The reactive nature of phosphorus (P) leads to the formation of insoluble Fe, Al and Ca phosphates in highly weathered tropical soils, thus reducing P availability for plant uptake. Biochar with its heterogeneous surface properties as influenced by feedstock and pyrolysis temperature can affect P retention and availability in tropical soils. In the present study, incubation studies were conducted for 90 days to investigate the effect of corn cob and rice husk biochar on P sorption and desorption in two acid (Typic Plinthustult-A & Plinthic Acrudox-B) and one neutral soil (Quartzipsamment-C). The biochars were pyrolyzed at varying temperatures (300 degrees C, 450 degrees C and 650 degrees C) and applied at a rate of 1% (w/w) to the soils. Phosphorus sorption data were fitted to Langmuir and Freundlich models. Phosphorus desorption was done on the residual samples that received initial P concentrations of 21.5 mg L-1, 43.0 mg L-1 and 86.0 mg L-1 solution using 10 mM KCl. The P sorption capacity of the two acid soils i.e. A (395 mg kg(-1)) and B (296 mg kg(-1)) were more than two fold that of the neutral soil (C) (105 mg kg(-1)). Addition of the biochar types to soil A raised the equilibrium P concentration in solution at decreasing pyrolysis temperature. Similar trend was observed in soil B with the exception of corn cob and rice husk biochar at 650 degrees C which increased the soil's (B) P sorption capacity. In soil C, both biochar types increased P sorption capacity with increasing pyrolysis temperatures. Phosphorus desorbability increased with increasing initial P concentrations in the three soils. Generally, P desorbability increased in the acid soils but decreased in the neutral soil upon biochar amendment. Decreases in P adsorption and consequently increases in P desorption were more pronounced when the 300 degrees C biochar types were amended with the soils. The study thus showed that biochar pyrolyzed at 300-450 degrees C could be used to reduce P sorption and increase P bioavailability especially in acid soils. The addition of biochar to neutral or alkaline soils might increase P retention possibly in the short-term, reducing P bioavailability.

The National Museum of Denmark and the Department of Geography at the University of Copenhagen have collaborated on a project investigating burial mounds near early Medieval churches. The aim was to identify a possible continuity in cult sites across the shift to Christianity in the late Viking Age. Charcoal samples from 18 mounds were radiocarbon dated but the results showed they were far older than expected. Control dating undertaken on burial mounds of known age confirmed that charcoal in the mound fill can at least be up to 3000?years older than the mound itself. As charcoal can survive in the surface soil layer for millennia, in spite of ploughing, bioturbation and frost, it may also dominate the charcoal pool of the grass or heather turfs used in the mound construction. Therefore, the article concludes, charcoal cannot be used to securely date archaeological features built with turfs and it is important to be aware of the possible presence of very old charcoal when selecting material for dating archaeological features, even those which otherwise would be judged unaffected by material from earlier archaeological periods.

In the tropical moist semi-deciduous forests of West Africa, soil catenas with extremely gravel-rich soil horizons at the summits and upper slopes and largely gravel-free profiles at the lower slope are common. Previous investigations have suggested that these gravel layers are the result of macro-invertebrates mining of fine-grained soil material from the subsoil leaving behind the gravel, to build galleries at the surface subsequently exposing it to water erosion transport downslope. We examined the indirect effect of this process on the distribution along a soil catena of crucial base cations (Ca2+, Mg2+, K+) and plant available phosphorous (P), which is often growth limiting in these tropical ecosystems. We found that the export of fine-grained soil material at the top of the catena reduces the soil stocks (to 1 m) of these elements by up to 60%, while the soil fertility downslope did not change significantly. This important long-term (100-1000 yr scale) reduction in soil fertility at the top of slopes resulting from bioturbation and water erosion is overlooked in contemporary literature, which primarily focus on the beneficial impact termites and ants have on ecosystem functioning in more level savannah landscapes. As the type of catena studied is widespread across tropical environments, this effect is likely ecologically substantial. Future research should aim at understanding such long-term consequences of bioturbation on landscape ecology as well as soil heterogeneity and fertility, so we do not overlook potential negative ecosystem effects.

Affiliations: 6 Department of Physical Geography and Ecosystem Science, Lund University, Sölvegatan 12, 7 223 62 Lund, Sweden. 8 Department of Geosciences and Natural Resource Management, University of Copenhagen, 9 1350 Copenhagen K, Denmark. 10 School of Energy, Environment and AgriFood, Cranfield University, College Road, Cranfield, 11 MK43 0AL, UK. 12 European Commission, DG Joint Research Centre, Via E. Fermi 2749, 21027 Ispra (VA), Italy. 13 Correspondence to jeppe.aa.kristensen@gmail.com 14 Deceased 15 These authors contributed equally to this work. 16

Leaching large amounts of acidity and metals into recipient watercourses and estuaries, acid sulfate (a.s.) soils constitute a substantial environmental issue worldwide. Mapping of these soils enables measures to be taken to prevent pollution in high risk areas. In Denmark, legislation prohibits drainage of areas classified as potential a.s. soils without prior permission from environmental authorities. The mapping of these soils was first conducted in the 1980’s. Wetlands, in which Danish potential a.s. soils mostly occur, were targeted and the soils were surveyed through conventional mapping. In this study, a probability map for potential a.s. soil occurrence was constructed for the wetlands located in Jutland, Denmark (c. 6500km2), using the digital soil mapping (DSM) approach. Among the variety of available DSM techniques, artificial neural networks (ANNs) were selected. More than 8000 existing soil observations and 16 environmental variables, including geology, landscape type, land use and terrain parameters, were available as input data within the modeling. Prediction models based on various network topologies were assessed for different selections of soil observations and combinations of environmental variables. The overall prediction accuracy based on a 30% hold-back validation data reached 70%. Furthermore, the conventional map indicated 32% of the study area (c. 2100km2) as having a high frequency for potential a.s. soils while the digital map displayed about 46% (c. 3000km2) as high probability areas for potential a.s. soil occurrence. ANNs, thus, demonstrated promising predictive classification abilities for the mapping of potential a.s. soils on a large extent.

Klangshøj is a flat-topped burial mound similar to the Royal Jelling mounds, although smaller. The myths tell that a well has existed on top of the mound as at Jelling and a spring had flown at the base of the mound. In order to verify the myths and a similar hydrology in Klangshøj as found in Jelling, several borings have been carried out in a north-south line across the mound. The investigation showed that Klangshøj is built of sods mainly harvested from heathland. The sods are of different grain sizes from fine sand to clay. The preservation conditions were excellent in three of the six borings, where undecomposed plant remnants, occasionally greenish, were observed. A 14C-dating showed that the mound was built in the Viking Age. The hydrology in Klangshøj is the same as in the Jelling mounds, with a permeable bioturbation zone covering almost impermeable, distinct sod layers. This form a perched groundwater table in the transition zone, which keeps the distinct sod layer below anaerobic, i.e. the preservation conditions extremely favourable. The perched water table drains internally as in the Jelling mounds, and there are no current nor fossil evidence to suggest a spring was ever present at the foot slope, as the local legend suggests. Moreover, it seems unlikely that a well, similar to the one on the Jelling mound, has existed on the top of the north-facing slope, as the amount of water the well would have been able to collect is little.

Every year in December to February the dust laden Harmattan blows from Sahara towards southwest to the countries along the east west going coast of the Gulf of Guinea. Only few investigations have quantified external dust deposition in forests and plantations in that region and hereby the nutrient input by dust. This paper investigates three different methods for sampling and quantifying dust deposition in natural forests and plantations in the moist semi-deciduous forest zone in Ghana. The three methods tested were the following: bowls with water, plastic mats with straws and funnels for determining canopy drip. The study shows that the canopy drip method overestimates the external dust deposition due to a significant enrichment of organic matter during the throughfall. Therefore, it is necessary to combine the canopy drip method with sampling from outside the forest to determine the external organic matter content in the dust deposited to a forest. The study showed that the dust deposition in the forest and in the rubber and cocoa plantations was 1.5 to 2.0 times bigger than on the plastic mats located outside the forested area and further, that the external nutrient input by dust to the natural forests is very low in the Kade area, only about 0.2 kg ha-1 total P and the readily available Ca, Mg and K are respectively 0.4 kg ha-1, 0.2 kg ha-1 and 0.7 kg ha-1.

In order to measure dust's nutrient input on farmland in different agro-ecological zones, Harmattan dust was sampled by mats with plastic straw in Ghana between 2002–2006. The inputs of total nutrients by Harmattan dust in Ghana per Harmattan period were about 1–2 kg Ca ha−1, 0.5–2 kg K ha−1, 0.5–1.5 kg Mg ha−1 and less than 0.5 kg P ha−1. Compared with the annual input of nutrients by precipitation, the dust accounted for 10% or less of Ca, Mg and K but approximately 20%–40% of P. The input of nutrients by dust was only valid for areas with vegetation, because in areas with none or sparse vegetation, loss of soil due to wind erosion and hereby loss of nutrients might be significant. In farmland areas with bare and vegetated fields there seemed to be an internal redistribution of the nutrients and not a net gain of nutrients from outside the area (long-range transported dust). The input of P by dust might be of some importance in the traditional shifting cultivation systems, while the inputs of other three nutrients of Ca, Mg and K were so low that they must be considered insignificant. In the intensive agriculture systems with huge inputs of manures and fertilizers the nutrient input by dust is insignificant and could be neglected.

Long-term application of manure to sandy soils to ensure high crop productivity may lead to phosphorus (P) leaching, which, in turn, may deteriorate the quality of recipient waters because of eutrophication. The risk of P leaching depends on contents of aluminum (Al) and iron (Fe) oxides that are strong P sorbents and of calcium (Ca) that can form sparingly soluble Ca-P compounds. While P sorption by Al and Fe oxides is limited, the formation of sparingly soluble Ca-P may theoretically continue as long as enough Ca is available, i.e. under such conditions long-term application of manure may be environmentally sustainable. This hypothesis has been tested on soils formed on beach sand at Keta, Ghana, which have been used since the 1930s for intensive cultivation based on manure application. Soil samples were collected from different depths down to 80 cm in profile pits on cultivated soils that received manure over 70 years and on uncultivated plots (controls). The samples were analyzed for texture, pH, total C and P and poorly ordered Al and Fe oxides as well as different P forms as assessed by the Hedley fractionation method. The results showed a fourfold P increase in the top 40 cm of the cultivated soils, whereas in the deepest soil layers the P contents in the cultivated and uncultivated soils were almost the same indicating very limited downward P transport despite long-term manure application. This was supported by comparable P concentrations in groundwater taken under cultivated and uncultivated sites. P fractionation showed that the cultivated top soils were dominated by sparingly soluble Ca-P compounds that accounted for about 70% of the P gain. Thus, the results suggest that intensive crop production under tropical semi-arid conditions may be environmentally sustainable when based on manure fertilization ensuring enough Ca supply and neutral to alkaline pH.

The presence of coarse fragments can have profound impact on soil moisture retention characteristics. The study was conducted to assess the effects of coarse fragments on the moisture retention characteristics of 16 soil series, developed over five different parent materials in the Densu basin. Soil profiles were excavated at five locations, to depths within 1.5 m in the field. Undisturbed soil core samples and disturbed samples were taken in triplicates from the major genetic horizons of each soil type within the effective root depth of 1 m. Coarse fragments content of soil more than 2 mm was measured on mass basis by sieving through a 2-mm mesh. Soil moisture retention was determined using the pressure plate apparatus at suctions of pF 1 (1.0 kPa), pF 2 (10.0 kPa), pF 2.5 (33.0 kPa) and pF 3 (100.0 kPa) for the undisturbed and pF 4.2 (1500 kPa) for the disturbed samples.The volumetric moisture content between field capacity (FC) pF 2.5 (33.0 kPa) and permanent wilting pointing (PWP) pF 4.2 (1500.0 kPa) was used to evaluate the available water content (AWC) by volume and then converted to root zone available water capacity (RZAWC) in millimetres (mm) assuming an effective root depth of 1 m within the basin. Results showed that soils formed over granite and its associations have high percentage of coarse fragments while soils developed over phyllites and its associations have high clay percentage. Soil organic matter was high in the topsoil of all profiles, ranging from 0.81 to 4.44% compared with the horizons below, and the bulk density of the topsoils were less than the limiting value of 1.6 Mg m-3. Site-specific moisture retention characteristics of the various soil series have been delineated. It was evident from the analyses that soils containing high clay content gave high RZAWC values compared with soils with high coarse fragments. Most of the topsoils of the profiles gave high RZAWC values compared with sub-layers with high amounts of coarse fragments. Critical water for plants establishment within the basin in the surface layer was quite favourable.

In the context of major global environmental challenges such as food security, climate change, fresh water scarcity and biodiversity loss, the protection and the sustainable management of soil resources in Africa are of paramount importance. To raise the awareness of the general public, stakeholders, policy makers and the science community to the importance of soil in Africa, the Joint Research Centre of the European Commission has produced the Soil Atlas of Africa. To that end, a new harmonised soil map at the continental scale has been produced. The steps of the construction of the new area-class map are presented, the basic information being derived from the Harmonized World Soil Database (HWSD). We show how the original data were updated and modified according to the World Reference Base for Soil Resources classification system. The corrections concerned boundary issues, areas with no information, soil patterns, river and drainage networks, and dynamic features such as sand dunes, water bodies and coastlines. In comparison to the initial map derived from HWSD, the new map represents a correction of 13% of the soil data for the continent. The map is available for downloading.

The Harmattan is a dry dust‐laden continental wind, and in the boreal winter Harmattan dust plumes affects many West African countries, including Ghana. When the Harmattan is strongest the southern part of Ghana is affected by the Inter Tropical Discontinuity (ITD). In this study, we investigate if the ITD functions as a barrier, preventing long transported Harmattan dust to settle south of, and below, it. This is done by analyzing a Harmattan dust outbreak, mapped using Earth observation (EO) data from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard the Meteosat Second Generation (MSG) platform, coupled with data from West African AERONET stations, and comparing these observations with wind data from NOAA's Air Resources Laboratory (ARL) program and the mineral suite of samples from seasonal dust deposits in north and south Ghana. In northern Ghana traces of minerals indicate a weak influence of particles from an arid environment, which is found consistent with the mapped dust plumes and NE wind directions. In southern Ghana the mineral composition show no sediments of an arid origin, the mapped dust plumes is less intense, and the surface wind directions and wind mass trajectories are more varying with lower wind speeds. Based on the results of this study it is concluded that dust deposited, or measured near ground, in the Harmattan period under the ITD, and south of it, does not contain material from the Chad Basin due to the local winds conditions.

•Carbon stock in Viking Age soils.•Carbon stock in modern soils with low and high inputs of manure.•Burial mounds as carbon stock archive.•Long term carbon loss/gain in farmland soils.

Soil organic carbon (SOC) in tropical forest soils is typically characterized by fast turnover rates but mineralization might be inhibited resulting in long term organic matter protection. This study focuses on SOC trapped in iron oxides in a Paleudult (chromic Acrisol) located in the moist semi-deciduous forest in Ghana by determining the age of the SOC, quantifying the geochemical characteristics of the SOC, and estimating how much of the SOC that is bound so tightly to the oxides that it does not participate in the normal turnover of organic matter in the soil. The SOC has been characterized by using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy and 14C dating. The turnover rate of the reactive SOC was determined by basal soil respiration (BSR) and a ratio between the SOC stock in the soil. The investigation shows that SOC associated with iron nodules has a 14C age of more than 21,000years and the absence of CO2 release during BSR measurements reveals that iron nodules have trapped and immobilized ~21% of SOC stored in the top 1m (~3.4kgCm−2). NMR results indicate that trapped SOC in the iron nodules is more decomposed than the organic matter in the topsoil.