Soil degradation assessment in mixed farming systems of southern Africa: use of nutrient balance studies for participatory degradation monitoring

Soil degradation assessments for mixed farming systems of the Molopo Basin (North West Province, South Africa and Southern District, Botswana) are provided from farmer–based research, designed to quantify nutrient fluxes across the farming system and to analyze the social, economic, political and environmental factors affecting nutrient management practices. This paper discusses the practical difficulties of how to use participatory farmer–led studies to assess soil degradation extent and its causes, and of then disseminating this information to farming communities and policymakers. Nutrient balance studies show that land degradation is evident on arable fields as soil nutrient depletion (the main threat to poorer manure–reliant farmers) and soil acidification (the main threat to wealthier farmers who have become dependent on inorganic fertilizer additions). Integrated nutrient management strategies involving both compound fertilizer additions and regular manure inputs can mitigate most soil degradation even on the sandy infertile Kalahari soils, but remain infrequently practised. The need to retain nutrient flows through the livestock sector from rangeland to arable land is thus vital to environmental sustainability and offers an applicable entry point for agricultural development initiatives and support. Factors identified as threatening the flow of nutrients from rangelands to arable lands include policy settings in terms of the different support programmes offered to communal farmers, village–level extension advice, household poverty levels and labour constraints.     

The SWAN model used to study wave evolution in a flume

The SWAN numerical model is used to model the evolution of JONSWAP wave spectra and hence the significant wave height of waves in a tank. Comparison with experiment has shown that modelling triad interactions in the numerical model leads to too low predictions of spectra and significant wave height and should therefore be excluded. The modelling of the breaking constant was also investigated, by looking at the use of a constant breaking constant, Nelson formula, and Goda formula (added into SWAN for this study). Using a constant value of 0.78 within SWAN gave the best comparison between theory and experiment.     

A comparison of black smoker hydrothermal plume behavior at Monolith Vent and at Clam Acres Vent Field: Dependence on source configuration

Quantitative visualization of acoustic images is used to compare the properties and behavior of high temperature hydrothermal plumes at two sites with different source configurations, increasing our understanding of how plume behavior reflects source configuration. Acoustic imaging experiments were conducted at the Clam Acres area of the Southwest Vent Field, 21°N East Pacific Rise and at Monolith Vent, North Cleft segment, Juan de Fuca Ridge. At Clam Acres, black smokers discharge from two adjacent chimneys which act as point sources, whereas multiple vents at Monolith Vent define a distributed elliptical source. Both plumes exhibit consistent dilution patterns, reasonable fits to the expected power law increase in centerline dilution with height, and simple bending of plume centerlines in response to ambient currents. Our data suggest that point source vents are associated with ordered plume structure, normal entrainment rates, and initial expansion of isosurfaces while distributed source vents are associated with disorganized plume structure, variable entrainment rates, and initial contraction of isosurfaces.     

America's Changing Farmscape: A Study of Agricultural Tourism in Michigan*

Agricultural tourism incorporates visits to farms for the purposes of on‐site retail purchases, enjoyment, and education. Long popular in the European Union (EU), agritourism is gaining popularity throughout the United States. Interest has grown as a result of stagnant grain prices, rising farm costs, and growing international competition. For rural areas seeking new economic options, the potential of these operations to generate new sources of income through sales and horizontal linkages to other tourism‐based activities has sparked interest beyond the farm gate. This article, based on a survey and a statistical analysis of 300 agritourism operations in Michigan, summarizes factors associated with successful operations.     

‘Where is the life in farming?’: The viability of smallholder farming on the margins of the Kalahari, Southern Africa

This paper investigates the ways in which national and regional policies relate to farming activities and concerns amongst the rural population in an area of southern Africa. The struggle to make a living through farming was a common theme to emerge from research about changing livelihoods in response to both variability in the environment and changes in policy. This local discourse echoed regional debates about land and agrarian reform in post-apartheid South Africa and the uncertain future of mixed farming in Botswana. It also raised broader questions about the viability of the future of small-medium-scale farming systems in rural areas in Africa, especially those within dynamic dryland environments such as the Kalahari. This paper looks specifically at the links between poverty and asset holding and aims to identify the ways in which people are or are not able to utilise or mobilise these assets in times of need. We argue that this can vary significantly between seemingly similar settlements, and similar households and that understanding this complexity is the key to recognising how future interventions many impact upon people’s lives. Too often, in the quest to produce understandings of poverty and livelihoods, the complexity, incongruity and reality of day-to-day practices are overlooked. Thus we seek to draw out the interactions between policy and natural resource use, and the capital asset changes involved in these interactions, which influence the sustainability of livelihoods and the differing levels of poverty and vulnerability.     

On the drag coefficient over shallow water

It is shown that if the wind-wave spectrum in shallow water is approximately independent of wind speed due to the combined effects of white-capping and bottom friction, then the wave-induced drag coefficient has a maximum value when the wind speed is twice the maximum wave-speed; as the wind speed increases further, the drag coefficient slowly decreases. This result is consistent with the observations of Hicks et al. (1974).     

Enriched H2, CH4, and He concentrations in hydrothermal plumes associated with the 1996 Gorda Ridge eruptive event

The rapid field response in March and April 1996 to diking-eruptive events on the Northern Gorda Ridge allowed the first detailed co-measurement of ³He, H₂, and CH₄ in plumes associated with a submarine eruption, and examination of the chemical transformation of these volatile species during aging of the plumes. The diking-eruptive event generated extensive event plumes with a low ³He/heat ratio of ∼0.4×10^-12 cm³ STP gm^-1 °C^-1, while chronic plumes had ³He/heat ratios of 2.1–6.2×10^-12 cm³ STP gm^-1 °C^-1. Hydrogen concentrations in the initial event plume (EP96A) reached a maximum value of 47 nM, 235 times that of ocean background concentration (0.2 nM). In contrast, roughly one month later H₂ concentrations, although still elevated, had drastically dropped to 40 times background values. Methane concentrations in both event and chronic plume fluids were also elevated and similar to stable hydrothermal systems in unsedimented ridge environments, reaching a maximum value of 7 nM. The intense rapid interaction at very high temperatures of hydrothermal fluids with the host rock during dike intrusion or with seawater during extrusion of the basalt likely generated high, localized concentrations of H₂. The data indicate, however, that maximum rates of H₂ production during event plume formation could have lasted a few days at most.     

Biogeochemical studies of a Native American runoff agroecosystem

Research on soil fertility is presented in the context of runoff agriculture, a venerable farming system that has been used for millennia in arid to semiarid regions, where water is a major limiting resource for crop production. The agroecology of runoff farming was studied with the Zuni to evaluate nutrient and hydrologic processes, management, maize productivity, and soil quality in some of the oldest recognized fields in the United States. This ancient Southwest agriculture has functioned without conventional irrigation or fertilization by tapping into biogeochemical processes in natural watersheds connected to fields. Carefully placed fields are managed on alluvial fans and other valley margin landforms to intercept runoff and associated sediment and organic debris transported from adjoining forested uplands. We report on research to evaluate and link nitrogen and phosphorus, two key nutrients for crop production, in watershed, soil, and crop components of this agroecosystem. Nutrient data have been collected by observational and experimental methods for each component and the transport of nutrients from watershed to field to maize. The condition of Zuni agricultural soils suggests that their knowledge and management of soils contributed to effective conservation. This study and others indicate the need for further long‐term monitoring and experimental research on watersheds, runoff processes, field soils, and crops across a range of arid to semiarid ecosystems. © 2007 Wiley Periodicals, Inc.