Our initial assessment took advantage of the large collection of soils that constitute part of the ISRIC - world soil information database (http://library.wur.nl/isric/). We obtained over 400 samples that span all major climate zones and soil types across most parts of the world (Fig. 1). Utilising the MIR/PLS technique, we developed a database that illustrates for the first time the predicted proportion of BC in this large soils dataset. We hope that this initial dataset may become an incentive for the rigorous establishment of a global BC map, data from which may then be incorporated into future IPCC reports and C cycle models.
Figure 2 shows the proportion of BC (as % of total SOC: BC%SOC) as well as total SOC contents across northern and southern hemispheres. These data show that BC in surface soils (A and A/B horizons) is ubiquitous in large parts of the world and occurs in the majority of the sampled locations. The variability was large, resulting in BC%SOC varying from over 50%
to almost 0%. The soils richest in BC occur in latitudes of 20-30° in both hemispheres, situated mostly in central and South America and southern parts of Africa. These areas correspond to tropical climates with a pronounced dry season in winter ('Aw' in the Koeppen classification). In higher latitudes in the northern hemisphere (60-70°), BC contents were also high and were associated with an increase in total SOC (Fig. 2). These areas are classified as humid-temperate, constantly moist climates (Cf in Koeppen classification). Higher SOC content at high latitudes results from cool climates and low decomposition rates, which promotes accumulation of organic matter.
Despite the high spatial variability of BC%SOC, soils with a high proportion of BC fell largely in the soil group of Vertisols. Vertisols occur worldwide in seasonal climate zones and in lower relief positions (Richardson and Vepraskas, 2000). Such areas of deposition would favour accumulation of BC through erosion and deposition. Another factor contributing to the high amounts of BC%SOC in Vertisols could be the comparably high proportion of clay (known to stabilize soil organic matter) and the type of clay (smectitic). The MIR-predicted data showed that Vertisols were amongst the soil types with the highest clay contents (average 44%). High clay content may promote retention of fine BC. However, clay alone is not a determining factor for high BC%SOC. Oxisols and Ultisols, having the highest clay content (from MIR prediction) of all analysed soil types (45%), had the lowest BC%SOC values. Oxisols, and to a lesser degree Ultisols, are mostly found in high-rainfall tropical climates, lacking seasonal rainfall (Af in Koeppen classification) and having a lower fire activity. While fire activity and BC content are both high in the southern hemisphere this relationship is not consistent and does not concur with findings in parts of the northern hemisphere where fire activity is lower, yet BC%SOC is still high (Carmona-Moreno et al.,
Thus, a combination of factors, such as climate (the necessity of a pronounced dry season to ensure fire occurrence), position in the landscape (areas of accumulation) and mineralogy (high amount of expansive clays) may be instrumental in promoting the formation and/or retention of BC%SOC in soils. As illustrated in Figure 1, net primary productivity alone did not appear to be a significant factor globally as it is highest in equatorial regions where BC%SOC was lowest.
While we show here that BC contents in surface soils are often significant, BC can also accumulate deeper in the soil. In addition, soil erosion and transport of BC may result in significant accumulation of BC in rivers, estuaries and off-shore sediments (Krull et al.,
2006).Thus, the inclusion of BC in global climate models will require a thorough assessment of BC contents not only in surface but in deeper soil horizons as well as aquatic sediments. The analyses conducted in this study indicate that methods exist to accomplish BC measurements for large areas. However, the high variability of the data indicates that broad empirical measurements and extrapolation over large areas are not sufficient for the aim of producing a global BC map. Thus, the next steps in a comprehensive assessment of global BC stocks and distribution have to include a detailed and consistent sampling format as well as a thorough assessment of the processes that control sources and sinks of BC.
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