Effects of Acacia seyal and biochar on soil properties and sorghum yield in agroforestry systems in South Sudan

We studied the effects of Acacia seyal Del. intercropping and biochar soil amendment on soil physico-chemical properties and sorghum (Sorghum bicolor L.) yields in a two-year field experiment conducted on a silt loam site near Renk in South Sudan. A split-plot design with three replications was used. The main factor was tree-cropping system (dense acacia + sorghum, scattered acacia + sorghum, and sole sorghum) and biochar (0 and 10 Mg ha(-1)) was the subplot factor. The two acacia systems had lower soil pH, N and higher C/N ratios compared to the sole sorghum system. Biochar significantly increased soil C, exchangeable K+ contents, field capacity and available water content, but reduced soil exchangeable Ca2+ and effective CEC, and had no effect on soil pH. Acacia intercropping significantly reduced sorghum grain yields while biochar had no significant effect on sorghum yields. The land equivalent ratio (LER) for sorghum yield was 0.3 for both acacia systems in 2011, with or without biochar, but increased in 2012 to 0.6 for the scattered acacia system when combined with biochar. The reduction in sorghum yields by the A. seyal trees was probably due to a combination of competition for water and nutrients and shading. The lack of a yield response to biochar maybe due to insufficient time or too low a dosage. Further research is needed to test for the effects of tree intercropping and biochar and their interactions on soil properties and crop yields in drylands.Peer reviewe

Biophysical interactions in tropical agroforestry systems

sequential systems, simultaneous systems Abstract. The rate and extent to which biophysical resources are captured and utilized by the components of an agroforestry system are determined by the nature and intensity of interac-tions between the components. The net effect of these interactions is often determined by the influence of the tree component on the other component(s) and/or on the overall system, and is expressed in terms of such quantifiable responses as soil fertility changes, microclimate modification, resource (water, nutrients, and light) availability and utilization, pest and disease incidence, and allelopathy. The paper reviews such manifestations of biophysical interactions in major simultaneous (e.g., hedgerow intercropping and trees on croplands) and sequential (e.g., planted tree fallows) agroforestry systems. In hedgerow intercropping (HI), the hedge/crop interactions are dominated by soil fertility improvement and competition for growth resources. Higher crop yields in HI than in sole cropping are noted mostly in inherently fertile soils in humid and subhumid tropics, and are caused by large fertility improvement relative to the effects of competition. But, yield increases are rare in semiarid tropics and infertile acid soils because fertility improvement does not offse

Historical Pollution: In Search of a Legal Definition

The chapter introduces and investigates the concept of \u201chistorical pollution\u201d in the context of environmental law. The subject is complex, as the principal consideration in our analysis concerns the role of time in the causation of harmful consequences (both for the environment and human health), notably in view of the prolonged timeframe that separates the conduct (pollution) and the effect (contamination). The phenomenon under scrutiny has significant implications for both the legal and the social fields. Indeed, legal enforcement of putative crimes against the environment is generally ineffective precisely because of this temporal factor, and this also has consequences for social perceptions and the dynamics of victimization. Yet these considerations could also contribute to a definition of historical pollution, as\u2014 given the complexity and uncertainties that surround the phenomenon\u2014they suggest ways in which material cases could be used to frame a general understanding of the concept. Attention here is focused on pollution related to industrial activities since such cases are prominent in the field. The proposed definition strives to establish a general framework within which domestic analyses can be located, consistent with the particularities of each legal system. It represents one of the first steps into the unexplored terrain of historical pollution

Nutrient enrichment shifts mangrove height distribution: Implications for coastal woody encroachment

<div><p>Global changes, such as increased temperatures and elevated CO₂, are driving shifts in plant species distribution and dominance, like woody plant encroachment into grasslands. Local factors within these ecotones can influence the rate of regime shifts. Woody encroachment is occurring worldwide, though there has been limited research within coastal systems, where mangrove (woody shrub/tree) stands are expanding into salt marsh areas. Because coastal systems are exposed to various degrees of nutrient input, we investigated how nutrient enrichment may locally impact mangrove stand expansion and salt marsh displacement over time. We fertilized naturally co-occurring <i>Avicennia germinans</i> (black mangrove) and <i>Spartina alterniflora</i> (smooth cordgrass) stands in Port Aransas, TX, an area experiencing mangrove encroachment within the Northern Gulf of Mexico mangrove-marsh ecotone. After four growing seasons (2010–2013) of continuous fertilization, <i>Avicennia</i> was more positively influenced by nutrient enrichment than <i>Spartina</i>. Most notably, fertilized plots had a higher density of taller (> 0.5 m) mangroves and mangrove maximum height was 46% taller than in control plots. Fertilization may promote an increase in mangrove stand expansion within the mangrove-marsh ecotone by shifting <i>Avicennia</i> height distribution. <i>Avicennia</i> individuals, which reach certain species-specific height thresholds, have reduced negative neighbor effects and have higher resilience to freezing temperatures, which may increase mangrove competitive advantage over marsh grass. Therefore, we propose that nutrient enrichment, which augments mangrove height, could act locally as a positive feedback to mangrove encroachment, by reducing mangrove growth suppression factors, thereby accelerating the rates of increased mangrove coverage and subsequent marsh displacement. Areas within the mangrove-marsh ecotone with high anthropogenic nutrient input may be at increased risk of a regime shift from grass to woody dominated ecosystems.</p></div

Fertiliser Trees for Sustainable Food Security in the Maize-Based Production Systems of East and Southern Africa

The negative effects of soil fertility depletion on food security, especially among smallholder farmers in Africa, is of economic importance, and may be worsened by climate change and rising global fertiliser prices. Substantial efforts and investment have gone into development of alternative soil fertility management options. These include vigorous research and development of N-fixing plants or “fertiliser trees”, that has been on-going in the last two decades in East and Southern Africa. In this paper, we review several studies conducted both on-station and on-farm and synthesise the results in terms of improvements in soil physical, chemical and biological properties, and crop yield in response to fertiliser trees. Our major findings are that (1) fertiliser trees add more than 60 kg N ha−1 per year through biological nitrogen fixation (BNF); (2) nutrient contributions from fertiliser tree biomass can reduce the requirement for mineral N fertiliser by 75%, translating to huge savings on mineral fertilisers; (3) fertiliser trees were also shown to substantially increase crop yield. A meta-analysis has further provided conclusive evidence that with good management, fertiliser trees can double maize yields compared with local farmer practices of maize cultivation without addition of external fertilisation. (4) Financial analyses showed that fertiliser tree systems are profitable and also have higher net returns than the farmers’ de facto practice, i.e. continuous maize cropping without fertiliser. We conclude that widespread adoption and scaling up of fertiliser trees can reduce the amount of mineral fertiliser needed, maintain the soil ecosystem, and positively impact on the livelihoods of farm households in southern Africa.www.agronomy-journal.or