Pergamon Soil Biol. Biochem. Vol. 21, No. 2, 229-240, 1995 pp. 0038-0717(94)00140-5 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP C o pyrig ht 0 1995 ElsevierScienceLtd Printe d in G re a t Britain. All rights reserved 0038-0717/ 95 $9.50 + 0.00 MICROBIAL COMMUNITY STRUCTURE AND pH RESPONSE IN RELATION TO SOIL ORGANIC MATTER QUALITY IN WOOD-ASH FERTILIZED, CLEAR-CUT OR BURNED CONIFEROUS FOREST SOILS ERLAND RAATH,’ ASA FROSTEGARD,’ TAINA PENNANEN2 and HANNU FRITZE’* ‘Department of Microbial Ecology, Ecology Building, Lund University, Helgonavlgen 5, 223 62 Lund, Sweden and 2Department of Forest Ecology, Finnish Forest Research Institute, P.O. Box 18, 01 301 Vantaa, Finland (Accepted I8 June 1994) Sunun~ry-Humus phospholipid fatty acid (PLFA) analysis was used in clear-cut, wood-ash fertilized (amounts applied: 1000, 2500, and 5000 kg ha-‘), or prescribed burned (both in standing and clear-cut) coniferous forests to study the effects of treatments on microbial biomass and community structure. The microbial biomass (total PLFAs) decreased significantly due to the highest rate of wood-ash fertilization, clear-cutting, and the two different fire treatments when compared to control amounts. Fungi appeared more seriously reduced by these treatments than bacteria, as revealed by a decreased index of fungal: bactexial PLFAs when compared to the controls. The community structure was evaluated using the PLFA pattern. The largest treatment effect was due to burning in both areas studied, which resulted in increases in 16: 105 and proportional decreases in 18:206. Clear-cutting and the different amounts of ash application resulted in similar changes in the PLFA pattern to the burning treatments, but these were less pro.nounced. Attempts to correlate the changes in the PLFA pattern to soil pH, bacterial pH response patterns (measured using thymidine incorporation), or substrate quality (measured using IR spectroscopy) were only partly successful. Instead, we hypothesize that the changes in the PLFA pattern of the soil organisms were related to an altered substrate quantity, that is the availability of substrates after the treatments. IYTRODUCI’ION Atmospheric acidic deposition is known to affect soil fertility, and in many countries, liming has been used to counteract anthrppogenic soil acidification in coniferous forest soils (Derome, 1990/1991; Huettl and Zoettl, 1993). Other measures used to improve the acid neutralization capacity of forest soils are wood-ash applicatison and prescribed burning. In both cases ash is deposited on the forest floor, resulting in a pH increase in the humus layer. These three treatments, liming, wood-ash application and prescribed burning, have all been shown to affect soil microbial biomass a.nd activity (e.g. Zelles et al., 1987; Persson et al., 1989; Illmer and Schinner, 1991; PietikSiinen and Fritze, 1993; Fritze et al., 1993; B&%th and Arnebrant, 1994). Changes in the total microbial community struc- ture can be detected by analysing the ester-linked phospholipid fatty acid (PLFA) composition of the soil, since different subsets of the community have different PLFA patterns (Tunlid and White, 1992). This technique has zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA ibeen used to indicate changes in the microbial species composition after liming and *Author for correspondence Fritze@metla.fi. (e-mail : Hannu, ash application in two coniferous forest soils (Frostegdrd et al., 1993a). These changes could be related to increases in the pH of the soil due to the treatments. At the same time, a pH related change in the response pattern of thymidine incorporation at different pH values was found for the bacterial community (B%th and Arnebrant, 1994). The influ- ence of the soil pH was further implicated by simul- taneous changes in the PLFA and pH response patterns in an area where pH had increased due to alkaline dust deposition (BMh et al., 1992). How- ever, although the qualitative changes in the PLFA pattern due to the pH-increasing treatments were similar for all sites studied, the quantitative effect of the pH changes differed between the sites, which was also the case with other biological measurements (BB?ith and Arnebrant, 1993, 1994). One explanation for these site differences could be that it was not pH per se that was the reason for the altered PLFA pattern, but instead that the pH changes altered some other property of the soil that in turn affected the microbial species composition. One soil property, which has been shown to affect the biological re- sponse (measured as soil respiration rate) after lim- ing, is the C:N ratio of the soil (Persson et al., 1990/1991). A low C:N ratio is usually found in more productive sites with higher soil fertility. 229