ALBERT

Notes: Nitrogen uptake can be modified by both frequency of defoliation and competing neighbour plants. The objective of this study was to investigate nitrogen uptake by Lolium perenne and Festuca rubra subject to weekly or monthly defoliation with five neighbours: either none, L perenne clipped weekly, L. perenne clipped monthly, F. rubra clipped weekly or F. rubra clipped monthly. Plants were grown in sand culture with nutrients supplied as solutions. Nitrogen uptake over a I-week period commencing at the time of a defoliation event was estimated using l5N as a tracer. L. perenne had greater dry weight and nitrogen uptake than F. rubra. Neighbours reduced nitrogen uptake per plant. When neighbours were present, clipping them more frequently resulted in greater uptake by sample plants. Over 57% of total plant uptake was partitioned towards the two youngest leaves. In mixtures of L. perenne with F. rubra, the percentages of the total pot dry weight and N content recovered in sample L. perenne plants increased when they were clipped less frequently, compared with when the accompanying F. rubra plants were clipped less frequently.

Notes: Abstract Accumulation of nitrogen (N) by plants in response to N supply outstripping demand is contrasted with storage of N, which implies that N in one tissue can be reused for the growth or maintenance of another. Storage can, therefore, occur in N-deficient plants; accumulation can not. The consequence of accumulation and storage of N is considered, particularly in relation to the reproductive growth of annual plants, which can often use a great deal of stored N. Nitrate and proteins are the forms of N most often stored in vegetative tissues and, quantitatively, ribulose 1,5-bisphosphate carboxylase/oxygenase is often the most important protein store. While storing nitrate will be less costly to the plant in terms of energy, protein stores offer several possible advantages. These advantages are (i) maximizing the potential for carbon assimilation, (ii) avoiding problems with the regulation of leaf turgor and (iii) allowing the reduction on nitrate to occur in the young, fully illuminated leaf.

Notes: We characterised the spatial structure of soil microbial communities in an unimproved grazed upland grassland in the Scottish Borders. A range of soil chemical parameters, cultivable microbes, protozoa, nematodes, phospholipid fatty acid (PLFA) profiles, community-level physiological profiles (CLPP), intra-radical arbuscular mycorrhizal community structure, and eubacterial, actinomycete, pseudomonad and ammonia-oxidiser 16S rRNA gene profiles, assessed by denaturing gradient gel electrophoresis (DGGE) were quantified. The botanical composition of the vegetation associated with each soil sample was also determined. Geostatistical analysis of the data revealed a gamut of spatial dependency with diverse semivariograms being apparent, ranging from pure nugget, linear and non-linear forms. Spatial autocorrelation generally accounted for 40–60% of the total variance of those properties where such autocorrelation was apparent, but accounted for 97% in the case of nitrate-N. Geostatistical ranges extending from approximately 0.6–6 m were detected, dispersed throughout both chemical and biological properties. CLPP data tended to be associated with ranges greater than 4.5 m. There was no relationship between physical distance in the field and genetic similarity based on DGGE profiles. However, analysis of samples taken as close as 1 cm apart within a subset of cores suggested some spatial dependency in community DNA-DGGE parameters below an 8 cm scale. Spatial correlation between the properties was generally weak, with some exceptions such as between microbial biomass C and total N and C. There was evidence for scale-dependence in the relationships between properties. PLFA and CLPP profiling showed some association with vegetation composition, but DGGE profiling did not. There was considerably stronger association between notional sheep urine patches, denoted by soil nutrient status, and many of the properties. These data demonstrate extreme spatial variation in community-level microbiological properties in upland grasslands, and that despite considerable numeric ranges in the majority of properties, overarching controlling factors were not apparent.