Current status, uncertainty and future needs in soil organic carbon monitoring
Currently, several efforts aimed at creating global soil C maps are underway. Conceptually, the approaches are different, as the SOC pool can be derived from soil morphological properties, or from measured SOC concentrations. The first approach is based on a higher density of data. Since the primary intention is the creation of a global soil map the derived soil C map can be seen as a secondary product. Because the displayed SOC stock is already an approximation, such maps are unsuitable for the assessment of SOC changes. However, they can – together with other elements of information – yield relevant information on potential hot-spots for SOC changes.
The second approach can yield more accurate SOC pool maps, if additional parameters, such as soil bulk density and rock content are available in a similar level of quality. In cases where the uncertainty of the SOC pool estimate is available, the assessment of SOC changes is possible.
Ideally, efforts between the soil mapping communities are joined. It takes a committed community of soil experts that is furnished with the required infrastructure and funding to accomplish the goal. Unlike in many other fields, earth observation systems are so far not offering a remedy. The required data are collected in programs where field work is having a major role. In recognition of these challenges the efforts of creating soil C maps are far from being completed. There are still strong discrepancies in the regional coverage of SOC information and combining information from regions with a high density of measured data with almost uncharted regions is a challenge. An important advantage of the maps already presented is to highlight which regions the most relevant information gaps exist.
A global SOC monitoring program needs to be driven by a clearly formulated need. A pre-requisite is the identification of regions where SOC changes are most likely, and of regions where the expected changes have a relevant extent. Therefore, a logical starting point is to focus on regions that at present have high SOC stocks. The second step is the identification of the main processes leading to SOC changes. These may be regions with accentuated rates of land-use change, or regions that are expected to be most strongly affected by climate change.
Another topic is the focus on lab procedures. A global SOC monitoring program needs standardized lab protocols. Presently, many methods for the analysis of C concentrations of the bulk soil or its fractions are available and all have their undisputed merits. With hindsight on the decentralized data generation it is feasible to suggest ISO-standardized methods for the measurement of the C concentration and to document them in a manual.
In a similar fashion modeling efforts need to be evaluated. Whereas the development of sophisticated models is well justified for the application in a regional context, the global soil C monitoring needs to rely – for the time being – on simulation models that can be parameterized with standardized input data that are globally available. All modeling exercises need to be validated with ground truth data. The existing LTSEs can have a decisive role for the quantification of the effect of soil processes leading to SOC changes, and for the validation of modeling results. In the last years significant progress has been made in assessing global SOC pools and in assembling comprehensive SOC databases.
However, in order to meet the information needs of the 21st century, an internationally organized and globally acting SOC network such as the International Soil Carbon Network will be required.
Besides its scientific challenges the establishment of a global SOC network is demanding with respect to the secondment of infrastructure and manpower. It requires a strong commitment by leading soil science institutions and the readiness for joining forces and overcoming institutional barriers. Establishing a global soil monitoring network will be a long and tedious process with many highs and lows. It will require the endurance of the involved scientists, and continuous funding in order to scrutinize and integrate emerging information in the already existing systems. The expected benefit of a functional global SOC information system will be tremendous, equally for policy makers, land managers, and the scientific community.
Adapted by Bruno Ferraz