The influence of soil moisture status on the soil microbial biomass was determined using three physiological methods, i.e. fumigation-incubation (using either k c = 0.41 or a different k c value determined for each treatment), substrate-induced respiration (SIR) method (with samples either remoistened or non-remoistened immediately prior to determination), and the oxygen-uptake method. These techniques were applied to soil samples that had been incubated at 15, 25, 35, 45 or 55% moisture content for 6 h or 3, 10 or 30 days. The fumigation-incubation technique (k c = 0.41) predicted that microbial biomass was maximal at intermediate soil moisture contents. Fumigation-incubation (varying k c) and SIR (non-remoistened samples) predicted that microbial biomass remained relatively constant across the entire soil moisture gradient. SIR (non-remoistened samples) and the O2 uptake curve method predicted that microbial biomass increased with increasing soil moisture content. However, relationships between pairs of methods for the same samples were extremely weak, with r 2 values never exceeding 0.50. The possible reasons for poor agreement between these methods are discussed.
No abstract is provided for this article.
Most of the earth's terrestrial species live in the soil. These organisms, which include many thousands of species of fungi and nematodes, shape aboveground plant and animal life as well as our climate and atmosphere. Indeed, all terrestrial ecosystems consist of interdependent aboveground and belowground compartments. Despite this, aboveground and belowground ecology have been conducted largely in isolation. This book represents the first major synthesis to focus explicitly on the connections between aboveground and belowground subsystems--and their importance for community structure and ecosystem functioning. David Wardle integrates a vast body of literature from numerous fields--including population ecology, ecosystem ecology, ecophysiology, ecological theory, soil science, and global-change biology--to explain the key conceptual issues relating to how aboveground and belowground communities affect one another and the processes that each component carries out. He then applies these concepts to a host of critical questions, including the regulation and function of biodiversity as well as the consequences of human-induced global change in the form of biological invasions, extinctions, atmospheric carbon-dioxide enrichment, nitrogen deposition, land-use change, and global warming. Through ambitious theoretical synthesis and a tremendous range of examples, Wardle shows that the key biotic drivers of community and ecosystem properties involve linkages between aboveground and belowground food webs, biotic interaction, the spatial and temporal dynamics of component organisms, and, ultimately, the ecophysiological traits of those organisms that emerge as ecological drivers. His conclusions will propel theoretical and empirical work throughout ecology.
The stability (resistance and resilience to disturbance) of a soil system is a key factor influencing ecosystem properties and processes. To compare the stability of different systems, it is necessary to have indices that provide a relative quantitative measure of both the resistance and resilience of a response variable in all possible scenarios. However, the indices currently in use are frequently unable to do this, or are difficult to interpret. Here, we present new indices that avoid these problems. We compare our indices with previously published indices of stability, and also test their performance by using a real data set. We show that our indices accurately represent the response of soil properties (e.g. soil microbial biomass) to a disturbance, and that they are capable of determining differences in stability between contrasting soils.
Physiological and biochemical methods for estimating soil microbial biomass are usually calibrated against other methods and parameters. However, while calibrations are usually made over soils with a very wide range of microbial biomass values (across a wide geographical range) they are often used to assess relatively small differences in microbial biomass across a narrower range of microbial biomass values (across a smaller geographical area, e.g. within a single field), where their reliability may be considerably less. We investigated the abilities of three methods of quantifying microbial biomass, i.e. substrate-induced respiration (SIR), fumigation-incubation (FI) and fumigation-extraction (FE) to serve as predictors of each other across two geographical scales, i.e. across 12 sites over an area of ≈ 100 × 100 km; and within each of these sites (12 samples per site) over an area of 0.3 ha each. Over the larger scale, relationships between pairs of methods were strong, with R 2 values always > 0.90. However, over the smaller scale, correlations between pairs of methods were variable and only significant for those sites in which spatial variability in soil organic matter was relatively high. Uncertain relationships between SIR and the fumigation-based methods may be expected because they apply to different subsets of the soil biomass (i.e. glucose-responsive vs chloroform-sensitive). However, we suggest that FI and FE are sometimes weakly correlated because the FI decomposition constant k c and the FE constant k EC vary differently relative to each other across underlying gradients. Calibration equations for estimating microbial biomass are most accurate when restricted to situations where the range of biomass values is comparable to that from which the calibration was first derived, and to similar soil types. In our study, it appears that calibration equations for predicting microbial biomass are only likely to provide reliable relative estimates in situations where the coefficient of variation (standard deviation/mean) of soil organic C is > 15%.
Three post-emergence herbicides (2,4-D, picloram and glyphosate) were applied to samples of an Alberta agricultural soil at concentrations of 0, 2, 20, and
No abstract is provided for this article.
In the past two centuries, anthropogenic fire suppression has affected many biomes, including boreal forests. Absence of fire in the boreal zone is often linked to declining soil fertility and increased carbon sequestration in the humus through changes in NPP and litter decomposition. We studied tree litterfall and litter decomposition for thirty lake islands in the boreal forested zone of Sweden, which differ naturally in fire regime: larger islands have burned far more frequently than smaller ones because they are intercepted more often by lightning. We used litter trays to show that the ratio of twig to foliar litterfall for Picea abies with prolonged absence of fire is largely responsible for the concomitant increased twig proportion in the total litterfall. We hypothesised that the increased twig proportion in the litterfall with time since fire would affect overall decomposition by reducing net litter quality and through impairing decomposition of associated foliar litters. We established a litter decomposition mesocosm experiment based on litter from the three main tree species found on the islands. From each of the thirty islands, we prepared a set of litterbags comprising three mixtures (all foliar litters combined, all twig litters combined, and all twig and foliar litters combined), plus monotypes (i.e. each of the separate foliage and twig litters for each species). Unlike most studies, we used the natural litterfall proportions in the mixtures. We found a (negative) effect of time since wildfire on decomposition rate for only the foliage‐twig mixtures, although twig litter did not inhibit the decomposition of foliar litter within these mixtures. The reduced decomposition rate in the foliage‐twig mixture from the small islands was therefore due to the increased proportion of twigs in the mixtures from these islands. Our results suggest that, with prolonged absence of fire, the increase of twig proportion in the foliage‐twig litterfall for P. abies combined with the shift to canopy dominance by P. abies may be important in contributing to reduced decomposition rates in boreal forest soils.
Summary 1. Non‐native invasive and nitrogen (N)‐fixing plant species can cause large ecosystem‐level impacts, particularly when they differ in functionally important plant traits from native and non N‐fixing species. However, it remains unclear as to whether and how plant invasion status and N fixation ability consistently influence key plant leaf and litter traits, and trait‐driven processes like herbivory and decomposition. 2. We compared leaf and litter traits, leaf palatability and litter decomposability for 41 co‐occurring woody species, including native N‐fixers, native non N‐fixers, invasive N‐fixers and invasive non N‐fixers, from a New Zealand floodplain. We tested the hypotheses that: (i) invasive and N‐fixing species have higher foliar N and specific leaf area, and lower concentrations of defensive phenolics and structural compounds than do native and non N‐fixing species, and (ii) invasive and N‐fixing species generally produce more decomposable litter and palatable foliage than do native and non N‐fixing species. 3. Consistent with our hypotheses, invaders had higher foliar N and N : P ratio, and lower C : N ratio, than did native species. However, in contrast to our hypotheses, foliar phenolics were higher for the invaders while other leaf and litter traits were unaffected by invasion status. Further, N‐fixers had higher N and N : P ratios, and lower C : N ratios than did non N‐fixers, but other leaf and litter traits were unaffected by N fixation ability. 4. Leaf palatability was unaffected by invasion status but was higher for N‐fixers than for non N‐fixers. Litter decomposability was unaffected both by invasion status and N fixation ability. We found a significant positive relationship between leaf palatability and litter decomposability across all species, because similar traits, particularly the C : P ratio and total phenolic concentrations of plant tissues, were correlated with both processes. 5. Our results demonstrate that a small number of key traits, such as C : P ratio and total phenolic concentrations, drive both herbivory and decomposition irrespective of plant invasion status or N fixation ability. As such, they highlight that interspecific differences in particular plant traits, rather than plant functional group memberships based on invasion status and N fixation ability, are more effective in predicting palatability and decomposability.
