Abstract
Burning
of the vegetation in the African savannahs in the dry season is
widespread and may have significant effects on soil chemical and
biological properties. A field experiment in a full factorial randomised
block design with fire, ash and extra grass biomass as main factors was
carried out in savannah woodland of the Gambella region in Ethiopia.
The microbial biomass C (Cmic) was 52%
(fumigation–extraction) and 20% (substrate-induced respiration) higher
in burned than unburned plots 12 d after burning. Both basal respiration
and potential denitrification enzyme activity (PDA) immediately
responded to burning and increased after treatment. However, in burned
plots addition of extra biomass (fuel load) led to a reduction of Cmic
and PDA due to enhanced fire temperature. Five days after burning,
there was a short-lived burst in the in situ soil respiration following
rainfall, with twice as high soil respiration in burned than unburned
plots. In contrast, 12 d after burning soil respiration was 21% lower in
the burned plots, coinciding with lower soil water content in the same
plots. The fire treatment resulted in higher concentrations of dissolved
organic C (24–85%) and nitrate (47–76%) in the soil until 90 d after
burning, while soil NH4+–N was not affected to the same extent. The increase in soil NO3−–N but not NH4+–N
in the burned plots together with the well-aerated soil conditions
indicated that nitrifying bacteria were stimulated by fire and
immediately oxidised NH4+–N to NO3−–N. In the subsequent rainy season, NO3−–N and, consequently, PDA were reduced by ash deposition. Further, Cmic
was lower in burned plots at that time. However, the fire-induced
changes in microbial biomass and activity were relatively small compared
to the substantial seasonal variation, suggesting transient effects of
the low severity experimental fire on soil microbial functioning.
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