Several studies have attempted to determine the age of the carbon being respired from soil if ambient temperatures increase. Basic thermodynamics predicts increased temperature sensitivity of the older organic matter, because the energy of activation is higher to begin with and as temperature is increased the chances of decomposition increase. The increased temperature allows the microorganisms’ enzymes to get over the high energy of activation of complex organic compounds more easily, making them more effective at decomposing the lower quality/ more complex substrates.
I am going to use enzyme assays to identify which carbon substrates are being decomposed and how decomposition of substrates responds to temperature. Extra-cellular enzymes are used by microbial communities to break down complex organic compounds into available subunits. The enzymes produced by the microbes are specific to the type of substrate they degrade, so knowing the amount of activity and how activity changes with temperature will indicate which substrates are being degraded and whether that changes as a function of temperature.
Enzyme assays will be used as indicators of substrate decomposition in response to temperature change. We are going to use enzymes that cover different levels of complexity for decomposition: glucose, cellulose, hemi-cellulose, and lignin, in order to determine temperature sensitivity for substrates of varying quality. We are going to use assays for three different comparisons:
(1) initial soil enzyme activity so that we know what was originally being decomposed and whether this varied due to land management,
(2) temperature effect on enzyme activity for a soil to see if substrate decomposition has changed,
(3) enzyme activity within a soil that has been at one temperature and increased to another to see if the type of substrate decomposition changes as temperature increases.

Meg

A general assumption in soil decomposition is that temperature affects the rate (k) of decomposition and that k increases with increasing temperature. Along these same lines it is generally assumed that the size of the different soil fractions is constant regardless of the decomposition temperature and that it is an inherent characteristic of the soil dependent on the quality of OM that is there. Microbes are generally assumed to prefer organic matter (OM) which is chemically uncomplex and more labile and that increases in temperature cause microbes to increase their rate of decomposition, but not what they decompose. Results from a soil incubation study we are currently working on seem to contradict with the above assumptions.

We have been incubating twelve different soils for over 400 days at four different temperature (5, 15, 25, 35° C) and measuring the CO2 coming off over time. We plotted respiration rate over time and used a three pool fist order kinetics model to determine the size and k of the active and slow fractions. The size of the active fraction in a particular soil was not the same size at different incubation temperatures and the size of the active fraction tended to increase with increasing incubation temperature. The decomposition rate of the active and slow fractions also tended to increase with increasing temperature, but this was much more variable. These results were consistent throughout our soils. It appears that at warmer temperatures more of the slow OM fraction is being decomposed like the labile fraction. This indicates that temperature not only affects the rate of decomposition, but also what is being decomposed.

Results from our study support the functional shift hypothesis that Dalias et al. (2003) proposes, where temperature doesn’t only affect the speed at which decomposition takes place and that carbon pools should not be defined exclusively by their chemical quality. Some of the mechanisms that Dalias uses to explain the differences he has observed are shifts in microbial community composition at different temperatures, shifts in the biochemical pathways of mineralization, or disproportional accumulation of secondary microbial products. This seems to add one more complication to the already complex process of decomposition.

Michelle

Dalias, P., G. D. Kokkoris, and A. Y. Troumbis. 2003. Functional shift hypothesis and the relationship between temperature and soil carbon accumulation. Biology and Fertility of Soils 37:90-95.

As part of an NACP project led by Steve Ogle, we plan are integrating remote sensing-based estimates of plant production (RSPP) into the Century model. The challenge of this activity is that the Century plant production (CPP) submodel is intricately linked to other components of the model: plant C and N inputs drive the soil C and N; plant produciton is limited by soil N and water availability; and plant biomass influences PET. Century was derived using CPP, but replacing CPP with anything else threatens to alter other output of interest - primarily soil C content.

Our first step is to run estimate RSPP and CPP for the same sites over the same time periods. We are in the process of doing this for experimental sites for which we also have ground-based aNPP estimates and/or other, related measurements over the course of the growing season. If the different sources of data agree closely with one another, we may simply be able to use RSPP in place of CPP. We'll want to make sure, though, that agreement is common across space and time.

If they tend to agree, our next step would be to insert RSPP in place of CPP. Century estimates potential PP based on latitude (surrogate for light input), climate, and genetic production potential (which is a funtion of crop type, and growing stages). This potential estimate is then down-regulated based on shading and water and nutrient limitations. If CPP agrees with RSPP, using RSPP will lead to reasonable amounts of N uptake and PET. If RSPP<CPP, N uptake will be too small and mineral N concentration will grow... too much. Also, plant tissue C:N would be narrower for RSPP than it would be for CPP. This would then go on to affect litter C:N and decomposition. If RSPP>CPP, the C:N ratio would widen. In extreme cases, the C:N ratio would bump up against crop-based limits. Similar conditions apply to water, though if RSPP>CPP and CPP is limited by water, greater biomass demands for water (based on RSPP) would exacerbate the difference between RSPP and CPP. We need to use the preliminary model runs to see just how often and how large these problems may be.

One interesting approach we have discussed is using data assimilation - a method in which observations model output and their associated uncertainties are used to modify state variables or model parameters. This approach has been used elsewhere and clearly demonstrates that frequent constraining data can enhance model performance. This approach, though, would still require us to figure out how to modify variables and/or parameters in the various Century sub-models to get the whole model running smoothly.

According to the BP carbon footprint calculator (click here), my family and I emit slightly more CO2 (23 tonnes/yr) than the average American family (18.58 tonnes/yr). Our emissions are from transportation (autos: ~9 tonnes/yr; airplanes: ~8 tonnes/yr), household energy use (5 tonnes/yr), and waste (<1 tonne/yr). How can we reduce these emissions: travel less or more efficiently, of decrease household energy use or use renewable energy (we've been on Xcel's wind energy waiting list for 9 months). Last week's economist had a leader about another option: buying carbon offsets. Though the article begins by explaining how carbon offsets are like indulgences (click here to see 'Sins of emission'; subscription required), it concludes rather optimistically that despite some problems carbon offset are a good idea. How much will it cost and how can I go about it? There are several web sites that will be happy to sell you emission reduction credits. One of them is carbonfund.org (click here). They sell offsets for $5.50/ton - to think that the emissions for my family could be offset for $126.5/yr! The Economist article warns that all of this offsetting business is rather nascent and that buyers should beware. But it turns out that buying credits will cost a lot less per tonne than buying a hybrid or even getting wind energy. I for one just might purchase an indulgence for my indulgence...

There is a news item in Nature (click here; subscription required; see also Agecoblog 7/25 entry) today reporting on the potential for C sequestration while building new terra preta soils. The potential is huge - if you look at how much C terra preta soils contain relative to other soils (250 versus 100 t C ha-1). The added char could come from any kind of organic waste or as a by product of biofuels. At one point the article states that all of the C emitted could be stored in terra preta nova soils (practical questions about how to get all of that stuff into the soil notwithstanding). Isn't it interesting to think of creating waste-deep repositories for char as an environmental solution!

The standard model is that leaf litter falls to the soil surface and their breaks down. During breakdown, some of it moves into the soil either as dissolved organic matter or as particulate organic matter. But there are a few lines of evidence suggesting that this isn't really the way things work. Stable isotopic labelling coupled with litter addition, removal, and switching experiments suggest that litter contributes only a small amount to belowground C (see some of Margaret Torn's recent work). A new paper in this week's Nature (Austin and Vivanco 2006) suggests that in some ecosystems almost all of the decomposition is done via photo-oxidation and that very little surface litter ever makes it to the soil. Austin and Vivanco (2006) screened out no, some (UV-B) or all solar radiation to see how it affected leaf litter breakdown on the soil surface. They found that screening UV-B and total solar radiation reduced decomposition by 33 and 60%! Again, most of the decomposition they observed was due to photo-oxidation, not breakdown by mirco- or macroorganisms. This process has been recognized, but still it runs counter to the standard model. Similar experiments at places like the CPER and analysis of big data sets like LIDET and CIDET should give us good information about how important photo-oxidation is at different sites. In the meantime, the main thing I take from this work is that if we're interested in following the carbon, we need to look belowground!

Austin, A.T., and L. Vivanco, 2006: Plant litter decomposition in a semi-arid ecosystem controlled by photodegradation. Nature, 442, 555-558.

Organic cropping systems cover less than three percent of farmland area, but acreage devoted to organic agriculture is growing by 12% per year and proceeds are growing by over 20% per year, making organic agriculture the fastest growing segment of US agriculture (Lotter 2003). In order to be certified Organic, use of genetic engineering, ionizing radiation, and application of sewage sludge are prohibited. Nutrients must be managed solely using tillage and cultivation practices, crop rotations, cover crops and animal and crop waste materials. Use of organic fertilizers will impact soil carbon stocks, but how much? How will the turnover of added carbon differ from SOM under conventional farming? Marriott and Wander (2006a, b) have recently published a couple of papers that investigate these questions. Both papers demonstrate that manure-legume- and legume-based organic systems increase soil C content across multiple fractions (whole soil [14% in 10 yrs!], POM, fPOM, iPOM). Interestingly the legume system lead to greater POM C enrichment and less POM N enrichment than the manure-legume system. I wonder how much this added OM impacts retention of mineral N...

An observation: seeing these two closely related papers with similar data from the same set of sites published in such close succession makes me wonder about our scientific system ostensibly structured to advance knowledge that simultaneously encourages scientists to parse one story into two parts.

Lotter, D. W. 2003. Organic agriculture. Journal of Sustainable Agriculture 21:59-128.

Marriott, E. E., and M. M. Wander. 2006a. Total and Labile Soil Organic Matter in Organic and Conventional Farming Systems. Soil Science Society of America Journal 70:950-959.

Marriott, E. E., and M. M. Wander. 2006b. Qualitative and quantitative differences in particulate organic matter fractions in organic and conventional farming systems. Soil Biology & Biochemistry 38:1527-1536.

Peer review is a foundation for scientific progression. Just like other interactions between people, it has its problems (cronyism, vendettas, sloth). But when peer review works well, it enables science to move forward conservatively and collectively. Email and the web have streamlined the jobs of editors, but what else might these tools lead to regarding peer review? Nature is currently running a web debate to discuss the issue (click here; subscription required).

I personally look forward to the day of more informal pre-publication via the web. The journal Biogeosciences (along with other EGU journals) has tried this using a new type of article called a 'discussion' paper. Papers are raplidly assessed for merit and, if acceptible, published on the web site. The review process is more open, with reviews posted to the web site too, with comments welcomed. After the review, the articles are published in the regular print and on-line journals. Apparently commentary on papers have been limited, so results are mixed: people seem to like the idea, but don't add much to the discussion. According to one of the Nature arcicles, this two stage, open review discourages inferior submissions.

Say you were asked to develop an index for aridity - how would you do it? I would use some measure of precipitation relative to evapotranspiration (i.e., water deficit)? What if climate data weren't available? Plant species composition is a good indicator and their photosynthetic pathways (C3 vs. C4 vs. CAM) are too. So too is 13C water-use-efficiency. What if you were looking for historical information for which there were no climate obsrevations or plant material? 18-O information from bioapatite (teeth and bones) has been used, but the relationship between 18-O and aridity is confounded because 18-O in some animals varies with aridity. An article in today's PNAS describes a the relationship between 18-O-derived aridity proxies and water deficit from 9 evaporation insensitive animals and 6 evaporation sensitive animals. There is a strong positive relationship between water deficit and the enamel-rain water 18-O difference. This information could be used to produce better aridity proxies in the region, provided the source of bioapatite could be identified. The interesting thing is that the authors can't identify a behavioral or physioligical mechanisms (panter vs. sweater, grazer vs. browser, large vs. small) to explain the difference. All in all rather obscure, but a nice sleuth job.

Levin, N.E., Cerling, T.E., Passey, B.H., Harris, J.M., Ehleringer, J.R., 2006. A stable isotope aridity index for terrestrial environments. Proceedings of the National Academy of Sciences. 103, 11201-11205.

The Amazonian black earth - Terra Preta - soils are something of a mystery. They are very rich soils (15%C compared with <2% in surrounding soils) that are unevenly distributed throughout Amazonia. The increased fertility is evident via huge improevments to nutrient supply and retention of added nutrients. The fertility doesn't seem to decline with cultivation either. The origin of Terra Preta soils is uncertain, but the current idea is that they are anthropogenic soils that were developed by some type of human influence. They could have been areas that received special attention to build soil fertility or possibly repositories for organic wastes (will outhouse sites be the Terra Preta soils of the future?). A recent short communication in SBB (Ponge et al. 2006) discusses the potential for earthworms to process charcoal to enhance soil fertility. Their research has shown that earthworms prefer mixtures of charcoal and soil mineral particles to either alone, and that the composition of the resultant products are similar to the "relative volume of components of the soil matrix, including plant tissues at varying stages of decomposition, mineral particles of varying size and nature, aggregates of varying colour, size and shape." Further, dark humus samples from modern shifting cultivation plots were mainly comprised of P. corethrurus fecal pellets. Field experiments demonstrated that introduction of earthworms enabled incorporation of charcoal and manioc peels into the soil, increasing soil fertility without cost.

So, score one more for the lowly earthworm! I for one am anxious to learn more about the qualities of Terra Preta soils.

Ponge, J.-F., Topoliantz, S., Ballor, S., Rossi, J.-P., Lavelle, P., Betsch, J.-M., Gaucher, P., 2006. Ingestion of charcoal by the Amazonian earthworm Pontoscolex corethrurus: A potential for tropical soil fertility. Soil Biol. Biochem. 38, 2008-2009.

For more info about Terra Preta soils, click here.