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 200Pg PyC for the uppermost 2 meters. These modelled estimates indicated a clear mismatch between the location of the current PyC studies and the geographical zones where we expect high PyC stocks.
Highlights
Fires affect about 4.64 million km2 of biomass per year, corresponding to about 4% of the earth’s vegetated surface (Randerson et al, 2012)
Our aims were: (1) to calculate the pyrogenic organic carbon (PyC) stocks in soils based on published data, (2) to investigate which of the three main drivers has the largest influence on the PyC content in soil organic carbon (SOC), and (3) to show a possible application of our database, by using it in combination with other global datasets to create a global estimation of PyC contents and stocks
Based on a large literature database, we assessed the content of PyC in SOC, investigated a variety of drivers related to PyC production and ecosystem properties to explain these contents
Summary
Fires affect about 4.64 million km of biomass per year, corresponding to about 4% of the earth’s vegetated surface (Randerson et al, 2012). A major part of the carbon involved in these vegetation fires is emitted as CO2 into the atmosphere, yet recent studies suggest up to 15% of fire affected biomass (Santín et al, 2015) is converted into pyrogenic organic carbon (PyC; known as fire-derived organic matter, charcoal or black carbon, Hammes and Abiven, 2013) This PyC has particular features: high relative carbon content, high chemical aromaticity, a comparably long mean residence time in the soil ranging from decades to millennia (Singh et al, 2012) and, under certain circumstances, it may have a variety of positive effects on soil properties e.g., increasing pH, water retention capacity, or nutrient availability and the retention of pollutants (Biederman and Harpole, 2013; Crane-Droesch et al, 2013). It may either physically erode, get transported by wind or water and leave the soil system or fragment into smaller pieces (Pignatello et al, 2015) and move down the soil profile where it can age, react and alter chemically and physically before being transferred to other potential pools including rivers, oceans, or sediments (Bird et al, 2015; Santín et al, 2016)
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