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Arme boeren in Sub-Sahara Afrika zitten vast in een vicieuze armoede cir kel omdat ze niet over de nodige middelen beschikken om voldoende te kun nen investeren in hun velden. Velen onder hen planten jaar in jaar uit z onder bemesting waardoor niet alleen hun oogsten mager zijn, maar ze hun bodems ook gradueel uitputten. Het Tropische Instituut voor bodembiolog ie en bodemvruchtbaarheid (TSBF-CIAT) introduceerde sojabonen in de regi o rond Migori (Zuid West Kenia) in 2005 om de levensconditie van arme bo eren te verbeteren. De dubbel-doel promiscue variëteiten die gebruikt we rden produceren niet alleen sojabonen voor een goede voeding en bron van inkomen maar kunnen ook de bodemvruchtbaarheid ten goede komen. Ze gebr uiken tijdens hun groei stikstof uit de lucht (i.p.v uit de bodems) en p roduceren zo grote hoeveelheden stikstofrijke bladeren die na de groei v an de plant op de bodem vallen. Zo verbeteren ze de kwaliteit van de bod ems wat een impact heeft op het volgend gewas in rotatie. Onder goede kl imaatsomstandigheden, voldoende bemesting en goed onderhoud van de velde n kunnen deze sojaplanten de volgende maïsoogst zelfs verdubbelen. Het probleem is dat arme boeren de middelen niet hebben om soja op een o ptimale manier te kweken. Het doel van dit onderzoek was om te analysere n of en hoe: 1) sojaproductie toegankelijk kan zijn voor arme boeren; en 2) de drie voordelen van sojaproductie (bodemvruchtbaarheid, voeding, i nkomen) tot uiting kunnen komen ondanks de suboptimale sojaproductiemeth odes. We analyseren tenslotte 3) de rol van samenwerking tussen boeren e n onderzoekers in het aanpassen van landbouw aan de situatie van arme bo eren. Dit onderzoek vond plaats tussen 2006 en 2008. 1) SOJAPRODUCTIE CONTEXTUALISEREN. De regio rond Migori heeft voldoende oppervlakte landbouwgrond, twee regenseizoenen per jaar, redelijke bodem s en de bevolking heeft een relatief hoog onderwijsniveau. Ondanks dit a lles leeft 49% van de bevolking in armoede (Hoofdstuk 2). De grootste be perkingen voor landbouw in de regio zijn: 1) gebrekkige toegang tot sted en, markten en goederen; 2) gebrekkige bemesting van de velden als resul taat van geldtekort; 3) tekort aan landbouw werkkrachten. Om de werkdruk en kost van meststoffen in sojaproductie te verminderen, hebben we coll aboratieve experimenten opgezet (Hoofdstuk 3). We ondervonden dat sojapl anten met lokale meststoffen (houtassen en koeienmest) evenveel konden p roduceren dan met minerale meststoffen (di-ammonium fosfaat). We bespaar den 1/3de van de planttijd door soja in geulen te planten in plaats van in individuele gaten zonder impact op de oogst. Door zaden uit te strooi en, bespaarden we 89% van de planttijd en verloren we slechts -15% oogst . Door slechts één keer onkruid te wieden in plaats van twee, bespaarden we 36% van de totale werktijd en verminderde de oogst slechts met -5%. De tijdsbesparing die deze plantmethodes opleveren kan vervolgens meer n uttig besteed worden om meststoffen te verzamelen. Deze resultaten beves tigen dat de productie van soja dus toegankelijk kan zijn voor arme boer en. 2) TOEGANKELIJKHEID VAN DE VOORDELEN VAN SOJA. Door maïs te planten na d e soja-experimenten konden we aantonen dat rotatie effecten tot uiting k omen, zelfs met suboptimale sojaproductiemethodes (Hoofdstuk 4). Na twee rotatiecycli bekwamen we zonder gebruik van meststoffen 90% meer maïsoo gst dan in een veld dat vier seizoenen met continue maïs beplant was. Ee n boer die het gewoon is maïs zonder meststoffen te planten kan dus via soja-maïs rotatie evenveel maïs bekomen in één seizoen dan wat hij norma al in twee seizoenen oogst. De soja oogst is dan een netto voordeel van het systeem. We ondervonden ook dat arme boeren sojavoedingsproducten ku nnen gebruiken als vervanging voor dure dierlijke producten (Hoofdstuk 5 ). In verband met het vermarkten van sojabonen merkten we op dat een kle inschalige arme boer een hogere productiekost heeft (0.40 US$ kg-1) dan de prijs die de Keniaanse olieindustrie in 2008 bood (0.31 US$ kg-1) (Ho ofdstuk 5). Sojaproducten die thuis kunnen bereid worden, zoals thee, no ten, melk, enz. kunnen echter verkocht worden aan de hogere prijs van 2. 10-3.54 US$ kg-1 (of een inkomen van 1190-2030 US$ ha-1). Deze producten kunnen een interessante bron van inkomen vormen voor vrouwen. Dit wijst op het belang van vormingsmateriaal dat leesbaar is door geletterde maa r ook ongeletterde vrouwen om zo deze vorm van inkomen toegankelijk te m aken voor de armste boeren. Boeren die meer financiële middelen hebben k unnen investeren in technologieën om de productiviteit van hun soja te v erhogen en zo aan een lagere kost te kunnen produceren. Dit geeft hen to egang tot grotere markten via de sojacoöperatieve of de industrie. 3) SAMENWERKING TUSSEN BOEREN EN ONDERZOEKERS. We vergeleken twee niveau s van samenwerking met boeren: de collaboratieve experimenten (CE) en de experimenten van boeren (FE). In de vier CE zones, hebben we de experim enten samen met de boeren ontworpen en geleid. De aanpak bleek efficiënt voor het identificeren van gepaste experimentele problemen en experimen tele behandelingen (Hoofdstuk 6). De samenwerking met de boeren liet ons toe om de behandelingen die niet adopteerbaar zijn voor arme boeren uit de experimenten weg te filteren zodat de behandelingen steeds beter pas ten op de lokale levencondities in de regio. De CE gaf ook een kans aan boeren om behandelingen die “archaïsch” lijken in een formeel kader te t esten zonder risico op commentaar van buren (indien ze die in hun eigen veld hadden getest). De samenwerking leidde dus tot meer aangepaste expe rimenten en tot meer aangepaste productiemethodes voor soja. In de FE zo nes, gaven we de boeren de basistraining rond het opzetten van hun eigen experimenten (Hoofdstuk 7). De experimenten van de boeren bevatten vele behandelingen die een bron kunnen zijn van inspiratie voor latere exper imenten. Tussen 2006 en 2008 vergrootte de diversiteit aan behandelingen in de sojavelden van boeren. Boeren bleven ook verder experimenteren. B oeren uit CE en FE zones hadden behandelingen uit de CE experimenten gea dopteerd in hun eigen velden, ondanks het feit dat de FE boeren nooit di rect contact hadden met de CE experimenten. We hadden hen enkel de oogst resultaten van de CE meegedeeld. We concludeerden dat de FE aanpak ertoe had geleid dat boeren meer vatbaar werden voor informatie van buitenuit en deze gingen testen in hun eigen velden. De aanpak had dus een kanaal gevormd voor het snel verspreiden van de resultaten van de CE. Tenslott e analyseerden we de evolutie van de relatie tussen boeren en onderzoeke rs tijdens de loop van het project tussen 2006 en 2008 (Hoofdstuk 8) en onderscheidden we hierin drie verschillende fases. 1) In de initiële fas e waren de boeren heel enthousiast, maar dit enthousiasme was gebaseerd op onrealistische verwachtingen; 2) hierop volgde een fase van teleurste llingen, waardoor de interesse van boeren voor het project verminderde; 3) in de derde fase onderging het project een nieuwe heropleving, gebase erd op nieuwe maar meer realistische verwachtingen. De negatieve effecte n hadden hun oorsprong in interculturele verschillen, vooroordelen en co mmunicatieproblemen. CONCLUSIES. We besluiten uit dit werk (Hoofdstuk 9) dat sojaproductie en zijn drie potentiële voordelen toegankelijk kunnen zijn voor arme boere n indien onderzoekers de nadruk leggen op technologieën die in hun conte xt toegankelijk zijn (onder andere lokale meststoffen, werkdruk verminde rende technieken, lokale recepten en lokale marktopties). Deze lokale op lossingen kunnen arme boeren de kans geven om gradueel hun inkomen te ve rhogen en zo geleidelijk aan over te schakelen naar meer productieve met hodes. We benadrukken ook het belang van “samenwerking tussen boeren en onderzoekers” om landbouwonderzoek beter te kunnen richten naar de omsta ndigheden van de boeren; “ontwikkeling met een focus op de armsten”. Om zulke samenwerking succesvol te maken moeten onderzoekers bereid zijn fr equent in contact te zijn met de boeren en de hiërarchie tussen hen en b oeren geleidelijk aan af te breken.

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Conservation agriculture, based on the combined implementation of (i) minimal soil disturbance, (ii) soil cover by crop residue retention or cover crops and (iii) crop diversification, has been identified as a sustainable improvement of conventional agriculture practices. It can decrease soil degradation and erosion, improve water conservation, enhance soil life, improve labour efficiency and decrease fossil fuel dependence. Despite these benefits, adoption of conservation agriculture among smallholder farmers has remained limited. The International Maize and Wheat Improvement Centre (CIMMYT) is investigating and disseminating conservation agriculture practices under small and medium scale maize based production systems in Mexico. In this research, eight experimental platforms were examined to evaluate the sustainability of conservation and conventional agriculture practices, based on the evaluation of the diagnostic criteria (i) soil quality, (ii) yield and (iii) profitability. The platforms are linked to CIMMYT’s conservation agriculture program and located in central and south Mexico. Physical soil parameters that were assessed are indicative for direct infiltration, aggregate distribution and stability and soil resistance to penetration. Chemical soil quality was evaluated based on the assessment of the soil pH, C/N ratio and soil organic C content. At the long-term experiment an in depth focus on plant available P was carried out, using the DGT method, that mimics diffusion processes at plant root level. Yield and profitability were determined. Zero tillage plots with crop residue retention resulted in the best infiltration. Aggregate distribution and stability did mostly improve under zero tillage with residue retention, although no positive effects of this treatment were observed at the two driest locations. Tillage with residue removal resulted in the easiest penetrable topsoil, while zero tillage with residue retention created in most platforms a topsoil with a higher resistance to penetration. At deeper layers (30-60 cm) this trend was reversed. Most significant treatment effects on the soil chemical characteristics were observed in the topsoil (0-5 cm) where both the C/N ratio and soil organic C content tended to increase under zero tillage with crop residue retention. A significant treatment effect on plant available P was observed while conclusions on the most P beneficial treatment are yet to be drawn. No treatment effect on yield and profitability was observed. The overall results indicate a net positive effect of conservation agriculture on the sustainability levels of the assessed parameters. However, not all benefits of conservation agriculture can be generalized across all assessed locations. This emphasizes the importance of continued field research on how best to adapt the general principles to local circumstances in order to increase adoption of conservation agriculture.

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Land use change is one of the major contributors to climate change. It is mainly caused by deforestation to expand the agricultural area. Since the attempts to cause a green revolution failed in sub-Saharan Africa (SSA), the large depletion in soil organic carbon (SOC) and nutrients has aggravated. This affects the yield of small-scale farmers. An approach for the agriculture in SSA to mitigate and adapt to climate change is the combined application of organic (OR) and mineral nitrogen (MR) inputs, which is a major principle of integrated soil fertility management. The combined application (OM) is hypothesized to have the largest potential to improve soil carbon sequestration. To verify this, the OR, MR and OM treatments are investigated in four long-term maize trials in Kenya. The MR is applied at a rate of 120 kg ha-1. The five OR quality classes are Tithonia diversifolia, Calliandra calothyrsus, maize stover, sawdust and farmyard manure. The classes have different amounts of polyphenol, lignin and nitrogen, which are the quality parameters of the OR. These classes are applied at a rate of 1.2 tons C ha-1 or 4 tons C ha-1. Therefore, the effect of both OR quality and OR quantity can be evaluated. The SOC stock is measured at a depth of 15 cm, 30 cm, 60 cm and 90 cm. Hence, the effect of the treatments in the subsoil can be analyzed. Additionally, the influence of clay and amorphous iron- (Feox) and aluminum- (Alox) oxyhydroxides to the SOC stock was investigated. These soil characteristics are expected to enhance the chemical stabilization of SOC resulting in better retention and, consequently, a larger SOC stock. A linear mixed effect model has been performed to evaluate the effect of treatments and soil characteristics to SOC stocks. Neither the treatment, nor the OR quality or the OR quantity turned out to be significantly contributing to increase SOC. Clay, Alox and Feox contents were significantly contributing to increase SOC due to chemical stabilization. The soils of Embu and Sidada contained a large amount of clay, Alox and Feox which resulted in a large SOC stock. The soils in Machanga and Aludeka had small amounts of clay, Alox and Feox and, consequently, small amounts of SOC.

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The highlands of the Upper Pangani Basin in Tanzania support a dense population of smallholder farmers. However, agriculture in the region is under increasing pressure due to population increase and climate change. In the future farmers must produce more food on less land under a changing and uncertain climate. Before advice can be given to farmers on how to cope, the current farming systems and their relation to the biotic and abiotic environment must first be understood. The overall objective of this thesis is to characterize the banana-based farming systems present on the southern slope of Mt. Kilimanjaro related to the different soil types present in the region. Three banana-based farming systems are characterized on the Southern slope of Mt. Kilimanjaro. In zone Up (1700 m) fields consist of a wide variety of crops and trees, and mainly food crops are grown. Thanks to abundant rainfall and fertile soils, households produce enough food to be self-sufficient year-round. The most important banana group is Mchare, which is cooked and consumed locally. In this elevation zone, an Umbric Nitisol with high organic carbon (TOC) is typical. Due to the high precipitation and weathering, generally a very low cation exchange capacity (CEC) and base saturation (BS) are found in the zone, which can be restricting for crop growth. The farming system in zone Mid (1300 m) is similar to zone Up, with a wide variety of crops grown in one field. However, rainfall is more seasonal, and dry seasons are more pronounced. As a result the majority of the households struggle to produce enough food for a few months per year, and must buy food. Banana production is 10 times higher than in zone Up and the Mchare and Cavendish form the two most important groups. Besides being a food crop, banana is also a cash crop along with coffee. At around 1400 m, a Silandic Andosol is analyzed, with low CEC and moderate BS and TOC. A major problem for the soils in Up and Mid is phosphorus (P) fixation, evident from low P content and deficiency in maize. At around 1000 m is an Ortho-eutric Mollic Nitisol. As elevation decreases, TOC decreases to low while BS increases to high, and CEC and P are moderate. Due to population pressure in zone Up and Mid people settled in the lowlands at the foot of Mt. Kilimanjaro, or zone Low (750m). Rainfall is scarce, so irrigation is needed to grow crops. By locating close to permanent water sources, food can be produced year-round, making farmers self-sufficient. Crops are grown in monoculture, and the major banana is Cavendish, produced for export. In this zone we find a Luvic Phaeozem. With less precipitation and higher temperature, TOC decreases to Low. High CEC and BS are typical of this generally fertile soil, where farming has not been present as long as in Mid and Up. This soil is suitable for irrigated agriculture and raising cattle, which are two major activities in the region.

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Rivers play an important role in the global carbon (C) cycle. They do not merely transport C from the terrestrial biome to the oceanic environment, but also bury and process organic matter, and thereby generally act as a source of CO2 to the atmosphere. This study focuses on riverine C cycling in Tana River Basin, Kenya, the longest river in Kenya (~1100 km), with a total catchment area of ~96,000 km². Data were collected during three basin-wide surveys along an altitudinal gradient, from 3600 m in headwaters to 8 m in lower Tana River in February 2008 (dry season), September-November 2009 (wet season), and June-July 2010 (end of wet season) as well as monthly sampling at three sites in lower Tana River between January 2009 and December 2011. A consistent downstream increase in total suspended matter (TSM, 0.6 to 7058 mg L-1) and particulate organic carbon (POC) was observed during all three sampling campaigns. With the exception of reservoir waters, POC was predominantly of terrestrial origin as indicated by generally high POC: Chlorophyll a (POC: Chl-a) ratios. Stable isotope signatures of POC (d13CPOC) ranged between -32‰ and -20‰ and increased downstream, reflecting an increasing contribution of C4-derived carbon. Dissolved organic carbon (DOC) concentrations were highest during the end of wet season (2.1 to 6.9 mg L-1), with stable isotope signatures generally between -28‰ and -22‰. A consistent downstream decrease in % organic carbon (%OC) was observed for soils, riverine sediments, and suspended matter. This was likely due to better preservation of the organic fraction in colder high altitude regions, with loss of carbon during downstream spiraling. d13C values for soil and sediment did not exhibit clear altitudinal patterns, but values reflect the full spectrum from C3-dominated to C4-dominated sites. With few exceptions, the entire riverine network was supersaturated in CO2. pCO2 values were generally higher in the lower main Tana River compared to headwater tributaries, opposite to the pattern typically observed in other river networks. First-order estimates show in-stream community respiration was responsible for the bulk of total CO2 evasion (59% to 89%) in the main Tana River while in tributaries respiration accounted for 4% to 52% of total CO2 evasion, suggesting CO2 evasion in tributaries was sustained by lateral CO2 inputs. While sediment loads increase downstream, both chlorophyll a (0.2 µg L-1 to 9.6 µg L-1) and primary production (0.004 µmol L-1 h-1 to 7.38 µmol L-1 h-1) increased consistently downstream. Diurnal fluctuations of biogeochemical parameters were found to be substantial only in the headwater stream, moderate in the reservoir and not detectable at main Tana River. The C sources supporting heterotrophic bacteria in both terrestrial soils and riverine sediments (bottom and suspended) were examined using d13C signatures of bacterial phospholipid-derived fatty acids (PLFA). Generally, bacterial communities appeared to use C3 and C4-derived carbon in the same proportions as they occur in the bulk organic matter pool across different environments. However, a positive correlation between d13C signatures of bacterial and green algal PLFA in the tributaries provides an indication for preferential use of this more labile C source when available. Annual fluxes of suspended sediments and different carbon species were quantified at different stations in the lower Tana River. Both TSM (24 to 9386 mg L-1) and POC (0.8 to 141.9 mg C L-1) concentrations were highly variable. DOC and DIC concentrations ranged from 0.8 to 5.2 mg C L-1 and 7.1 to 23.5 mg C L-1, respectively, but their seasonal variations were not systematic. A strong shift in the origin of POC was observed, being dominated by C3-derived C during dry conditions (low d13CPOC signatures between -28‰ and -25‰), but with significant C4 contributions during higher flow events (d13CPOC up to -19.5 ‰). In contrast, d13CDOC did not show any clear pattern with discharge despite the high range (-27.2‰ to -17.4‰). Over 80% of the sediment and organic carbon was transported during the high flow seasons (March to May and October to December combined). The floodplains along the main Tana River appear to play a critical role in regulating the transport of sediments and organic carbon as reflected by a significant reduction (35-65%) in the sediment, POC and DOC delivery along the stretch between Garissa and Tana River Primate Reserve (~350 km). Considering the additional trapping of sediments and carbon in the upstream reservoirs, and in floodplains downstream of the studied river section, it becomes evident that a major fraction of the sediment and C inputs into the Tana River network are retained and/or processed prior to entering the Indian Ocean.