Fire effects on soils: the human dimension

Abstract

Soils are among the most valuable non-renewable resources on the Earth. They support natural vegetation and human agro-ecosystems, represent the largest terrestrial organic carbon stock, and act as stores and filters for water. Mankind has impacted on soils from its early days in many different ways, with burning being the first human perturbation at landscape scales. Fire has long been used as a tool to fertilize soils and control plant growth, but it can also substantially change vegetation, enhance soil erosion and even cause desertification of previously productive areas. Indeed fire is now regarded by some as the seventh soil-forming factor. Here we explore the effects of fire on soils as influenced by human interference. Human-induced fires have shaped our landscape for thousands of years and they are currently the most common fires in many parts of the world. We first give an overview of fire effect on soils and then focus specifically on (i) how traditional land-use practices involving fire, such as slash-and-burn or vegetation clearing, have affected and still are affecting soils; (ii) the effects of more modern uses of fire, such as fuel reduction or ecological burns, on soils; and (iii) the ongoing and potential future effects on soils of the complex interactions between human-induced land cover changes, climate warming and fire dynamics.This article is part of the themed issue 'The interaction of fire and mankind'.

Keywords: bushfire; climate change; land cover; prescribed fire; wildfire; wildland fire.

Figure 1.

Effects on the biological, chemical and physical properties of soil and associated temperature ranges reached near the mineral soil surface for different types of human-induced fires (slash-and-burn, underburning, pile burning and ecological burning). See box 1 for a definition of ‘human-induced’ fires. The temperature ranges provided are broad estimates. Specific temperatures and associated soil effects will depend on the characteristics of each fire and soil. Temperature scale is nonlinear. SOM: soil organic matter; PyOM: pyrogenic organic matter; SWR: Soil water repellency. Data derived from [–24].

Figure 2.

Current (a) and potential natural (b) global land cover under present climatic conditions, showing the extent of forests and woodlands (dark green), shrublands and open woodlands (pale green), grass biomes (orange), croplands and urban areas (red), croplands and mixed vegetation (pink) and bare land or ice/snow (grey). Data source: Current land cover map derived from GLCNMO2008 [36]; Global potential natural vegetation map derived from Ramankutty & Foley [37].

Figure 3.

(a) Soil erosion caused by recurrent burning for pasture generation (Asturias, northwest Spain). (b) Terra preta soil, made by addition of charcoal and ash, versus a natural oxisol in Central Amazonia (see box 2; Photo courtesy of B. Glaser). (c) Underburning in a dry eucalypt forest with very limited effect on soil (September 2014, southeast Australia). (d) High-intensity ecological burn in Jasper National Park (May 2015, Alberta, Canada; Photo by Parks Canada).