SOIL BIOLOGY & BIOCHEMISTRY Soil Scarifi cation and Wildfi re Interactions and Effects on Microbial Communities and Carbon

111 Soil Sci. Soc. Am. J. 72:111-118 doi:10.2136/sssaj2006.0292 Received 21 Aug. 2006. *Corresponding author (mary.stromberger@colostate.edu). © Soil Science Society of America 677 S. Segoe Rd. Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Soil scarifi cation is any technique that prepares a site to improve accessible seedbed and thus promotes growth of disseminated seed (Archibold et al., 2000). It may involve tilling, disk trenching, roller-chopping, blading, or mounding of inverted humus (Johansson, 1994), but it always entails the removal or mixing of the top organic layer of a soil with mineral soil. This is a common silvicultural practice used in parts of Canada, Europe, and the United States that has been found to signifi cantly improve seed establishment in numerous forest tree species (Burns, 1983; Örlander et al., 1996; Karlsson and Örlander, 2000), including naturally regenerating seedlings of ponderosa pine in the Colorado Rocky Mountains (Shepperd et al., 2006). Scarifi cation allows successful seed establishment because once the mineral soil is exposed, seedling roots can penetrate faster than if they had to pass through a thick organic layer. It can also benefi t seedlings by increasing soil temperature and aeration status, and by reducing competition between a seedling and understory vegetation for water and nutrients (Thompson, 1984; Örlander et al., 1996). It also has been shown, however, that removal or incorporation of the organic horizon from the soil surface by scarifi cation impacts soil C dynamics, nutrient cycling, and nutrient storage (Vitousek and Matson, 1985; Rauni et al., 1992; Johansson, 1994). For example, changes brought about by scarifi cation (e.g., warmer soil temperatures and mixing of the forest fl oor with mineral soil) can enhance microbial decomposition of forest fl oor residues, as demonstrated by Johansson (1994). While accelerated microbial activity could benefi t seedlings by increasing nutrient turnover rates, it could lead to reduced levels of nutrients and soil C in the long term (Wilson and Pyatt, 1984; Johnson, 1992; Örlander et al., 1996) as soluble nutrients are leached away and soil organic matter is respired to CO2. The latter is known as “priming” and occurs when relatively labile substrates (e.g., dissolved organic C in incorporated forest fl oor litter) contribute to the degradation of more resistant forms of C in soils (Bingemann et al., 1953). This effect is probably due to increased extracellular enzyme production and microbial biomass fueled by the addition of labile dissolved organic C (Shen and Bartha, 1996; Hamer and Marschner, 2002; Fontaine et al., 2004). This could explain why scarifi cation reduced total forest soil C levels by 6 to 41% in a long-term (>20-yr) study by Örlander et al. (1996). Few studies have examined the effects of scarifi cation on soil microbial biomass and community structure. Short-term studies (<5 yr post-scarifi cation) have found no effect of scarifi cation on soil microbial biomass C (Ohtonen et al., 1992) Aida E. Jiménez Esquilín Dep. of Soil and Crop Sciences Colorado State Univ. Fort Collins, CO 80523-1170