Although factors affecting plant growth and plant carbon/nutrient balance – e.g., light availability and defoliation by herbivores – may also propagate changes in below‐ground food webs, few studies have aimed at linking the above‐ground and below‐ground effects. We established a 29‐week laboratory experiment (∼one growing season) using autotrophic microcosms to study the effects of light and defoliation on plant growth, plant carbon/nutrient balance, soil inorganic N content, and microbial activity and biomass in soil. Each microcosm contained three substrate layers – mineral soil, humus and plant litter – and one Nothofagus solandri var. cliffortioides seedling. The experiment constituted of the presence or absence of two treatments in a full factorial design: shading (50% decrease in light) and artificial defoliation (approximately 50% decrease in leaf area in the beginning of the growing season). At the end of the experiment a range of above‐ground and below‐ground properties were measured. The shading treatment reduced root and shoot mass, root/shoot ratio and leaf production of the seedlings, while the defoliation treatment significantly decreased leaf mass only. Leaf C and N content were not affected by either treatment. Shading increased NO 3 –N concentration and decreased microbial biomass in humus, while defoliation did not significantly affect inorganic N or microbes in humus. The results show that plant responses to above‐ground treatments have effects which propagate below ground, and that rather straightforward mechanisms may link above‐ground and below‐ground effects. The shading treatment, which reduced overall seedling growth and thus below‐ground N use and C allocation, also led to changes in humus N content and microbial biomass, whereas defoliation, which did not affect overall growth, did not influence these below‐ground properties. The study also shows the carbon/nutrient balance of N. solandri var. cliffortioides seedlings to be highly invariant to both shading and defoliation.
Reindeer grazing in the Fennoscandian area has a considerable influence on the ground vegetation, and this is likely in turn to have important consequences for the soil biota and decomposition processes. The effects of reindeer grazing on soil biota, decomposition and mineralization processes, and ecosystem properties in a lichen‐dominated forest in Finnish Lapland were studied inside and outside a large long term fenced reindeer exclosure area. Decomposition rates of Vaccinium myrtillus leaves in litter bags were retarded in the grazed area relative to the ungrazed area, as well as in subplots from which lichens had been artificially removed to simulate grazing. The effect of reindeer grazing on soil respiration and microbial C was positive in the lichen and litter layers of the soil profile, but retarded in the humus layer. There was no effect of grazing on gross N mineralization and microbial biomass N in the humus and upper mineral soil layer, but net N mineralization was increased by grazing. In these layers soil respiration was reduced by grazing, indicating that reindeer effects reduce the ratio of C to N mineralized by soil microorganisms. Grazing stimulated populations of all trophic groupings of nematodes in the lichen layer and microbe feeding nematodes in the litter layer, indicating that grazing by reindeer has multitrophic effects on the decomposer food‐web. Grazing decreased lichen and dwarf shrub biomasses and increased the mass of litter present in the litter layer on an areal basis, but did not significantly alter total C storage per unit area in the humus and mineral soil layers. The N concentration of lichens was increased by grazing, but the N concentrations of both living and dead Pinus sylvestris needles and Empetrum hermaphroditum leaves were not affected. There was some evidence for each of three mechanisms which could account for the grazing effects that we observed in our study. Firstly, reindeer may have changed the composition and quality of litter input by affecting plant species composition and through addition of N from urine and faeces, resulting in a lack of available C relative to N for decomposer organisms. Secondly, the organic matter in the soil may be older in the grazed area, because of reduction of recent production of lichen litter relative to the ungrazed area. The organic matter in the grazed area may have been in a different phase of decomposition from that in the exclosure. Thirdly, the soil microclimate is likely to be affected by reindeer grazing through physical removal of lichen cover on the ground, and this can have a significant influence on soil microbial processes. This is supported by the strong observed effects of experimental removal of lichens on decomposer processes. The impact of reindeer grazing on soil processes may be a result of complex interactions between different mechanisms, and this could help to explain why the below‐ground effects of reindeer grazing have different consequences to those which have been observed in recent investigations on other grazing systems.
We find the suggestion that substantial charcoal loss occurred in the humus-charcoal mixtures implausible and discuss why complexing of soluble carbon released from the mixtures by underlying mineral soil should be minor. This exchange highlights our limited knowledge about charcoal effects on native soil carbon, indicating that strong advocacy for charcoal addition to offset CO 2 emissions remains premature.
All terrestrial ecosystems consist of an explicit producer subsystem and a decomposer subsystem. While traditionally these components have usually been considered in isolation from one another, they are obligatory dependent upon each other. Producers provide the organic carbon sources that drive the decomposer community and decomposer activity is in turn responsible for mineralizing nutrients required for maintaining growth of the producers. Mutualists, herbivores, pathogens and parasites affect producer-decomposer interactions both by directing changes in the flow of energy and resources, and by imposing selective forces that lead to evolutionary changes in individual producer and decomposer populations. The direct and indirect interactions between above-ground and below ground communities therefore have the potential to operate as major drivers of population-, community- and ecosystem-level processes (Hooper et al. 2000; Van der Putten et al. 2001; Wardle 2002).
In the long-term absence of rejuvenating disturbances, forest succession frequently proceeds from a maximal biomass phase to a retrogressive phase characte
Soil microorganisms and the processes that they govern are essential for long-term sustainability of agricultural systems, but most studies on agricultural effects on the soil microflora are inherently short-term. We investigated the effects of three aspects of agricultural intensification, i.e. cultivation (disturbance), herbicide addition (modification of plant composition) and mulching (resource addition) on soil biological properties such as microbial biomass and activity over 7 yr in annual (maize) and perennial (asparagus) cropping systems. The mulching treatment had strong, usually positive effects on both substrate-induced respiration (SIR) and CO2–C release from chloroform-fumigated soil throughout the study. In the perennial crop, treatments allowing high weed biomass caused large increases in microbial biomass and respiration after yr 3, and in both sites microbial biomass was positively correlated with weed biomass and negatively with crop plant biomass. This latter effect appears due to the high decomposability of weed residues relative to those from crop plants. Microbial biomass was also enhanced in atrazine-treated plots in the annual crop but only during the final year, presumably due to beneficial effects of plot invasion by herbicide-tolerant weeds. Mulching often also enhanced the microbial metabolic quotient (qCO2), the bacteria-to-fungal biomass ratio and within-year temporal variability of the microbial biomass, all of which are indicative of greater turnover and instability of the microbial biomass. Other treatments generally had smaller effects on these properties, although in the perennial crop an intense summer drought in yr 4 caused a large elevation in the metabolic quotient in the herbicide-treated (low weed) plots relative to the other plots, suggesting that high quality weed residues have stabilising effects. Temporal variability across years of both SIR and CO2–C release from fumigated soil was greatest in the herbicide-treated plots in the perennial crop, suggesting that high weed biomass (producing easily degradable organic matter) has stabilising effects. Decomposition rates of added litter were partially consistent with the microbial biomass data, with the highest litter breakdown rates usually occurring in the mulched plots. Our study shows that soil biological properties such as microbial biomass and activity are not necessarily adversely affected by agricultural intensification and that consequences of intensification mainly depend upon practices which alter the quality and quantity of residue inputs. Further, our results underline the need for long,-term field experiments, and several of the effects we identified could only have been detected through an experiment of several years duration.
No abstract is provided for this article.
Preface 1. The intellectual and social climate of the nineteenth century 2. Recurrent reasons for educational expansion in England 3. The raw material 4. The changing school: administrative and organizational changes, 1800-1939 5. Changes in curricula and teaching methods 6. The role and status of the teacher 7. Secondary education 8. Recent developments and their implications 9. 1970-1975 Conclusion Glossary Further reading Chronological table Index.
No abstract is provided for this article.
1 Following major disturbances ecosystem development occurs but in the prolonged absence of disturbance a decline (retrogressive) phase follows in which productivity and nutrient availability diminishes. Although it is recognized that litter quality and decomposition rates decrease as retrogression proceeds, little is known about the extent to which this is driven among- vs. within-species variation across these sequences. 2 We selected six long-term chronosequences that each included retrogressive stages, in New Zealand, Hawaii, Sweden, Alaska and Australia. Two involve significant species turnover across the sequence so that different species dominate at different stages, two involve low species turnover so that the same dominant species occur at all stages, and two involve some turnover of species but with certain species persisting throughout most of the sequence. 3 For each chronosequence, we collected litter from each dominant plant species at each stage of that sequence. For each litter collection we measured concentrations of N and P, and performed laboratory decomposition bioassays to measure mass loss, N and P loss, and the response of mass loss to mixture with litters of coexisting species. 4 We found that litter N and P concentrations often declined with increasing ecosystem age, both among- and within-species. However, the relative importance of among- and within-species effects varied across the six chronosequences. Rates of litter mass, N, and P loss during decomposition sometimes decreased with increasing ecosystem age, but most often at the among-species rather than the within-species level. 5 Litter mixing effects often varied across chronosequence stages, but the magnitude and direction of these effects was inconsistent among sequences. Variation in litter mixing effects across chronosequence stages was driven mainly by among- rather than within-species variation. 6 Although several recent studies have emphasized the role of within-species variation on ecosystem properties, our results point to among-species variation as a consistently important ecological driver, with within-species variation being important only for some variables and in some instances. As such they highlight that decomposition processes are most likely to be highly responsive to gradients of soil fertility (such as across chronosequences) when significant species turnover occurs across the gradient.
