CONVERSION OF NATURAL FOREST TO COCOA AGROFOREST IN LOWLAND HUMID GHANA: IMPACT ON PLANT BIOMASS PRODUCTION, ORGANIC CARBON AND NUTRIENT DYNAMICS A Thesis submitted to the Department of Agroforestry, Faculty of Renewable Natural Resources, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY IN AGROFORESTRY BY EVANS DAWOE MPhil. (Agroforestry), MSc. (Appropriate Rural Technology & Extension Skills) Faculty of Renewable Natural Resources College of Agriculture and Natural Resources JUNE 2009 DECLARATION I hereby declare that this work, submitted for the degree of PhD (Agroforestry) is the result of my own investigations conducted under supervision and that, to the best of my knowledge, it contains no material previously published by another person nor material which has been accepted for the award of any other degree of the university, except where due acknowledgement has been made in the thesis. Evans Dawoe ... ………..…….……. …….……………… (Student’s name) Signature Date Certified by: Prof. Semion J. Quashie-Sam ..…….…....…………. …………..………… (Supervisor) Signature Date Prof. Samuel K. Oppong ................…….…..… ..…………………… (Supervisory committee Signature Date Member) Certified by: Dr. Olivia Agbenyegah …………….…………. ……………………... (Head of Department) Signature Date ii Abstract This study was conducted to assess the effects of forest conversion to shaded-cocoa system on plant biomass, nutrient fluxes and soil physico-chemical properties along a chronosequence (forest, 3, 15 and 30-year-old cocoa farms) in the Moist Semi-deciduous Forest Zone of the Ashanti Region, Ghana. It also explored farmer indigenous knowledge and perceptions of soils and soil fertility dynamic processes. Plant biomass and above- ground organic carbon and nutrient pools significantly declined following changes in land- use compared to soil pools. Tree biomass constituted the largest pool ranging from 12.7 ±1.6 Mg ha -1 for the 3-year-old cocoa system to 209.3±33.3 Mg ha -1 in the forest. Soil Organic Carbon (SOC) in 0-60 cm soil depth did not change significantly over a 30-year period and ranged from 49.0±2.3 to 67.4±1.1 Mg C ha-1 in 3 year-old shaded cocoa system and forest respectively. SOC significantly declined only in the top (0-10 cm) soil at 3 years after conversion but recovered at 15 years. Thirty-year-old shaded-cocoa systems yielded up to 151 Mg C ha -1 primarily stored in established trees (both cocoa and shade trees) and soil pools. Total N declined only in the 10-20 cm soil depth in 3 and 15 year-old treatments but remained stable in all other soil depths across the chronosequence while available P stocks declined significantly. Soil exchangeable Ca, K and Mg stocks remained relatively stable with a tendency to improve, and cation exchange capacity (CEC) and base saturation increased more or less along the chronosequence. Soil bulk density (gm cm -3) increased significantly with increasing age of plantation only for the top 0-10 cm soil layer but did not differ among sites for similar depths. Despite the apparent stability of soil C stocks and nutrients (0-60 cm) along the chronosequence, soil quality declined under cocoa land-use at 3 years. Microbial biomass demonstrated a strong seasonal variation. However, conversion of forest did not result in a significant decline in microbial biomass. iii Mean annual litterfall and stand litterstocks differed significantly among land-uses. Litterfall ranged from 5.0 Mg ha -1 in 3-year-old cocoa to 10.4 Mg ha -1 forest systems while stand litterstocks were from 3.6 to 5.9 Mg ha -1 in 3 and 15-year-old farms respectively. Annual decomposition coefficients ( kL) were similar in cocoa systems (0.221-0.227) but greater under forests (0.354). Estimated nutrient inputs from litterfall was 4 to 165 kg ha - 1yr -1 of P and Ca respectively in 15-year-old and forest plots respectively. Turnover of fine roots was 3,591, 1,427, 2,466 and 4,066 kg ha -1yr -1 for forest, 3, 15 and 30-year-old plots respectively. Nutrient inputs through turnover of fine roots were estimated to be 16-31 kg N ha –1year –1, 2 -5 kg P ha –1year –1, 9-36 kg K ha –1 year –1, 18-47 kg Ca ha –1year –1 and 3-25 kg Mg ha –1year –1 across the chronosequence. There were significant differences in incident rainfall, throughfall and stemflow chemistry. Mean annual inputs of nutrients fluxes in incident rainfall were 5.7 kg N, 0.14 kg P, 13.6 kg K, 9.43 kg Ca and 5.6 kg Mg ha -1 yr -1. Rainfall loading or net canopy exchange was negative for total N at all sites while concentrations of P and the basic cations increased in throughfall relative to incident rainfall. Throughfall on average constituted about 95% of the total solute inputs of rainfall origin to forest floor. The mean N and P input-output balances were negative showing the system’s ‘no external input’ character. Farmers in the study had a well-developed knowledge system of their soils and related fertility processes. They derived their knowledge from observable plant and soil characteristics namely; soil color, crop yield, water retention capacity, difficulty to work soil, type and abundance of indicator weeds, leaf color or deficiency symptoms observed on crops and presence and abundance of soil macro-fauna. The qualitative perceptions of farmers matched scientific assessment of fertile or infertile soils. The results suggest the integration of local and scientific knowledge to facilitate the processes for formulating iv policies and development plans for agriculture truly participatory, gender sensitive and collaborative approaches. Enhancement farmers’ capability to adopt improved farm management and land preparation methods is required to conserve the soil and sustain long-term productivity. Key words: Litterfall, stand litterstocks, forest conversion, litter quality, nutrient fluxes, indigenous soil knowledge v Acknowledgements This research would not have been carried out without the contribution of several individuals and institutions. I would like to express my sincere thanks to Prof. S. J. Quashie-Sam for accepting the theme of my research as well as for his support, and reading through the draft as the lead-supervisor. In spite of his tight schedule, he always found time for the numerous progress meetings we had. Special gratitude to Professor S. K. Oppong member of the Supervisory Committee for his critical comments, and Professors C. Quansah and E.Y. Sarfo of the Crop Science Department of the Faculty of Agriculture who read through the first draft of the research proposal and made useful recommendations for its improvement. Thanks to Dr. Charles Oti-Boateng for reading through the manuscript and making useful suggestions. Many senior researchers from the Soil Research Institute (SRI) provided moral, academic and technical support at all stages of this study: Dr. F. Tetteh and Dr. S. Antwi advised on the soil sampling protocol, while Dr. R. Issaka shared his statistical skills with me for the data analysis. I wish to express my sincere gratitude to Research and Conference Committee of the Kwame Nkrumah University of Science and Technology (KNUST) for providing the financial support. The Teaching and Learning Innovation Fund (TALIF) also provided financial support to enable me travel to Cornell University, USA, where the study was designed and the research proposal finalized. Special thanks are due to Prof. Johannes Lehmann and Adjunct Prof. Peter Hobbs, Research Associate Dr. Dawit Solomon and graduate student James Kiyanga all of the Crop and Soil Science Department, Cornell University. I greatly benefited from their useful comments, guidance and suggestions. Special mention needs to be made of the Director of the Technology Consultancy Centre Dr. (Mrs.) Peggy Oti-Boateng for her support during the fieldwork. I do also acknowledge vi the technical support of Dr. Marney Isaac of the University of Toronto (U of T). Her support with field equipment, literature and suggestions during the thesis write-up are greatly appreciated. Professor Vic Timmer also of U of T deserves special mention. I profited so much from his profound knowledge and experience about chronosequence research. The short time he spent with me out on the research site opened a completely new understanding to me, providing useful comments and background theories on chronosequence studies. I thank the members of the Sustainable Tree Crop Programme (STCP) Farmers’ Cocoa School Groups in Amankyia, and farmers from Kobeng, Appahkrom, Seidi and Nkonteng who were my research partners for their co-operation and support during the fieldwork. I thank ‘Coach Coach’ Kwaku Oduro for assisting me with the fieldwork. I thank Messrs Senaya, Owusu-Ansah, Ofori and Owiredu all technicians of the Soil Research Institute (SRI) of Ghana for assisting in the collection of soil samples and supporting the analysis of soil, plant tissue and water samples. My sincere thanks go to Dr. Kenneth Gbeddy, the District Director, Ministry of Food and Agriculture, Nkawie and his entire staff especially Mrs. Juliana Kumatse, for their assistance and cooperation during the field identification and subsequent fieldwork. Finally, my affectionate thanks go to my wife Cecilia and our son Makafui who have made this achievement possible by their prayers and endured my long stay away from them. To them I dedicate this thesis. vii TABLE OF CONTENTS Declaration …. …. …. …. …. …. …. …. ii Abstract .... …. …. …. …. …. …. …. …. iii Acknowledgements…. …. …. …. …. …. …. …. vi Table of Contents …. …. …. …. …. …. …. …. viii List of Tables …. …. …. …. …. …. …. …. xiv List of Figures …. …. …. …. …. …. …. …. xvii List of Plates …. …. …. …. …. …. …. …. xviii List of Abbreviations and Symbols …. …. …. …. …. xix Chapter 1 Introduction .... .... .... .... .... .... .... 1 1.1 The agricultural sector in Ghana …. …. …. …. …. 2 1.2 Traditional soil fertility regenerating strategies …. …. …. 3 1.3 Soil fertility dynamics under cocoa land-use …. …. …. 4 1.4 Research problems and justification for the study …. …. …. 5 1.4.1 Low inherent soil fertility ….