Burning causes long-term changes in soil organic matter content of a South African grassland

Published source details Fynn R.W.S., Haynes R.J. & O'Connor T.G. (2003) Burning causes long-term changes in soil organic matter content of a South African grassland. Soil Biology & Biochemistry, 35, 677-687.
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Summary

In African savannas and grasslands, fire is frequently used by livestock farmers and wildlife managers to control bush and to remove dead and dying vegetation of low forage quality. In this study, the effects of burning native ‘southern tall grassveld’ grassland (an open savanna of Acacia sieberiana) on soil organic matter content was investigated in a 50 year field experiment burning trial at Ukulinga, a research farm near the city of Pietermaritzburg (29°24′E, 30°24′S), Kwa-Zulu Natal, South Africa.

The experiment was established in 1950. Treatments (three replicates each) were applied to plots (18.3 x 13.7 m separated by 4.5 m, in a randomized block, comprising: annual burning in winter (first week of August) or spring (i.e. after first rains of at least 12.5 mm in 24 h), biennial and triennial burning in winter, spring or autumn (April), plus no burn (control) and annual mowing.

Soil samples were collected in July 2000 in 2 cm layers down to 10 cm depth. These were analysed for carbon (C) and nitrogen (N). Plant litter and root biomass was also assessed.

Significant decreases in organic C only occurred in the 0-2 cm layer under annual and biennial winter burning and biennial and triennial autumn burning. Spring burning had less affect, attributable to much litter decomposing and/or being incorporated by soil invertebrates, prior to burns. Total N content was decreased substantially to 6 cm depth by all burn treatments (thus the C:N ration widened). Burning also resulted in increased microbial biomass C and root density in the 4-10 cm layer (attributed to decreased litter input but increased root turnover in the soil). Although organic C and total N content decreased with increasing depth, potentially mineralizable N increased (in both unburnt and burnt plots) indicating a shortage of N in these native grasslands.

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