Soil properties and combustion temperature: Controls on the decomposition rate of pyrogenic organic matter

Pyrogenic carbon (PyC) is the material left behind after incomplete combustion of biomass, including a spectrum of materials ranging from charcoal to lightly charred biomass. We investigated the roles of combustion temperature and properties of soil at two distinct landform positions on carbon dioxide (CO2) flux from soil and decomposition of PyC. Bark from Pinus jeffreyi was charred at three temperatures (200 °C, 350 °C, and 500 °C) for an hour to create PyC before it was incubated with soil collected from eroding and depositional landform positions along the same hillslope. We determined the fraction of PyC in soil C at the beginning and end of the incubation using the Kurth-MacKenzie-Deluca digestion method. We found rates of microbial respiration in soil from the depositional landform position were over three times higher than from soil from the eroding landform position. The soil from the eroding landform position had less optimal decomposition conditions, including water holding capacity and organic matter content, compared with the soil from the depositional landform position. Furthermore, over the length of the incubation experiment, PyC concentrations decreased only in depositional soils that contained lower temperature chars, indicating that low temperature PyC has shorter residence times in soils from depositional landform positions, compared to eroding ones. Using scanning electron microscopy, we also observed physical decay of the low temperature chars. Previous research has shown that PyC is preferentially eroded from hillslopes post-fire. Here we show that erosion of PyC and deposition into depositional landform positions can cause the decay rate of PyC to significantly increase, exerting a significant control on the long-term fate of PyC in the soil system.