Soil fertility gradients and production constraints for coffee and banana on volcanic mountain slopes in the east African rift: a case study of Mt. Elgon

Volcanic mountains in the East African Rift (e.g. Mt. Kenya, Mt. Kilimanjaro, Mt. Elgon) are some of the most productive agricultural regions, often dominated by coffee and banana cultivation. Consequently, these regions suffer from a high and increasing population density with a declining soil fertility status imposing pressure on the available land, which in turn results in encroaching into the national forests. This study documents the soil fertility constraints along the slopes of Mt. Elgon and explores its corresponding gradients in plant nutritional status. This research links the topography of Mt. Elgon to the prevailing soil types and their current fertility status. It reveals important relations and gradients between soil fertility parameters and its corresponding environment along the slope. Soil pH, soil available P and exchangeable K, Ca and Mg are significantly decreasing with elevation. Thereby, gradients and constraints in macro- and micro-nutrient uptake by coffee and banana are revealed along the toposequence and different altitude-specific nutrient limitations are determined for both crops. K, Mn and Si uptake in both crops is decreasing with elevation along the slope, while the Mo and Ni uptake in both crops is increasing. With increasing elevation, B uptake is only decreasing in coffee and P uptake is only decreasing in banana. In addition, the antagonistic interaction between K and Mg limits the Mg uptake of both crops in the lower areas, while in the high region the Mg uptake is simply limited by low soil availability. It follows that a general fertilizer recommendation cannot be made in these regions and that the soil fertility problems along these slopes should be specifically addressed and appropriately managed according to the local requirements

Soil mobility of surface applied polyaromatic hydrocarbons in response to simulated rainfall

Polyaromatic hydrocarbons (PAHs) are emitted from a variety of sources and can accumulate on and within surface soil layers. To investigate the level of potential risk posed by surface contaminated soils, vertical soil column experiments were conducted to assess the mobility, when leached with simulated rainwater, of six selected PAHs (naphthalene, phenanthrene, fluoranthene, pyrene, benzo(e)pyrene and benzo(ghi)perylene) with contrasting hydrophobic characteristics and molecular weights/sizes. The only PAH found in the leachate within the experimental period of 26 days was naphthalene. The lack of migration of the other applied PAHs were consistent with their low mobilities within the soil columns which generally parallelled their log Koc values. Thus only 2.3% of fluoranthene, 1.8% of pyrene, 0.2% of benzo(e)pyrene and 0.4% of benzo(ghi)perylene were translocated below the surface layer. The PAH distributions in the soil columns followed decreasing power relationships with 90% reductions in the starting levels being shown to occur within a maximum average depth of 0.94 cm compared to an average starting depth of 0.5 cm. A simple predictive model identifies the extensive time periods, in excess of 10 years, required to mobilise 50% of the benzo(e)pyrene and benzo(ghi)perylene from the surface soil layer. Although this reduces to between 2 and 7 years for fluoranthene and pyrene, it is concluded that the possibility of surface applied PAHs reaching and contaminating a groundwater aquifer is unlikely

Root performance and root responses of rice to cope with drought events and low soil phosphorus availability

status: publishe

Architectural root responses of rice to reduced water availability can overcome phosphorus stress.

Drought and low phosphorus (P) availability are major limitations for rainfed rice production. Crop roots are important for soil resource acquisition and tolerance to P and water limitations. Two pot and two field trials were conducted to evaluate architectural root responses of contrasting rice varieties to combinations of different levels of P (deficient to non-limiting) and water availability (water stressed to submergence) and to identify the interactions with different varieties. Root development was then related to drought and/or low P tolerance. Although shoot and root growth responded more to P than to water availability, architectural root responses to water were much more prominent than responses to P availability. Reduced water availability decreased nodal thickness and increased secondary root branching, both factors partially enhancing P uptake efficiency and even overcoming a decreased root:shoot ratio under reduced water availability. In contrast to root thickness and secondary branching, basal lateral root density was strongly determined by variety and was related to enhanced P uptake. Reduced water availability induces root modifications whichapart from enhancing drought resiliencealso affect P uptake efficiency. Future research on rice roots and nutrient uptake may hence take into account the large effects of water on root development

Combining phosphorus placement and water saving technologies enhances rice production in phosphorus-deficient lowlands

Lowland rice production in sub-Saharan Africa (SSA) is often limited by water supply and low phosphorus (P) availability and efforts are needed towards more efficient management of both resources. Field and pot experiments were set up to evaluate combinations of water saving technologies and micro-dose P placement methods (i.e. the localized application of a small P dose to a sub-surface area, often combined with seeds into the planting hole) with due attention to treatment effects on root architecture. A two-year field experiment was set up in a lowland rice field in Tanzania with factorial combinations of different levels of water supply (field capacity, alternating wetting drying, permanent flooding) and P application (no P; 3.45 and 6.90 kg ha−1 placement versus 25 kg ha−1 broadcast), thereby testing residual effects in year 2. A trial in pots (10.5 L) was additionally performed with equivalent treatments and allowing measurements of soil solution composition, apparent fertilizer efficiency, and root density versus depth. Rice grain yields ranged 0–5 ton ha−1 and mainly responded to P application. The P placement at the lowest P rate resulted in higher grain yield at field capacity (2.0–2.5 ton ha−1) than in flooded rice (1.2–1.6 ton ha−1), whereas these differences were absent at higher P rates. Lower water supply at field capacity enhanced root growth and rooting depth, decreased nodal root thickness and enhanced root P uptake efficiency compared to flooded condition. Modelling P diffusion outwards the granules showed more restricted P diffusion under reduced water supply and, therefore, less P immobilization in the soil under field capacity. These differences between water treatments were more pronounced at lower than at higher P supply. This study shows that both root responses and P diffusion outwards placed granules explain rice development and yields under micro-dose P placement and water saving technologies. P placement can contribute to intensify rice production while countering soil P decline in P deficient lowlands when resources are limited

Anatomical root responses of rice to combined phosphorus and water stress - relations to tolerance and breeding opportunities.

Drought and low P availability are major limitations for rainfed rice (Oryza spp.) production. Root anatomy plays a key role in resource acquisition and tolerance to P and water limitations. Root anatomical responses of three contrasting rice varieties to combinations of different levels of P (deficient to non-limiting) and water availability (water stress to submergence) were evaluated in two pot trials. P availability was the dominant growth-limiting factor, but anatomical root responses to water availability were more prominent than responses to P availability. Cortical cell file number and number of xylem vessels decreased as a response to water stress, but stele and xylem diameter increased. Low P availability induced thinner xylem vessels and a thinner stele. Drought tolerance related to an overall thicker root stele, thicker xylem vessels and a larger water conductance. Some root traits were observed to be more responsive to water and P availability, whereas other traits were more robust to these environmental factors but highly determined by variety. The observed genotypic variation in root anatomy provides opportunities for trait-based breeding. The plasticity of several traits to multiple environmental factors highlights the need for strategic trait selection or breeding adapted to specific target environments

A thiol protease and an anionic peroxidase are induced by lowering cytokinins during callus growth in Petunia.

We previously identified a group of proteins that increase early in Petunia hybrida calli subcultured on a low-cytokinin medium, unlike the calli subcultured on a high-cytokinin medium. The calli on the low-cytokinin medium do not regenerate (J.-P. Renaudin, C. Tournaire, B, Teyssendier de la Serve [1991] Physiol Plant 82: 48-56). Two of these proteins, P21 and P17, have been identified by peptide sequencing and cloned. P21 is highly homologous to a group of thiol proteases, including barely aleurain, rice oryzain gamma, Arabidopsis SAG2, and mammalian cathepsin H. P17 is highly homologous to a group of anionic peroxidases from potato and tomato. A study of their expression in two P. hybrida lines, PC6 and St40 which differ in their ability to regenerate, showed that the genes for P21 and P17 are differentially expressed depending on the type and the age of the organ, with the highest expression in senescing leaves and in aged calli. The data are in favor of these genes being associated with an early step of senescence, which may be due, in part, to a reduction in total cytokinin. The two Petunia lines are, thus, functionally different concerning the action of cytokinin in two developmental phenomena: in vitro organogenesis and senescence