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Contrasting effects of repeated summer drought on soil carbon efflux in hydric and mesic heathland soils

GLOBAL CHANGE BIOLOGY, Issue 10 2008
ALWYN SOWERBY
Abstract Current predictions of climate change include altered rainfall patterns throughout Europe, continental USA and areas such as the Amazon. The effect of this on soil carbon efflux remains unclear although several modelling studies have highlighted the potential importance of drought for carbon storage. To test the importance of drought, and more importantly repeated drought year-on-year, we used automated retractable curtains to exclude rain and produce repeated summer drought in three heathlands at varying moisture conditions. This included a hydric system limited by water-excess (in the UK) and two mesic systems with seasonal water limitation in Denmark (DK) and the Netherlands (NL). The experimental rainfall reductions were set to reflect single year droughts observed in the last decade with exclusion of rain for 2,3 months of the year resulting in a 20,26% reduction in annual rainfall and 23,38% reduction in mean soil moisture during the drought period. Unexpectedly, sustained reduction in soil moisture over winter (between drought periods) was also observed at all three sites, along with a reduction in the maximum water-holding capacity attained. Three hypotheses are discussed which may have contributed to this lack of recovery in soil moisture: hydrophobicity of soil organic matter, increased water use by plants and increased cracking of the soil. The responses of soil respiration to this change in soil moisture varied among the sites: decreased rates were observed at the water-limited NL and DK sites whilst they increased at the UK site. Reduced sensitivity of soil respiration to soil temperature was observed at soil moisture contents above 55% at the UK site and below 20% and 13% at the NL and DK sites, respectively. Soil respiration rates recovered to predrought levels in the NL and DK sites during the winter re-wetting period that indicates any change in soil C storage due to changes in soil C efflux may be short lived in these mesic systems. In contrast, in the hydric UK site after 2 years of drought treatment, the persistent reduction in soil moisture throughout the year resulted in a year-round increase in soil respiration flux, a response that accelerated over time to 40% above control levels. These findings suggest that carbon-rich soils with high organic matter content may act as a significant source of CO2 to the atmosphere following repeated summer drought. Nonrecovery of soil moisture and a persistent increase in soil respiration may be the primary mechanism underlying the reported substantial losses of soil carbon from UK organic soils over the last 20 years. These findings indicate that the water status of an ecosystem will be a critical factor to consider in determining the impact of drought on the soil carbon fluxes and storage. [source]

Dispersal characteristics and management of a rare damselfly

JOURNAL OF APPLIED ECOLOGY, Issue 4 2003
Bethan V. Purse
Summary 1Coenagrion mercuriale is a rare damselfly in Britain and mainland Europe and has been declining in the last 30 years. It has specialized habitat requirements and has been viewed, traditionally, as a poor disperser. Knowledge of its dispersal ability was considered in its Biodiversity Species Action Plan as essential for the formulation of appropriate conservation management strategies. 2Mark,release,recapture (MRR) studies of C. mercuriale in two large UK heathland populations were undertaken. Mature adults had a low rate of movement within continuous areas of habitat (average < 25 m movement), low emigration rates (1·3,11·4%) and low colonization distances (maximum 1 km), all comparable to similarly sized coenagrionids. 3Movements were more likely within than between patches of suitable habitat over short to medium distances (50,300 m). Between-patch movements were more likely between patches that were close together. Scrub barriers reduced dispersal. 4The probability of dispersal between two recaptures depended on the length of the time interval between them. Coenagrion mercuriale performed considerable between-patch movements within a small fraction (1,2 days) of its mean mature adult life span (7,8 days). 5Qualitative comparison of field colonization distances measured here and distances between UK sites occupied by C. mercuriale revealed that empty sites within large clusters of sites would probably be recolonized rapidly and dispersal events would be frequent. However, such events would occur rarely within small isolated sites or clusters of sites, leaving local populations prone to extinction. 6Synthesis and applications. These data show that management effort should be directed towards maximizing the likelihood of C. mercuriale recolonizing sites naturally within 1,3 km of other populations (particularly within large clusters). Scrub boundaries should be removed between existing populations and empty, but suitable, sites to facilitate stepping-stone dispersal movements. [source]

A metabolomic study of substantial equivalence of field-grown genetically modified wheat

PLANT BIOTECHNOLOGY JOURNAL, Issue 4 2006
John M. Baker
Summary The ,substantial equivalence' of three transgenic wheats expressing additional high-molecular-weight subunit genes and the corresponding parental lines (two lines plus a null transformant) was examined using metabolite profiling of samples grown in replicate field trials on two UK sites (Rothamsted, Hertfordshire and Long Ashton, near Bristol) for 3 years. Multivariate comparison of the proton nuclear magnetic resonance spectra of polar metabolites extracted with deuterated methanol,water showed a stronger influence of site and year than of genotype. Nevertheless, some separation between the transgenic and parental lines was observed, notably between the transgenic line B73-6-1 (which had the highest level of transgene expression) and its parental line L88-6. Comparison of the spectra showed that this separation resulted from increased levels of maltose and/or sucrose in this transgenic line, and that differences in free amino acids were also apparent. More detailed studies of the amino acid composition of material grown in 2000 were carried out using gas chromatography-mass spectrometry. The most noticeable difference was that the samples grown at Rothamsted consistently contained larger amounts of acidic amino acids (glutamic, aspartic) and their amides (glutamine, asparagine). In addition, the related lines, L88-6 and B73-6-1, both contained larger amounts of proline and ,-aminobutyric acid when grown at Long Ashton than at Rothamsted. The results clearly demonstrate that the environment affects the metabolome and that any differences between the control and transgenic lines are generally within the same range as the differences observed between the control lines grown on different sites and in different years. [source]

Erratic judgements: re-evaluating solutional erosion rates of limestones using erratic-pedestal sites, including Norber, Yorkshire

AREA, Issue 4 2005
H S Goldie
The re-interpretation of surface karst landforms in Northern England has led to a re-examination of well-known erratic-pedestal sites that were the origin for karst denudation rates applied extensively, on the supposition that erratics protect underlying limestone from rainwater solution. Height of the pedestal has been used to calculate long-term solutional-lowering, as much as 50 cm in 15 ka (33.3 mm/ka) from UK sites. The sites include Norber and Scar Close, Yorkshire, UK. This paper shows that the sites have been misinterpreted, in particular at Norber where the erratics lie on a pre-existing structurally stepped surface. Norber and several other sites also experience much mechanical weathering, in relatively weak, well-fractured limestones, a process which must be distinguished from solution. Sites in strong, less-fractured limestones demonstrate lower rates, which are sounder indications of land surface lowering. Some pedestals have been confirmed as partly resulting from solutional weathering in surrounding soil and vegetation. Re-evaluation reduces solution rates to 3,13 cm in 15 ka. Applying such rates has profound implications for understanding limestone landscapes, challenging orthodox views. [source]