Due to their extreme aridity, high rate of UV irradiation and low soil carbon (C) content, the soils of the Atacama Desert represent one of the world's most hostile environments for microbial life and its survival. Although infrequent, climatic conditions may, however, prevail which temporarily remove these stresses and allow life to briefly flourish. In this study we investigated the response of soil microbial communities to water and C availability across an aridity gradient (semi-arid, arid, hyper-arid) within the Atacama Desert. We simulated the impact of hyper-dry spells, humid fogs and precipitation events on the activation of the microbial community and the subsequent mineralization of low (glucose) and high (plant residues) molecular weight C substrates. Our results showed that mineralization rate followed the trend: semi-arid > arid > hyper-arid. Some glucose mineralization was apparent under hyper-arid conditions (water activity, a w = 0.05), although this was 10-fold slower than under humid conditions and ca. 200-fold slower than under wet conditions. A lag phase in CO2 production after glucose-C addition in the hyper-arid soils suggested that mineralization was limited by the low microbial biomass in these soils. No lag phase was apparent in the corresponding semi-arid or arid soils. In contrast, the breakdown of the plant residues was initially much slower than for glucose and involved a much longer lag phase in all soils, suggesting that mineralization was limited by low exoenzyme activity, particularly in the humid and hyper-dry soils. Our results also showed that microbial C use efficiency followed the trend: hyper-arid > arid > semi-arid. In conclusion, we have shown that even under hyper-arid conditions, very low levels of microbial activity and C turnover do occur. Further, the microbial communities are capable of rapidly responding to available C once water becomes more abundant, however, this response is both biomass and metabolically limited in hyper-arid soils.
The aims of this study were to assess changes in heavy metal availability in two contrasting feedstocks during aerobic composting, and the availability of said metals in the finished composts. A high C-to-N ratio mixed biodegradable municipal solid waste (MSW) feedstock was successfully composted on its own and in combination with green waste. Changes in heavy metal speciation throughout the composting process were studied using the modified BCR sequential extraction protocol. It was found that total Cu, Pb and Zn concentrations increased over time due to the progressive mineralization of the compost feedstock. Metals were fractionated differently within the two feedstocks, although only Cu showed significant redistribution (mostly to the oxidisable fraction) over the 5month composting period. The MSW-derived composts performed comparably with other commercially-available composts in a series of plant growth trials. Plant metal accumulation was not influenced by the heavy metals present in the MSW-derived compost implying that they are not plant available. It is recommended that these relatively low value/quality composts may be used for remediation of acidic heavy metal contaminated sites.
. 1. Saltmarsh conservation management often involves livestock grazing to maximise plant diversity and provide suitable breeding habitat for over‐wintering coastal birds. The effect of grazing on invertebrates is rarely quantified, but results from limited studies of terrestrial and coastal grasslands demonstrate greater abundance and species richness in un‐grazed grassland. 2. The impact of short sward (<8 cm) cattle grazing on the ground dwelling invertebrate community was assessed on an English inter‐tidal upper salt marsh using pitfall traps. Abundance, species richness, functional group structure, abundance of coastal specialists, environmental factors that influence invertebrate habitat choice and food web composition were compared for grazed and un‐grazed marsh. 3. In total, 90 000 invertebrates were sampled. Predatory, zoophagus and detritivorous Coleoptera were significantly more abundant on the un‐grazed marsh. In contrast, predatory Hemiptera and Araneae were significantly more abundant on the grazed marsh. Sheet weaver spiders were significantly more abundant on the grazed marsh, foliage running hunters and space web builders more abundant on the un‐grazed marsh. Most inter‐tidal coastal specialist species exhibited clear habitat preference for the grazed marsh. Total species richness was not significantly different between grazing treatments. 4. Linear direct gradient analysis showed that two environmental variables influenced by grazing intensity, soil temperature and vegetation height, significantly explained the composition of invertebrate functional groups. Larger bodied invertebrates dominated the un‐grazed food web. 5. We conclude that both short sward cattle grazed and un‐grazed saltmarsh habitat should be maintained to maximise invertebrate abundance and diversity and provide suitable habitat for coastal specialists.
No abstract is provided for this article.
In this paper, linear and spherical semivariogram models were determined for use in kriging hourly and daily solar irradiation for every season of the year. The data used to generate the models were from 18 weather stations in western Nebraska. The models generated were tested using cross validation. The performance of the spherical and linear semivariogram models were compared with each other and also with the semivariogram models based on the best fit to the sample semivariogram of a particular day or hour. There were no significant differences in the performance of the three models. This result and the comparable errors produced by the models in kriging indicated that the linear and spherical models could be used to perform kriging at any hour and day of the year without deriving an individual semivariogram model for that day or hour. The seasonal mean absolute errors associated with kriging, within the network, when using the spherical or the linear semivariograms models were between 10% and 13% of the mean irradiation for daily irradiation and between 12% and 20% for hourly irradiation. These errors represent an improvement of 1%–2% when compared with replacing data at a given site with the data of the nearest weather station.
An accurate assessment of soil organic matter dynamics is critical for understanding and predicting ecosystem responses to anthropogenic perturbation such as climate change, pollution and agriculture. These measurements of carbon (C) turnover are frequently made in the laboratory where it is easier to control the experimental conditions. However, this could potentially bias the results due to a range of factors including the absence of plants and associated mycorrhizas, physical disturbance, alteration in redox status etc. The aim of this study was therefore to directly compare the turnover of organic acids under field and laboratory conditions in three contrasting land use regimes (grassland, woodland and arable). The method involved the injection of 14C-labeled citric acid into the top soil and monitoring of its mineralization by capturing 14CO2 evolved from the soil over a 7 day period. Laboratory climate conditions were matched to those in the field. In both the laboratory and the field we showed that 14CO2 evolution followed a bi-phasic pattern and conformed extremely well to a double first-order kinetic model. While the first rapid mineralization phase showed a similar half-life for citrate under both laboratory and field conditions (4.9 ± 0.7 h), the second slower mineralization phase had a significantly longer half-life when performed in the field. Overall, our results suggest that the first rapid mineralization phase was largely independent of our experimental conditions whilst the turnover of the citrate-derived C immobilized in the microbial biomass was significantly affected by the incubation conditions either due to differences in substrate utilization and storage pathways or due to faster microbial turnover in the laboratory.