America, and confirmed the glassy-eyed enthusiasm of the terra preta crowd.

With global carbon budgets in mind, most of us were impressed with the idea of a carbon-negative process (making soils a net carbon sink, away from earth's atmosphere) that both provides usable fuel and improves soil characteristics.

We debated whether the terra preta phenomenon could be duplicated elsewhere, for example in Australian environments (or Chinese, or anywhere non-tropical) with lower primary productivity. Or, whether terra preta formation could be induced in the short term, given that these soils formed over centuries or millennia. Related to this was whether charring different materials would produce a charcoal with similar properties - and exactly what were the key properties of this stuff, anyway? A comparison was made with soil amendments like zeolites which had no carbon-cycle impact. We wondered about the sorption properties of charcoals in soils for nutrients, trace elements and organic compounds, and how these properties might change during pedogenesis.

The manufacture of charcoal by modern terra preta proponents approximates an efficient cycle by not being optimal for any one component. We wondered whether the same principle could be applied to other systems, like agricultural production.

We wondered and pondered over many things - with no clear answers. We asked excellent questions, though. We'll try to do it again at our next meeting.

PS there is a conference on this stuff coming up soon. See http://www.iaiconference.org/ - commercial entities openly trying to develop systems to mimic terra preta soils.