I am trying to compare soil carbon concentrations from different studies.
The problem is that most of them use different sampling depths:
0-10cm
0-15cm
0-20cm
0-30cm
...
Is there an acceptable technique to mathematically adjust concentrations to other sampling depths?
I understand that it will be a rough estimate since it depends on the distribution on C.
The concentration of C can decrease rapidly with depth, or conversely decrease only slowly...
Is it tolerable to define a model distribution to convert concentrations to subsequently make comparisons?
(Example for model distributions on the attached picture)
Dear Thomas,
I strongly suggest not to convert. Indeed soil organic carbon concentration typically decreases with increasing soil depths, but the degree varies strongly between soil types. Thus, it does not excist a general rule or equation that you can apply. However, converting carbon concentrations into carbon stocks (stock=carbon concentration*thickness of soil layer/horizon*soil density) works well and is widely accepted in soil science.
In case soil density (or bulk density) has not been reported you can apply a pedo-transfer function introduced by Post & Kwon 2000 (doi: 10.1046/j.1365-2486.2000.00308.x ). Land use change may have an impact soil mass, which you can address by applying an approach introduced by Ellert & Bettany 1995 (doi: 10.4141/cjss95-075 ).
BR Nils
Dear Thomas,
I strongly suggest not to convert. Indeed soil organic carbon concentration typically decreases with increasing soil depths, but the degree varies strongly between soil types. Thus, it does not excist a general rule or equation that you can apply. However, converting carbon concentrations into carbon stocks (stock=carbon concentration*thickness of soil layer/horizon*soil density) works well and is widely accepted in soil science.
In case soil density (or bulk density) has not been reported you can apply a pedo-transfer function introduced by Post & Kwon 2000 (doi: 10.1046/j.1365-2486.2000.00308.x ). Land use change may have an impact soil mass, which you can address by applying an approach introduced by Ellert & Bettany 1995 (doi: 10.4141/cjss95-075 ).
BR Nils
The anisotropy of soil with depth means that the soil has a unique profile. Therefore, naturally every soil property has its specific depth function. Organic matter contributes to nutrient retention and turnover, soil structure, moisture retention and availability, degradation of pollutants, carbon sequestration and soil resilience. Estimating the soil organic carbon (SOC) pool account for a soil depth of about 1 m. Deeper soil horizons, however, may have a high capacity to sequester significant amounts of SOC as the turnover time and chemical recalcitrance of soil organic matter(SOM) increases with depth. The subsoil below 1‐m depth may have the potential to sequester between 760 and 1520 Pg C. Total SOC varied between land use systems and ranged from 3.1 kg C m(-2) in croplands to 23.9 kg C m(-2) in natural forest, and average N stock ranged from 0.4 kg N m(-2) in croplands to 2.1 kg N m(-2) in natural forest. Microorganisms digest up to 90% of the organic carbon that enters a soil in organic residues. In doing so, they respire the carbon back into the atmosphere as carbon dioxide. While up to 30% of organic inputs can eventually be converted to humus, depending on soil type and climate, in Australian agricultural soils this value is often significantly less.
https://www.researchgate.net/publication/300327557_Measuring_and_Modelling_Soil_Depth_Functions?_sg=NgrOq-gDuSYX2O3grDhxj-8TqB_3nqBFCKupMB4Lhay9ffXEKIXsU47_yAOkmN1B3RHW6X6u6DJ4i2AfLsuuxDGvu0fhGRzcYhUp5qy2.88RltsFDPLTdNc6ihxv9r1CIDqHSV4qYRbAcVODGlhmsWKZ5iJLwQ_gXTisr7-cZxDRoyVpTbPzzIljfAFg7Bw
https://www2.nrel.colostate.edu/projects/irc/public/Documents/Software/Century5/Reference/html/Century/exp-c-distrib.htm
https://tigerprints.clemson.edu/cgi/viewcontent.cgi?article=3548&context=all_theses
http://eprints.gla.ac.uk/110310/
https://www.googleadservices.com/pagead/aclk?sa=L&ai=DChcSEwj6kPy_lMTiAhWrve0KHX4DBdAYABAAGgJkZw&ohost=www.google.com&cid=CAESEeD2UqkuhwfG1eFw7OqOuCib&sig=AOD64_3YnVtCEVZ_nFgFy2o86fYA1XhK_w&q=&ved=2ahUKEwie1fW_lMTiAhW1sHEKHWYYAxAQ0Qx6BAgKEAE&adurl=
I think if you have 2 depths i.e. 0-10 and 10-20 cm, you can merge as 0-20 cm for presentation or to develop any figure/table, however in result interpretation you must mention it, as Nils has mentioned the OC has strong variation with depth.
Natural soils can be more carbonated in the depth than hardly used agricultural soils with high pressure of heavy tractors. So experimented, we did find in first type until 12% organic substances in 120 cm and in second type there was 0% because the earthworms could not transport their organic products deeper than 40 cm in compacted soils.
It depends on your depth distribution, which is mainly determined by your land use (but not only). If you like to get some more insights on how depth distributions depends on land use and soil type settings, you can have for example a look at one of my papers about modelling the 3D distribution of SOC in Belgium:
Article Modelling the three-dimensional spatial distribution of soil...
But, I'm of course more than happy to have a chat. :-)
Dear Thomas,
the answer from Nils Borchard is correct and comprehensive, but in case you have additional data helping in simulation of organic C distribution with depth you could use an approach we proposed in recent paper:
Article A mixed model for landscape soil organic carbon prediction a...
No, this can not be accepted. Distribution of OC depends on the biotic activities on the surface soil and along with depth it may be decreased in normal agricultural field or increased in forest floor. So it is very difficult to represent any formula or statistical relationships between OC and depth of soil.
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