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Phosphorus-efficient soybean germplasm as an entry point to integrated soil fertility management in Western Kenya
A spiral of ever-decreasing soil fertility is threatening the livelihoods of smallholder farmers in Western Kenya. While high population pressure leads to the cutback of fallow periods, the use of large amounts of fertilizers is neither within reach of resource-poor farmers nor sustainable on the long term because of finite phosphate rock reserves. A realistic approach to increase production levels and to counteract soil fertility decline is integrated soil fertility management (ISFM), which combines improved germplasm with judicious use of mineral fertilizers and organic amendments. As nitrogen (N) is the most limiting nutrient in most agricultural systems, the cultivation of legumes, which can fix N from atmospheric N2, may reverse the downward trend in soil fertility, and legumes therefore play an important role in ISFM. However, many soils in Western Kenya are characterized by low P availability and high P-sorption. Legumes such as soybean (Glycine max</>) are particularly sensitive to P deficiency as nodulation and N fixation largely depend on P supply. In this context, the selection and development of soybean genotypes that areadapted to low P conditions is highly needed. Plants can adapt to P-limiting conditions through the development of P-uptake enhancing root traits (P uptake efficiency) or through a change in physiological processes that affect the use of P in the plant tissue (P utilization efficiency).The objectives of this study were to assess variation in P uptake and utilization efficiencies among soybean genotypes, to investigate the occurrence of root characteristics that contribute to P uptake efficiency, and to evaluate how the incorporation of P-efficient soybean germplasm into ISFM systems may improve soil fertility and P fertilizer use efficiency. </>
First, a methodological framework was implemented to compare plant growth at low P supply among soybean genotypes that largely differin yield potential at high P supply. A distinction was made between P productivity, measured as absolute biomass or grain yield at a given suboptimal P level, and P efficiency, measured as biomass or grain yieldat suboptimal P relative to corresponding values at optimal P. In addition, a new index, the suboptimal P tolerance index (SPTIbiomass), was introduced by multiplying P productivity and P efficiency. With regard to P uptake, a similar terminology was adopted (P uptake productivity, P uptake efficiency and SPTIuptake). A pot trial was set up to evaluate variation in these low P tolerance indices among 40 soybean genotypes at early growth stages. Further, the response of 20 soybean genotypes to suboptimal and optimal levels of P fertilizer application was evaluated in a multi-location field trial in Western Kenya. In the pot trial, a 2-fold variation in P uptake productivity and a 2.5-fold variation in P uptake efficiency among genotypes was observed, but no variation in P efficiency and the SPTIbiomass could be demonstrated. In the field trial, a significant 2-fold variation in P efficiency and 6-fold variation in SPTIbiomass among genotypes was observed. P uptake efficiency and P utilization efficiency correlated negatively in the pot trial, which indicated that a trade-off exists between both components of P efficiency. Strong positive correlations between P efficiency and P uptake efficiency in both the pot and the field trial demonstrated that variation in overall P efficiency is determined by factors affecting P uptake rather than P utilization. Furthermore, we showed that the use of different indices results in a different ranking of genotypes. In the pot trial, variation in biomass production attributed to the inherent potential (biomass yield underoptimal conditions) was eliminated by ranking genotypes in terms of theSPTI, while the same was achieved in the field by ranking in terms of Pefficiency.</>
The contribution of different root traits to variation in P uptake efficiency among soybean genotypes grown in a P-deficient Ferralsol was investigated by regression analysis and mechanistic modeling. Eight genotypes were grown in a pot trial at suboptimal and optimal P availability. Root hair growth was visualized by growing plants in a novel agar system where P intensity was buffered by Al2O3 nanoparticles. In the pot trial, P uptake did not vary among the genotypes at optimal Psupply but differed about 2-fold at suboptimal P. The genotypes differed in P uptake efficiency but not in P utilization efficiency. Variation among genotypes was observed for several root characteristics that affect P uptake, such as the root to shoot ratio, root diameter, colonizationby arbuscular mycorrhizal fungi, and root hair length and density. Regression analysis and mechanistic modeling indicated that P uptake efficiencies were to a large extent related to root hair development (length and density) and, to a lower extent, to colonization by mycorrhizal fungi.Sensitivity analysis showed that root hairs can directly increase P uptake with maximally 36% (through P uptake at the root hair level). Indications were found that the indirect effect of root hairs on P uptake (improved P uptake leading to improved root growth, in turn leading to enhanced P uptake) exceeded the direct effect. Breeding for improved root hair development is suggested as a promising way to increase P uptake efficiency of soybean.</>
Growth and P uptake of 5 soybean genotypes at a suboptimal rate of poorly soluble rock P was compared to that at an equal rate of water-soluble P in 2 moderately acid, P-deficient soils with different P buffering capacity. Mucuna (Mucuna pruriens</>), which is known for its ability to mobilize sparingly available P, was grown as a reference species. Depending on the genotype, the biomass of soybean at a suboptimal amount of rock P relative to that at an equal rate of soluble P varied between 53 and 86%. The results suggested that mucuna mobilizessparingly available P through rhizosphere-modifying processes, while soybean is able to take up similar amounts of P through the development ofa larger root system. A different ranking of the genotypes in terms of P efficiency was obtained in the 2 soils, indicating that several P uptake-enhancing mechanisms play a role while the relative benefits of such mechanisms depend on the soil physicochemical characteristics.</>
Lastly, we evaluated the response of 5 soybean genotypes to increasing levels of P fertilizer under field conditions in terms of P uptake, N fixation and grain yields, and determined the rotation effect of these genotypes on a subsequent maize crop in terms of N and P nutrition (the N effect and the P effect). P application increased P uptake, nodulation, Nfixation and grain yields, and the soybean genotypes differed largely in P productivity. The results demonstrated that N balances after a season of soybean cropping and N effects of soybean on a subsequent maize crop do not necessarily increase when P rates are increased. Further, no differences in N effects among genotypes and no P effects of soybean on maize were observed. However, we showed that even on strongly P-fixing soils, residual effects of P applied to soybean can largely overcome P limitations to a subsequent maize crop. When P was applied solely to the soybean crop, the total P recovery (P taken up by soybean and maize in percentage of applied P), was significantly higher when P-productive genotypes were incorporated in the rotation. Hence, despite the absence of differences among genotypes in effects on the N and P nutrition of a subsequent maize crop, P fertilizer use efficiency in a soybean-maize rotation can be increased by incorporating P-productive soybean genotypes.</>
First, a methodological framework was implemented to compare plant growth at low P supply among soybean genotypes that largely differin yield potential at high P supply. A distinction was made between P productivity, measured as absolute biomass or grain yield at a given suboptimal P level, and P efficiency, measured as biomass or grain yieldat suboptimal P relative to corresponding values at optimal P. In addition, a new index, the suboptimal P tolerance index (SPTIbiomass), was introduced by multiplying P productivity and P efficiency. With regard to P uptake, a similar terminology was adopted (P uptake productivity, P uptake efficiency and SPTIuptake). A pot trial was set up to evaluate variation in these low P tolerance indices among 40 soybean genotypes at early growth stages. Further, the response of 20 soybean genotypes to suboptimal and optimal levels of P fertilizer application was evaluated in a multi-location field trial in Western Kenya. In the pot trial, a 2-fold variation in P uptake productivity and a 2.5-fold variation in P uptake efficiency among genotypes was observed, but no variation in P efficiency and the SPTIbiomass could be demonstrated. In the field trial, a significant 2-fold variation in P efficiency and 6-fold variation in SPTIbiomass among genotypes was observed. P uptake efficiency and P utilization efficiency correlated negatively in the pot trial, which indicated that a trade-off exists between both components of P efficiency. Strong positive correlations between P efficiency and P uptake efficiency in both the pot and the field trial demonstrated that variation in overall P efficiency is determined by factors affecting P uptake rather than P utilization. Furthermore, we showed that the use of different indices results in a different ranking of genotypes. In the pot trial, variation in biomass production attributed to the inherent potential (biomass yield underoptimal conditions) was eliminated by ranking genotypes in terms of theSPTI, while the same was achieved in the field by ranking in terms of Pefficiency.</>
The contribution of different root traits to variation in P uptake efficiency among soybean genotypes grown in a P-deficient Ferralsol was investigated by regression analysis and mechanistic modeling. Eight genotypes were grown in a pot trial at suboptimal and optimal P availability. Root hair growth was visualized by growing plants in a novel agar system where P intensity was buffered by Al2O3 nanoparticles. In the pot trial, P uptake did not vary among the genotypes at optimal Psupply but differed about 2-fold at suboptimal P. The genotypes differed in P uptake efficiency but not in P utilization efficiency. Variation among genotypes was observed for several root characteristics that affect P uptake, such as the root to shoot ratio, root diameter, colonizationby arbuscular mycorrhizal fungi, and root hair length and density. Regression analysis and mechanistic modeling indicated that P uptake efficiencies were to a large extent related to root hair development (length and density) and, to a lower extent, to colonization by mycorrhizal fungi.Sensitivity analysis showed that root hairs can directly increase P uptake with maximally 36% (through P uptake at the root hair level). Indications were found that the indirect effect of root hairs on P uptake (improved P uptake leading to improved root growth, in turn leading to enhanced P uptake) exceeded the direct effect. Breeding for improved root hair development is suggested as a promising way to increase P uptake efficiency of soybean.</>
Growth and P uptake of 5 soybean genotypes at a suboptimal rate of poorly soluble rock P was compared to that at an equal rate of water-soluble P in 2 moderately acid, P-deficient soils with different P buffering capacity. Mucuna (Mucuna pruriens</>), which is known for its ability to mobilize sparingly available P, was grown as a reference species. Depending on the genotype, the biomass of soybean at a suboptimal amount of rock P relative to that at an equal rate of soluble P varied between 53 and 86%. The results suggested that mucuna mobilizessparingly available P through rhizosphere-modifying processes, while soybean is able to take up similar amounts of P through the development ofa larger root system. A different ranking of the genotypes in terms of P efficiency was obtained in the 2 soils, indicating that several P uptake-enhancing mechanisms play a role while the relative benefits of such mechanisms depend on the soil physicochemical characteristics.</>
Lastly, we evaluated the response of 5 soybean genotypes to increasing levels of P fertilizer under field conditions in terms of P uptake, N fixation and grain yields, and determined the rotation effect of these genotypes on a subsequent maize crop in terms of N and P nutrition (the N effect and the P effect). P application increased P uptake, nodulation, Nfixation and grain yields, and the soybean genotypes differed largely in P productivity. The results demonstrated that N balances after a season of soybean cropping and N effects of soybean on a subsequent maize crop do not necessarily increase when P rates are increased. Further, no differences in N effects among genotypes and no P effects of soybean on maize were observed. However, we showed that even on strongly P-fixing soils, residual effects of P applied to soybean can largely overcome P limitations to a subsequent maize crop. When P was applied solely to the soybean crop, the total P recovery (P taken up by soybean and maize in percentage of applied P), was significantly higher when P-productive genotypes were incorporated in the rotation. Hence, despite the absence of differences among genotypes in effects on the N and P nutrition of a subsequent maize crop, P fertilizer use efficiency in a soybean-maize rotation can be increased by incorporating P-productive soybean genotypes.</>
Date:1 Oct 2008 → 18 Apr 2013
Keywords:ISFM (Integrated Soil Fertility Manageme, Efficiency, Roots, Kenya, Phosphorus, Legumes, Soil Fertility, Soybean
Disciplines:Agricultural plant production, Agricultural animal production, Agriculture, land and farm management, Other agriculture, forestry, fisheries and allied sciences, Soil sciences, challenges and pollution
Project type:PhD project