Effects of exotic and native tree leaf litter on soil properties of two contrasting sites in the Iberian Peninsula

Aims

We assessed the effects of native and exotic tree leaf litter on soil properties in two contrasting scenarios. The native Quercus robur and Pinus pinaster tree species coexist with the aliens Eucalyptus globulus and Acacia dealbata in acid soils of NW Spain. The native trees Fraxinus angustifolia and Ulmus minor coexist with the aliens Ailanthus altissima, Robinia pseudoacacia and Ulmus pumila in eutrophic basic riparian soils in Central Spain.

Methods

Four plastic trays per species were filled with homogenized top-soil of the site and covered with leaf litter. Before and after 9?months of incubation, litter mass, soil pH, organic matter, mineral and total N were measured. Available mineral N (NO ₃ ^? -N and NH ₄ ⁺ -N) was assessed every 2?months.

Results

Soil biological activity was higher in the basic than in the acid soil. Litter of the exotic trees tended to decompose less than litter of native species, probably due to the presence of secondary metabolites in the former. Soil pH, mineral and total N responded differently to different litter types, irrespective of their exotic or native origin (acid soil), or was similar across litter treatments (basic riparian soil). The similar response of the basic soil to the addition of different litter types may be due to the low contrast of litter quality between the species. E. globulus litter inhibitied soil microbial activity much more than the rest of the studied litter types, leading to a drastic impoverishment of N in soils.

Conclusion

Litter of exotic N-fixing trees (A. dealbata and R. pseudoacacia) did not increase soil N pools because of the inhibition of microbial activity by secondary compounds. Therefore, secondary metabolites of the litter played a major role explaining exotic litter impact on soil properties.     

Differential responses of native and exotic coastal sage scrub plant species to N additions and the soil microbial community

Background and aims

Native shrub species of southern California have a long history of displacement by exotic annual herbs and forbs. Such invasions may be mediated by interactions with the microbial community and changes in the N cycle as a result of N pollution. However, the simultaneous effects of the soil microbial community status and N fertilization on dominant native and exotic plant species growth have not been thoroughly explored in this ecosystem.

Methods

Three species of native shrubs and of exotic annuals were grown in an orthogonal two-factor greenhouse experiment. To assess the importance of the soil microbial community pre-sterilized soils were inoculated with sterilized or non-sterilized field soil; to assess the importance of N type pots were fertilized with nitrate, ammonium or glycine solutions. Plant shoot and root biomass was measured after harvesting.

Results

The natives Artemisia californica and Eriogonum fasciculatum had lower growth in sterilized soil, suggesting microbial facilitation of these species, and E. fasciculatum higher growth with ammonia than either nitrate or glycine. Salvia apiana had equal growth under all conditions. The exotics Brassica nigra and Bromus madritensis grew equally in sterilized and unsterilized soil, and B. madritensis greater growth with ammonia fertilizer. Centaurea melitensis had greater growth in sterilized soil, and with either form of inorganic N.

Conclusions

These results highlight the importance of the soil microbial community in contributing to relative success of native vs. exotic species, and could inform restoration approaches for these species.     

Temperature sensitivity of soil carbon and nitrogen mineralization: impacts of nitrogen species and land use type

Background and aims

Climate warming, nitrogen (N) deposition and land use change are some of the drivers affecting ecosystem processes such as soil carbon (C) and N dynamics, yet the interactive effects of those drivers on ecosystem processes are poorly understood. This study aimed to understand mechanisms of interactive effects of temperature, form of N deposition and land use type on soil C and N mineralization.

Methods

We studied, in a laboratory incubation experiment, the effects of temperature (15 vs. 25 °C) and species of N deposition (NH₄ ⁺-N vs. NO₃ ^?-N) on soil CO₂ efflux, dissolved organic C (DOC) and N (DON), NH₄ ⁺-N, and NO₃ ^?-N concentrations using intact soil columns collected from adjacent forest and grassland ecosystems in north-central Alberta.

Results

Temperature and land use type interacted to affect soil CO₂ efflux, concentrations of DON, NH₄ ⁺-N and NO₃ ^?-N in most measurement times, with the higher incubation temperature resulted in the higher CO₂ efflux and NH₄ ⁺-N concentrations in forest soils and higher DON and NO₃ ^?-N concentrations in grassland soils. Temperature and land use type affected the cumulative soil CO₂ efflux, and DOC, DON, NH₄ ⁺-N and NO₃ ^?-N concentrations. The form of N added or its interaction with the other two factors did not affect any of the C and N cycling parameters.

Conclusions

Temperature and land use type were dominant factors affecting soil C loss, with the soil C in grassland soils more stable and resistant to temperature changes. The lack of short-term effects of the deposition of different N species on soil C and N mineralization suggest that maybe there was a threshold for the N effect to kick in and long-term experiments should be conducted to further elucidate the species of N deposition effects on soil C and N cycling in the studied systems.     

C and N allocation in soil under ryegrass and alfalfa estimated by 13C and 15N labelling

Background and Aims

Below-ground translocated carbon (C) released as rhizodeposits is an important driver for microbial mobilization of nitrogen (N) for plants. We investigated how a limited substrate supply due to reduced photoassimilation alters the allocation of recently assimilated C in plant and soil pools under legume and non-legume species.

Methods

A non-legume (Lolium perenne) and a legume (Medicago sativa) were labelled with ¹⁵N before the plants were clipped or shaded, and labelled twice with ¹³CO₂ thereafter. Ten days after clipping and shading, the ¹⁵N and ¹³C in shoots, roots, soil, dissolved organic nitrogen (DON) and carbon (DOC) and in microbial biomass, as well as the ¹³C in soil CO₂ were analyzed.

Results

After clipping, about 50 % more ¹³C was allocated to regrowing shoots, resulting in a lower translocation to roots compared to the unclipped control. Clipping also reduced the total soil CO₂ efflux under both species and the ¹³C recovery of soil CO₂ under L. perenne. The ¹⁵N recovery increased in the shoots of M. sativa after clipping, because storage compounds were remobilized from the roots and/or the N uptake from the soil increased. After shading, the assimilated ¹³C was preferentially retained in the shoots of both species. This caused a decreased ¹³C recovery in the roots of M. sativa. Similarly, the total soil CO₂ efflux under M. sativa decreased more than 50 % after shading. The ¹⁵N recovery in plant and soil pools showed that shading has no effect on the N uptake and N remobilization for L. perenne, but, the ¹⁵N recovery increased in the shoot of M. sativa.

Conclusions

The experiment showed that the dominating effect on C and N allocation after clipping is the need of C and N for shoot regrowth, whereas the dominating effect after shading is the reduced substrate supply for growth and respiration. Only slight differences could be observed between L. perenne and M. sativa in the C and N distribution after clipping or shading.     

Differences in nitrogen use strategies between native and exotic tree species: predicting impacts on invaded ecosystems

Background and aim

Exotic plant species can alter the nitrogen cycle in invaded ecosystems. We assess the differences in nitrogen use strategies and litter production and dynamics among three native riparian trees (Fraxinus angustifolia, Populus alba and Ulmus minor) and three co-occurring exotics (Ailanthus altissima, Robinia pseudoacacia and Ulmus pumila), currently spreading throughout river banks in inner Spain. We aim to predict the ecological consequences of a replacement of the natives by the exotics.

Methods

We compared the leaf lifespan, nitrogen resorption efficiency in leaves, nitrogen mean residence time, amount and timing of litter production and amount of nitrogen returned to soils between these native and exotic species.

Results

We found differences among species in all the variables measured, but not between native or exotic origins. Species were ranked from the most to the least conservative nitrogen use strategy as follows: U. pumila was the most conservative species, followed by the three natives (with an intermediate strategy), A. altissima and finally by the nitrogen-fixer R. pseudoacacia. The studied exotic species would produce contrasting impacts on the nitrogen cycle upon invasion.

Conclusions

On the basis of our results, we predict little impacts on the nitrogen cycle if U. pumila dominates the landscape. Despite being nitrogen-fixer R. pseudocacia would not increase soil nitrogen availability in the study area due to its low litter production and litter decomposition rates. In contrast, we predict an increase in nitrogen availability of soils upon A. altissima invasion, as this species produces a high amount of nitrogen rich and labile litter. This study offers a striking example of the contingencies involved in predicting the ecosystem impacts of exotic plant invasion.     

Degradability, molecular weight and adsorption properties of dissolved organic carbon and nitrogen leached from different types of decomposing litter

Aims

We characterized dissolved organic matter (DOM) leached during decomposition of deciduous silver birch litter (Betula pendula Roth.), coniferous Norway spruce litter (Picea abies (L.) Karst.) and a mixture of these litters in order to find out whether the properties of DOM would explain the earlier observed signs for higher microbial activity in soil under birch than spruce.

Methods

DOM leached from decomposing litters was collected in a litter-column experiment in the laboratory. Adsorption properties (XAD-8 resin fractionation) and molecular weight as well as the degradability of dissolved organic carbon (DOC) and nitrogen (DON) were measured three times during decomposition: 1) in the early stages, 2) after the mass loss reached 20–30 % and 3) when the mass loss reached 30–40 %.

Results

The leaching of DOC hydrophilic neutrals and bases, regarded easily degradable, decreased during decomposition. The leaching of DOC in hydrophobic acids, regarded refractory, increased from spruce and especially from the mixture litter during decomposition and may be connected to the degree of litter decomposition that was highest for the mixture. Unexpectedly, the degradability of DOC differed only slightly between the litters but the degradability of DON was substantially higher for spruce than birch. Spruce DOM seemed to be more N-rich than birch DOM in the early stages of decomposition and it seemed that labile DON was mobilized earlier from spruce than birch litter.

Conclusions

We conclude that the decomposition degree of litter determines largely the properties of DOM. The observed differences in the properties of DOM sampled during the litter decomposition cannot explain differences in C and N cycling between birch and spruce.     

Competitive interaction between the exotic plant Rhus typhina L. and the native tree Quercus acutissima Carr. in Northern China under different soil N:P ratios

Background and aims

Anthropogenic nitrogen (N) and phosphorus (P) input has changed the relative importance of nutrient elements. This study aimed to examine the effects of different nutrient conditions on the interaction between exotic and native plants.

Methods

We conducted a greenhouse experiment with a native species Quercus acutissima Carr. and an exotic species Rhus typhina L. grown in monocultures or mixtures, under three N:P ratios (5, 15 and 45 corresponding to N-limited, basic N and P supply and P-limited conditions, respectively). After 12 weeks of treatment, traits related to biomass allocation, leaf physiology and nutrient absorption were determined.

Results

R. typhina was dominant under competition, with a high capacity for carbon assimilation and nutrient absorption, and the dominance was unaffected by increasing N:P ratios. R. typhina invested more photosynthate in leaves and more nutrients in the photosynthetic apparatus, enabling high biomass production. Q. acutissima invested more photosynthate in roots and more nutrients in leaf persistence at the expense of reduced carbon assimilation capacity.

Conclusions

Different trade-offs in biomass and nutrient allocation of the two species is an important reason for their distinct performances under competition and helps R. typhina to maintain dominance under different nutrient conditions.     

Effects on nutrient cycling of conifer restoration in a degraded tropical montane forest

Background and aims

Exotic coniferous species have been used widely in restoration efforts in tropical montane forests due to their tolerance to adverse conditions and rapid growth, with little consideration given to the potential ecological benefits provided by native tree species. The aim of this study was to elucidate differences in litterfall and nutrient flow between a montane oak forest (Quercus humboldtii Bonpl.) and exotic coniferous plantations of pine (Pinus patula Schltdl. & Cham.) and cypress (Cupressus lusitanica Mill.) in the Colombian Andes.

Methods

Litter production, litter decomposition rate, and element composition of leaf litter were monitored during 3 years.

Results

Litter production in the oak forest and pine plantation was similar, but considerably lower in the cypress plantation . Similar patterns were observed for nutrient concentrations in litterfall, with the exception of Ca which was three times higher in the cypress plantation. The annual decay rate of litter was faster in the montane oak forest than in either of the exotic coniferous plantations. The potential and net return of nutrients to the forest floor were significantly higher in oak forest than in the exotic coniferous plantations.

Conclusions

Future restoration programs should consider new species that can emulate the nutrient flow of native broadleaf species instead of exotic species that tend to impoverish soil nutrient stocks in tropical montane forests.     

Nitrate-use traits of understory plants as potential regulators of vegetation distribution on a slope in a Japanese cedar plantation

Background and aims

Plant physiological traits and their relation to soil N availability was investigated as regulators of the distribution of understory shrub species along a slope in a Japanese cedar (Cryptomeria japonica) plantation in central Japan.

Methods

At the study site, previous studies demonstrated that both net and gross soil nitrification rates are high on the lower slope and there are dramatic declines in different sections of the slope gradient. We examined the distributions of understory plant species and their nitrate (NO ₃ ^? -N) use traits, and compared the results with the soil traits.

Results

Our results show that boundaries between different dominant understory species correspond to boundaries between different soil types. Leucosceptrum stellipilum occurs on soil with high net and gross nitrification rates. Hydrangea hirta is dominant on soil with high net and low gross nitrification rates. Pieris japonica occurs on soil with very low net and gross nitrification rates. Dominant understory species have species-specific physiological traits in their use of NO ₃ ^? -N. Pieris japonica lacks the capacity to use NO ₃ ^? -N as a N source, but other species do use NO ₃ ^? -N. Lindera triloba, whose distribution is unrelated to soil NO ₃ ^? -N availability, changes the extent to which it uses NO ₃ ^? -N in response to soil NO ₃ ^? -N availability.

Conclusions

Our results indicate that differences in the physiological capabilities and adaptabilities of plant species in using NO ₃ ^? -N as a N source regulate their distribution ranges. The identity of the major form of available soil N is therefore an environmental factor that influences plant distributions.     

Growth, carboxylate exudates and nutrient dynamics in three herbaceous perennial plant species under low, moderate and high phosphorus supply

Background and aims

Australian herbaceous native species have evolved in phosphorus (P) impoverished soils. Our objective was to explore shoot and root adaptations of two of these species with potential to be developed as pasture plants, at low, moderate and high P supply after 4 and 7?weeks of growth.

Methods

A glasshouse experiment examined the effect of 5, 20 and 80?mg?P?kg^?1 air-dry soil on growth, rhizosphere carboxylate content, and mineral nutrition of two Australian native perennials, Kennedia nigricans (Fabaceae) and Ptilotus polystachyus (Amaranthaceae), and the exotic Medicago sativa (Fabaceae).

Key results

Leaf P concentrations at P80 were 6, 14 and 52?mg?P?g^?1 leaf dry weight for M. sativa, K. nigricans and P. polystachyus, respectively. As soil P concentration increased, rhizosphere carboxylate content decreased for M. sativa, increased and then decreased for K. nigricans and was unchanged for P. polystachyus. For all species, the contribution of malic acid declined at the second harvest. For all species and P treatments, the amount of rhizosphere carboxylates per unit root length decreased as root length of a plant increased. Plant P content was determined more by P uptake rate per unit root length and time than by root length. Uptake of Mo for all species, and uptake of K, Mg and Mn for P. polystachyus, increased with soil P concentration. Uptake of Fe and S was higher when the content of carboxylates in the rhizosphere was higher.

Conclusion

Root physiological adaptations (i.e. rhizosphere carboxylate content and P-uptake rate) are more important than morphological adaptations (i.e. root length and diameter) to enhance the uptake of P and cations.     

Long-term impact of elevated CO<Subscript>2</Subscript> on phosphorus fractions varies in three contrasting cropping soils

Background and aims

Leaf litters commonly interact during decomposition in ways that can synergistically increases rates of decay. These interactions have been linked to moisture availability, suggesting that drought could slow decomposition rates by disrupting litter interactions. Slowed decomposition may reduce competitive ability of exotic species that exploit rapid decomposition rates as part of niche construction mechanisms. Here, we evaluated the impacts of drought on interactions between native and exotic species’ litter decomposition.

Methods

We considered litter mixtures of Lupinus polyphyllus (exotic N-fixing forb), Trifolium pratense (native N-fixing forb), Senecio inaequidens (exotic non-N-fixing forb), and Senecio jacobaea (native non-N-fixing forb) with the native grass Alopecurus pratensis and evaluated the difference between the observed rate of decay and the one expected based on species decomposing in monocultures. Litters were deployed in Belgium and Germany and exposed to a 56 day drought, which resembled local millennium drought (statistical recurrence of duration in local precipitation series >1000 years).

Results

Litter interactions reduced mass remaining by 81% in Belgium and 15% in Germany, averaged across mixtures. Similarly, litter interactions reduced N remaining by 93% in Belgium and 14% in Germany. Drought consistently removed these interactions and resulted in additive litter decay. Litters of native and exotic species did not differ in their response to drought.

Conclusions

These findings support moisture availability as a key regulator of interactions between litters during decomposition. Thus, increasing frequency of drought may slow nutrient cycling to a greater extent than previously thought.

     

Spatiotemporal variations affect uptake of inorganic and organic nitrogen by dominant plant species in an alpine wetland

Aims

Dominant plant species may coexist and maintain high productivity in alpine wetland through available nitrogen (N) niche differentiation over time and space. We tested the hypotheses that dominant plant species differ in uptake of inorganic and organic N and that such differences depend on soil depth and season.

Methods

We conducted a short-term ¹⁵N-labeling experiment in an alpine wetland on the Tibetan Plateau. The experiment used a factorial design with three N forms (nitrate, ammonium and glycine), three soil depths (0–5, 5–10 and 10–15 cm), two seasons (May and July) and three dominant species (Carex muliensis, C. lasiocarpa and Potentilla anserina).

Results

All three species took up organic N (glycine), but showed different patterns over seasons and depths. ¹⁵N uptake rate was higher in May than in July in C. muliensis and C. lasiocarpa, but lower in May than in July in P. anserina. C. muliensis took up more ¹⁵NH₄ ⁺ and ¹⁵NO₃ ^? than glycine-¹⁵N at all soil depths. C. lasiocarpa took up more glycine-¹⁵N than ¹⁵NH₄ ⁺ or ¹⁵NO₃ at 5–10 cm depth. P. anserina showed little difference in uptake at any soil depths.

Conclusions

Dominant species in alpine wetland are able to take up both organic and inorganic N, but show different patterns depending on N form, soil depth, season and their interactions.     

Can perennial dominant grass <Emphasis Type="Italic">Miscanthus sinensis</Emphasis> be nurse plant in recovery of degraded hilly land landscape in South China?

Perennial C₄ grasses, especially Miscanthus sinensis, are widely distributed in the degraded lands in South China. We transplanted native and exotic tree seedlings under the canopy of M. sinensis to assess the interaction (competition or facilitation) between dominant grass M. sinensis and tree seedlings. The results of growth, chlorophyll fluorescence, and ultrastructure showed that negative effects may be stronger in perennial dominant grass M. sinensis. Although M. sinensis buffered the air temperature, improved soil structure, and increased soil phosphorus content, these beneficial effects were outweighed by the detrimental effect, especially overshading. To ensure the establishment of target native species in M. sinensis communities in degraded lands of South China, restoration strategies should include removing aboveground vegetation, planting target species seedlings in openings to reduce the effects of canopy shading, and/or selecting competition-tolerant target species. Also, seedlings of exotic species used in restoration engineering cannot be directly planted under the canopy of M. sinensis.     

High precipitation causes large fluxes of dissolved organic carbon and nitrogen in a subtropical montane <Emphasis Type="Italic">Chamaecyparis</Emphasis> forest in Taiwan

Fluxes of dissolved organic carbon (DOC) and nitrogen (DON) may play an important role for losses of C and N from the soils of forest ecosystems, especially under conditions of high precipitation. We studied DOC and DON fluxes and concentrations in relation to precipitation intensity in a subtropical montane Chamaecyparis obtusa var. formosana forest in Taiwan. Our objective was, to quantify DOC and DON fluxes and to understand the role of high precipitation for DOC and DON export in this ecosystem. From 2005 to 2008 we sampled bulk precipitation, throughfall, forest floor percolates and seepage (60 cm) and analyzed DOC, DON and mineral N concentrations. Average DOC fluxes in the soil were extremely high (962 and 478 kg C ha^?1 year^?1 in forest floor percolates and seepage, respectively) while DON fluxes were similar to other (sub)tropical ecosystems (16 and 8 kg N ha^?1 year^?1, respectively). Total N fluxes in the soil were dominated by DON. Dissolved organic C and N concentrations in forest floor percolates were independent of the water flux. No dilution effect was visible. Instead, the pool size of potentially soluble DOC and DON was variable as indicated by different DOC and DON concentrations in forest floor percolates at similar precipitation amounts. Therefore, we hypothesized, that these pools are not likely to be depleted in the long term. The relationship between water fluxes in bulk precipitation and DOC and DON fluxes in forest floor percolates was positive (DOC r = 0.908, DON r = 0.842, respectively, Spearman rank correlation). We concluded, that precipitation is an important driver for DOC and DON losses from this subtropical montane forest and that these DOC losses play an important role in the soil C cycle of this ecosystem. Moreover, we found that the linear relationship between bulk precipitation and DOC and DON fluxes in forest floor percolates of temperate ecosystems does not hold when incorporating additional data on these fluxes from (subtropical) ecosystems.     

The effects of snowpack properties and plant strategies on litter decomposition during winter in subalpine meadows

Background and aims

Approximately 50 % of belowground organic carbon is present in the northern permafrost region and due to changes in climate there are concerns that this carbon will be rapidly released to the atmosphere. The release of carbon in arctic soils is thought to be intimately linked to the N cycle through the N cycle’s influence on microbial activity. The majority of new N input into arctic systems occurs through N₂-fixation; therefore, N₂-fixation may be the key driver of greenhouse gases from these ecosystems.

Methods

At Alexandra Fjord lowland, Ellesmere Island, Canada concurrent measurements of N₂-fixation, N mineralization and nitrification rates, dissolved organic soil N (DON) and C, inorganic soil N and surface greenhouse gas fluxes (CO₂, N₂O and CH₄) were taken in two ecosystem types (Wet Sedge Meadow and Dryas Heath) over the 2009 growing season (June-August). Using Structural Equation Modelling we evaluated the hypothesis that CO₂, CH₄ and N₂O flux are linked to N₂-fixation via the N cycle.

Results

The soil N cycle was linked to CO₂ flux in the Dryas Heath ecosystem via DON concentrations, but there was no link between the soil N cycle and CO₂ flux in the Wet Sedge Meadow. Methane flux was also not linked to the soil N cycle, nor surface soil temperature or moisture in either ecosystem. The soil N cycle was closely linked to N₂O emissions but via nitrification in the Wet Sedge Meadow and inorganic N in the Dryas Heath, indicating the important role of nitrification in net N₂O flux from arctic ecosystems.

Conclusions

Our results should be interpreted with caution given the high variability in both the rates of the N cycling processes and greenhouse gas flux found in both ecosystems over the growing season. However, while N₂-fixation and other N cycling processes may play a more limited role in instantaneous CO₂ emissions, these processes clearly play an important role in controlling N₂O emissions.     

The natural abundance of 15N in litter and soil profiles under six temperate tree species: N cycling depends on tree species traits and site fertility

Aims

We investigated the influence of tree species on the natural ¹⁵N abundance in forest stands under elevated ambient N deposition.

Methods

We analysed δ¹⁵N in litter, the forest floor and three mineral soil horizons along with ecosystem N status variables at six sites planted three decades ago with five European broadleaved tree species and Norway spruce.

Results

Litter δ¹⁵N and ¹⁵N enrichment factor (δ¹⁵N_litter–δ¹⁵N_soil) were positively correlated with N status based on soil and litter N pools, nitrification, subsoil nitrate concentration and forest growth. Tree species differences were also significant for these N variables and for the litter δ¹⁵N and enrichment factor. Litter from ash and sycamore maple with high N status and low fungal mycelia activity was enriched in ¹⁵N (+0.9 delta units) relative to other tree species (European beech, pedunculate oak, lime and Norway spruce) even though the latter species leached more nitrate.

Conclusions

The δ¹⁵N pattern reflected tree species related traits affecting the N cycling as well as site fertility and former land use, and possibly differences in N leaching. The tree species δ¹⁵N patterns reflected fractionation caused by uptake of N through mycorrhiza rather than due to nitrate leaching or other N transformation processes.     

Intermediate herbivory intensity of an aboveground pest promotes soil labile resources and microbial biomass via modifying rice growth

Background and Aims

The importance of aboveground herbivores for modifying belowground ecosystems has prompted numerous studies; however, studies can be biased by context dependent conditions which lead to extremely inconsistent results. So far, the impacts of herbivory intensity by important rice pests on rice paddy soil ecosystems are lacking. The aim of this study was to test the hypothesis that intermediate herbivory intensity of the brown planthopper (Nilaparvata lugens Stål) could promote soil labile resources and microbial biomass, while high intensity would show a reverse pattern, by mediating rice plant growth. This study will also help the development of integrative pest management.

Methods

Four hopper infestation density treatments (0, 4, 8 and 12 nymphs per rice plant) and two infestation duration treatments (9 and 15 days after N. lugens infestation, DAI 9 and DAI 15) were established in a glasshouse experiment. Soil and plant were sampled destructively from four replicates and analysed for soil labile resources availability, soil microbial biomass and plant performance, respectively.

Results

The infestation density significantly affected both shoot and root mass of rice (P?<?0.05), soil dissolved organic carbon (DOC) and nitrogen (DON), and microbial biomass carbon (MBC) and nitrogen (MBN), and the effects were further enhanced by prolonged infestation duration. Compared to the control (CK) without N. lugens, plant dry mass, DOC, DON, MBC and MBN increased under low (LD) and moderate hopper densities (MD) but decreased under high density (HD) on DAI 9. Moreover, the LD treatment exerted the most promotional effects on DAI 15. Rice root to shoot ratio generally increased in treatments subjected to herbivory. The labile resources and microbial biomass showed close relationships with both shoot and root mass across treatments, in particular with root mass on DAI 15. Such a trend indicated that the shift of photosynthate allocation to belowground contributed to changes of soil resource availability and microbial biomass.

Conclusions

Intermediate herbivory intensity showed positive effects on rice seedling performance and, further, promoted soil labile resource availability and microbial biomass. The importance of extrapolating temporal and spatial scale, i.e. from the short-term greenhouse experiment to an entire rice growing season in the field, was highlighted.     

Growth characteristics of ectomycorrhizal seedlings of Quercus glauca,Quercus salicina,and Castanopsis cuspidata planted on acidic soil

Key message

Seedlings of three Fagaceae species planted on acidic, infertile colluvial soil showed accelerated growth when inoculated with ectomycorrhizal fungi.

Abstract

We conducted a study with seedlings of Fagaceae species inoculated with ectomycorrhizal fungi to estimate their utility for growth in acidic soil conditions. We selected Quercus glauca, Quercus salicina and Castanopsis cuspidata as typical evergreen, broad-leaved, woody species of southwestern Japan. Seedlings were inoculated with Astraeus hygrometricus or Scleroderma citrinum, and planted in acidic, infertile colluvial soil collected from an abandoned site. Six months after planting, seedlings of the three species inoculated with A. hygrometricus were growing well, especially, Q. salicina. The growth of seedlings inoculated with S. citrinum was inferior to seedlings inoculated with A. hygrometricus. In contrast, seedlings without ectomycorrhizal fungi did not grow well. Differences in growth among the three types of seedlings were related to differences in the levels of nutrient acquisition. We concluded that Fagaceae seedlings inoculated with A. hygrometricus were best suited for acidic, infertile environments.     

Optimal prescribed burn frequency to manage foundation California perennial grass species and enhance native flora

Grasslands can be diverse assemblages of grasses and forbs but not much is known how perennial grass species management affects native plant diversity except in a few instances. We studied the use of late-spring prescribed burns over a span of 11 years where the perennial grass Poa secunda was the foundation species, with four additional years of measurements after the final burn. We evaluated burn effects on P. secunda, the rare native annual forb Amsinckia grandiflora and local native and exotic species. Annual burning maintained P. secunda number, resulted in significant expansion, the lowest thatch and exotic grass cover, the highest percentage of bare ground, but also the lowest native forb and highest exotic forb cover. Burning approximately every 3 years maintained a lower number of P. secunda plants, allowed for expansion, and resulted in the highest native forb cover with a low exotic grass cover. Burning approximately every 5 years and the control (burned once from a wildfire) resulted in a decline in P. secunda number, the highest exotic grass and thatch cover and the lowest percentage of bare ground. P. secunda numbers were maintained up to 4 years after the final burn. While local native forbs benefited from burning approximately every 3 years, planted A. grandiflora performed best in the control treatment. A. grandiflora did not occur naturally at the site; therefore, no seed bank was present to provide across-year protection from the effects of the burns. Thus, perennial grass species management must also consider other native species life history and phenology to enhance native flora diversity.     

Effect of four plant species on soil 15N-access and herbage yield in temporary agricultural grasslands

Background and aims

We carried out field experiments to investigate if an agricultural grassland mixture comprising shallow- (perennial ryegrass: Lolium perenne L.; white clover: Trifolium repens L.) and deep- (chicory: Cichorium intybus L.; Lucerne: Medicago sativa L.) rooting grassland species has greater herbage yields than a shallow-rooting two-species mixture and pure stands, if deep-rooting grassland species are superior in accessing soil ¹⁵N from 1.2 m soil depth compared with shallow-rooting plant species and vice versa, if a mixture of deep- and shallow-rooting plant species has access to greater amounts of soil ¹⁵N compared with a shallow-rooting binary mixture, and if leguminous plants affect herbage yield and soil ¹⁵N-access.

Methods

¹⁵N-enriched ammonium-sulphate was placed at three different soil depths (0.4, 0.8 and 1.2 m) to determine the depth dependent soil ¹⁵N-access of pure stands, two-species and four-species grassland communities.

Results

Herbage yield and soil ¹⁵N-access of the mixture including deep- and shallow-rooting grassland species were generally greater than the pure stands and the two-species mixture, except for herbage yield in pure stand lucerne. This positive plant diversity effect could not be explained by complementary soil ¹⁵N-access of the different plant species from 0.4, 0.8 and 1.2 m soil depths, even though deep-rooting chicory acquired relatively large amounts of deep soil ¹⁵N and shallow-rooting perennial ryegrass when grown in a mixture relatively large amounts of shallow soil ¹⁵N. Legumes fixed large amounts of N₂, added and spared N for non-leguminous plants, which especially stimulated the growth of perennial ryegrass.

Conclusions

Our study showed that increased plant diversity in agricultural grasslands can have positive effects on the environment (improved N use may lead to reduced N leaching) and agricultural production (increased herbage yield). A complementary effect between legumes and non-leguminous plants and increasing plant diversity had a greater positive impact on herbage yield compared with complementary vertical soil ¹⁵N-access.