Gert Nyberg

Large-scale forest girdling shows that current photosynthesis drives soil respiration

The respiratory activities of plant roots, of their mycorrhizal fungi and of the free-living microbial heterotrophs (decomposers) in soils are significant components of the global carbon balance, but their relative contributions remain uncertain. To separate mycorrhizal root respiration from heterotrophic respiration in a boreal pine forest, we conducted a large-scale tree-girdling experiment, comprising 9 plots each containing about 120 trees. Tree-girdling involves stripping the stem bark to the depth of the current xylem at breast height terminating the supply of current photosynthates to roots and their mycorrhizal fungi without physically disturbing the delicate root–microbe–soil system. Here we report that girdling reduced soil respiration within 1–2 months by about 54% relative to respiration on ungirdled control plots, and that decreases of up to 37% were detected within 5 days. These values clearly show that the flux of current assimilates to roots is a key driver of soil respiration; they are conservative estimates of root respiration, however, because girdling increased the use of starch reserves in the roots. Our results indicate that models of soil respiration should incorporate measures of photosynthesis and of seasonal patterns of photosynthate allocation to roots.

Effects of planted tree fallows on soil nitrogen dynamics, above-ground and root biomass, N2-fixation and subsequent maize crop productivity in Kenya

The effects of planted fallows of Sesbania sesban (L.) Merr. and Calliandra calothyrsus (Meissner) on soil inorganic nitrogen dynamics and two subsequent maize crops were evaluated under field conditions in the highlands of eastern Kenya. Continuous unfertilised maize, maize/bean rotation and natural regrowth of vegetation (weed fallow) were used as control treatments. The proportion of symbiotic N2-fixation was estimated by measuring both leaf 15N enrichment and whole-plant 15N enrichment by the 15N dilution technique for Sesbania and Calliandra, using Eucalyptus saligna (Sm.) and Grevillea robusta (A. Cunn) as reference species. Above- and below-ground biomass and N contents were examined in Sesbania, Calliandra, Eucalyptus and Grevillea 22 months after planting. Both the content of inorganic N in the topsoil and the quantity of N mineralised during rainy seasons were higher after the Sesbania fallows than after the other treatments. Compared to the continuous unfertilised maize treatment, both residual crop yields were significantly higher when mineral N (one application of 60 kg N ha−1) was added. Furthermore, the second crop following the Sesbania fallow was significantly higher than the continuous maize crop. The above-ground biomass of the trees at final harvest were 31.5, 24.5, 32.5 and 43.5 Mg ha−1 for the Sesbania, Calliandra, Grevillea and Eucalyptus, respectively. For the total below-ground biomass the values for these same tree species were 11.1, 15.5, 17.7, and 19.1 Mg ha−1, respectively, of which coarse roots (>2 mm), including tap roots, amounted to 70–90%. About 70–90% of the N in Sesbania, and 50–70% in Calliandra, was derived from N2-fixation. Estimates based on leaf 15N enrichment and whole-plant 15N enrichment were strongly correlated. The N added by N2-fixation amounted to 280–360 kg N ha−1 for Sesbania and 120–170 kg N ha−1 for Calliandra, resulting in a positive N balance after two maize cropping seasons of 170–250 kg N ha−1 and 90–140 kg N ha−1, for Sesbania and Calliandra, respectively. All the other treatments gave negative N balances after two cropping seasons. We conclude that Sesbania sesban is a tree species well suited for short duration fallows due to its fast growth, high nutrient content, high litter quality and its ability to fix large amounts of N2 from the atmosphere.

Intermediate tree cover can maximize groundwater recharge in the seasonally dry tropics

Water scarcity contributes to the poverty of around one-third of the world’s people. Despite many benefits, tree planting in dry regions is often discouraged by concerns that trees reduce water availability. Yet relevant studies from the tropics are scarce, and the impacts of intermediate tree cover remain unexplored. We developed and tested an optimum tree cover theory in which groundwater recharge is maximized at an intermediate tree density. Below this optimal tree density the benefits from any additional trees on water percolation exceed their extra water use, leading to increased groundwater recharge, while above the optimum the opposite occurs. Our results, based on groundwater budgets calibrated with measurements of drainage and transpiration in a cultivated woodland in West Africa, demonstrate that groundwater recharge was maximised at intermediate tree densities. In contrast to the prevailing view, we therefore find that moderate tree cover can increase groundwater recharge, and that tree planting and various tree management options can improve groundwater resources. We evaluate the necessary conditions for these results to hold and suggest that they are likely to be common in the seasonally dry tropics, offering potential for widespread tree establishment and increased benefits for hundreds of millions of people.

Respiration from C3 plant green manure added to a C4 plant carbon dominated soil

Application of tree leaves (C3 plants) on maize (Zea mays L.) (C4 plant) fields is an agroforestry management technology to restore or maintain soil fertility. The rate at which the tree leaves decompose is crucial for the nutrient supply to the crop. We studied the in situ decomposition of Sesbania sesban (L.) Merr. leaves or C3 sugar for 4 – 8 days after application to a maize field in Kenya. By using the difference of around 10‰ in natural abundance of 13C between the endogenous soil C (mainly C4) and the applied C (C3), we could calculate the contributions of the two C sources to soil respiration. The δ13C value of the basal respiration was from –15.9 to –16.7‰. The microbial response to the additions of leaves and sugar to this tropical soil was immediate. Application of sesbania leaves gave an initial peak in respiration rates that lasted from one to less than 6 days, after which it levelled off and remained about 2 – 3 times higher (230–270 mg C m-2 h-1) than the control respiration rates throughout the rest of the experiment (5 – 8 days). In the sugar treatment, there was no initial peak in respiration rate. The respiration rate was 170 mg C m-2 h-1 after 4 days. At the end of the experiments, after 4–8 days, as much as 14–17% of the added C had been respired and about 60% of the total respiration was from the added sesbania leaves or C3 sugar. This non-destructive method allows repeated measurements of the actual rate of C mineralisation and facilitates decomposition studies with high temporal resolution in the field.

Enclosures in West Pokot, Kenya: Transforming land, livestock and livelihoods in drylands

Dryland livestock production systems are changing in many parts of the world, as a result of growing human populations and associated pressure on water and land. Based on a combination of social and natural science methods, we studied a 30-year transformation process from pastoralism to a livestock-based agro-pastoral system in northwestern Kenya, with the overall aim to increase the understanding of the ongoing transition towards intensified agro-pastoralist production systems in dryland East Africa.Key to this transformation was the use of enclosures for land rehabilitation, fodder production, and land and livestock management. Enclosures have more soil carbon and a higher vegetation cover than adjacent areas with open grazing. The level of adoption of enclosures as a management tool has been very high, and their use has enabled agricultural diversification, e.g. increased crop agriculture, poultry production and the inclusion of improved livestock. Following the use of enclosures, livelihoods have become less dependent on livestock migration, are increasingly directed towards agribusinesses and present new opportunities and constraints for women. These livelihood changes are closely associated with, and depend on, an ongoing privatization of land under different tenure regimes.The results indicate that the observed transformation provides opportunities for a pathway towards a sustainable livestock-based agro-pastoral system that could be valid in many dryland areas in East Africa. However, we also show that emergent risks of conflicts and inequalities in relation to land, triggered by the weakening of collective property rights, pose a threat to the sustainability of this pathway.

Performances of Cotton–Maize Rotation System as Affected by Ploughing Frequency and Soil Fertility Management in Burkina Faso

On-farm experiments were conducted on two soil types (Lixisol and Luvisol) in the western cotton area of Burkina Faso with the objective to develop sustainable water and soil fertility management techniques that improve cotton–maize productivity. The hypothesis that reducing ploughing frequency with addition of organic and mineral fertilizers may improve cotton (Gossypium hirsutum) and maize (Zea mays L.) productions was tested. The treatments were combination of two tillage regimes (annual oxen ploughing, AP and ploughing/manual scarifying, RT) with compost (Co) and without compost (nCo) application. The treatment annual ploughing with compost addition (AP + Co) had the highest soil water content (WC) on both the Lixisol and the Luvisol. The cotton yield increase was 46 and 36% on reduced tillage plot with compost additions (RT + Co) compared to the control in the Lixisol and the Luvisol, respectively. In the Lixisol the highest maize grain yield was recorded in the annual ploughing plot with the additional amount of nitrogen equivalent to the compost nitrogen content. Reduced tillage together with compost additions had the highest maize yield in the Luvisol. These results confirmed the hypothesis that reduced tillage with organic and mineral fertilization improved cotton and maize productions.

Assessing gender roles in a changing landscape: diversified agro-pastoralism in drylands of West Pokot, Kenya

Previous studies in drylands have shown that while gender roles are becoming more flexible, privatization and formalization of land tenure tends to marginalize women in drylands while environmental degradation leads to differential changes in gender workload. Chepareria, a ward in West Pokot County, has undergone the above-mentioned tenure and environmental changes and is nowadays dominated by private enclosures as a land management approach. This study reviewed in which ways these rangeland enclosures have influenced gender roles in the dryland systems of Chepareria in West Pokot, with specific regard to division of labour, financial responsibilities and decision-making processes on the household level, and also tries to identify the underlying driving forces that have contributed to the observed changes.Our results indicate that the workload of women has increased under new land fragmentation processes in rangelands due to increased responsibilities in cattle herding and income generation, but also that due to this, women have gained higher influence in household decisions concerning the family economy. It was found that women to a larger extent than before are engaged in small-scale business such as the selling of farm products and poultry keeping, hence getting financial resources under their own control. In this way, women are involved in decision-making processes and income generation to a higher extent than before, though they are still excluded from certain traditionally male-dominated spheres both within and beyond the household, such as the sale of cattle and the handling of larger amounts of financial resources.Under the new land fragmentation processes in Chepareria, women are increasingly seen as crucial income earners and they are enjoying a higher degree of financial independence, more decision-making power and a louder voice than before in household and public matters.

Agroforestry extension and dietary diversity – an analysis of the importance of fruit and vegetable consumption in West Pokot, Kenya

Worldwide, the challenges of nomadic, pastoralist systems are causing their slow but steady disappearance in favour of sedentary agropastoralism. This paper draws upon an existing household data set from a survey collected and organized by the Swedish non-governmental organization (NGO) Vi Agroforestry, directed at a livestock-based, agro-pastoralist area in West Pokot County, western Kenya. The study focuses on the question of food diversity and malnutrition and the role of agroforestry extension services, i.e. knowledge spread, transfer and development. Our basic hypothesis is that certain fruit and vegetable related food groups are under-consumed in West Pokot, especially in the dryland areas. The results of the study shows that agroforestry, combined with advice through extension efforts can imply a transition path for pastoralists which involves improved dietary diversity, especially concerning food groups that include roots, tubers, fruits and leafy vegetables. From the results certain restrictions that hinder this transition become clear. An important but often overlooked factor is lack of information and knowledge as a determinant of household behavior in developing countries. NGOs such as Vi Agroforestry can play an important role in overcoming this restriction by providing extension services. Developing countries in general are not information-rich environments, a fact that is especially the case for poor citizens living in rural areas. The paper illustrates that careful attention to the information and knowledge available to households is necessary when designing development cooperation.