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Title: Nutrient dynamics in miombo woodlands in Zimbabwe
Author: Wuta, Menas
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2003
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available