H.A. Verhoef

Soil invertebrate fauna enhances grassland succession and diversity

One of the most important areas in ecology is to elucidate the factors that drive succession in ecosystems and thus influence the diversity of species in natural vegetation. Significant mechanisms in this process are known to be resource limitation and the effects of aboveground vertebrate herbivores. More recently, symbiotic and pathogenic soil microbes have been shown to exert a profound effect on the composition of vegetation and changes therein. However, the influence of invertebrate soil fauna on succession has so far received little attention. Here we report that invertebrate soil fauna might enhance both secondary succession and local plant species diversity. Soil fauna from a series of secondary grassland succession stages selectively suppress early successional dominant plant species, thereby enhancing the relative abundance of subordinate species and also that of species from later succession stages. Soil fauna from the mid-succession stage had the strongest effect. Our results clearly show that soil fauna strongly affects the composition of natural vegetation and we suggest that this knowledge might improve the restoration and conservation of plant species diversity.

Positive effects of organic farming on below‐ground mutualists: large‐scale comparison of mycorrhizal fungal communities in agricultural soils

*The impact of various agricultural practices on soil biodiversity and, in particular, on arbuscular mycorrhizal fungi (AMF), is still poorly understood, although AMF can provide benefit to plants and ecosystems. Here, we tested whether organic farming enhances AMF diversity and whether AMF communities from organically managed fields are more similar to those of species-rich grasslands or conventionally managed fields. *To address this issue, the AMF community composition was assessed in 26 arable fields (13 pairs of organically and conventionally managed fields) and five semi-natural grasslands, all on sandy soil. Terminal restriction fragment length polymorphism community fingerprinting was used to characterize AMF community composition. *The average number of AMF taxa was highest in grasslands (8.8), intermediate in organically managed fields (6.4) and significantly lower in conventionally managed fields (3.9). Moreover, AMF richness increased significantly with the time since conversion to organic agriculture. AMF communities of organically managed fields were also more similar to those of natural grasslands when compared with those under conventional management, and were less uniform than their conventional counterparts, as expressed by higher beta-diversity (between-site diversity). *We suggest that organic management in agro-ecosystems contributes to the restoration and maintenance of these important below-ground mutualists.

Efficiency of lead and cadmium excretion in populations of Orchesella cincta (Collembola) from various contaminated forest soils

(1) Population differentiation in the collembolan Orchesella cincta (L.) under the influence of metal soil pollution was studied by measuring Pb and Cd excretion efficiency in individual Collembola from various sites. (2) Total and exchangeable fractions of Pb, Cd, Zn, Cu, Fe and Ca were determined in the A-horizon of forest soils at ten locations in the Netherlands, Belgium and Germany. Concentrations of Pb and Cu were higher in the AO-horizon than in the A0o-layer. Mobility of the elements decreased in the order Cd, Pb, Zn, Cu. (3) Pb and Cd concentrations in 0. cincta varied with site contamination, while Zn and Cu concentrations were more constant. The metal concentrations in Collembola were not clearly related to any particular chemical fraction of the soil metals. (4) Resistance of 0. cincta to Pb and Cd was studied by budget experiments and standardized assays of metal excretion. (5) Intestinal assimilation from dietary exposure was 0-4% for Pb and 8-3%/, for Cd. From the assimilated amount, 48% of Pb and 30% of Cd was excreted per moulting interval by intestinal exfoliation. (6) Although there was considerable interindividual variation, the average excretion efficiency was not influenced by physiological effects of such factors as degree of exposure, weight or sex. (7) Comparison of ten populations showed that high soil contamination in mining areas resulted in populations of 0. cincta having an increased average excretion efficiency for Pb and Cd.

Dynamics and stratification of bacteria and fungi in the organic layers of a scots pine forest soil

Abstract The abundance and micro-stratification of bacteria and fungi inhabiting the organic layers of a Scots pine forest (Pinus sylvestris L.) were investigated. An experiment using stratified litterbags, containing organic material of four degradation stages (fresh litter, litter, fragmented litter and humus) was performed over a period of 2.5 years. Dynamics and stratification of fluorescent stained bacteria and fungi, ratios between bacterial and fungal biomass, and relationships with moisture and temperature are described. Average bacterial counts in litter and fragmented litter were similar, i.e., approximately 5×109 bacteriag–1 (dry weight) organic matter, and significantly exceeded those in humus. The mean bacterial biomass ranged from 0.338 to 0.252mg carbon (C) g–1 (dry weight) organic matter. Lengths of mycelia were significantly below the usually recorded amounts for comparable temperate coniferous forests. The highest average hyphal length, 53mg–1 (dry weight) organic matter, was recorded in litter and decreased significantly with depth. The corresponding mean fungal biomass ranged from 0.050 to 0.009mg Cg–1 (dry weight). The abundance of bacteria and fungi was influenced by water content, that of fungi also by temperature. A litterbag series with freshly fallen litter of standard quality, renewed bimonthly, revealed a clear seasonal pattern with microbial biomass peaks in winter. The mean hyphal length was 104mg–1 (dry weight) and mean number of bacteria, 2.40×109 bacteria g–1 (dry weight). Comparable bacterial and fungal biomass C were found in the freshly fallen litter [0.154 and 0.132mgCg–1 (dry weight) organic material, respectively]. The ratio of bacterial-to-fungal biomass C increased from 1.2 in fresh litter to 28.0 in humus. The results indicate the existence of an environmental stress factor affecting the abundance of fungi in the second phase of decomposition. High atmospheric nitrogen deposition is discussed as a prime factor to explain low fungal biomass and the relatively short lengths of fungal hyphae in some of the forest soil layers under study.

Dynamics and stratification of functional groups of micro- and mesoarthropods in the organic layer of a Scots pine forest

Abstract This paper addresses the abundance, biomass and microstratification of functional groups of micro- and mesoarthropods inhabiting the organic layers of a Scots pine forest (Pinus sylvestris L.). An experiment using stratified litterbags, containing organic material of four degradation stages, i.e., freshly fallen litter, litter, fragmented litter and humus, was performed over a period of 2.5 years. Statistical data analysis revealed that each organic layer had a different, characteristic species composition that changed with time following successive degradation stages. Species of Acari, Araneae and Collembola were assigned to different functional groups based on taxonomy, microstratification, food type or feeding mode. The abundance and biomass carbon of functional groups were dependent on the organic layer and most functional groups showed a particular preference for one of the upper organic layers. Temporal and spatial differences in density and biomass carbon of functional groups could partly be related to fluctuations in the soil climate, although effects of trophic interactions could not be ruled out. A general decline in abundance and biomass, especially in populations of fungal feeders, during the last year of the study could not be explained by a reduction in litterbag volume, changed litter chemistry or soil climate, but was attributed to an indirect effect of a remarkable increase in soil coverage by wavy hair grass, Deschampsia flexuosa (L.). The analysis demonstrated that species diversity, microhabitat specification, soil fauna succession, and degradation stages of organic material are interrelated. The results obtained indicate that both the chemistry of organic matter and decomposition rates have an important effect on trophic relationships and community structure.

Water balance in Collembola and its relation to habitat selection; cuticular water loss and water uptake

Abstract In the order Collembola a clear relationship was found between overall cuticular water loss and water conditions of the habitat. The different transpiration rates were negatively correlated with the haemolymph osmotic pressure, but there was no clear causal relationship. In two species, Orchesella cincta and Tomocerus minor, which live sympatric but have a different micro-distribution (partly due to small scale heterogeneity in water conditions), important differences exist both in rate of water loss and in speed of water uptake: Orchesella cincta had a significantly lower transpiration and a higher speed of water uptake than Tomocerus minor. The transpiration rates of both species were linearly related to the vapour pressure deficit of the ambient air. Contrary to Orchesella cincta, Tomocerus minor lost water in saturated conditions. Freshly-killed Orchesella cincta had a higher transpiration rate than living individuals, but in Tomocerus minor there was no such difference. It is suggested that the main integumentary resistance against water loss in Orchesella cincta is the epidermal cell and in Tomocerus minor the epicuticle. The important role of the ventral vesicles in the water relations of Collembola was confirmed.

Spatiotemporal stability of an ammonia oxidizing community in a nitrogen-saturated forest soil

Elevated levels of nitrogen input into various terrestrial environments in recent decades have led to increases in soil nitrate production and leaching. However, nitrifying potential and nitrifying activity tend to be highly variable over space and time, making broad-scale estimates of nitrate production difficult. This study investigates whether the high spatiotemporal variation in nitrate production might be explained by differences in the structure of ammonia-oxidizing bacterial communities in nitrogen-saturated coniferous forest soils. The diversity of ammonia-oxidizing bacteria of the β-subgroup Proteobacteria was therefore investigated using two different PCR-based approaches. The first targeted the 16S rRNA gene and involved temporal temperature gradient electrophoresis (TTGE) of specifically amplified PCR products, with subsequent band excision and nucleotide sequence determination. The second approach involved the cloning and sequencing of PCR-amplified amoA gene fragments. All recovered 16S rDNA sequences were closely related to the culture strain Nitrosospira sp. AHB1, which was isolated from an acid soil and is affiliated with Nitrosospira cluster 2, a sequence group previously shown to be associated with acid environments. All amoA-like sequences also showed a close affinity with this acid-tolerant Nitrosospira strain, although greater sequence variation could be detected in the amoA analysis. The ammonia-oxidizing bacterial community in the nitrogen-saturated coniferous forest soil was determined to be very stable, showing little variation between different organic layers and throughout the year, despite large differences in the total Bacterial community structure as determined by 16S rDNA DGGE community fingerprinting. These results suggest that environmental heterogeneity affecting ammonia oxidizer numbers and activity, and not ammonia oxidizer community structure, is chiefly responsible for spatial and temporal variation in nitrate production in these acid forest soils.

Functional differences between closely-related soil arthropods with respect to decomposition processes in the presence or absence of pine tree roots

Abstract The effect of three collembolan species on decomposition and N mobilization was studied in a Pinus nigra plantation, using field enclosures and by trenching tree roots. The fauna were added to defaunated organic material. Estimates were made of weight and N losses from fresh litter, fragmented litter and humus after 7 months exposure. N mobilization was also measured as leachable mineral N using suction cup lysimcters. and was found to be dominated by NO3−-N. In the presence of the surface dwelling Orcheselln cincta (L.) a net loss of 310 mmol m−2 of total N in humus was observed, as opposed to a small gain of 76 mmol N m−2 in the control. Tomocerus minor (Lubbock) induced a nearly two-fold increase in the mobilization of NO3−-N to the leachate compared to Isotoma nolabilis Schaffer. possibly as a consequence of differences in grazing saprotrophic fungi. The influence of I. notabilis on the experimental systems differed from the effects of the other Collembola in effecting an increased mass loss of humus material. Tree roots suppressed fragmented litter decomposition significantly by 2.0%. and had considerable effect on N dynamics. In the presence of roots, leachable quantities of NO3−-N were reduced to half the amounts that were collected in their absence. The amount of total-N of fragmented litter was nearly unchanged in non-rooted plots, but was reduced at a rale of 274 mmol m−2 after 7 months. It was concluded that C and N mineralization in soils is influenced by the ecological characteristics of the dominant species and is not simply a function of trophic group or biomass.

Temporal and spatial variation of nitrogen transformations in a coniferous forest soil

Forest soils show a great degree of temporal and spatial variation of nitrogen mineralization. The aim of the present study was to explain temporal variation in nitrate leaching from a nitrogen-saturated coniferous forest soil by potential nitrification, mineralization rates and nitrate uptake by roots. Variation in nitrate production in time and space, between the different organic horizons, has been related to temperature, moisture content, substrate availability and pH. Temporal variation in concentrations of nitrate and ammonium in the forest floor was significant during a one-year cycle, when randomly taken samples were pooled. Nitrogen concentrations differed between the different organic horizons with highest concentrations found in the litter layer, decreasing with increasing depth. Ammonium concentrations always exceeded nitrate concentrations by a factor ten, indicating that ammonium was not limiting nitrification. Nitrification potential, the nitrate production at field moisture at 25°C, was highest in the litter layer, lower in the fragmentation layer and hardly measurable in the mineral soil. Uptake of nitrate by roots and changes in mineralization rates turned out to be unimportant to explain variation in time, as seasonal fluctuations seem to be less important than spatial variation. We found that horizontal spatial variation in potential nitrate production, leaching of nitrate and nitrogen concentrations from non-pooled field samples was higher than variation in time. All this reflects the actual spatial variation in the field, which is not explained by differences in moisture content or temperature. Overall neither pH nor substrate availability could explain this observed variation, however, local variation in microsites may be responsible for small-scale spatial variation. Allelopathic compounds and/or the composition of the microbial community are suggested as factors possibly affecting nitrate production.

Long-term decomposition of successive organic strata in a nitrogen saturated Scots pine forest soil

Abstract The objective of this research was to investigate possible destabilising effects of high deposition of inorganic nitrogen on long-term decomposition of organic matter in forest soil. Degradation of successive organic layers, representing different stages in the process of decomposition, was studied in a nitrogen-saturated, Pinus sylvestris forest soil with a long history of high atmospheric-N deposition. Stratified litterbag sets, filled with litter, fragmented litter, or humus, were used to obtain information on decay rates, in addition to changes in C and N concentration of the residues, during a two-and-a-half-year period. Time series with contrasting incubation–recovery sequences of litterbag sets, where equal decay stages in each sequence experienced different environmental conditions, were used to untangle the effects of time initiation from length of incubation. A series of two monthly renewed litterbags, filled with freshly fallen needles, was used as a measure of the potential influence of season on decomposition rate. Decay rate of litter was tightly linked to changes in C and N concentration and the C/N ratio of the residues. These relationships were independent of differences in introduction and recovery sequences of the litterbags and stress the importance of C and N concentration as the major control of the rate of litter decomposition. Mass loss, residual N concentration, and C/N ratio approach rapidly to asymptotic values, almost within the first year of incubation; this may be seen in the light of C or nutrient requirements for the growth of the microbial community during the decomposition of recalcitrant organic compounds. All observations support the hypothesis that enhanced atmospheric-N deposition aggravates C-limitation for microbial degradation which may explain the strong reductions in long-term decomposition.