Pyrogenic organic matter in soil: Its origin and occurrence, its chemistry and survival in soil environments

Abstract

Since geological times, vegetation fires have shaped Earth and produced charcoal, which is accumulating in sediments and soils. Only recently the role of pyrogenic organic matter (PyOM) as an important constituent of slow turning soil organic matter (SOM) has received more attention. Whereas in some environments, considerable long residence times of millennia and more were determined for PyOM, newer field and laboratory studies determined turnover times that were in the range of humified fire-unaffected SOM. In order to obtain an explanation of those contradictory observations, a better understanding of the nature of PyOM and its stability in different soil environments is needed. Therefore, after emphasizing the importance of fire and charcoal production during history, the present knowledge about occurrence, chemistry of PyOM formation and its recalcitrance is reviewed in the present work. Summarizing this information, it is revealed that PyOM can be decomposed by physical, chemical and biological means, and that its resistance against degradation in soils largely depends upon its chemistry and the environment in which it is accumulating. Comparable to humification of fire-unaffected SOM, different mechanisms including O 2-deficiency, unfavorable conditions for microorganisms, or interaction with the mineral phase seem to be involved in PyOM preservation and stabilization. Depending on soil conditions, the efficiency of the single pathway varies, resulting in a PyOM pattern which is typical for a certain soil, and it may even represent a typical feature of it. This concept could explain the varying abundance and recalcitrance of PyOM in fire-affected soils, some of which are presented in more detail within this work. In turn, the respective PyOM pattern, which could have been generated over decades and millennia, determines typical soil properties. Thus, PyOM can be seen to be actively involved in the pedogenic process, leading to soil classes such as Terra preta soils in the Amazon region, and possibly also to Chernozems in Central Europe or other Black Soils all over the world.