Pyrogenic Carbon in Soils: A Literature-Based Inventory and a Global Estimation of Its Content in Soil Organic Carbon and Stocks

Abstract

Pyrogenic carbon (PyC) is considered one of the most stable components in soil and can represent more than 30% of total soil organic carbon (SOC). However, few estimates of global PyC stock or distribution exist and thus PyC is not included in any global carbon cycle models, despite its potential major relevance for the soil pool. To obtain a global picture, we reviewed the literature for published PyC content in SOC data. We generated the first PyC database including more than 560 measurements from 55 studies. Despite limitations due to heterogeneous distribution of the studied locations and gaps in the database, we were able to produce a worldwide PyC inventory. We found that global PyC represent on average 13.7% of the SOC and can be even up to 60%, making it one of the largest groups of identifiable compounds in soil, together with polysaccharides. We observed a consistent range of PyC content in SOC, despite the diverse methods of quantification. We tested the PyC content against different environmental explanatory variables: fire and land use (fire characteristics, land use, net primary productivity), climate (temperature, precipitation, climatic zones, altitude), and pedogenic properties (clay content, pH, SOC content). Surprisingly, soil properties explain PyC content the most. Soils with clay content higher than 50% contain significantly more PyC (>30% of the SOC) than with clay content lower than 5% (<6% of the SOC). Alkaline soils contain at least 50% more PyC than acidic soils. Furthermore, climatic conditions, represented by climatic zone or mean temperature or precipitation, correlate significantly with the PyC content. By contrast, fire characteristics could only explain PyC content, if site-specific information was available. Datasets derived from remote sensing did not explain the PyC content. To show the potential of this database, we used it in combination with other global datasets to create a global worldwide PyC content and a stock estimation, which resulted in around 200 Pg PyC for the uppermost 2 m. These modeled estimates indicated a clear mismatch between the location of the current PyC studies and the geographical zones where we expect high PyC stocks.

Long term change in chemical properties of preindustrial charcoal particles aged in forest and agricultural temperate soil

“…However, the smaller content of the O-rich, thermally less stable fraction of charcoal for the long cultivation times and the relative enrichment of the O-poor, thermally stable fraction might also result from the slowdown of the weathering of charcoal related to a better protection of charcoal particles by association with minerals. This is in line with the storage of large amounts of BC in some soils with large clay content and high pH (Czimczik and Masiello, 2007;Reisser et al, 2016). Even though both mechanisms are sound for explaining the relative enrichment of the thermally more stable fraction of charcoal over time of cultivation, they have opposite implications for the persistence of charcoal in cultivated soil.…”

“…Association of charcoal with minerals was shown to increase with time of cultivation, which might improve the protection of charcoal against microbial decomposition and therefore balance the lower chemical resistance of charcoal aged in agricultural soil. This assumption is consistent with the high degree of storage of BC in clay rich soils and in soils with high pH (Czimczik and Masiello, 2007;Reisser et al, 2016). Our data do not allow determining which of the two antagonist effects predominates (decrease of chemical resistance vs. increase of association with minerals) for the persistence of charcoal in agricultural soil.…”

“…According to recent estimates, soil stores ca. 200 Pg of BC in the uppermost 2 m (Reisser et al, 2016), which corresponds to ca. 10% of global soil organic carbon (SOC) stocks calculated by Batjes (2016).…”

“…Soil conditions are also expected to have an influence. For instance, preferential accumulation of BC was observed in clayrich soils (Reisser et al, 2016), which suggests that association with minerals might be an important mechanism of its stabilization in soil (Czimczik and Masiello, 2007).…”

Quantifying Changes in Total and Pyrogenic Carbon Stocks Across Fire Severity Gradients Using Active Wildfire Incidents

“…The lack of available data for PyC from a full range of fire-prone forest types has been a challenge identified in the literature, especially in regards to data needed for modeling fire and carbon dynamics (North et al, 2009;Liu et al, 2011). Important advancements in providing this muchneeded data Santín et al, 2015a;Reisser et al, 2016) led to a recent modeling study of global PyC TABLE 4 | Post-fire PyC (estimated as PyC CTR via chemical oxidation) proportions of post-fire C, C affected, and post-fire total ecosystem C for plots measured before and after fire in mixed-conifer forest type.…”