The Biogeochemistry of Nitrogen and Phosphorus Cycling in Native Shrub Ecosystems in Senegal
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AN ABSTRACT OF THE DISSERTATION OF Ekwe L. Dossa for the degree of Doctor of Philosophy in Soil Science presented on December 28, 2006. Title: The Biogeochemistry of Nitrogen and Phosphorus Cycling in Native Shrub Ecosystems in Senegal Abstract approved: Richard P. Dick Two native shrub species (Piliostigma reticulatum and Guiera senegalensis) are prominent vegetation components in farmers’ fields in Senegal. However, their role in nutrient cycling and ecosystem function has largely been overlooked. A study including both laboratory and field experiments was conducted to evaluate potential biophysical interactions of the two shrub species with soils and crops in Senegal. Carbon (C), nitrogen (N) and phosphorus (P) mineralization potential of soils incubated with residues of the two shrubs species was studied in laboratory conditions. Additionally, the effect of shrub-residue amendment on P sorption by soils was examined. Under field conditions, the effect of presence or absence of shrubs on crop productivity and nutrient recycling in soil was investigated. Another study examined shrub species effect on spatial distribution of nutrients and P fractions. Results showed shrub residues used as amendments immobilized N and P, which suggested these residues have limited value as immediate nutrient sources for crops. However, soils amended with shrub residues sorbed less P than unamended soils, indicating that when added to P-fixing soils, shrub residues could improve P availability to crops. In the absence of fertilization or when water was limiting, shrubs increased crop yield, likely through a combination of improved soil quality and water conditions associated with the shrub canopy and rhizosphere. The presence of shrubs increased nutrient-use efficiency over sole crop systems. Shrubs were shown to create “islands of fertility” that had greater C, N and P concentrations under their canopies than in open areas. Shrubs differed in their ability to modify soil properties. G. senegalensis had higher soil C and N under its canopy and lower pH than P. reticulatum. Sequential extraction of P fractions showed that NaOH-P, most of which is organic P, was the dominant P fraction in the soils. These results showed that, in the Sahel, shrubs are important components of cropping systems that should be conserved and actively managed. ©Copyright by Ekwe L. Dossa December 28, 2006 All Rights Reserved The Biogeochemistry of Nitrogen and Phosphorus Cycling in Native Shrub Ecosystems in Senegal by Ekwe L. Dossa A DISSERTATION submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Presented December 28, 2006 Commencement June 2007 Doctor of Philosophy dissertation of Ekwe L. Dossa presented on December 28, 2006 APPROVED: Major Professor, representing Soil Science Head of the Department of Crop and Soil Science Dean of the Graduate School I understand that my dissertation will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my dissertation to any reader upon request. Ekwe L. Dossa, Author ACKNOWLEDGEMENTS I wish to express my gratitude to persons and institutions for their contribution towards completion of this dissertation. I would like to thank my major advisor, Dr. Richard Dick, who provided the means for this doctoral opportunity and who guided me through my research and data analyses. My appreciation goes to my committee members Drs. Peter Bottomley, Jana Compton, Jeffrey Jenkins, Jeffrey Stone and also to Drs. John Baham, Jennifer Field, Kim Anderson and Maria Dragila for their time, attention, critical comments and technical support throughout my graduate program. Gratitude goes to many persons from the host Institution ISRA in Senegal, including Drs. Khouma Mamadou, Modou Sene, Diedhiou Ibrahima, Samba Ndiaye and Badiane Aminata. My sincere appreciation goes to Dr. Komi Assigbetse for providing laboratory facilities at the IRD in Senegal and for enabling the working environment for productive and satisfactory laboratory work. Special mention goes to Mrs. Fatou Gueye for all the effort in subtly arranging for the necessary chemicals and laboratory supplies needed for my soil and plant analyses. My appreciation extends to Dr. Saidou Sall, Omar Faye, and technicians of IRD who helped perform some of the soil analyses. Many thanks and sympathy to colleagues of the NSF project, Dr. Abel Lufafa, Dr. Fred Kizito and Sire Diedhiou for the common path we walked along and the many thoughts we shared together. Thanks to fellow graduate students and departmental administrative staff including Peggy Mullet and Tracy Mitzel for their support. My sincere gratitude goes to Joan Sandeno for proof-reading this work and assisting in many ways throughout my graduate program. Many from my family have encouraged me all the way long. Thanks to Mom, Eugenie, Henriette, Richard and Casimir for their moral support. Special attention and thoughts go to Romuald, Christelle and Naomie for their sacrifice and patience. May you find inspiration in this work. Raoul Gomez, at the end of the day, I remember the fascinating story of our life, and all mates involved. This work is dedicated to late Louis Dossa, a self-educated man who inspired me. Last but not least, thanks to the Lord Who made all things possible and “Who lifted me out of the ash heap”. CONTRIBUTION OF AUTHORS Ekwe L Dossa conducted research, analyzed data and wrote each manuscript. Dr. Richard Dick assisted with research design, data analyses and reviewed each manuscript. Drs. Khouma Mamadou, Ibrahima Diedhiou, Fred Kizito, Modou Sene, Aminata Badiane and Ndiaye A. Samba were involved in research design and data collection for manuscript 1 and manuscript 2. Dr. John Baham was involved in research design, and review of manuscript 3. Dr. Jana Compton and Dr. Komi Assigbetse contributed to study design and were involved in data analysis of manuscript 4. TABLE OF CONTENTS Page CHAPTER 1. General Introduction ................................................................................1 References...........................................................................................................5 CHAPTER 2. Carbon, nitrogen and phosphorus mineralization in soils incubated with native shrub residues in semiarid Senegal .............................................................7 Abstract ...............................................................................................................8 2.1. Introduction.................................................................................................9 2.2. Materials and Methods..............................................................................11 2.3. Results.......................................................................................................15 2.4. Discussion .................................................................................................18 2.5. Conclusions and Perspectives ...................................................................23 Acknowledgements...........................................................................................24 References.........................................................................................................25 CHAPTER 3. Crop productivity, nitrogen and phosphorus cycling in shrub-based cropping systems in semiarid Senegal ..........................................................................38 Abstract .............................................................................................................39 3.1. Introduction...............................................................................................40 3.2. Materials and Methods..............................................................................41 3.3. Results and Discussion..............................................................................45 3.4. Conclusions and Perspectives ...................................................................50 Acknowledgements...........................................................................................51 References.........................................................................................................52 CHAPTER 4. Phosphorus sorption and desorption characteristics of soils incubated with native shrub residues in semiarid Senegal ............................................................70 Abstract .............................................................................................................71 4.1. Introduction...............................................................................................72 4.2. Materials and Methods..............................................................................73 4.3. Results and Discussion..............................................................................77 TABLE OF CONTENTS (Continued) Page 4.4. Conclusions...............................................................................................83 Acknowledgements...........................................................................................83 References.........................................................................................................84 CHAPTER 5. Phosphorus fractions and nutrient patterns in two native shrublands of a semiarid ecosystem ................................................................................................96 Abstract .............................................................................................................97