Inter‐specific variability in organic nitrogen uptake of three temperate grassland species

We tested the inter‐specific variability in the ability of three dominant grasses of temperate grasslands to take up organic nitrogen (N) in the form of amino acids in soils of differing fertility. Amino acid uptake was determined by injecting dual labeled glycine‐2‐¹³C‐¹⁵N into the soil, and then measuring the enrichment of both ¹³C and ¹⁵N in plant tissue after 50 hours. We found enrichment of both ¹³C and ¹⁵N in root and shoot material of all species in both soils, providing first evidence for direct uptake of glycine. We show that there was considerable inter‐specific variability in amino acid uptake in the low fertility soil. Here, direct uptake of amino acid was greater in the grass Agrostis capillaris, which typically dominates low fertility grassland, than Lolium perenne, which inhabits more fertile sites. Direct uptake of amino acid for Holcus lanatus. was intermediate between the above two species. Unlike in the low fertility soil, there was no difference in uptake of either ¹³C or ¹⁵N by grasses in the high fertility soil, where uptake of mineral N is thought to be the major mechanism of N uptake of these grasses. Overall, our findings may contribute to our understanding of differences in competitive interactions between grasses in soils of different fertility status.     

The impacts of individual plant species on rhizosphere microbial communities in soils of different fertility

To investigate the effects of individual plant species on microbial community properties in soils of differing fertility, a microcosm experiment was carried out using plant species representative of the dominant flora in semi-fertile temperate grasslands of northern England. Soil microbial biomass and activity were found to be significantly greater in the more fertile, agriculturally improved soil than in the less productive unimproved meadow soil. Differences in microbial community structure were also evident between the two soils, with fungal abundance being greater in the unimproved soil type. Individual plant species effects significantly differed between the two soils. Holcus lanatus and Anthoxanthum odoratum stimulated microbial biomass in the improved soil type, but negatively affected this measure in the unimproved soil. In both soil types, herb species generally had negative effects on microbial biomass. Patterns for microbial activity were less consistent, but as with microbial biomass, A. odoratum and H. lanatus promoted respiration, whereas the herbs negatively affected this measure. All plant species grown in the improved soil increased the abundance of fatty acids synthesised by bacteria (bacterial phospholipid fatty acid analysis) relative to bare soil, but they negatively impacted on this group of fatty acids in unimproved soil. Similarly, the abundance of the fungal fatty acid 18:26 was increased by all plants in the more fertile improved soil only, albeit non-significantly. Our data indicate that effects of plant species on microbial properties differ markedly in soils of differing fertility, making general predictions about how individual plants impact on soil properties difficult to make.     

Seasonal variation in soluble soil carbon and nitrogen across a grassland productivity gradient

Understanding the fate and turnover of the pools that comprise dissolved organic nitrogen (DON) in soil is key to determining its role in ecosystem functioning. We investigated seasonal changes of dissolved organic carbon (DOC) and nitrogen (DON) concentrations within four molecular weight (MW) size fractions across an altitudinal gradient (from lowland to montane systems), and quantified individual amino acids and amino acid constituents of oligopeptidic-N, as well as nitrate and ammonium. We tested two ideas: first, that DON is more abundant than DIN in low-productivity relative to high-productivity grassland ecosystems; and second, that the abundance of peptides and amino acids is likewise greater in low- than high-productivity grassland. The most productive site had a history of inorganic fertiliser application, and hence in this site alone DIN was more abundant than DON. Plant productivity varied 3-fold between the other sites, and DON was generally at higher concentrations in the sites of lower productivity both in absolute terms as well as relative to DIN, with a large increase observed in spring. The fraction containing the highest concentration of the DON had a MW of >100 kDa, and in summer and autumn this fraction was more abundant at the lowest productivity site. We conclude that relationships between the abundance of DON relative to DIN and ecosystem productivity is dependent on season, and hence more complex than previously suggested, and that peptides are a dynamic and potentially nutritionally significant component of DON.     

Do plant species encourage soil biota that specialise in the rapid decomposition of their litter?

Plants are often nutrient limited and soil organisms are important in mediating nutrient availability to plants. Thus, there may be a selective advantage to plants that alter the soil community in ways that enhance the decomposition of their litter and, hence, their ability to access nutrients. We incubated litter from three tree species (Fagus sylvatica, Acer pseudoplatanus and Picea sitchensis) in the presence of biota extracted from soil beneath a stand of each species to test the hypothesis that litter decomposes fastest in the presence of biota derived from soil where that species is locally abundant. We found that respiration rate, a measure of decomposer activity and carbon mineralisation, was affected by litter type and source of soil biota, whereas, mass loss was only affected by litter type. However, litter from each tree species did not decompose faster in the presence of indigenous soil biota. These findings, therefore, provide no support for the notion that woodland plants encourage the development of soil communities that rapidly decompose their litter.     

Ecosystem properties and forest decline in contrasting long-term chronosequences

During succession, ecosystem development occurs; but in the long-term absence of catastrophic disturbance, a decline phase eventually follows. We studied six long-term chronosequences, in Australia, Sweden, Alaska, Hawaii, and New Zealand; for each, the decline phase was associated with a reduction in tree basal area and an increase in the substrate nitrogen-to-phosphorus ratio, indicating increasing phosphorus limitation over time. These changes were often associated with reductions in litter decomposition rates, phosphorus release from litter, and biomass and activity of decomposer microbes. Our findings suggest that the maximal biomass phase reached during succession cannot be maintained in the long-term absence of major disturbance, and that similar patterns of decline occur in forested ecosystems spanning the tropical, temperate, and boreal zones.     

The short-term effects of cessation of fertiliser applications,liming, and grazing on microbial biomass and activity in a reseeded upland grassland soil

A field study was conducted to determine the influence of a short-term (2 year) cessation of fertiliser applications, liming, and sheep-grazing on microbial biomass and activity in a reseeded upland grassland soil. The cessation of fertiliser applications (N and NPK) on a limed and grazed grassland had no effect on microbial biomass measurements, enzyme activities, or respiration. Withholding fertiliser and lime from a grazed grassland resulted in significant reductions in both microbial biomass C (P<0.05) and dehydrogenase activity (P<0.05) by approximately 18 and 21%, respectively. The removal of fertiliser applications, liming, and grazing resulted in even greater reductions in microbial biomass C (44%, P<0.001) and dehydrogenase activity (31%, P<0.001), and significant reductions in microbial biomass N (P<0.005), urease activity (P<0.05), phosphatase activity (P<0.001), and basal respiration (P<0.05). The abundance of culturable bacteria and fungi and the soil ATP content were unaffected by changes in grassland managements. With the cessation of liming soil pH fell from 5.4 to 4.7, and the removal of grazing resulted in a further reduction to pH 4.5. A significant negative linear relationship (r ²=0.97; P<0.01) was found between increasing soil acidity and dehydrogenase activity. Possible mechanisms influencing these changes are discussed.     

Changes in tree growth resulting from simulated browsing have limited effects on soil biological properties

Herbivores can indirectly affect ecosystem productivity by modifying feedbacks that occur between dominant plants and below-ground properties, especially through altering nutrient availability in soil. The aim of this study was to examine, under controlled conditions, the effect of simulated browsing by large herbivores on the growth characteristics of downy birch (Betula pubescens), a dominant tree species of native regenerating forests in northern Britain, and to determine how effects of browsing on tree growth cascaded through to soil microbial communities, thereby affecting nutrient availability in soil. Downy birch seedlings were grown in mesocosms for 2 years and subjected to simulated browsing in the form of defoliation and clipping treatments. Upon destructive harvest, a number of measures of both tree growth characteristics and soil biological and nutrient properties were made. Clipping of birch trees significantly reduced total root biomass (27%), fine root biomass (29%), coarse root biomass (27%) and above-ground biomass (18%), whereas defoliated trees were significantly shorter than non-defoliated trees. Despite these significant and negative effects of browsing on tree growth, soil biological properties remained largely unaffected, other than rates of N mineralisation, which were greater under defoliated trees. We conclude that other factors, such as herbivore effects on litter quantity and quality which feedback to soil biological properties in the longer-term are more important in determining ecosystem responses to browsing.     

Influence of single trees on spatial and temporal patterns of belowground properties in native pine forest

Linkages between forest dynamics and ecosystem processes are poorly understood and this limits our ability to adequately estimate future changes in forest ecosystems due to human-induced global change. In particular at the single tree level, our understanding of temporal and spatial changes of belowground properties during forest succession is limited. Thus, our aim was to test whether we find a spatial and temporal gradient in nutrient availability and an associated shift in microbial community structure with increasing distance and age of single trees. We found that inorganic nitrogen was less available below the crown of single trees, while soluble organic carbon (DOC) was much more abundant, in particular in the inner zone of influence, i.e. close to the stem. The fungal:bacterial PLFA ratio was greater while microbial biomass carbon (MicC) was lower below the tree crown, indicating a strong influence of trees on spatial patterns of microbial biomass and community structure. Moreover, the positive correlation between MicC and total extractable N, and the negative correlation between fungal:bacterial biomass and δ¹⁵N, suggested that the microbial biomass was N limited below the tree crown and as a consequence nutrient cycling was presumably decelerated compared to open conditions. We also found a temporal pattern of increasing surface soil C and N content with increasing tree age (up to 250 years), underlining the significant role of single trees in creating spatial and temporal heterogeneity in forests.     

Influence of disturbance and nitrogen addition on plant and soil animal diversity in grassland

Two key determinants of biological diversity that have been examined in aboveground and aquatic systems are productivity, or resource supply, and physical disturbance. In this study, we examined how these factors interact under field conditions to determine belowground diversity using microarthropods (mites and Collembola) as our test community. To do this, we established a field manipulation experiment consisting of crossed, continuous gradients of nitrogenous (N) fertilizer addition (up to 240 kg N ha^?1) and disturbance (imitated trampling by cattle) to produce a gradient of soil nutrient availability and disturbance. Due to the relatively short-term nature of our study (i.e. 2 years), we only detected minimal changes in plant diversity due to the experimental manipulations; in the longer term we would expect to detect changes in plant diversity that could potentially impact on soil fauna. However, disturbance reduced, and additions of N increased, aboveground biomass, reflecting the potential effects of these manipulations on resource availability for soil fauna. We found that disturbance strongly reduced the abundance, diversity, and species richness of oribatid mites and Collembola, but had little effect on predatory mites (Mesostigmata). In contrast, N addition, and therefore resource availability, had little effect on microarthropod community structure, but did increase mesostigmatan mite richness and collembolan abundance at high levels of disturbance. Oribatid community structure was mostly influenced by disturbance, whereas collembolan and mesostigmatan diversity were responsive to N addition, suggesting bottom-up control. That maximal species richness of microarthropod groups overall occurred in undisturbed plots, suggests that the microarthropod community was negatively affected by disturbance. We found no change in microarthropod species richness with high N additions, where plant productivity was greatest, indicating that soil biotic communities are unlikely to be strongly regulated by competition. We conclude that the diversity of soil animals is best explained as a combination of their many varied life history tactics, phenology and the heterogeneity of soils that enable so many species to co-exist.     

Effect of density and species richness of soil mesofauna on nutrient mineralisation and plant growth

The aim of this study was to test the relative importance of changes in density and species richness of soil mesofauna as determinants of nutrient mineralisation and plant growth. The experiment was carried out using microcosms containing a mixture of plant litter and soil in which seedlings of Lolium perenne were planted, and a range of combinations of levels of density and species richness of microarthropods added. Over the duration of the experiment, nutrient release, measured as concentrations of NO₃ ^--N and total N in leachates, increased significantly with increasing microarthropod density, but decreased with increasing species richness. Leachate concentrations of NH₄ ⁺-N, dissolved organic N and C (DON and DOC) were not affected by the faunal treatments. Soil respiration, a measure of microbial activity, decreased with increasing density of microarthropods, whereas microbial biomass was not affected by microarthropods. Increasing density of soil animals had a negative effect on the shoot biomass of L. perenne while the effect of species richness was positive. Neither the species richness nor density of soil microarthropods was found to significantly influence root biomass. We conclude that variations in animal density had a greater influence on soil nutrient mineralisation processes than did species richness. Possible reasons for these opposing effects of animal density and diversity on soil N mobilization are discussed.