A rapid assessment of anthropogenic disturbances in East African wetlands

The use of East African freshwater wetlands for agriculture has increased in recent decades, raising concerns about potential impacts on wetlands and the long-term sustainability of such land use trends. WET-health is an indicator-based rapid wetland assessment approach developed in South Africa. It allows determining the conditions of wetlands in four assessment modules (hydrology, geomorphology, vegetation, and water quality) by observing the degree of deviation of a wetland from its anticipated natural reference state. We tested the transferability of the WET-health concept for East African inland valley swamps and floodplain wetlands based on 114 assessment units at four study sites. Due to large wetland areas and different environmental settings in East Africa, we modified the original approach using a random selection of assessment units and an assessment scheme based on disturbance types (Appendices A and B). Estimated WET-health impact scores were matched with biophysical and socioeconomic variables using a generalized linear mixed model. Land use included largely undisturbed wetland units occurring side by side with seasonally cropped or grazed units, and drained, permanently cultivated units. A strong differentiation of impact scores between the four assessment modules was apparent with highest scores for vegetation and lowest scores for geomorphology. Vegetation and water quality responded most sensitively to land use changes. The magnitude of wetland disturbance is predominantly determined by management factors such as land use intensity, soil tillage, drainage intensity, and the application of agrochemicals and influences vegetation attributes and the provision of ecosystem services. The proposed modification of WET-health enables users to assess large wetland areas during relatively short periods of time. While further studies will be required, WET-health appears to be a promising concept to be applied to wetlands in East Africa and possibly beyond.     

Effects of kelp (Macrocystis integrifolia) on soil chemical properties and crop response

In 1981 a two-year field plot experiment was established to assess the effects of quantities (0, 7.5, 15, 30, 60 and 120 t ha⁻¹) of fresh kelp (Macrocystis integrifolia) on crop growth and nutritional response and chemical properties of a fine-textured soil. Soil was analyzed for NO₃−N, NH₄−N, electrical conductivity, pH, Cl and exchangeable cations (K, Mg, Ca, Mn and Na). The plots were planted to beans (Phaseolus vulgaris) in the first year and peas (Pisum sativum) in the second year. Marketable bean yields increased in the first year with kelp applications up to 60 t ha⁻¹, with yields, emergence and flowering being reduced by the 120 t ha⁻¹ application. Soluble salts (EC) and Cl concentrations in the soil eight days after application increased linearly and sharply with increasing quantities of kelp. Increased K concentration and moisture content, characteristics of plants growing in a salt-stressed soil environment, were measured. A subsequent companion greenhouse experiment confirmed that the reduced bean emergence and growth with 120 t ha⁻¹ applications of kelp were primarily due to soluble salts. The only growth effects upon peas in the second year was a slight reduction in leaf plus stem yields with increasing applications of kelp.     

Modeling long-term crop response to fertilizer and soil nitrogen

A simple nitrogen balance model to calculate long-term changes in soil organic nitrogen, nitrogen uptake by the crop and recovery of applied nitrogen, is presented. It functions with time intervals of one year or one growing season. In the model a labile and a stable pool of soil organic nitrogen are distinguished. Transfer coefficients for the various inputs of nitrogen are established that specify the fractions taken up by the crop, lost from the system, and incorporated in soil organic nitrogen. It is shown how input data, model parameters and initial pool sizes can be derived and how the model can be used for calculating long-term changes in total soil organic nitrogen and uptake by the crop. For nitrogen applied annually as fertilizer or organic material the time course of nitrogen uptake and recovery of applied nitrogen is calculated. To test the sensitivity of the model, calculations have been performed for different environmental conditions with higher or lower risks for losses. The model has also been applied to establish fertilizer recommendations for a certain target nitrogen uptake by the crop. Finally, for agricultural systems where periods of cropping alternate with peroids of green fallow the time course of nitrogen uptake by the crop is calculated.     

Above- and belowground insect herbivores differentially affect soil nematode communities in species-rich plant communities

Interactions between above‐ and belowground invertebrate herbivores alter plant diversity, however, little is known on how these effects may influence higher trophic level organisms belowground. Here we explore whether above‐ and belowground invertebrate herbivores which alter plant community diversity and biomass, in turn affect soil nematode communities. We test the hypotheses that insect herbivores 1) alter soil nematode diversity, 2) stimulate bacterial‐feeding and 3) reduce plant‐feeding nematode abundances. In a full factorial outdoor mesocosm experiment we introduced grasshoppers (aboveground herbivores), wireworms (belowground herbivores) and a diverse soil nematode community to species‐rich model plant communities. After two years, insect herbivore effects on nematode diversity and on abundance of herbivorous, bacterivorous, fungivorous and omni‐carnivorous nematodes were evaluated in relation to plant community composition. Wireworms did not affect nematode diversity despite enhanced plant diversity, while grasshoppers, which did not affect plant diversity, reduced nematode diversity. Although grasshoppers and wireworms caused contrasting shifts in plant species dominance, they did not affect abundances of decomposer nematodes at any trophic level. Primary consumer nematodes were, however, strongly promoted by wireworms, while community root biomass was not altered by the insect herbivores. Overall, interaction effects of wireworms and grasshoppers on the soil nematodes were not observed, and we found no support for bottom‐up control of the nematodes. However, our results show that above‐ and belowground insect herbivores may facilitate root‐feeding rather than decomposer nematodes and that this facilitation appears to be driven by shifts in plant species composition. Moreover, the addition of nematodes strongly suppressed shoot biomass of several forb species and reduced grasshopper abundance. Thus, our results suggest that nematode feedback effects on plant community composition, due to plant and herbivore parasitism, may strongly depend on the presence of insect herbivores.     

Standing crop densities and distribution of Spirulina and benthic diatoms in East African alkaline saline lakes

SUMMARY. The diversity of Eastern Rift Valley alkaline saline lakes is illustrated and their temporal fluctuations in ionic concentration are emphasized.
The standing crop densities of phytoplanktonic blue-green algae of the genus Spirulina and benthic diatoms were measured. Spirulina abundance was measured as dry mass density and the validity of the gravimatric estimates was checked by chlorophyll analysis and microscopic counting. Benthic diatom standing crops were assessed by chlorophyll determinations. Spirulina density is extremely variable in time and may fluctuate over more than an order of magnitude. High density blooms of Spirulina are not a common characteristic feature of these lakes. The causes of changes in Spirulina standing crop and the associated alterations in the structure of the primary producer community are discussed and three qualitative states which may occur are described.
Similar standing crops of benthic diatoms (mean 45 mg m⁻² of chlorophyll-α) were found at all lakes wherever the water depth was shallow enough to allow light penetration to the sediment surface.     

Soil and total ecosystem respiration in agricultural fields: effect of soil and crop type

A study was made of the effect of soil and crop type on the soil and total ecosystem respiration rates in agricultural soils in southern Finland. The main interest was to compare the soil respiration rates in peat and two different mineral soils growing barley, grass and potato. Respiration measurements were conducted during the growing season with (1) a closed-dynamic ecosystem respiration chamber, in which combined plant and soil respiration was measured and (2) a closed-dynamic soil respiration chamber which measured only the soil and root-derived respiration. A semi-empirical model including separate functions for the soil and plant respiration components was used for the total ecosystem respiration (TER), and the resulting soil respiration parameters for different soil and crop types were compared. Both methods showed that the soil respiration in the peat soil was 2–3 times as high as that in the mineral soils, varying from 0.11 to 0.36 mg (CO₂) m^–2 s^–1 in the peat soil and from 0.02 to 0.17 mg (CO₂) m^–2 s^–1 in the mineral soils. The difference between the soil types was mainly attributed to the soil organic C content, which in the uppermost 20 cm of the peat soil was 24 kg m^–2, being about 4 times as high as that in the mineral soils. Depending on the measurement method, the soil respiration in the sandy soil was slightly higher than or similar to that in the clay soil. In each soil type, the soil respiration was highest on the grass plots. Higher soil respiration parameter values (R_s0, describing the soil respiration at a soil temperature of 10°C, and obtained by modelling) were found on the barley than on the potato plots. The difference was explained by the different cultivation history of the plots, as the potato plots had lain fallow during the preceding summer. The total ecosystem respiration followed the seasonal evolution in the leaf area and measured photosynthetic flux rates. The 2–3-fold peat soil respiration term as compared to mineral soil indicates that the cultivated peat soil ecosystem is a strong net CO₂ source.     

Vegetation and soil properties in restored wetlands near Lake Taihu,China

Riparian wetlands are important components of the lake ecosystem, and they play essential roles in maintaining system health. Remediation of degraded lakeshore wetlands is an essential component of lake restoration. A study was conducted to investigate the restoration of lakeshore wetlands, which were converted to rice fields and then abandoned for 2, 5, 10 and 15 years, near Lake Taihu. Soil samples (0–20 cm and 20–40 cm) were taken and plant species were investigated. The carbon content in the soil had increased significantly, rising from 0.71% to 1.85% between 2 and 15 years. Organic matter accumulation improved soil texture, and water stable aggregate content (>0.25 mm) and soil porosity increased. Total nitrogen in the soil increased from 0.06% to 0.13%, and total Kjeldahl nitrogen increased from 124.4 mg kg⁻¹ to 351.5 mg kg⁻¹. Total phosphorus in the soil increased from 0.045% to 0.071%, and the Olsen-P value increased from 5.13 mg kg⁻¹ to 16.0 mg kg⁻¹. Results showed that phosphorous did not increase as much as nitrogen. In the vegetation restoration process, plant species composition moved towards a natural wetland community, and spatial heterogeneity and landscape diversity increased. The richness of plant biodiversity increased rapidly in the first 2 years, then more slowly in later restoration stages. The wetlands recovery process may be complicated by interactions of biota and soil and hydrological conditions.     

Variation of microbial communities in soil, rhizosphere, and rhizoplane in response to crop species, soil type, and crop development.

We investigated the influence of plant species, soil type, and plant development time on the shaping of microbial communities in soil and in association with roots. The sample group consisted of a total of 32 microcosms in three habitats: soil, rhizosphere, and rhizoplane. Communities were represented by the patterns of a sequence-specific separation of rRNA target sequences. Effects of experimental parameters were classified by a cluster analysis of pattern similarities. The type of plant species (clover, bean, or alfalfa) had the greatest effect in plant-associated habitats and also affected soil patterns. Plant development had a minor habitat-dependent effect that was partly obscured by replicate variation. The results stress the applicability of biased community representations in an analysis of induced variation.     

Wood chip soil amendments in restored wetlands affect plant growth by reducing compaction and increasing dissolved phenolics

Adding chipped wood to soil ameliorates compaction, allowing faster plant growth that is critical to successful wetland restorations. Following the filling and planting of an erosion gully in Halstead Meadow, Sequoia National Park, the tallest leaf height and maximum clone width of transplanted Scirpus microcarpus seedlings were negatively correlated with soil compaction. Plant height decreased by 9.8 cm and width decreased by 11.9 cm per MPa of soil compaction (range of 0.74–4.50 MPa). We experimentally amended mineral soil in a test trench and found that every 0.10 cm³/cm³ addition of wood chips (range of 0.00–0.75 cm³/cm³) reduced compaction by 0.174 MPa. Had the Halstead Meadow gully fill contained an equivalent volume of wood chips to the reference area soil organic matter content (0.64 cm³/cm³), we predict compaction would have been reduced by 1.11 MPa, increasing individual transplant width spread by 36%, approximately doubling the vegetated area after two growing seasons. In a greenhouse phytometer experiment, conifer bark leachate (phenolics 211 mg/L) significantly reduced plant growth and, in the presence of added nutrients, increased the production of the enzyme polyphenol oxidase (PPO). However, phenolics concentration in bark‐free conifer wood leachate (12 mg/L), similar to field‐sampled concentrations, did not affect plant growth or PPO production. Pure conifer bark is not recommended as a soil amendment, but the addition of low‐bark‐content wood chips to gully fill may be a feasible and effective means of reducing soil compaction, accelerating plant establishment, and lowering wetland restoration project costs.     

高原湿地沼泽化草甸土壤真菌与理化性质的关系

以高原湿地纳帕海沼泽化草甸为研究对象, 采用稀释培养结合形态鉴定比较分析0—20 cm、20—40 cm土层的土壤真菌多样性及群落结构组成, 以及土壤理化性质对土壤真菌多样性及群落结构组成的影响。结果表明: 0—20 cm和20—40 cm土层中的真菌数量、Shannon-Wiener多样性指数(H')、均匀度指数(JSW)和丰富度指数(DMA)均表现为0—20 cm土层> 20—40 cm土层; 分别分离得到土壤真菌12属和10属, 其中木霉属、青霉属、腐霉属、曲霉属同为沼泽化草甸两个土层的优势类群, 表现出较高的相似性, 同时20—40 cm未发现枝孢菌属和壳囊孢属, 又表现出一定的差异性。经RDA冗余分析, 土壤有机质、全氮、速效钾、速效磷、pH、容重和自然含水率可能是影响沼泽化草甸土壤真菌群落结构组成的主要因子。     

Macroinvertebrate abundance, water chemistry, and wetland characteristics affect use of wetlands by avian species in Maine

Our objective was to determine use by avian species (e.g., piscivores, marsh birds, waterfowl, selected passerines) of 29 wetlands in areas with low (<200 μeq l⁻¹) acid-neutralizing capacity (ANC) in southeastern Maine. We documented bird, pair, and brood use during 1982–1984 and in 1982 we sampled 10 wetlands with a sweep net to collect invertebrates. We related mean numbers of invertebrates per wetland to water chemistry, basin characteristics, and avian use of different wetland types. Shallow, beaver (Castor canadensis)-created wetlands with the highest phosphorus levels and abundant and varied macrophyte assemblages supported greater densities of macroinvertebrates and numbers of duck broods (88.3% of all broods) in contrast to deep, glacial type wetlands with sparse vegetation and lower invertebrate densities that supported fewer broods (11.7%). Low pH may have affected some acid-intolerant invertebrate taxa (i.e., Ephemeroptera), but high mean numbers of Insecta per wetland were recorded from wetlands with a pH of 5.51. Other Classes and Orders of invertebrates were more abundant on wetlands with pH > 5.51. All years combined use of wetlands by broods was greater on wetlands with pH ≤ 5.51 (77.4%) in contract to wetlands with pH > 5.51 that supported 21.8% of the broods. High mean brood density was associated with mean number of Insecta per wetland. For lentic wetlands created by beaver, those habitats contained vegetative structure and nutrients necessary to provide cover to support invertebrate populations that are prey of omnivore and insectivore species. The fishless status of a few wetlands may have affected use by some waterfowl species and obligate piscivores.     

Long-term fertilisation strategies and form affect nutrient budgets and soil test values,soil carbon retention and crop yield resilience

Plant and Soil - The aim of this study was to evaluate the effects of long-term mineral and organic fertilisation on crop performance and soil fertility. The Long-Term Nutrient Depletion Trial...     

Dynamics of the linuron hydrolase libA gene pool size in response to linuron application and environmental perturbations in agricultural soil and on-farm biopurification systems

libA, a gene encoding a novel type of linuron hydrolase, was recently identified in the linuron-mineralizing Variovorax sp. strain SRS16. In order to assess the contribution of libA to linuron degradation in environmental settings, libA abundance was monitored in response to the application of linuron and to environmental perturbations in agricultural soil microcosms and microcosms simulating the matrix of on-farm biopurification systems. libA numbers were measured by real-time PCR and linked to reported data of Variovorax community composition and linuron mineralization capacity. In the soil microcosms and one biopurification system setup, libA numbers responded to the application of linuron and environmental changes in congruency with the modulation of linuron mineralization capacity and the occurrence of a particular Variovorax phylotype (phylotype A). However, in another biopurification system setup, no such correlations were found. Our data suggest that in the simulated environmental settings, the occurrence of libA can be linked to the linuron mineralization capacity and that libA is primarily hosted by Variovorax phylotype A strains. However, the results also suggest that, apart from libA, other, as-yet-unknown isofunctional genes play an important role in linuron mineralization in the environment.     

Field assessment of soil biological and chemical quality in response to crop management practices

Soil microbiological and chemical aspects were evaluated to determine the effects of conservation tillage and crop rotation on soil fertility over a 16-year period. A field trial was established to compare two cropping systems (continuous soybean and maize/soybean, soybean/maize rotation). In addition, maize (Zea mays L.) and soybean (Glycine max L., Merr) were grown in two different tillage systems: no tillage and reduced tillage. Soil populations of Trichoderma spp., Gliocladium spp. and total fungi were more abundant when maize or soybean were under conservation tillage and in the maize/soybean and soybean/maize rotation, than in continuous soybean. Furthermore, higher levels of microbial respiration and fluorescein diacetate hydrolysis (FDA), were recorded under no tillage systems. However, soil counts of Actinomycetes and Pythium spp., and Pythium diversity together with soil microbial biomass were not affected by the field treatments. To establish a correlation with soil biological factors, soil chemical parameters, such as pH, organic matter content, total N, electrical conductivity, N–NO₃ ⁻ and P were also quantified, most of the correlations being significantly positive. Under no tillage there was a clear increase of the amount of crop residues and the C and N soil content due to the presence of residues. Also the distribution of crop residues in surface soil due to zero tillage and the quality of these residues, depending on the crop rotation employed, improved on soil biological and chemical characteristics. Crop yield was also enhanced by zero tillage through the management of residues. Although yield values were not directly associated with the development of microorganisms, both yield and microorganisms were influenced by crop management. These results suggest that measuring soil properties over a long period helps to define effective management strategies in order to preserve soil conditions.     

How soil characteristics and water quality influence the biogeochemical response to flooding in riverine wetlands

Although phosphate concentrations have been reduced, the rivers Meuse and Rhine are still polluted with sulphate, which most probably affects vegetation development in newly created riverine wetlands. The influence of flooding with river water rich in sulphate was tested on three soil types from floodplains of the river Meuse using flow-through and batch experiments. Soils were selected for contrasting concentrations of iron and organic matter and originated from a floating fen (iron-poor, organic), an alder carr (iron-rich, organic) and a clay pit (iron-rich, low in organic matter). Flooding induced mobilisation of phosphate. Sulphate only enhanced this effect in the alder carr soil, where sulphide and phosphate competed for binding to iron. Only in the floating fen soil did the addition of sulphate result in the formation of free sulphide, which reduced the growth of Glyceria maxima, serving as a phytometer. In addition, the floating soil started to sink, due to falling methane concentrations. In the different soil types methane production was hampered by the presence of more favourable electron acceptors such as sulphate in the water and Fe(III) in the soil. It was concluded that the effects of inundation with sulphate-polluted water strongly depend on the soil type: under iron-poor circumstances, free sulphide may accumulate, leading to phytotoxicity, while in soils rich in iron, sulphide toxicity is prevented, but phosphate availability may be increased. In addition, shortage of easily degradable organic matter can limit the formation of potential toxicants such as ammonium, iron and sulphide. Results are discussed in terms of their implications for nature management.