Effects of prescribed fire regime on soil carbon and nitrogen in forest soils

© 2015 Dr. Hari Ram ShresthaPrescribed burning is an effective management tool to reduce forest fuel loads and associated risks of wildfire in seasonally dry forests in Australia and elsewhere around the world. Both the area and frequency of prescribed burning are likely to increase in the future to tackle the increasing threat of damaging wildfires under climate change. However, present knowledge on effects of frequent prescribed burning on carbon (C) storage and productivity of soil in Australian temperate forests is limited. This study investigated the effects of 27 years of repeated low-intensity prescribed burning in dry sclerophyll eucalypt forests in south-eastern Australia on the forest floor and surface soil, focusing particularly on organic matter (OM) and C, and the key nutrients nitrogen (N) and phosphorus (P). Four prescribed fire treatments designed as factorial combination of two fire seasons (autumn and spring) and two fire frequencies (high frequency - burning in 3-year rotation and low frequency - burning in 10-year rotation) and un-burnt Control were studied. Litter (>2 mm and <2 mm size fractions) and soil in depth increments (0-2, 2-5, 05-10, 10-20 and 20-30 cm) were analysed for OM, total C, total N, charcoal C (acid-peroxide digestion), mineral N (ammonium and nitrate), potentially mineralizable N (PMN by hot KCl extraction) and available P (Bray-1). The impacts of repeated burning on the distribution of C and N among coarse (2000-50 µm) and fine (<50 µm) soil organic matter (SOM) fractions were characterized to 10 cm depth (0-2, 2-5 and 5-10 cm increments). Overall, repeated prescribed burning affected soil physical and chemical properties; soil BD and pH were increased but soil EC was decreased, particularly in 0-2 and 2-5 cm depths. Among the burn treatments, high frequency (3-year) burning increased soil BD, soil pH was unaffected but soil EC decreased compared with low frequency (10-year) burning. In autumn burning, soil EC increased but soil BD and soil pH were not affected as compared with spring burning. Concentrations and stocks of total C and total N in litter and soil were affected by repeated prescribed burning. Total C in Litter >2 mm was not affected due to burning. Concentrations and stocks of total C were decreased in Litter 2 mm). The C:N ratios were not affected in Litter 2mm. Under high frequency burning, concentrations and stocks of total C and total N in Litter <2 mm and in soil to 30 cm depth were less than in low frequency burning. In autumn burning, concentrations of total C and total N in Litter <2 mm were higher but their stocks were less as compared with spring burning. In dry sclerophyll eucalypt forests, charcoal C stock in soil to 20 cm depth was 7.55 Mg ha-1, comprising about 12% of total C. There was a strong correlation between concentration of total C and charcoal C in