Florian Wichern

Field measurements of the CO2 evolution rate under different crops during an irrigation cycle in a mountain oasis of Oman

For millennia oasis agriculture has been the backbone of rural livelihood in the desertic Sultanate of Oman. However, little is known about the functioning of these oasis systems, in particular with respect to the C turnover. The objective was to determine the effects of crop, i.e. alfalfa, wheat and bare fallow on the CO 2 evolution rate during an irrigation cycle in relation to changes in soil water content and soil temperature. The gravimetric soil water content decreased from initially 24% to approximately 16% within 7 days after irrigation. The mean CO2 evolution rates increased significantly in the order fallow (27.4 mg C m −2 h −1 ) < wheat (45.5 mg C m −2 h −1 ) < alfalfa (97.5 mg C m −2 h −1 ). It can be calculated from these data that the CO2 evolution rate of the alfalfa root system was nearly four times higher than the corresponding rate in the wheat root system. The decline in CO2 evolution rate, especially during the first 4 days after irrigation, was significantly related to the decline in the gravimetric water content, with r = 0.70. CO2 evolution rate and soil temperature at 5 cm depth were negatively correlated (r =− 0.56, n = 261) due to increasing soil temperature with decreasing gravimetric water content.

Soil Microbial Properties Along a Precipitation Transect in Southern Africa

Soil samples were collected from 24 sites along a precipitation transect in semi-arid and arid southern Africa (Botswana and Namibia), ranging from approximately 550 mm average annual rainfall to less than 50 mm and grouped into four precipitation classes. The objective was to determine whether soil microbial biomass and activity mainly in these classes reflects changes in water availability or to some extent changes in soil chemical properties and texture. The texture of the soils belonging to the four precipitation classes was generally dominated by sand. In the wettest precipitation class (>400 mm), mean contents of soil organic C (5.6 mg g−1 soil), total N (0.51 mg g−1 soil), and microbial biomass C (112 μg g−1 soil), as well as the mean basal respiration (5.6 μg CO2–C g−1 soil d−1) were always highest, exceeding the mean values of the driest precipitation class (<50 mm) 4- to 8-fold. However, the mean microbial biomass C/soil organic ratio reached the highest value (4.3%) in this latter class. Soil organic C, total N, and basal respiration all showed highly significant correlations with the mean annual precipitation. Soil organic C and total N, but not basal respiration, were also significantly correlated with clay content and cation exchange capacity. Microbial biomass C revealed a strong correlation with total N, but especially with soil organic C. Small changes in clay content are just as important as the mean annual precipitation in the present sandy soils. However, significant amounts of soil microorganisms exist even in the driest areas.

Evaluation of the wick method for in situ 13C and 15N labelling of annual plants using sugar-urea mixtures

To investigate the amount and fate of root-derived C and N, often tracer techniques are used, where plants are labelled with isotopes. In the present study, we evaluated the suitability of the cotton wick method for in situ labelling of peas (Pisum sativum L.) and oats (Avena sativa L.) with 13C and 15N simultaneously. With two greenhouse experiments we investigated how the wick method and aqueous urea and sugar solutions at a variety of concentrations affected plant development. In addition, we investigated the distribution of 13C and 15N in plants from column experiments under outdoor conditions. Solution was taken up by the plant from a small vial connected to the stem by a cotton wick which was passed through a hole in the stem of the plants. Generally, solution uptake varied between individual plants and decreased with increasing sugar concentrations. Below-ground, above-ground and total plant dry matter, were not significantly affected by the wick method and the applied solutions. Mixtures of aqueous glucose solutions at 2 to 4% and aqueous urea solutions at 1% are useful carriers of 13C and 15N. However, in the investigated plants isotopes were not homogeneously distributed among plant parts. Above-ground plant biomass was preferentially enriched with 13C and 15N, whereas below-ground plant biomass was generally lower enriched. Moreover, isotope distribution ratio of individual plants varied considerably, independent of plant part or timing of labelling. This must be taken into account when estimating root-derived C and N. Future studies comparing labelling methods need to present the isotope distribution ratios among plant parts to allow a true comparison of the methods and the evaluation of their suitability for estimating rhizodeposition.

Catch crops store more nitrogen below-ground when considering rhizodeposits

Background and aimsOne means of reducing nitrate leaching in temperate farming is to include catch crops in crop rotations, which immobilize residual nitrogen (N) in their biomass. For an accurate quantification of the N stored in catch crops and subsequently released from residues, their total biomass, including roots and rhizodeposits has to be assessed.MethodsIn a pot experiment under controlled conditions, oil and forage radish (Raphanus sativus L. var. oleiformis Pers.) and winter turnip rape (Brassica rapa L. var. silvestris [Lam.] Briggs) plants were leaf-labelled every five to seven days with 15N–urea (99at%) five times during the vegetation. At harvest, plants were separated into shoot, coarse, medium and fine roots by hand picking and wet sieving, respectively. The amount of N derived from rhizodeposition (NdfR) was calculated using two different calculation approaches. In addition to the pot experiment, a field experiment with unlabelled plants was set up to extrapolate the results from the pot experiment to the field scale, at the same time evaluating the influence of additional mineral N fertilization on biomass distribution. The contribution of rhizodeposition to total N in the field was estimated by extrapolating the root-N-to-rhizodeposition-N ratio of the pot experiment to the field experiment.ResultsIn the pot trial, between 4.6 and 10.3% of the total assimilated nitrogen of the catch crops was found as rhizodeposits, which is at the lower end of values from other studies on legumes and non-legumes. In the field experiment the shoot-to-root ratio was lower compared to the pot experiment. Thus, the contribution of rhizodeposition to total N under field conditions is substantially higher. Fertilization in the field trial mostly influenced the formation of above-ground plant biomass.ConclusionsConsidering the rhizodeposition reveals, that the investigated catch crops store more N than previously assumed. As a consequence, catch crops have to be evaluated with a stronger focus on below-ground biomass to make sure the right amounts of N are considered for fertilization schemes in crop rotations.

Dose-dependent reactions of Aporrectodea caliginosa to perfluorooctanoic acid and perfluorooctanesulfonic acid in soil

As a consequence of their widespread use, e.g. as protective coatings for fabrics, and their resistance to thermal and biological breakdown, perfluorinated compounds are increasingly found in the environment, but little is known about their ecotoxicological properties. A 40-day microcosm experiment was carried out to examine the effects of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) on the endogeic geophagus earthworm Aporrectodea caliginosa, its survival and feeding on soil organic C and microbial biomass C. Three levels of concentration (1, 100, and 500 mg kg(-1)) were chosen. The lowest represented the maximum found in sediments and soils and the other two are extreme concentrations that might occur in pollution hotspots and that have been shown to poison organisms. Earthworms promoted the production of CO2 and decreased microbial biomass C in soil, regardless of the presence of PFOA or PFOS. Both compounds significantly decreased the surviving numbers and dry weight of earthworms at concentrations of 100 mg kg(-1). No earthworms survived at PFOA and PFOS concentrations of 500 mg kg(-1). At concentrations of 1 mg kg(-1), no negative effects were observed. The δ(13)C values of A. caliginosa did not differ between treatments. In contrast, the δ(15)N values were significantly increased after adding 1 mg kg(-1) of PFOA, reflecting elevated portions of soil-derived N in the earthworm tissue. In contrast, these portions of soil-derived N were lower in the earthworms after addition of 100 mg kg(-1) of PFOA and PFOS. In conclusion, extreme concentrations of PFOA and PFOS negatively affected endogeic A. caliginosa, whereas a concentration of 1 mg kg(-1) of PFOA and PFOS was related to an increased uptake of soil N by the earthworms.

Der Baobab (Adansonia digitata L.): Wildobst aus Afrika für Deutschland und Europa?!

ZusammenfassungExotische Früchte aus fernen Ländern erlangen in Deutschland und Europa immer mehr an Bedeutung. Gerade in den Tropen und Subtropen ist die Vielfalt an Fruchtarten enorm groß. Das getrocknete Fruchtfleisch des Baobabs (Adansonia digitata L.) ist seit 2008 als neuartige Lebensmittelzutat für die Europäische Union zugelassen und mehr und mehr Produkte kommen auf den Markt.In seiner Heimat Afrika ist der Baobab ein imposanter Baum der Savannen. Aufgrund seines ausgedehnten Wurzelsystems und der hohen Wasserspeicherkapazität im Stamm übersteht er Trockenzeiten und Dürreperioden. In Afrika ist der Baum Nahrungsgrundlage und Bestandteil für Kleidung und Medizin, er bietet den Menschen Schutz und ist Kultstätte, liefert Viehfutter und Material für Gebrauchsgegenstände aller Art.Im Rahmen einer Studie konnten für Europa insgesamt schon mehr als 300 Produkte des Baobabs bzw. mit Baobab-Zutaten identifiziert werden. In Deutschland sind als Lebensmittel sowohl naturbelassenes Baobab-Fruchtpulver als auch Verarbeitungsprodukte wie Getränke, Brotaufstriche, Müsliriegel, Konfekte und Schokoladen mit Baobab-Fruchtfleisch erhältlich. Darüber hinaus ist eine Vielzahl von pharmazeutischen und kosmetischen Produkten mit Extrakten des Baobabs in unseren Geschäften verfügbar. Das Produktspektrum reicht von After Shave und Parfum über Shampoo und Duschgel bis hin zu Beinlotion und Fußspray.Baobab-Setzlinge und -Samen werden in Deutschland für die Anzucht und Kultivierung als Zimmerpflanze verkauft. Der Senegal produziert kommerziell Baobab-Bonsais für den europäischen Markt.In Afrika werden die Früchte fast ausschließlich von wildwachsenden Bäumen geerntet. Jedoch könnte der erwerbsmäßige Anbau des Baobabs eine wichtige Einkommensquelle für die ländliche Bevölkerung darstellen. Dafür sind die weitere Erforschung und Domestizierung dieser Wildobstart sowie der ausdrückliche Schutz der natürlichen Bestände als wichtige genetische Ressource zwingend notwendig.AbstractIn Germany and Europe exotic fruits from distant places are becoming more and more important. In the tropics and subtropics an enormous variety of edible fruits exists. In 2008 the dry fruit pulp of the baobab (Adansonia digitata L.) was approved as novel food for the EU and thus entered the formal food market. Hence, an increasing number of products became available in Europe.The baobab is an impressive tree in the savannas of Africa. Due to its dense and wide spreading root system and its enormous water storing capacity in the stem, the tree can grow under very dry environmental conditions. In many regions of Africa it is a multipurpose indigenous fruit tree species which offers shelter and provides food, fodder and medicine as well as raw material for many purposes.However, even in Europe a wide range of products is now available as shown in the present study, which identified more than 300 baobab products or products with baobab as an ingredient. In Germany, not only the natural baobab pulp but also ready-made foodstuffs such as soft drinks, sandwich spreads, cereal bars, sweets and chocolates are available. Furthermore, a diverse range of pharmaceutical and cosmetic products with extracts from the baobab is available in the market. The products range from after shave and perfume to shampoo and shower gel to leg lotion and foot spray.In Germany baobab saplings and seeds are offered for planting and cultivation as an indoor pot plant. Commercially marketed baobab bonsai seedlings for the European market are produced in Senegal.In Africa the baobab fruits are mainly harvested from wild stands. However, the commercial production of baobab fruits in orchards could be an important future opportunity for income generation to many local households. More research on baobab, including the domestication of this wild fruit tree species, and the conservation of its natural stands as important genetic resource are prerequisites for sustainably increased fruit production.

Africa’s wooden elephant: the baobab tree (Adansonia digitata L.) in Sudan and Kenya: a review

Wild edible fruits hold great potential for improving human diets, especially in agricultural societies of the developing world. In Africa, a well-known supplier of such fruits is the baobab (Adansonia digitata L., Malvaceae), one of the most remarkable trees of the world. Several studies in different African countries have highlighted this indigenous fruit tree as a priority species for domestication and expanded use. However, internationally available information on baobab in East Africa, particularly in Sudan and Kenya, remains scarce. This review aims to shed light on the ecology, diversity and current level of utilization of baobab in East Africa in order to facilitate domestication and conservation of the species. A list of priority research areas is provided at the end of the review to encourage further studies and investment in this unique plant taxon.

In the land of plenty: catch crops trigger nitrogen uptake by soil microorganisms

Background and aimsCatch crops (CC) reduce nitrate leaching, and may resolve a major concern in nitrogen (N) intensive agriculture. CC efficiency depends on N uptake ability, which is related to root development, biomass partitioning, and competition with soil microbes. We investigated the effect of N addition on this with three CC species.MethodsThree CC species were grown in pots with three N concentrations. Shoot and root biomass, C:N content, and specific root length were determined, whereas residual N, dissolved organic N (DON) and C, and microbial biomass N and C were measured.ResultsAddition of N did not consistently effect plant biomass nor its partitioning, probably because of overall high N. However, CC did reduce residual N, and so did soil microorganisms, likely facilitated by C-release from roots. Moreover, plant presence reduced DON, likely through uptake by soil microorganisms, partly followed by plant uptake.ConclusionsCC not only take up residual N themselves, but also trigger considerable N uptake by soil microorganisms that thrive on C-release from roots. This plant-microbe-nitrogen interaction has to be considered when evaluating CC systems. It remains unclear to which extent soil microorganisms immobilise inorganic N and mineralise or take up DON.

Relationship between Remote Sensing Data, Plant Biomass and Soil Nitrogen Dynamics in Intensively Managed Grasslands under Controlled Conditions

The sustainable use of grasslands in intensive farming systems aims to optimize nitrogen (N) inputs to increase crop yields and decrease harmful losses to the environment at the same time. To achieve this, simple optical sensors may provide a non-destructive, time- and cost-effective tool for estimating plant biomass in the field, considering spatial and temporal variability. However, the plant growth and related N uptake is affected by the available N in the soil, and therefore, N mineralization and N losses. These soil N dynamics and N losses are affected by the N input and environmental conditions, and cannot easily be determined non-destructively. Therefore, the question arises: whether a relationship can be depicted between N fertilizer levels, plant biomass and N dynamics as indicated by nitrous oxide (N2O) losses and inorganic N levels. We conducted a standardized greenhouse experiment to explore the potential of spectral measurements for analyzing yield response, N mineralization and N2O emissions in a permanent grassland. Ryegrass was subjected to four mineral fertilizer input levels over 100 days (four harvests) under controlled environmental conditions. The soil temperature and moisture content were automatically monitored, and the emission rates of N2O and carbon dioxide (CO2) were detected frequently. Spectral measurements of the swards were performed directly before harvesting. The normalized difference vegetation index (NDVI) and simple ratio (SR) were moderately correlated with an increasing biomass as affected by fertilization level. Furthermore, we found a non-linear response of increasing N2O emissions to elevated fertilizer levels. Moreover, inorganic N and extractable organic N levels at the end of the experiment tended to increase with the increasing N fertilizer addition. However, microbial biomass C and CO2 efflux showed no significant differences among fertilizer treatments, reflecting no substantial changes in the soil biological pool size and the extent of the C mineralization. Neither the NDVI nor SR, nor the plant biomass, were related to cumulative N2O emissions or inorganic N at harvesting. Our results verify the usefulness of optical sensors for biomass detection, and show the difficulty in linking spectral measurements of plant traits to N processes in the soil, despite that the latter affects the former.