两种代表性蚯蚓对设施菜地土壤微生物群落结构及理化性质的影响

不同生态型蚯蚓的取食偏好和生境有所差异,因此蚯蚓的生态型差异可能关乎其对土壤性质的不同影响;有关不同生态型蚯蚓对土壤性质尤其是微生物学性质影响的研究有助于了解蚯蚓生态功能的作用机制。在野外调控试验的第4年采集土壤,研究了牛粪混施和表施处理下内层种威廉腔环蚓(Metaphire guillelmi)和表层种赤子爱胜蚓(Eisenia foetida)对设施菜地土壤微生物群落结构和主要理化性质的影响。结果表明,土壤微生物群落结构同时受到蚯蚓种类和牛粪施用方式的影响。牛粪表施时,两种蚯蚓均显著降低了菌根真菌、真菌生物量和原生动物生物量(P0.05);牛粪混施时,不同蚯蚓的影响有所差异,威廉腔环蚓明显增加了菌根真菌、真菌生物量和放线菌生物量,而赤子爱胜蚓的作用不明显。此外,两种蚯蚓均提高了土壤孔隙度、团聚体稳定性和土壤p H、矿质氮以及微生物生物量碳氮水平,但提高幅度取决于蚯蚓种类和牛粪施用方式。冗余分析表明蚯蚓影响下土壤微生物群落结构的变化与团聚体稳定性、p H、速效磷、矿质氮呈正相关,而与土壤容重呈负相关。     

Microbial and decomposition efficiencies of monoculture and polyculture vermireactors,based on epigeic and anecic earthworms

Efforts have been made to evaluate the microbial and decomposition efficiency of three different vermireactors: (i) polyculture (introducing equal numbers of anecic and epigeic earthworms), (ii) monoculture (anecic) and (iii) monoculture (epigeic), designed by using earthworms of two different ecological categories i.e. anecic (Lampito mauritii Kinberg) and epigeic (Eisenia fetida (Savigny)). The microbial load of vermireactors was measured through substrate-induced respiration rate (SIR), microbial biomass N content and rate of dehydrogenase activity, while mineralization rate was evaluated measuring some chemical parameters of the substrate. Earthworms caused a decrease (as compared to initial value) in pH (41.9–80.7%), organic C (10.3–14.2%) and C:N ratio (41.9–80.7%) and an increase in total N (29.1–58.8%), NH₄-N (876.1–1485.7%), NO₃-N (29081.8–56792.6%), available P (16–19.4%) and exchangeable K (9.8–13.5%) contents of the substrate. The mineralization efficiency of the reactors was in the order: polyculture (epigeic + anecic) > monoculture (anecic) > monoculture (epigeic). The polyculture reactor showed the maximum rate of SIR (2.91 ± 0.2 mg CO₂ g⁻¹ substrate), microbial biomass N (3108.1 ± 289.2 mg N g⁻¹ substrate), and dehydrogenase activity (2453.3 ± 379.8 μg g⁻¹ substrate 24 h), while in the monoculture (epigeic) the lowest values of the same parameters were observed. It is concluded that the observed differences among reactors were due to different feeding behaviour and niche structures of epigeic and anecic earthworms. Data suggests that burrowing earthworms in waste-decomposing-system not only enhance the microbial efficiencies, but at the same time also accelerate the organic matter mineralization in a vermireactor. However, most of the previous studies were based on monoculture reactors (using epigeic earthworms) which have been recommended for waste decomposition operations, but this study revealed that polyculture vermicomposting (adding of burrowing worms with epigeic earthworms in vermicomposting system) might be beneficial for rapid decomposition of organic wastes.     

模拟氮沉降和降雨对华西雨屏区常绿阔叶林土壤呼吸的影响

从2013年12月至2014年11月,通过野外原位试验,对华西雨屏区常绿阔叶林进行了模拟氮沉降和降雨试验,采用LI-8100土壤碳通量分析系统(LI-COR Inc.,USA)测定了对照(CK)、氮沉降(N)、减雨(R)、增雨(W)、氮沉降+减雨(NR)、氮沉降+增雨(NW)6个处理水平的土壤呼吸速率,并通过回归方程分析了温度和湿度与土壤呼吸速率间的关系。结果表明:(1)氮沉降和增雨抑制了常绿阔叶林土壤呼吸速率,减雨促进了常绿阔叶林土壤呼吸速率。(2)减雨使华西雨屏区常绿阔叶林土壤呼吸年通量增加了258 g/m~2,而模拟氮沉降和增雨使华西雨屏区常绿阔叶林土壤呼吸年通量分别减少了321g/m~2和406g/m~2。(3)减雨增加了土壤呼吸的温度敏感性,模拟氮沉降和增雨降低了土壤呼吸的温度敏感性。(4)模拟温度和湿度与土壤呼吸速率间回归方程分析表明,土壤水分对土壤呼吸速率的影响较小。(5)模拟氮沉降和增雨处理减少土壤微生物生物量碳、氮的含量,减雨处理增加了土壤微生物生物量碳、氮的含量。(6)模拟氮沉降和降雨对华西雨屏区土壤CO_2释放的影响未表现出明显的交互作用。     

Population- and ecosystem-level effects of predation on microbial-feeding nematodes

We studied the role of nematode predation in the functioning of detrital food webs assembled in microcosms. The microcosms contained defaunated humus and litter materials, a diverse microbial community with bacteria, fungi and protozoa, and a birch (Betula pendula) seedling infected with mycorrhizal fungi. Different levels of top-down control upon microbivorous nematodes were set up by assembling food webs either without predators, or in combinations with a specialist and a non-specialist predatory mite (Mesostigmata). The nematode community was composed of either (1) three species of bacterivorous, or (2) three species of fungivorous nematodes or (3) both groups together. After two growing periods for the birch (38 weeks), the microcosms were destructively sampled for animal and microbial biomasses, concentration of mineral N in the soil, plant biomass and plant N concentration. The specialist predator reduced biomasses of both bacterial- and fungal-feeding nematodes by more than 50%, whereas the non-specialist predator weakly increased the biomass of fungivorous nematodes. Thus, under high predation pressure, the biomass of microbivores changed as predicted by trophic dynamic models assuming strong top-down control and uniformly behaving trophic levels. Despite this, microbial biomass was unaffected by the predators. However, microbial respiration increased slightly in the presence of predators. Assuming that microbial respiration correlates with microbial productivity, the increase in microbial respiration indicates a cascading productivity regulation. The composition of the microbivore community had only a minor effect on the outcome of the top-down control on microbes. The >50% reduction in nematode biomass and respiration coincided with <16% increase in microbial respiration and did not affect microbial biomass. Presence of the specialist predator slightly reduced soil NH⁺ ₄ concentration in communities with fungivore nematodes but plant growth and N uptake remained unchanged. Thus, the structure of the community only weakly controlled nutrient mineralisation. Received: 18 May 1998 / Accepted: 3 May 1999     

Root-Parasitic Nematodes Enhance Soil Microbial Activities and Nitrogen Mineralization

Obligate root-parasitic nematodes can affect soil microbes positively by enhancing C and nutrient leakage from roots but negatively by restricting total root growth. However, it is unclear how the resulting changes in C availability affect soil microbial activities and N cycling. In a microplot experiment, effects of root-parasitic reniform nematodes (Rotylenchulus reniformis) on soil microbial biomass and activities were examined in six different soils planted with cotton. Rotylenchulus reniformis was introduced at 900 nematodes kg^–1 soil in May 2000 prior to seeding cotton. In 2001, soil samples were collected in May before cotton was seeded and in November at the final harvest. Extractable C and N were consistently higher in the R. reniformis treatments than in the non-nematode controls across the six different soils. Nematode inoculation significantly reduced microbial biomass C, but increased microbial biomass N, leading to marked decreases in microbial biomass C:N ratios. Soil microbial respiration and net N mineralization rates were also consistently higher in the nematode treatments than in the controls. However, soil types did not have a significant impact on the effects of nematodes on these microbial parameters. These findings indicate that nematode infection of plant roots may enhance microbial activities and the turnover of soil microbial biomass, facilitating soil N cycling. The present study provides the first evidence about the direct role of root-feeding nematodes in enhancing soil N mineralization.     

Root, mycorrhiza and earthworm interactions: their effects on soil structuring processes, plant and soil nutrient concentration and plant biomass

Earthworms, arbuscular mycorrhiza fungi (AMF) and roots are important components of the belowground part of terrestrial ecosystem. However, their interacting effects on soil properties and plant growth are still poorly understood. A compartmental experimental design was used in a climate chamber in order to investigate, without phosphorus (P) addition, the single and combined effects of earthworms (Allolobophora chlorotica), AMF (Glomus intraradices) and roots (Allium porrum) on soil structure, nutrient concentration and plant growth. In our experimental conditions, plant roots improved soil structure stability (at the level of macroaggregates) whereas earthworms decreased it. AMF had no effect on soil structure stability but increased P transfer from the soil to the plant and significantly increased plant biomass. Earthworms had no direct influence on P uptake or plant biomass, and the N/P ratio measured in the shoots indicated that P was limiting. Interactions between AMF and earthworms were also observed on total C and N content in the soil and on total root biomass. Their effects varied temporally and between the different soil compartments (bulk soil, rhizosphere and drilosphere). After comparison with other similar studies, we suggest that effects of earthworms and AMF on plant production may depend on the limiting factors in the soil, mainly N or P. Our experiment highlights the importance of measuring physical and chemical soil parameters when studying soil organism interactions and their influence on plant performance.     

Tree leaf litter composition and nonnative earthworms influence plant invasion in experimental forest floor mesocosms

Dominant tree species influence community and ecosystem components through the quantity and quality of their litter. Effects of litter may be modified by activity of ecosystem engineers such as earthworms. We examined the interacting effects of forest litter type and earthworm presence on invasibility of plants into forest floor environments using a greenhouse mesocosm experiment. We crossed five litter treatments mimicking historic and predicted changes in dominant tree composition with a treatment of either the absence or presence of nonnative earthworms. We measured mass loss of each litter type and growth of a model nonnative plant species (Festuca arundinacea, fescue) sown into each mesocosm. Mass loss was greater for litter of tree species characterized by lower C:N ratios. Earthworms enhanced litter mass loss, but only for species with lower C:N, leading to a significant litter × earthworm interaction. Fescue biomass was significantly greater in treatments with litter of low C:N and greater mass loss, suggesting that rapid decomposition of forest litter may be more favorable to understory plant invasions. Earthworms were expected to enhance invasion by increasing mass loss and removing the physical barrier of litter. However, earthworms typically reduced invasion success but not under invasive tree litter where the presence of earthworms facilitated invasion success compared to other litter treatments where earthworms were present. We conclude that past and predicted future shifts in dominant tree species may influence forest understory invasibility. The presence of nonnative earthworms may either suppress of facilitate invasibility depending on the species of dominant overstory tree species and the litter layers they produce.     

Effects of elevated atmospheric CO2 on protozoan abundance in soil planted with wheat and on decomposition of wheat roots

The effect of elevated CO₂ on growth of wheat plants (Triticum aestivum cv. Minaret) and soil protozoan and bacterial populations was investigated in soil pots placed in open top chambers fumigated with ambient air or air enriched with CO₂ (ambient + 320 l L^–1 CO₂). We harvested plants two times during the growing season and measured the biomass and the C and N content of roots and shoots. The soil was divided into bulk and rhizosphere soil and the number of bacteria (colony-forming units, CFU) and protozoa was determined. There was no effect of atmospheric CO₂ content on the number of bacteria, but the total number of bacterivorous protozoa was higher in pots from the elevated CO₂ treatment. This increase was mainly due to an increase in the number of protozoa in the bulk soil. Density of protozoa in the rhizosphere was not affected by elevated CO₂. This suggests that the increase in protozoan numbers was a result of a general increase in rhizodeposition, presumably caused by increased root production, and not to an increased root exudation per root mass. After harvest, soil from the two treatments was incubated with and without roots and the respiration rate was estimated at intervals for 200 days. During the first 55 days, the specific root induced respiration rate was not affected by the CO₂ level at which the plants had been grown, indicating that the quality of the easily decomposable components of the roots was not affected by CO₂ level.     

Decomposers and root feeders interactively affect plant defence in <Emphasis Type="Italic">Sinapis alba</Emphasis>

Aboveground herbivory is well known to change plant growth and defence. In contrast, effects of soil organisms, acting alone or in concert, on allocation patterns are less well understood. We investigated separate and combined effects of the endogeic earthworm species Aporrectodea caliginosa and the root feeding nematode species Pratylenchus penetrans and Meloidogyne incognita on plant responses including growth and defence metabolite concentrations in leaves of white mustard, Sinapis alba. Soil biota had a strong impact on plant traits, with the intensity varying due to species combinations. Nematode infestation reduced shoot biomass and nitrogen concentration but only in the absence of earthworms. Earthworms likely counteracted the negative effects of nematodes. Infestation with the migratory lesion-nematode P. penetrans combined with earthworms led to increased root length. Earthworm biomass increased in the presence of this species, indicating that these nematodes increased the food resources of earthworms—presumably dead and decaying roots. Nitrogen-based defence compounds, i.e. glucosinolates, did not correlate with nitrogen levels. In the presence of earthworms, concentrations of aromatic glucosinolates in leaves were significantly increased. In contrast, infection with P. penetrans strongly decreased concentrations of glucosinolates (up to 81%). Infestation with the sedentary nematode M. incognita induced aromatic glucosinolates by more than 50% but only when earthworms were also present. Myrosinase activities, glucosinolate-hydrolysing enzymes, were unaffected by nematodes but reduced in the presence of earthworms. Our results document that root-feeding nematodes elicit systemic plant responses in defence metabolites, with the responses varying drastically with nematode species of different functional groups. Furthermore, systemic plant responses are also altered by decomposer animals, such as earthworms, challenging the assumption that induction of plant responses including defence traits is restricted to herbivores. Soil animals even interact and modulate the individual effects on plant growth and plant defence, thereby likely also influencing shoot herbivore attack.     

Inhibition of growth,and effects on nutrient uptake of arctic graminoids by leaf extracts — allelopathy or resource competition between plants and microbes?

Previous research has shown that plant extracts, e.g. from boreal dwarf shrubs and trees, can cause reduced growth of neighbouring plants: an effect known as allelopathy. To examine whether arctic and subarctic plants could also be affected by leaching of phytochemicals, we added extracts from the commonly occurring arctic dwarf shrubs Cassiope tetragona and Empetrum hermaphroditum, and from mountain birch, Betula pubescens ssp. tortuosa to three graminoid species, Carex bigelowii, Festuca vivipara and Luzula arcuata, grown in previously sterilized or non-sterilized arctic soils. The graminoids in non-sterilized soil grew more slowly than those in sterilized soil. Excised roots of the plants in non-sterilized soil had higher uptake rate of labelled P than those in sterilized soil, demonstrating larger nutrient deficiency. The difference in growth rate was probably caused by higher nutrient availability for plants in soils in which the microbial biomass was killed after soil sterilization. The dwarf shrub extracts contained low amounts of inorganic N and P and medium high amounts of carbohydrates. Betula extracts contained somewhat higher levels of N and much higher levels of P and carbohydrates. Addition of leaf extracts to the strongly nutrient limited graminoids in non-sterilized soil tended to reduce growth, whereas in the less nutrient limited sterilized soil it caused strong growth decline. Furthermore, the N and P uptake by excised roots of plants grown in both types of soil was high if extracts from the dwarf shrubs (with low P and N concentrations) had been added, whereas the P uptake declined but the N uptake increased after addition of the P-rich Betula extract. In contrast to the adverse extract effects on plants, soil microbial respiration and soil fungal biomass (ergosterol) was generally stimulated, most strongly after addition of the Betula extract. Although we cannot exclude the possibility that the reduced plant growth and the concomitant stimulation of microbial activity were caused by phytochemicals, we believe that this was more likely due to labile carbon in the extracts which stimulated microbial biomass and activity. As a result microbial uptake increased, thereby depleting the plant available pool of N and P, or, for the P-rich Betula extract, depleting soil inorganic N alone, to the extent of reducing plant growth. This chain of events is supported by the negative correlation between plant growth and sugar content in the three added extracts, and the positive correlation between microbial activity, fungal biomass production and sugar content, and are known reactions when labile carbon is added to nutrient deficient soils.     

Endogeic earthworms modify soil phosphorus,plant growth and interactions in a legume–cereal intercrop

Background and aims

Intercropping of legumes and cereals appears as an alternative agricultural practice to decrease the use of chemical fertilizers while maintaining high yields. A better understanding of the biotic and abiotic factors determining interactions between plants in such associations is required. Our study aimed to analyse the effect of earthworms on the legume–cereal interactions with a focus on the modifications induced by earthworms on the forms of soil phosphorus (P).

Methods

In a glasshouse experiment we investigated the effect of an endogeic earthworm (Allolobophora chlorotica) on the plant biomass and on N and P acquisition by durum wheat (Triticum turgidum durum L.) and chickpea (Cicer arietinum L.) either grown alone or intercropped. The modifications of the different organic and inorganic P forms in the bulk soil were measured.

Results

There was no overyielding of the intercrop in the absence of earthworms. Earthworms had a strong influence on biomass and resource allocation between roots and shoots whereas no modification was observed in terms of total biomass production and P acquisition. Earthworms changed the interaction between the intercropped species mainly by reducing the competition for nutrients. Facilitation (positive plant–plant interactions) was only observed for the root biomass and P acquisition in the presence of earthworms. Earthworms decreased the amount of organic P extracted with NaOH (Po NaOH), while they increased the water soluble inorganic P (Pi H₂O) content.

Conclusions

In this experiment, earthworms could be seen as “troubleshooter” in plant–plant interaction as they reduced the competition between the intercropped species. Our study brings new insights into how earthworms affect plant growth and the P cycle.     

Symposium on “Protozoan Ecology”: The Role of Protozoa in Litters and Soils1

Soil is the focus of organic matter turnover in terrestrial ecosystems and is an interstitial mosaic of microsites composed of particle aggregates and pore spaces, where transformation, decomposition, mineralization, and humification of organic matter takes place. Microorganisms and animals are scattered discontinuously in these microsites. Microarthropods and larger fauna increase the rate and amount of mineralization by comminution of organic matter and by redistribution of microsites through movements of earthworms and large arthropods; however, mineralization and return of nutrients to plants occurs in the community of bacteria, fungi, protozoa, and nematodes living in the water films covering aggregates and filling pore spaces. Protozoa, especially small amoebae, are important bacterial grazers because they can enter tiny spaces unavailable to nematodes. The latter graze bacteria, fungi, and protozoa. Protozoan and nematode predation increase the amounts of soluble nutrients and decrease the competitive abilities of bacteria, thus making these nutrients more available to plants. Protozoa enhance nutrient recycling out of proportion to their biomass.     

How does glutamine synthetase activity determine plant tolerance to ammonium?

The wide range of plant responses to ammonium nutrition can be used to study the way ammonium interferes with plant metabolism and to assess some characteristics related with ammonium tolerance by plants. In this work we investigated the hypothesis of plant tolerance to ammonium being related with the plants’ capacity to maintain high levels of inorganic nitrogen assimilation in the roots. Plants of several species (Spinacia oleracea L., Lycopersicon esculentum L., Lactuca sativa L., Pisum sativum L. and Lupinus albus L.) were grown in the presence of distinct concentrations (0.5, 1.5, 3 and 6 mM) of nitrate and ammonium. The relative contributions of the activity of the key enzymes glutamine synthetase (GS; under light and dark conditions) and glutamate dehydrogenase (GDH) were determined. The main plant organs of nitrogen assimilation (root or shoot) to plant tolerance to ammonium were assessed. The results show that only plants that are able to maintain high levels of GS activity in the dark (either in leaves or in roots) and high root GDH activities accumulate equal amounts of biomass independently of the nitrogen source available to the root medium and thus are ammonium tolerant. Plant species with high GS activities in the dark coincide with those displaying a high capacity for nitrogen metabolism in the roots. Therefore, the main location of nitrogen metabolism (shoots or roots) and the levels of GS activity in the dark are an important strategy for plant ammonium tolerance. The relative contribution of each of these parameters to species tolerance to ammonium is assessed. The efficient sequestration of ammonium in roots, presumably in the vacuoles, is considered as an additional mechanism contributing to plant tolerance to ammonium nutrition.     

Muscodor albus, a potential biocontrol agent against plant-parasitic nematodes of economically important vegetable crops in Washington State, USA

The fungus, Muscodor albus, was tested for nematicidal and nematostatic potential against four plant-parasitic nematode species with three different feeding modes on economically important vegetable crops in the Pacific Northwest. Meloidogyne chitwoodi, Meloidogyne hapla, Paratrichodorus allius, and Pratylenchus penetrans were exposed for 72 h to volatiles generated by M. albus cultured on rye grain in sealed chambers at 24 °C in the laboratory. In addition, the nematodes were inoculated into soil fumigated with M. albus, and incubated for 7 days prior to the introduction of host plants under greenhouse conditions. The mean percent mortality of nematodes exposed to M. albus in the chamber was 82.7% for P. allius, 82.1% for P. penetrans, and 95% for M. chitwoodi; mortality in the nontreated controls was 5.8%, 7%, and 3.9%, respectively. Only 21.6% of M. hapla juveniles died in comparison to 8.9% in controls. However, 69.5% of the treated juveniles displayed reduced motility and lower response to physical stimulus by probing, in comparison to the control juveniles. This is evidence of nematostasis due to M. albus exposure. The greenhouse study showed that M. albus caused significant reduction to all nematode species in host roots and in rhizosphere soil. The percent mortality caused by M. albus applied at 0.5% and 1.0% w/w in comparison to the controls was as follows: 91% and 100% for P. allius in the soil; 100% for P. penetrans in bean roots and soil; 85% and 95% for M. chitwoodi in potato roots, and 56% and 100% in the soil; 100% for M. hapla both in pepper roots and soil. In this study, M. albus has shown both nematostatic and nematicidal properties.     

Effects of earthworms and organic litter distribution on plant performance and aphid reproduction

Human management practices and large detritivores such as earthworms incorporate plant litter into the soil, thereby forming a heterogeneous soil environment from which plant roots extract nutrients. In a greenhouse experiment we investigated effects of earthworms and spatial distribution of ¹⁵N-labelled grass litter on plants of different functional groups [Lolium perenne (grass), Plantago lanceolata (forb), Trifolium repens (legume)]. Earthworms enhanced shoot and root growth in L. perenne and P. lanceolata and N uptake from organic litter and soil in all plant species. Litter concentrated in a patch (compared with litter mixed homogeneously into the soil) increased shoot biomass and ¹⁵N uptake from the litter in L. perenne and enhanced root proliferation in P. lanceolata when earthworms were present. Growth of clover (T. repens) was rather independent of the presence of earthworms and organic litter distribution: nevertheless, clover took up more nitrogen in the presence of earthworms and exploited more ¹⁵N from the added litter than the other plant species. The magnitude of the effects of earthworms and organic litter distribution differed between the plant species, indicating different responses of plants with contrasting root morphology. Aphid (Myzus persicae) reproduction was reduced on P. lanceolata in the presence of earthworms. We suggest that earthworm activity may indirectly alter plant chemistry and hence defence mechanisms against herbivores.     

降雨量改变对常绿阔叶林干旱和湿润季节土壤呼吸的影响

通过野外原位试验,研究降雨量改变对华西雨屏区常绿阔叶林干旱和湿润季节土壤呼吸速率的影响。采用LI-8100土壤碳通量分析系统(LI-COR Inc.,USA)测定干旱和湿润季节对照(CK)、增雨10%(LA)、增雨5%(TA)、减雨10%(LR)、减雨20%(MR)、减雨50%(HR)6个处理水平的土壤呼吸速率,并通过回归方程分析温度和湿度与土壤呼吸速率间的关系。结果表明:湿润季节土壤呼吸速率高于干旱季节,HR处理对干旱季节土壤呼吸速率影响较大,而LA处理对湿润季节土壤呼吸速率的影响较大。TA和LR处理使土壤呼吸的温度敏感性增加,而HR、LA和MR处理使土壤呼吸的温度敏感性降低,干旱季节Q10值高于湿润季节。各处理湿润季节土壤微生物量碳氮含量显著高于干旱季节,HR、MR和LA处理减少土壤微生物生物量碳、氮的含量,而TA和LR处理增加土壤微生物生物量碳、氮的含量。与湿润季节相比,干旱季节土壤水分对土壤呼吸速率的影响较大;而与土壤温度相比,土壤水分对土壤呼吸速率的影响较小。在降雨量改变的背景下,华西雨屏区常绿阔叶林无论是干旱还是湿润季节,适当增雨和减雨都会促进土壤呼吸速率,而较高量的增雨和减雨会抑制土壤呼吸速率。     

Liming and nitrogen fertilization affects phosphatase activities, microbial biomass and mycorrhizal colonisation in upland grassland

We have studied the effects of factorial combinations of lime and N additions on soil microbial biomass, respiration rates and phosphatase activity of an upland grassland. We also used an Agrostis capillaris seedling bioassay to assess the effect of the treatments on the activity of arbuscular-mycorrhizal (AM) fungi and root surface phosphatase enzymes and the concentrations of N and P in the bioassay plant shoots. In the F and H horizons, soil microbial biomass carbon (C^mic) decreased in response to the liming, while addition of lime and N together reduced basal respiration rates. In the Ah horizon, C^mic was unaffected by the treatments but basal respiration rates decreased in the plots receiving nitrogen. Soil phosphatase activity decreased only in the Ah horizon in plots receiving lime, either in combination with N or alone. The mass of root fwt. colonized by AM fungi increased in response to the treatments in the order nitrogenR²=28.7%, P=0.004). The results demonstrate the sensitivity of both free-living heterotrophic microorganisms and symbiotic mycorrhizal fungi to short-term (2 years) applications of lime and N to long-term upland grassland, particularly in relation to the key P cycling activities undertaken by these organisms.     

Soil plus root respiration and microbial biomass following water, nitrogen, and phosphorus application at a high arctic semi desert

In order to investigate the effects of anticipated increased precipitation and changing soil nutrient levels on soil CO₂ efflux from high arctic semi desert, a field experiment was carried out in Northeast Greenland. Water, phosphorus, and nitrogen were added to plots in a fully factorial design. Soil microbial biomass carbon was analysed after one year, and respiration from soil plus roots was measured in situ throughout the third growing season after initiation of the experiment. Soil plus root respiration was enhanced by up to 47%, and the microbial biomass by 24%, by the weekly water additions, but not by nutrient additions. The direct effect of increased soil moisture on CO₂ efflux suggests that future changes of precipitation levels and patterns may strongly affect below-ground respiration in arctic semi deserts, with direction of responses depending upon amounts and frequencies of precipitation events. Morover, low CO₂ emission at low light intensities regardless of treatment suggests that the major part of the below-ground respiration originated from turnover of recently fixed C. Hence, the more recalcitrant soil organic matter C pool may not change in proportion to changes in below-ground respiration rate.     

Effect of nitrogen form at various levels of potassium,on the Meloidogyne-Fusarium wilt complex in muskmelon

Summary The influence of various potassium concentration and of nitrate or ammonium was evaluated on non inoculated muskmelon plants and on plants inoculated with theFusarium wilt pathogen (F), the root-knot nematode (Meloidogyne javanica) (Nem), or a combination of both pathogens (F+Nem). Increasing potassium concentrations raised top fresh weights (TFW) in all four groups. Nitrate fertilized plants weighed more than plants receiving ammonium, independent of the K⁺ level in the medium. In the Nem+F infected plants TFW values were one and half to two times greater in those receiving the nitrate than in those receiving the ammonium. Number of nematodes in the roots were not affected by nutritional difference. In the ammonium fertilized F and Nem+F plants 30% more wilting was found than after nitrate application. Irrespective of the form of nitrogen that was applied accumulation of N, P and K was found in the roots of the Nem+F and/or Nem plants, while in the shoots Ca, Mg, Na and P accumulated and K was depleted.Contribution from the Agricultural Research Organization, (ARO), No. 944-E, 1983 series.     

Carbon balance in a monospecific stand of an annual herb <Emphasis Type="Italic">Chenopodium album</Emphasis> at an elevated CO<Subscript>2</Subscript> concentration

Elevated CO₂ enhances carbon uptake of a plant stand, but the magnitude of the increase varies among growth stages. We studied the relative contribution of structural and physiological factors to the CO₂ effect on the carbon balance during stand development. Stands of an annual herb Chenopodium album were established in open-top chambers at ambient and elevated CO₂ concentrations (370 and 700 μmol mol⁻¹). Plant biomass growth, canopy structural traits (leaf area, leaf nitrogen distribution, and light gradient in the canopy), and physiological characteristics (leaf photosynthesis and respiration of organs) were studied through the growing season. CO₂ exchange of the stand was estimated with a canopy photosynthesis model. Rates of light-saturated photosynthesis and dark respiration of leaves as related with nitrogen content per unit leaf area and time-dependent reduction in specific respiration rates of stems and roots were incorporated into the model. Daily canopy carbon balance, calculated as an integration of leaf photosynthesis minus stem and root respiration, well explained biomass growth determined by harvests (r ² = 0.98). The increase of canopy photosynthesis with elevated CO₂ was 80% at an early stage and decreased to 55% at flowering. Sensitivity analyses suggested that an alteration in leaf photosynthetic traits enhanced canopy photosynthesis by 40–60% throughout the experiment period, whereas altered canopy structure contributed to the increase at the early stage only. Thus, both physiological and structural factors are involved in the increase of carbon balance and growth rate of C. album stands at elevated CO₂. However, their contributions were not constant, but changed with stand development.