Habitat‐specific positive and negative effects of soil biota on seedling growth in a fragmented tropical montane landscape

Soil biota, in particular fungi and other microbes, are known to interactively influence plant community structure. However, soil biota effects that can be isolated in a greenhouse experiment may be overridden by other biotic and abiotic factors in the field. Here we conducted parallel greenhouse and field experiments and quantified how soil biota sampled at the habitat level affect the performance of potential host plants within and across neighboring habitat types (pastures, coffee plantations and forest fragments) in a montane region in Colombia. We hypothesized that the direction and strength of soil‐biota effects depend on the habitat where soil is sampled, focal plant's life history, and field environmental characteristics (soil nutrients, light). In a greenhouse experiment, we compared growth of 10 plant species with soil from home (where species typically occur) and foreign (where conspecific adults rarely occur) habitats, with or without soil sterilization. In the field, we conducted a reciprocal transplant experiment in which we suppressed soil fungi with the application of fungicide. In the greenhouse experiment, fast‐growing pasture grass and pioneer trees performed less well with live soil from their home, compared to foreign habitats, and such home disadvantage was reduced following soil sterilization. Home disadvantage associated with live soil biota was also detectable in the field experiment, although light conditions of grasslands and coffee plantations benefited growth of these fast‐growing species. In contrast, coffee and shade‐tolerant trees performed similarly or better with their home soils, and showed no response to soil biota suppression. Overall, the species‐and‐habitat specific soil biota effects detectable in the field experiment were similar in direction and relative strength to those from the greenhouse experiments. Our findings highlight the importance of habitat‐level plant–soil interactions and plant life history for the regeneration of natural forests and agricultural production in human‐modified landscapes.     

Nonlinearity of effects of invasive ecosystem engineers on abiotic soil properties and soil biota

Invasions of non‐indigenous species into natural communities are currently rated as one of the most important threats to biodiversity. Particularly exotic ecosystem engineers such as earthworms potentially have profound impacts on community assembly and functioning. We investigated the impact of invasion by the lumbricid earthworms into an aspen forest of the Canadian Rocky Mountains on soil organic matter, microorganisms and microarthropod communities. Building on the results of previous studies in this forest, we expected positive effects of Lumbricus terrestris middens and negative effects of Octolasion tyrtaeum on soil biota (increase and decrease in soil nutrient concentrations, microbial parameters and soil microarthropod density and diversity, respectively). Further, we expected that earthworm effects change with time. Combined results of previous and the present study suggest a wavelike colonization pattern for Dendrobaena octaedra and O. tyrtaeum and that indeed the impact of earthworms on soil biota changed with time, likely due to changes in earthworm density. Unexpectedly, L. terrestris middens neither affected soil abiotic nor soil biotic properties. By contrast and in contrast to our hypothesis, carbon and nitrogen concentration and C‐to‐N ratio in deeper soil layers increased in presence of O. tyrtaeum, thereby likely enhancing nutrient availability for soil microorganisms and microarthropods. Even though the density of this endogeic species was rather low, presence of O. tyrtaeum resulted in increased densities of a number of microarthropod taxa and increased microarthropod diversity. The results suggest that at low density, invasive ecosystem engineers, such as O. tyrtaeum, cause disturbances of intermediate strength thereby beneficially affecting soil microorganisms and most microarthropods. This contrasts earlier effects during the wavelike invasion of O. tyrtaeum into the aspen forest when densities of O. tyrtaeum were high resulting in generally detrimental effects on soil biota. The results emphasize the nonlinearity of earthworm effects on abiotic and biotic soil properties and call for further long‐term investigations.     

Moisture enhances the positive effect of leaf‐cutting ant refuse dumps on soil biota activity

Soil biota activity in arid lands is often limited by the availability of water and organic matter. We experimentally explored whether small changes in soil moisture affect the activity of soil biota in external refuse dumps of the leaf‐cutting ant Acromyrmex lobicornis, one of the most important sources of organic matter in a semi‐arid land of north‐western Patagonia. We estimated CO₂ consumption in refuse dumps and in adjacent, non‐nest soil samples at two moisture levels, after 48 and 72 h. Soil biota activity, estimated by respiration rates, was up to 160 times greater in refuse dumps than in adjacent, non‐nest soils. Activity of soil biota in non‐nest soil did not change through time and was not affected by moisture. Conversely, soil biota increased their activity in refuse dump samples only at high moisture condition after 72 h. As the activity of microorganisms is key for soil nutrient generation and availability, refuse dumps may be considered as ‘islands of fertility’ for plants. This effect may be especially important after sporadic spring rainfalls, when the beneficial effect of refuse dumps on soil biota is enhanced. In addition, as refuse dumps generate several times more CO₂ than non‐nest soils, nest areas may be considered also as hot spots of CO₂ emissions. These results illustrate the potential importance of ant nests for nutrient cycling, maintenance of plant cover and carbon balance in arid ecosystems.     

The effects of mycorrhizal roots on litter decomposition, soil biota, and nutrients in a spodosolic soil

We studied the effects of mycorrhizal pitch pine (Pinus rigida) roots on litter decomposition, microbial biomass, nematode abundance and inorganic nutrients in the E horizon material of a spodosolic soil, using field microcosms created in a regenerating pitch pine stand in the New Jersey Pinelands. Pine roots stimulated litter decomposition by 18.7% by the end of the 29 month study. Both mass loss and N and P release from the litter were always higher in the presence of roots than in their absence. Nutrient concentrations in decomposing litter were similar, however, in the presence and absence of roots, which suggests that the roots present in the with-root treatment did not withdraw nutrients directly from the litter. The soil was slightly drier in the presence of roots, but there was no discernible effect on soil microbial biomass. The effects of roots on soil extractable inorganic nutrients were inconsistent. Roots, however, were consistently associated with higher numbers of soil nematodes. These results suggest that, in soils with low total C and N contents, roots stimulate greater activity of the soil biota, which contribute, in turn, to faster litter decomposition and nutrient release.Contribution No. 95-22 from the Institute of Marine and Coastal Sciences.Contribution No. 95-22 from the Institute of Marine and Coastal Sciences.     

Harvester ant nests, soil biota and soil chemistry

Many ant species accumulate organic debris in the vicinity of their nests. These organic materials should provide a rich resource base for the soil biota. We examined the effect of harvester ant nests (Pogonomyrmex barbatus) on the soil community and soil chemistry. Ant nest soils supported 30-fold higher densities of microarthropods and 5-fold higher densities of protozoa than surrounding, control soils. The relative abundances of the major groups of protozoa differed as well: amoebae and ciliates were relatively overrepresented, and flagellates underrepresented, in ant nest versus control soils. Densities of bacteria and fungi were similar in the two soil types. Concentrations of nitrate, ammonium, phosphorus, and potassium were significantly higher in ant nest soils, while concentrations of magnesium, calcium, and water were similar in nest and control soils. Ant nest soils were marginally more acidic than controls. The results demonstrate that P. barbatus nests constitute a significant source of spatial heterogeneity in soil biota and soil chemistry in arid grasslands. Received: 17 March 1997 / Accepted: 10 June 1997     

A meta‐analysis of leaf‐cutting ant nest effects on soil fertility and plant performance

1. Leaf‐cutting ants (LCAs) are considered as one of the most important agents of soil disturbances that affect vegetation patterns, but these assertions are based on isolated studies or anecdotal data. In this study, meta‐analysis techniques were used to quantitatively analyse the generality of these effects and determine some of their sources of variation. 2. The results reveal the following: (i) LCA nest sites showed higher levels of soil fertility than control sites, but the key source of these nutrients is the refuse material rather than the nest soil itself; (ii) refuse material from external piles tended to be richer in nutrient content than refuse material from internal refuse chambers; (iii) nest sites from temperate habitats showed higher cation content than those located in tropical/subtropical habitats; and (iv) nest sites showed higher plant growth than adjacent non‐nest sites (especially if plants have access to the refuse) but similar plant density and plant richness. 3. As LCAs improve nutrient availability in nest sites through the accumulation of refuse material, the location of the refuse will have a relevant role affecting vegetation. LCA species with external refuse dumps could benefit herbs, early vegetation stages and short‐living plants, whereas those with internal refuse chambers could benefit long‐living, large trees. However, the positive effect on individual plants does not extend to population and community levels. The foraging preferences of ants and the changes in microclimatic conditions around nests could act as selective ecological filters. 4. As refuse material from external piles and nest sites in temperate habitats tend to show higher fertility than refuse material from internal nest chambers and nest sites in tropical/subtropical habitats, LCA species with external refuse dumps in temperate regions could be of particular relevance for nutrient cycling and vegetation patterns.     

Exotic cheatgrass and loss of soil biota decrease the performance of a native grass

Soil disturbances can alter microbial communities including arbuscular mycorrhizal (AM) fungi, which may in turn, affect plant community structure and the abundance of exotic species. We hypothesized that altered soil microbial populations owing to disturbance would contribute to invasion by cheatgrass (Bromus tectorum), an exotic annual grass, at the expense of the native perennial grass, squirreltail (Elymus elymoides). Using a greenhouse experiment, we compared the responses of conspecific and heterospecific pairs of cheatgrass and squirreltail inoculated with soil (including live AM spores and other organisms) collected from fuel treatments with high, intermediate and no disturbance (pile burns, mastication, and intact woodlands) and a sterile control. Cheatgrass growth was unaffected by type of soil inoculum, whereas squirreltail growth, reproduction and nutrient uptake were higher in plants inoculated with soil from mastication and undisturbed treatments compared to pile burns and sterile controls. Squirreltail shoot biomass was positively correlated with AM colonization when inoculated with mastication and undisturbed soils, but not when inoculated with pile burn soils. In contrast, cheatgrass shoot biomass was negatively correlated with AM colonization, but this effect was less pronounced with pile burn inoculum. Cheatgrass had higher foliar N and P when grown with squirreltail compared to a conspecific, while squirreltail had lower foliar P, AM colonization and flower production when grown with cheatgrass. These results indicate that changes in AM communities resulting from high disturbance may favor exotic plant species that do not depend on mycorrhizal fungi, over native species that depend on particular taxa of AM fungi for growth and reproduction.     

Species-specific effects on topsoil development affect Quercus ilex seedling performance

We illustrate experimentally the importance of species-specificity in the soil-mediated interactions among plants inhabiting heterogeneous landscapes. Topsoils from nine microhabitats were used to experimentally grow Quercus ilex acorns in the laboratory. We recorded germination, emergence, acorn left and seedling performance (HGR, biomass and root allocation). Different topsoils had very contrasting physico-chemical properties, creating a heterogeneous arena. This heterogeneity translated to differences in seedling performance, since emergence and survival differed among topsoils. However, nutrient content in topsoils did not appear to benefit seedling performance. In addition, there were strong correlations among parameters of plant performance indicating a consistency of the effect of topsoil type on Q. ilex seedlings. This implies that it is crucial to consider the specificity of the interaction between plants to wholly understand the structure of plant communities. Species-specific interactions may play an important role in the organization and dynamic of plants spatial distribution.     

The interaction of soil biota and soil structure under global change

The structural framework of soil mediates all soil processes, at all relevant scales. The spatio-temporal heterogeneity prevalent in most soils underpins the majority of biological diversity in soil, providing refuge sites for prey against predator, flow paths for biota to move, or be moved, and localized pools of substrate for biota to multiply. Just as importantly, soil biota play a crucial role in mediating soil structure: bacteria and fungi aggregate and stabilize structure at small scales (μm–cm) and earthworms and termites stabilize and create larger-scale structures (mm–m). The stability of this two-way interaction of structure and biota relations is crucial to the sustainability of the ecosystem. Soil is constantly reacting to changes in microclimates, and many of the soil–plant–microbe processes rely on the functioning of subtle chemical and physical gradients. The effect of global change on soil structure–biota interactions may be significant, through alterations in precipitation, temperature events, or land-use. Nonetheless, because of the complexity and the ubiquitous heterogeneity of these interactions, it is difficult to extrapolate from general qualitative predictions of the effects of perturbations to specific reactions. This paper reviews some of the main soil structure–biota interactions, particularly focusing on soil stability, and the role of biota mediating soil structures. The effect of alterations in climate and land-use on these interactions is investigated. Several case studies of the effect of land-use change are presented.     

土壤生物及其对土壤生态学发展的影响

土壤生物区系、土壤生物多样性和全球变化已成为土壤生态学研究的肖沿领域,土壤生物以不同的方式改变着土壤的物理、化学和生物学特性,某一等级层次上的土壤生物群落的组成和结构可以对其它等级层次上的资源空间异质性产生影响,而这种空间异质性受到许多生物圈层－－土壤功能区域所维持。本文评价了土壤生物区系在土壤生态系统过程中的作用,论述了土壤生物多样性与生态系统功能的关系,讨论了土壤生态系统对全球变化的影响。     

Defoliation reduces soil biota – and modifies stimulating effects of elevated CO2

To understand the responses to external disturbance such as defoliation and possible feedback mechanisms at global change in terrestrial ecosystems, it is necessary to examine the extent and nature of effects on aboveground–belowground interactions. We studied a temperate heathland system subjected to experimental climate and atmospheric factors based on prognoses for year 2075 and further exposed to defoliation. By defoliating plants, we were able to study how global change modifies the interactions of the plant–soil system. Shoot production, root biomass, microbial biomass, and nematode abundance were assessed in the rhizosphere of manually defoliated patches of Deschampsia flexuosa in June in a full‐factorial FACE experiment with the treatments: increased atmospheric CO₂, increased nighttime temperatures, summer droughts, and all of their combinations. We found a negative effect of defoliation on microbial biomass that was not apparently affected by global change. The negative effect of defoliation cascades through to soil nematodes as dependent on CO₂ and drought. At ambient CO₂, drought and defoliation each reduced nematodes. In contrast, at elevated CO₂, a combination of drought and defoliation was needed to reduce nematodes. We found positive effects of CO₂ on root density and microbial biomass. Defoliation affected soil biota negatively, whereas elevated CO₂ stimulated the plant–soil system. This effect seen in June is contrasted by the effects seen in September at the same site. Late season defoliation increased activity and biomass of soil biota and more so at elevated CO₂. Based on soil biota responses, plants defoliated in active growth therefore conserve resources, whereas defoliation after termination of growth results in release of resources. This result challenges the idea that plants via exudation of organic carbon stimulate their rhizosphere biota when in apparent need of nutrients for growth.     

Effect of soil biota on growth and allocation by Eucalyptus microcarpa

We examined growth of Eucalyptus microcarpa seedlings in soil collected from four sites in southeastern Australia, in which retired pasture land has been revegetated with mixed plantings of Eucalyptus and Acacia species. Revegetation of farm land in southeastern Australia is an area of major investment. The focus of the study was to examine the influence of soil biota on seedling growth and its possible interaction with soil enrichment from a legume (Acacia) and decomposition rates. We used a soil freezing treatment (−80°C for 3 days) to retard the soil biota, with the expectation that invertebrates in particular would be killed. Soil freezing did not cause a nutrient pulse, but did reduce the level of ammonium in soil. Nitrate levels increased with time in pots, regardless of the soil treatment. Decomposition rates measured using cellulose substrate were significantly reduced by the freeze treatment, but only for approximately 90 days. Eucalyptus microcarpa seedlings grown in freeze-treated soil were approximately 40% smaller (total biomass), had marginally lower LAR (leaf area ratio), and significantly lower LMA (leaf mass per area). Low LMA indicates that leaves are either thinner in cross-section or less dense. We hypothesise that both the poor growth of seedlings and production of less robust leaves are consequences of reduced availability of soil nutrients due to the diminished soil biota after freeze treatment. Litter under Acacia was richer in nitrogen than litter under Eucalyptus but there was no difference in nitrogen content of soil, and consequently no soil source effects on plant growth or decomposition. We suggest that variation in the soil biota has the potential to greatly enhance or hinder the success of revegetation on retired agricultural land, but enrichment of soil by decomposition of nitrogen rich litter in these sites requires longer than the 8–15 years since they were revegetated.     

Soil steaming effects on weed seedling emergence under the influence of soil type,soil moisture,soil structure and heat duration

Soil steaming applied in bands is a new technology with the potential to radically lower the burden of hand‐weeding intra‐row weeds in non‐herbicidal vegetable cropping. Preliminary studies with band‐steaming have shown effective control of viable weed seeds when the maximum soil temperatures reach 60–80°C. This temperature range has a particular agronomic interest, and the present study aimed at investigating the influence of soil factors and heat duration on weed seed mortality of soil steaming targeting 60–80°C. Two soil types (sand versus sandy loam) and two moisture levels (moist versus dry) were studied in one experiment (expt) while two levels of structure of a sandy loam (coarse versus fine) were included in a second experiment. A third experiment was focussing on the significance of heat duration expressed as the speed of cooling‐down after steaming had been stopped. Weed control efficacy was generally greater in sand than in sandy loam and soil irrigation further improved weed control. Steam application to the finely structured soil improved weed control efficacy relative to that obtained with the coarse soil of larger aggregate size. The rapidity of cooling from the maximum temperature did not affect the efficacy of the treatment on weed seed mortality. Based on these experiments, a maximum soil temperature of 80°C should ensure satisfactory weed control under moist soil conditions, especially if the soil is cultivated prior to steaming.     

Distance‐dependent effects of soil‐derived biota on seedling survival of the tropical tree legume Ormosia semicastrata

Question: How do seed germination and subsequent seedling survival of O. semicastrata (Hance forma litchiifolia How) vary with respect to distance from parent trees and conspecific density in different types of tropical forest? Are there effects of soil biota on O. semicastrata that systematically depend on distance from parent trees and conspecific density? Do soil pathogens differently affect survival of O. semicastrata in different types of tropical forest? Location: Tropical lowland rain forest and tropical montane rain forest in Jianfengling National Nature Reserve, Hainan Island, China. Methods: Individual adult O. semicastrata trees were selected in lowland rain forest and montane rain forest. Soil was collected at a distance of 0‐5 m or 15‐20 m from the parent tree. Soil samples from each distance were combined into a bulk sample. Half of the soil sample was sterilized by autoclaving. Surface‐sterilized seeds were then added to the soil material in shade‐houses at both forests. Results: Germination of O. semicastrata seeds at low‐ or high‐seed density was barely affected by the sterilization procedure. In both forests, seedlings grown in non‐sterilized soil collected close to parent trees had significantly higher mortality compared to those in sterilized soil. In contrast, seedling survival with soil collected far from parent trees was not affected by the soil sterilization procedure. Conclusions: Host‐specific pathogens concentrated in the soil around parent trees may regulate community structure of tropical trees at the stage of seedling development.     

Soil fertility effects on tree seedling performance are light‐dependent: evidence from a long‐term soil chronosequence

Soil chronosequences are a powerful tool for understanding how limitation of plant growth by nutrients and light changes throughout ecosystem development, but experimental tests of how availability of these resources interact to influence plant performance as ecosystem development proceeds are rare. We utilise the well‐characterised Franz Josef soil chrononosequence in New Zealand, a sequence of sites caused by a retreating glacier that spans ca 120 000 years and that includes all stages of ecosystem development from primary succession through to retrogression. Soil fertility is relatively low at either end of the sequence due to limitation of biological processes initially by N and ultimately by P whereas light availability is lowest at intermediate stages of the sequence dominated by tall forest. Growth and leaf traits of nine woody plant species, including those that occur widely along the chronosequence and those that are restricted to short portions of it, were quantified in a mesocosm experiment. Phytometers of these species were each grown in each of nine soils collected from throughout the chronosequence at either high (30%) or low (2%) light levels; these soil and light conditions represent the full variation observed along the sequence. Plant growth and biomass were greatest in soils from intermediate stages of the chronosequence and in high light. However, the stimulatory effects of soil fertility largely disappeared under shaded conditions that are characteristic of intermediate stages of ecosystem development. Our results demonstrate that long‐term changes in soil fertility and light availability that occur throughout ecosystem development had direct effects on plant species performance, but that there were stronger interactive effects of soils and light availability. Because light and soil resource availability shift predictably but have different trajectories throughout ecosystem development, our results help to understand variation in plant species performance and community assembly along complex environmental gradients.     

Regeneration of mature Norway spruce stands: early effects of selective cutting and clear cutting on seepage water quality and soil fertility

The cutting of trees influences element turnover in the forest ecosystem. The reduction of plant uptake, as well as an increased mineralization and nitrification due to higher soil temperature and soil moisture, can lead to considerable losses of nutrients from the main rooting zone. This may result in a reduced soil fertility and a decrease in drinking water quality due to high nitrate concentrations in the seepage water. In Bavaria (Germany) selective cutting is preferred to clear cutting when initiating the regeneration of Norway spruce stands with European beech. This paper summarizes the early effects of both forest management practices on soil fertility and seepage water quality for three different sites. Shown are the concentrations of nitrogen and base cations in the seepage water as well as the water and ion fluxes during the first year after tree cut. Nutrient inputs decreased on thinned plots and even more at clear-cuts. Nitrate concentrations in the seepage water are hardly affected by moderate thinning; however, on clear-cuts, the nitrate concentration increases significantly, and base cations are lost from the upper mineral soil. This effect is less obvious at sites where a dense ground vegetation, which is able to take up excess nitrogen, exists.     

土壤微生物对不同种源乌桕生长的影响

【背景】土壤微生物对植物成功入侵具有重要影响,研究中国本地土壤微生物对美国入侵种乌桕的净生长效应有利于进一步理解乌桕成功入侵的机理。【方法】以本地和入侵2个种源多个种群的乌桕为试验材料,通过对土壤进行灭菌处理,研究土壤微生物对不同种源乌桕幼苗生长的影响。【结果】土壤微生物对入侵种乌桕具有正效应,与本地种相比,入侵种乌桕幼苗总生物量、相对生长速度、根冠比以及总叶面积、叶面积比、比叶面积等生物学指标在新鲜土壤中均表现出明显优势。种源和土壤处理间存在交互效应的指标显示土壤微生物去除对入侵种乌桕的抑制作用更为明显。这一结果与土壤自然天敌逃逸假说相背,入侵种乌桕在一定程度上具有较本地种更强的土壤微生物利用能力,在非根际土壤微生物的作用下入侵种乌桕具有较强的生长能力和明显的竞争优势。【结论与意义】本研究证实土壤微生物可能对乌桕成功入侵具有不可忽略的作用。由于该试验在乌桕起源地进行,结合在乌桕入侵地进行的研究有助于进一步分析土壤微生物对入侵的贡献作用;研究土壤微生物与入侵植物的互作关系有助于对入侵物种进行准确预测和有效控制,减少生物入侵对生态系统功能的破坏。