Miombo woodlands are tropical savanna formations dominated by genera Brachystegia,
Julbernardia and Isoberlinia and are widespread in central and southern Africa. These
woodlands are an important resource providing among other things, grazing and browse
for livestock and wildlife, medicines for local people, food like mushroom and honey
and timber for building and fuel. Understanding the structural and functional
characteristics of miombo ecosystems and the factors affecting and controlling
production is therefore important, both for their significance in understanding the
environment and also for their value to local communities. In addition to soil moisture,
herbivory and fire, soil nutrients are known to be an important determinant of savanna
ecosystems.
Nutrient dynamics have been studied at two Zimbabwean sites providing a range of
representative miombo environments - the Henderson Research Station near Mazowe
and the Mukuvisi Woodlands in Harare. The aim of the research has been to assess the
major inputs and outputs of macronutrient nutrients (principally N, P, K, Ca and Mg) in
miombo woodlands and to determine the effects of fire on nutrient cycling.
The quantities of nutrients added in rainfall and throughfall were measured during the
1999/2000 and 2000/2001 rainy seasons. Additions of mineral N in rainfall were 14.7
and 7.4 kg/ha/yr at the Mukuvisi Woodlands experimental sites for the 1999/2000 and
2000/2001 rainy seasons respectively. Mineral N added at the Henderson Research
Station experimental sites was 12.3 and 5.7 kg/ha/yr for the 1999/2000 and 2000/2001
rainy seasons respectively. Cation input in rainfall was very low at Henderson
compared to Mukuvisi, a result which can be explained by the high level of pollution in
and around Harare. The amount of cations added during the 2000/2001 ranged from 0.8
to 7.8 kg/ha/yr and 0.2 to 1.2 kg/ha/yr for Mukuvisi Woodlands and Henderson
Research Station experimental sites respectively, with K being the highest added in
rainfall at both study areas. Nutrients were altered significantly by the canopy. Most of
the N was absorbed and/or adsorbed whereas cations were significantly increased. Stem
flow was measured only at Henderson Research Station sites and nutrient additions
were very low (<0.3 kg/ha/yr) compared to throughfall.
Dominant miombo tree species were found to conserve nutrients by re-absorbing them
from senescing leaves. Phosphorus had the largest percentage withdrawn for all the
dominant tree species and this ranged from 48 to 75 % of the total P in mature leaves (in
November 1999). Total N, K and Mg withdrawn ranged from 22 to 33, 22 to 31 and 12
to 21 % respectively. Litterfall at the study sites ranged from 2.20 to 4.44 t/ha/yr.
Litterfall (<2 cm) is the largest nutrient cycling pathway in miombo woodlands,
transferring between 36.6 to 65.2 kg N/ha/yr; 5.5 to 10.2 kg P/ha/yr; 15.3 to 26.7 kg
K/ha/yr; 28.7 to 53.8 kg Ca/ha/yr; 4.9 to 8.6 kg Mg/ha/yr and 1.2 to 2.1 kg Na/ha/yr to
the woodland floor. Litter decomposition was faster at the Henderson sites where there
is evidence of high termite activity. K and Na were released fastest from decomposing
litter compared to other nutrients.
Nutrients leached from miombo soils were in the order K (1.24-2.52 kg/ha/yr) > N03_-N
(1.11-2.30 kg/ha/yr) > Ca (0.82-1.49 kg/ha/yr) > NH4+-N (0.39-0.83 kg/ha/yr) > Na
(0.28-0.54 kg/ha/yr) « Mg (0.32-0.52 kg/ha/yr). Potassium was the most easily leached
nutrient from litter and it was also found to be the highest cation in leachate collected
from 100 cm depth. Losses of N in the form of N2O were also measured and were found
to range from 0.29 to 0.60 kg/ha/yr and 0.27 to 0.62 kg/ha/yr at Mukuvisi Woodlands
and Henderson Research Station experimental areas respectively. Compared to nutrient
additions, losses through this pathway are low. Early burning resulted in loss of
nutrients N, P, Ca, Mg, K and Na from herbaceous vegetation and, from litter, only N
was lost in significant amounts.
From the study it can be concluded that rainfall is an important nutrient input.
Throughfall also contributes substantially to nutrients added to soils in miombo
woodlands especially bases. The results from this study seem to indicate that miombo
woodlands cycle nutrients efficiently with minimum losses. The internal nutrient
cycling comprising mainly litterfall is able to re-circulate the largest proportion of
nutrients. Losses through gaseous N20 emissions and leaching losses relative to the sum
of throughfall and stem flow were found to be low. Fire resulted in some nutrient losses
confirming the hypothesis that burning miombo woodlands results in significant loss of
nutrients.