Tropical dryland agroforestry on clay soils: : Analysis of systems based on Acacia senegal in the Blue Nile region, Sudan

Acacia senegal, the gum arabic producing tree, is the most important component in traditional dryland agroforestry systems in the Blue Nile region, Sudan. The aim of the present study was to provide new knowledge on the potential use of A. senegal in dryland agroforestry systems on clay soils, as well as information on tree/crop interaction, and on silvicultural and management tools, with consideration on system productivity, nutrient cycling and sustainability. Moreover, the aim was also to clarify the intra-specific variation in the performance of A. senegal and, specifically, the adaptation of trees of different origin to the clay soils of the Blue Nile region. In agroforestry systems established at the beginning of the study, tree and crop growth, water use, gum and crop yields, nutrient cycling and system performance were investigated for a period of four years (1999 to 2002). Trees were grown at 5 x 5 m and 10 x 10 m spacing alone or in mixture with sorghum or sesame; crops were also grown in sole culture. The symbiotic biological N2 fixation by A. senegal was estimated using the 15N natural abundance (δ15N) procedure in eight provenances collected from different environments and soil types of the gum arabic belt and grown in clay soil in the Blue Nile region. Balanites aegyptiaca (a non-legume) was used as a non-N-fixing reference tree species, so as to allow 15N-based estimates of the proportion of the nitrogen in trees derived from the atmosphere. In the planted acacia trees, measurements were made on shoot growth, water-use efficiency (as assessed by the δ13C method) and (starting from the third year) gum production. Carbon isotope ratios were obtained from the leaves and branch wood samples. The agroforestry system design caused no statistically significant variation in water use, but the variation was highly significant between years, and the highest water use occurred in the years with high rainfall. No statistically significant differences were found in sorghum or sesame yields when intercropping and sole crop systems were compared (yield averages were 1.54 and 1.54 ha-1 for sorghum and 0.36 and 0.42 t ha-1 for sesame in the intercropped and mono-crop plots, respectively). Thus, at an early stage of agroforestry system management, A. senegal had no detrimental effect on crop yield, but the pattern of resource capture by trees and crops may change as the system matures. Intercropping resulted in taller trees and larger basal and crown diameters as compared to the development of sole trees. It also resulted in a higher land equivalent ratio. When gum yields were analysed it was found that a significant positive relationship existed between the second gum picking and the total gum yield. The second gum picking seems to be a decisive factor in gum production and could be used as an indicator for the total gum yield in a particular year. In trees, the concentrations of N and P were higher in leaves and roots, whereas the levels of K were higher in stems, branches and roots. Soil organic matter, N, P and K contents were highest in the upper soil stratum. There was some indication that the P content slightly increased in the topsoil as the agroforestry plantations aged. At a stocking of 400 trees ha-1 (5 x 5 m spacing), A. senegal accumulated in the biomass a total of 18, 1.21, 7.8 and 972 kg ha-1of N, P, K and OC, respectively. Trees contributed ca. 217 and 1500 kg ha-1 of K and OC, respectively, to the top 25-cm of soil over the first four years of intercropping. Acacia provenances of clay plain origin showed considerable variation in seed weight. They also had the lowest average seed weight as compared to the sandy soil (western) provenances. At the experimental site in the clay soil region, the clay provenances were distinctly superior to the sand provenances in all traits studied but especially in basal diameter and crown width, thus reflecting their adaptation to the environment. Values of δ13C, indicating water use efficiency, were higher in the sand soil group as compared to the clay one, both in leaves and in branch wood. This suggests that the sand provenances (with an average value of -28.07 ) displayed conservative water use and high drought tolerance. Of the clay provenances, the local one (Bout) displayed a highly negative (-29.31 ) value, which indicates less conservative water use that resulted in high productivity at this particular clay-soil site. Water use thus appeared to correspond to the environmental conditions prevailing at the original locations for these provenances. Results suggest that A. senegal provenances from the clay part of the gum belt are adapted for a faster growth rate and higher biomass and gum productivity as compared to provenances from sand regions. A strong negative relationship was found between the per-tree gum yield and water use efficiency, as indicated by δ13C. The differences in water use and gum production were greater among provenance groups than within them, suggesting that selection among rather than within provenances would result in distinct genetic gain in gum yield. The relative δ15N values ( ) were higher in B. aegyptiaca than in the N2-fixing acacia provenances. The amount of Ndfa increased significantly with age in all provenances, indicating that A. senegal is a potentially efficient nitrogen fixer and has an important role in t agroforestry development. The total above-ground contribution of fixed N to foliage growth in 4-year-old A. senegal trees was highest in the Rahad sand-soil provenance (46.7 kg N ha-1) and lowest in the Mazmoom clay-soil provenance (28.7 kg N ha-1). This study represents the first use of the δ15N method for estimating the N input by A. senegal in the gum belt of Sudan. Key words: Acacia senegal, agroforestry, clay plain, δ13C, δ15N, gum arabic, nutrient cycling, Ndfa, Sorghum bicolor, Sesamum indicum

Nile Basin Initiative diskursiivisena areenana

Pro gradu -tutkielma käsittelee Niilin alueen poliittista kehitystä käyttäen viitekehyksenä Barry Buzanin, Ole Wæverin sekä Jaap de Wilden muodostaman Kööpenhaminan koulukunnan teoriaa turvallistamisesta sekä turvallisuuskomplekseista. Niilin kymmenen valtiota kattavan jokialueen poliittista historiaa on vuosikymmenet hallinnut valtioiden välinen epäluottamus, ja erityisesti Egypti, joka on täysin riippuvainen Niilin vesivaroista, on julistanut Niilin vesivarojen olevan sille kansallinen turvallisuuskysymys, ja se on uhannut käyttää myös voimakeinoja puolustaakseen vedenkäyttöoikeuksiaan. Egyptiä on myös voitu Niilin alueen hegemonina, sillä vanhat, alun perin siirtomaa-ajalta peräisin olevat sopimukset ovat antaneet sille lähes yksinoikeuden Niilin vesivarojen käyttöön. Samanaikaisesti erityisesti Etiopia, jonka alueelta suurin osa joen vesivaroista on peräisin, on pitänyt vanhoja sopimuksia epäreiluina, ja vedonnut maassa kasvavaan nälänhätään ja ruokapulaan, mutta myös useat muut alueen valtiot ovat esittäneet tyytymättömyytensä vanhoihin sopimuksiin. Vuonna 1999 kehityksessä tapahtui kuitenkin käänne, sillä jokilaakson valtiot perustivat tuolloin Niilin jokialueen yhteistyöjärjestön Nile Basin Initiativen (NBI). Tutkielma keskittyy NBI:n perustamisen jälkeiseen ajanjaksoon. Keskeinen tutkimuskysymys on, millä tavalla Niilin alueen jäsenvaltiot pyrkivät 2000-luvun alkupuolella keskinäisellä toiminnallaan muuttamaan alueella vallitsevaa kilpailu- ja konfliktiasetelmaa rauhanomaiseen suuntaan. Aineistona käytetään NBI:n omia kausijulkaisuja vuosilta 2006–2010, ja tutkimuksen menetelmänä käytetään diskurssianalyysia. Tutkitun aineiston perusteella on esitettävissä, että Niilin alueen poliittisesta kehityksestä on selvästi erotettavissa diskursseja, jotka ovat NBI:n toiminnan taustalla. Nämä ovat luottamuksen rakentamisen, yhteisten haasteiden voittamisen, yhteistyön tärkeyden sekä saavutusten diskurssit. Aineiston perusteella on nähtävissä, että valtioiden välisen luottamuksen vähitellen lisääntyessä yhteistyötä aletaan yhä enemmän legitimoida aikaisemmin yhteistyöstä saavutetuilla hyödyillä. Toisaalta diskurssianalyysin perusteella on myös nähtävissä, että alueella on ratkaisematta yhä fundamentaalisia ongelmia; uutta vesisopimusta ei ole vielä vuoteen 2010 mennessä aikaansaatu ja NBI:n jäsenvaltioiden suhteissa on myös nähtävissä kiistaa periaatteista, joihin yhteistyön olisi pohjauduttava. Tutkielma pyrkii analyysin avulla kehittämään myös Kööpenhaminan koulukunnan teoriaa turvallistamisesta. Kööpenhaminan koulukunta jättää turvallistamisen purkamiskehityksen kirjoituksissaan taka-alalle keskustelun painottuessa siihen, kuinka jokin asia tuodaan esille julkiseen turvallisuuskeskusteluun. Tältä osin tutkimuksen tavoitteena on esittää mahdollinen kehityskulku turvallistamisen purkamisdiskurssista. Teoriaosuudessa käytetyt keskeiset lähteet: Allan 2001,2009; Buzan 1991; Buzan ja Wæver 2003; Buzan, Wæver ja de Wilde 1998; Jutila 2008; Schulz 1995; Stritzel 2007; Turton 2001; Wæver 1995; Wendt 1992, 1999.