Effects of soil acidity on nodulation ofAlnus glutinosa and viability ofFrankia

Summary Black alder seedlings were grown from seed for 7 weeks in six soils limed to various pH levels and inoculated withFrankia in two inoculation-seeding time combinations (inoculated and seeded concurrently; inoculated then seeded 5 weeks after inoculation). Three mine soils and three non-mine soils were used. Soil pHs in the study ranged from 3.6 to 7.6. In the second inoculation-seeding time combination, a series of soil samples at each of the pH levels below 7.0 were relimed to pH 7.0 immediately prior to seeding. The purpose of the study was to examine the effects of soil acidity on the nodulation of black alder byFrankia and the viability ofFrankia in acid soils. Based on the average number of nodules established per seedling, soil pH was determined to be a significant factor affecting nodulation in the mine soils. The highest levels of nodulation occurred between soil pH 5.5 and 7.2. Below pH 5.5, nodulation was reduced. There was also evidence of decreased viability of the endophyte below pH 4.5.     

Early colonization of red alder and Douglas fir by ectomycorrhizal fungi and Frankia in soils from the Oregon coast range

The potential for mycorrhizal formation and Frankia nodulation were studied in soils from six sites in the Pacific Northwest. The sites included young and old alder stands, a 1-year-old conifer clear-cut, a young conifer plantation, and rotation-aged and old-growth conifer stands. A bioassay procedure was used with both red alder and Douglas fir seedlings as hosts. After 6 weeks growth, seedlings of both hosts were harvested every 3 weeks for 21 weeks and numbers of nodules and ectomycorrhizal types estimated. Nodules formed on red alder and ectomycorrhizae formed on both alder and Douglas fir in soil from all sites. Nodulation potential was highest in soil from the alder stands and the conifer plantation. Seven morphologically distinct ectomycorrhizal types were recovered on Douglas fir and five on alder. Only Thelephora terrestris, a broad-host-range mycobiont, formed mycorrhizae on both hosts. New ectomycorrhizal types formed on both hosts throughout the bioassay. Ectomycorrhizal colonization of alder was greatest in the alder and clear-cut soils. Low ectomycorrhizal colonization on alder was found in soils from sites where conifers were actively growing. Ectomycorrhizal colonization of Douglas fir was highest in the young alder and conifer plantation soils and was low in the rotation-aged conifer soil. The highest diversity of ectomycorrhizal types was found on alder in the conifer clear-cut soil and on Douglas fir in the rotation-aged conifer soil. Effects of host specificity, nodulation and mycorrhiza-forming potential and nodule-mycorrhiza interactions on seedling establishment are discussed in relation to seral stage dynamics and attributes of pioneer ectomycorrhizal fungal species.     

Effect of juglone on growthin vitro ofFrankia isolates and nodulation ofAlnus glutinosa in soil

Summary In vitro growth (total protein content) of 5Frankia isolates was significantly inhibited at 10^–4 M juglone (5-hydroxy-1, 4-napthoquinone) concentration, but the degree of inhibition varied with theFrankia isolate. Isolates fromAlnus crispa [Alnus viridis ssp.crispa (Ait.) Turril] were most tolerant of 10^–4 M juglone relative to controls, while an isolate fromPurshia tridentata (Pursh.) D.C. was most inhibited, displaying a dramatic decrease in growth and greatly altered morphology.Nodulation of black alder [Alnus glutinosa L. (Gaertn.)] in an amended prairie soil inoculated with aFrankia isolate from red alder (Alnus rubra Bong.) was significantly decreased by the addition of aqueous suspensions of 10^–3 M and 10^–4 M juglone. This decrease was partially independent of decreased plant growth. The addition of an equal volume of sand to the soil mixture further decreased nodulation of black alder.Frankia inoculation of the soil mixtures significantly increased the total number of nodules formed per seedling, and the degree of differences in seedling nodulation owing to juglone and soil treatments.     

Foliar chemical concentrations in red alder stands of various ages

Summary Six stands of red alder (Alnus rubra Bong.) were sampled to assess the relationship between stand age and foliar chemical concentrations. The stands ranged in age from 9 to 45 years, and were located near Olympia, Washington, on sites of similar soil, aspect, and slope. Foliage was collected in July, and concentrations of most essential nutrients and aluminum were determined. Foliar concentrations of these elements varied among the different stands. Highest concentrations of N, P, Ca, S, Fe, and Al were found in trees of the youngest stand. Concentrations of N, P, S, and Fe were negatively correlated with stand age. The strongest correlation was obtained with P (r=–0.96,P<0.01), followed by N (r=–0.85,P=0.03). Results suggest that declining growth and deterioration of red alder stands with age may be related, at least in part, to limiting supplies of essential nutrients, especially P, N, and S.     

Survival ofFrankia strains introduced into soil

Factors affecting the establishment of Alnus/Frankia symbioses were studied partly by following the survival ofFrankia strains exposed to different soil conditions, and partly by investigating the effect of pH on nodulation. TwoFrankia strains were used, both of the Sp⁻ type (sporangia not formed in nodules). One of the strains sporulated heavily, while the other formed mainly hyphae. The strains originated fromAlnus incana root nodules growing in soils of pH 3.5 and 5.0. The optimum pH for their growth in pure culture was found to be 6.7 and 6.2, respectively. The strains were introduced into twoFrankia-free soils, peat and fine sand. Their survival, measured as the persistance of nodulation capacity using the plant infection technique, was followed for 14 months. The survival curves of the strains were similar despite the morphological differences between the strains in pure culture. The nodulation capacities declined over time both at 14 and 22°C. Survival was better in soils limed to a pH above 6 than in soils at their original pH (peat 2.9, fine sand 4.2). The effect of pH on nodule formation in Alnus seedlings by theFrankia strains was studied in liquid culture. The number of nodules increased linearly within the pH range studied (3.5–5.8). No nodules were formed at pH 3.5.     

Frankia inoculation,soil biota,and host tissue amendment influence Casuarina nodulation capacity of a tropical soil

The effects of soil biota, Frankia inoculation and tissue amendment on nodulation capacity of a soil was investigated in a factorial study using bulked soil from beneath a Casuarina cunninghamiana tree and bioassays with C. cunninghamiana seedlings as capture plants. Nodulation capacities were determined from soils incubated in sterile jars at 21 °C for 1, 7, and 28 days, after receiving all combinations of the following treatments: ± steam pasteurization, ± inoculation with Frankia isolate CjI82001, and ± amendment with different concentrations of Casuarina cladode extracts. Soil respiration within sealed containers was determined periodically during the incubation period as a measure of overall microbial activity. Soil respiration, and thus overall microbial activity, was positively correlated with increasing concentrations of Casuarina cladode extracts. The nodulation capacity of soils inoculated with Frankia strain Cj82001 decreased over time, while those of unpasteurized soils without inoculation either increased or remained unaffected. The mean nodulation capacity of unpasteurized soil inoculated with Frankia CjI82001 was two to three times greater than the sum of values for unpasteurized and inoculated pasteurized soils. Our results suggest a positive synergism between soil biota as a whole and Frankia inoculum with respect to host infection.     

Alder-Frankia interaction and alder-poplar association for biomass production

Summary Alders have an important role to play in biomass producing stands because of their N₂-fixing ability and their capacity to withstand soils having an excess of moisture. The objectives of preliminary trials were (1) to find if there is any alder-genotype xFrankia-strain interaction when the effect of inoculating the bacteria was compared to no inoculation in seed beds of different species and provenances of alder, (2) to measure the possible effect of black alders interplanted in poplars compared to pure poplar plots. Two trials were laid out to study the alder-Frankia interaction. Both produced interaction. In the first one the inoculation had a favorable effect onAlnus glutinosa at age 2 years andA. cordata at age 1 and 2 and no effect onA. rubra. In the second one the inoculation had a depressive effect at age 1 on 2 of 3 provenances ofA. rubra and no effect on 1A. rubra, 3A.glutinosa and 3A. cordata provenances.A closely spaced field trial associating one black alder provenance and the poplar clone UNAL gives no superiority of mixed plots compared to pure plots. The results suggest that the N₂-fixation of alders is not profitable to poplars at age 3 with a 1.5×2 m spacing.     

Inorganic N turnover and availability in annual- and perennial-dominated soils in a northern Utah shrub-steppe ecosystem

The exotic annual grass Bromus tectorum has replaced thousands of hectares of native perennial vegetation in semi-arid ecosystems of the western United States. Inorganic N availability and production were compared in soil from monodominant patches of Bromus tectorum, the perennial bunchgrass Elymus elymoides, and the shrub Artemisia tridentata, in Curlew Valley, a salt-desert shrub site in Northern Utah. Bromus-dominated soil had greater %N in the top 10 cm than Artemisia or Elymus-dominated soils. As determined by spring isotope-dilution assays, gross mineralization and nitrification rates were higher in Bromus-dominated than Artemisia-dominated soils, but gross rates of NH₄ ⁺ and NO₃ ^– consumption were also higher. Litterbags had greater mass loss and N mineralization when buried in Bromus stands than in Artemisia stands, indicating the soil environment under the annual grass promotes decomposition. As determined by nitrification potential assays, nitrifier populations were higher under Bromus than under Artemisia and Elymus. Soil inorganic N concentrations were similar among vegetation types in the spring, but NO₃ ^– accumulated under Bromus once it had senesced. An in situ net mineralization assay conducted in autumn indicated that germinating Bromus seedlings are a strong sink for soil NO₃ ^–, and that net nitrification is inherently low in soils under Artemisia and Elymus. Results of the study suggest that differences in plant uptake and the soil environment promote greater inorganic N availability under Bromus than under perennial species at the site.     

Chemical composition of soil solutions extracted from New Zealand beech forests and West German beech and spruce forests

Concentrations of ions were measured in soil solutions from beech (Nothofagus) forests in remote areas of New Zealand and in solutions from beech (Fagus sylvatica) and Norway spruce (Picea abies) forests in North-East Bavaria, West Germany, to compare the chemistry of soil solutions which are unaffected by acid deposition (New Zealand) with those that are affected (West Germany). In New Zealand, soil solution SO₄ ^2– concentrations ranged between <2 and 58 mol L^–1, and NO₃ ^– concentrations ranged between <1 and 3 mol L^–1. In West Germany, SO₄ ^2– concentrations ranged between 80 and 700 mol L^–1, and NO₃ ^– concentrations at three of six sites ranged between 39 and 3750 mol L^–1, but was not detected at the remaining three sites. At all sites in New Zealand, and at sites where the soil base status was moderately high in West Germany, pH levels increased, and total Al (Al_t) and inorganic monomeric Al (Al_i) levels decreased rapidly with increasing soil depth. In contrast, at sites on soils of low base status in West Germany, pH levels increased only slightly, and Al levels did not decline with increasing soil depth.Under a high-elevation Norway spruce stand showing severe Mg deficiency and dieback symptoms in West Germany, soil solution Mg²⁺ levels ranged between 20 and 60 mol L^–, and were only half those under a healthy stand. Al_t and Al_i levels were substantially higher the healthy stand than under the unhealthy stand, indicating that Al toxicity was not the main cause of spruce decline.     

Inoculation and production of container-grown red alder seedlings

Summary The inoculation ofAlnus rubra (red alder) withFrankia sp. can lead to a highly efficient symbiosis. Several factors contribute to the successful establishment of nitrogenfixing nodules: (1) quantity and quality ofFrankia inoculant; (2) time and method of inoculation; (3) nutritional status of the host plant.Frankia isolates were screened for their ability to nodulate and promote plant growth of container-grown red alder. Inoculations were performed on seedlings and seeds. Apparent differences in symbiotic performance could be seen when seeds or seedlings were inoculated. Plants inoculated at planting performed significantly better than those inoculated four weeks later in terms of shoot height, nodule number and shoot dry weight. If inoculation was delayed further, reduction in shoot height, nodule number and shoot dry weight resulted. The effect of fertilizer was also investigated with regard to providing optimal plant growth after inoculation. Plants receiving 1/5 Hoagland's solution minus nitrogen showed maximal plant growth with abundant nodulation. Plants receiving 1/5 Hoagland's solution with nitrogen showed excellent plant growth with significantly reduced nodulation.     

Litterfall and nutrient returns in red alder stands in western Washington

Summary Litterfall was collected over 1 year from eight natural stands of red alder growing on different sites in western Washington. The stands occurred at various elevations and on different soils, and differed in age, basal area, and site index. Most litterfall was leaf litter (average 86 percent). Amounts of litterfall and leaf litter varied significantly (P<0.05) among the sites. Average weights of litterfall and leaf litter in kg ha^–1 yr^–1, were 5150 and 4440, respectively. Weight of leaf litter was not significantly (P<0.05) related to site index, stand age, or basal area. The sites varied significantly (P<0.05) in concentrations of all elements determined in the leaf litter, except Zn. Average chemical concentrations were: N, 1.98 percent; P, 0.09 percent; K, 0.44 percent; Ca, 1.01 percent; Mg, 0.21 percent; S, 0.17 percent; SO₄–S, nil; Fe, 324 ppm; Mn, 311 ppm; Zn, 53 ppm; Cu, 13 ppm; and Al, 281 ppm. There were significant correlations between some stand characteristics and concentrations of some elements, and among the different chemical components of the leaf litter. Important correlations were found between stand age and P concentration (r=–0.84,P<0.01); weight of leaf litter and P concentration (r=0.74,P<0.05); weight of leaf litter and K concentration (r=0.71,P<0.05); concentrations of N and S (r=0.81,P<0.05); and concentrations of Fe and Al (r=0.98,P<0.01). Returns of the different elements to the soil by leaf litter varied among the different sites. Average nutrient and Al returns, in kg ha^–1 yr^–1, were: N, 82; Ca, 41; K, 19; Mg, 8; S, 7; P, 4; Fe, 1; Mn, 1; Al, 1; Zn, 0.2, and Cu, <0.1.     

Spatial variability of NO and NO2 flux rates from soil of spruce and beech forest ecosystems

In order to evaluate differences in the magnitude of NO and NO₂ flux rates between soil areas in direct vicinity to tree stems and areas of increasing distance to tree stems, we followed in 1997 at the Höglwald Forest site with a fully automated measuring system a complete annual cycle of NO and NO₂ fluxes from soils of an untreated spruce stand, a limed spruce strand, and a beech stand using at each stand measuring chambers which were installed onto the soils in such a way that they formed a stem to stem gradient. Flux data obtained since the end of 1993 from measuring chambers placed at the interstem areas of the stands, which had been used for the calculation of the long year annual mean of NO and NO₂ flux rates from soils of the stands, are compared to both (a) those obtained from the interstem chambers in 1997 and (b) those from the stem to stem gradient chambers. Daily mean NO fluxes obtained in 1997 were in a range of 0.3 – 280.1 g NO-N m^–2 h^–1 at the untreated spruce stand, 0.5 – 273.2 g NO-N m^–2 h^–1 at the limed spruce stand and 0.5 - 368.8 g NO-N m^–2 h^–1 at the beech stand, respectively. Highest NO emission rates were observed during summer, lowest during winter. Daily mean NO₂ fluxes were in a range of –83.1 – 7.6 g NO₂-N m^–2 h^–1 at the untreated spruce stand, -85.1 – 2.1 g NO₂-N m^–2 h^–1 at the limed spruce stand and –77.9 to –2.0 g NO₂-N m^–2 h^–1 at the beech site, respectively. As had been observed for the years 1994–1996, also in 1997 NO emission rates were highest at the untreated spruce stand and lowest at the beech stand and liming of a spruce stand resulted in a significant decrease in NO emission rates. For NO₂ no marked differences in the magnitude of flux rates were found between the three different stands. Results obtained from the stem to stem gradient experiments revealed that at all stands studied NO emission rates were significantly higher (between 1.6- and 2.6-fold) from soil areas close to the tree stems and decreased – except at the beech stand - with increasing distance from the stems, while for NO₂ deposition no marked differences were found. Including the contribution of soil areas in direct vicinity to the beech stems in the estimation of the annual mean NO source strength revealed that the source strength has been underestimated by 40% in the past.     

Dynamics of leaf minerals,bdleaf area,and biomass from hardwoods intensively grown on a peat bog

Summary Water willow and grey alder were grown on a raised sphagnum bog in central Sweden. The stands were intensively treated by daily irrigation and fertilization during the growing period in order to improve site fertility. After a 2-year establishment period high production rates were achieved in willow stands, 0.8 kg stem dry weight m^–2 year^–1 on current plus one (C+1) year old shoots. In these stands the canopy was closed with a leaf area index (LAI) that peaked at approximately 7. The canopy in the alder stand did not close during the initial 3 years of growth and the measured production rate was relatively low, at approximately 0.4 kg dry weight m^–2 year ^–1 in the last year. The leaf nitrogen content was 3%–4% of dry weight during the summer and the other studied mineral elements were in almost optimal proportion to nitrogen. This was considered to be an effect of the intensive fertilization regime. Above-ground production close to maximum yield was attained in the prevailing conditions of soil, climate and biomass partitioning.     

pH buffering in acidic soils developed under Picea abies and Quercus robur – effects of soil organic matter, adsorbed cations and soil solution ionic strength

Soil solution chemistry, soil acidity andcomposition of adsorbed cations were determinedin two soil profiles developed under a mixedspruce (Picea abies and Piceasitchensis) stand and in one soil profiledeveloped under an oak (Quercus robur)stand. Soils under spruce were classified asSpodosols and soils under oak were classifiedas Inceptisols. All profiles were developed inthe same parent material; a Saahlian sandy tillcontaining less than 2% clay. In the mineralsoil, the contribution from mineral surfaces tothe total cation-exchange capacity (CEC_t)was estimated to be less than 3%. Soilsolution pH and the percent base saturation ofCEC_t [%BS = 100 (2Ca + 2Mg + Na + K)CEC_t ^–1] were substantially lower inthe upper 35–40 cm of the two Spodosols, ascompared to the Inceptisol. The total amount ofsoil adsorbed base cations (BC) did not differamong the three profiles on an area basis downto 1 m soil depth. Thus, soil acidification ofCEC_t due to net losses of BC could notexplain differences in soil pH and %BS amongthe soil profiles. A weak acid analogue, takingthe pH-effect of metal complexation intoconsideration, combined with soil solutionionic strength as a covariate, could describeboth the pH variation by depth within soilprofiles and pH differences between theInceptisol and the two Spodosol profiles. Ourresults confirm and extend earlier findingsfrom O and E horizons of Spodosols that theextent to which organic acid groups react withAl minerals to form Al-SOM complexes is a majorpH-buffering process in acidic forest soils. Wesuggest that an increasing Al-saturation of SOMis the major reason for the widely observed pHincrease by depth in acidic forest soils with apH less than approximately 4.5. Our resultsstrongly imply that changes in mass of SOM, theionic strength in soil solution and therelative composition of soil adsorbed Al and Hneed to be considered when the causality behindchanges in pH and base saturation isinvestigated.     

Effects of Zn<Superscript>2+</Superscript> on nodulation and growth of a South American actinorhizal plant, <Emphasis Type="Italic">Discaria americana</Emphasis> (Rhamnaceae)

Discaria americana is a xerophytic shrub which lives in symbiosis with an actinomycete of the genus Frankia. The objective of this paper was to investigate the effects of high soil Zn²⁺ concentrations on growth and nodulation on the association Discaria americana–Frankia with the aim of determining if this association is suitable for improving contaminated soils. Two experiments were performed in 1 dm³ pots containing soil and different Zn additions, from 0 to 2,000 mg Zn²⁺ kg⁻¹ dry soil, with or without N fertilization. Zn additions strongly delayed shoot and root growth, but once growth was initiated, the biomass production of the plants supplied with moderate Zn amounts did not differ from the control plants. Zn reduced the final nodule number, but not the total nodule biomass. At the end of the experiment only the highest Zn treatments showed a lower nodule weight than the control plants, while N addition completely inhibited nodulation. It is concluded than Zn reduces the number of Frankia infections, but once the actinomycete is inside the roots, nodules can continue growing according to plant demand for N, compensating the reduced nodule number with more biomass. On the other hand, there is a toxic effect of Zn itself on plants when present in very high concentrations.     

重庆酸雨区不同林型对土壤酸化和真菌群落的影响

不同林型土壤的酸化缓冲能力不同,真菌在土壤系统中扮演着重要的角色,而对土壤真菌群落结构和组成与土壤酸化的关系缺乏深入研究。以重庆铁山坪林场的马尾松纯林（Pi）和经马尾松纯林改造后的香樟纯林（Ci）、木荷纯林（Sc）、马尾松-香樟混交林（Pi_Ci）以及马尾松-木荷混交林（Pi_Sc）为研究对象,每个林型分别设置4个20 m×20 m的样地,分别采集腐殖质层（O层）和淋溶层（A层）土壤进行土壤性质及真菌群落分析,以探讨酸雨区森林土壤真菌群落与缓解土壤酸化的关系。研究表明：（1）与Pi相比,Ci土壤酸化明显缓解（高pH低NH₄ ： NO₃）,且能有效提高土壤全磷（TP）含量;而Sc虽然土壤pH值与Pi没有显著差异,但显著（P<0.05）提高了NH₄ ： NO₃,且显著降低土壤TP和全钾（TK）含量（P<0.05）;（2）不同林型土壤真菌群落多样性以Ci最为丰富,且表征土壤酸化的指标pH值、阳离子交换量（CEC）与真菌多样性显著正相关（P<0.05）,NH₄ ： NO₃与多样性显著负相关（P<0.05）;（3）林型和土层都对真菌群落结构有显著影响（P<0.001）,且林型的影响大于土层的影响;而土壤酸化程度将五个林型的土壤真菌群落区分成两个大类：Ci和Pi_Ci;Pi,Sc以及Pi_Sc。（4） Ci中有益菌（如Mortierella）更多,Pi以外生菌根真菌占优势（Russulaceae、Russula、Tomentella以及Sebacina）;Sc以及Pi_Sc则含有更多的植物病原菌（Cladophialophora,Paecilomyces,Venturiales）、嗜酸菌及产酸菌（Paecilomyces,Penicillium）。在酸雨区受损马尾松林地种植香樟促进土壤真菌多样性提高,且产酸真菌、嗜酸菌丰度降低,而有益真菌丰度增加,可有效缓解土壤酸化;而种植木荷后土壤中的病原菌、嗜酸菌和产酸菌相对丰度增加,导致土壤进一步酸化。因此,通过将受酸雨损害严重的马尾松纯林改造成香樟纯林或马尾松-香樟混交林,有助于缓解土壤的酸化,实现酸雨区森林生态系统的可持续发展。     

Distribution ofFrankia in soils from forest and afforestation sites in northern Sweden

Summary The presence in soil ofFrankia, capable of forming nitrogen-fixing root nodules onAlnus incana (L.) Moench, was investigated. Intact soil cores from forested as well as disturbed sites were sampled and both alder-rich and alder-free sites were included in the study. Surface-sterilized alder seeds were sown in the soil cores which were kept in sterile culture tubes in a growth chamber. Root nodules with nitrogenase activity developed in soil cores from all sites studied. Thus, infective and effectiveFrankia was present in all of the soils sampled, even from sites free from actinorhizal plants and irrespective of pH and nitrogen content of the soils.     

Oxidation of elemental-S in coastal-dune sands and soils

Summary S-oxidation was studied in samples of (a) coastal sands lacking vegetation; (b) sands from beneath isolated stands ofAmmophila arenaria andHippophaë rhamnoides; and (c) dune soils obtained from beneath vegetation growing on mature dunes. S-oxidation in samples taken from dune environments was compared with the process in a fertile garden soil.Elemental-S was oxidized to SO ₄ ^2– in all samples, with S₂O ₃ ^2– being formed as intermediates. S-oxidation was most pronounced in the dune soil, followed by the garden soil,Ammophila arenaria andH. rhamnoides rhizospheres and finally the non-vegetated sand. The rate of S-oxidation thus generally increased with increasing C and N content, increasing vegetation cover and decreasing soil-sand pH.Maximum S-oxidation occurred at 30–37°C, but some of the intermediates appeared even at 45°C, presumably indicating abiotic S-oxidation at high temperatures. S-oxidation decreased the pH of the two soils studied, but did not markedly acidify the unvegetated or rhizosphere sands.     

Soil Organic Carbon and Water Retention after Conversion of Grasslands to Pine Plantations in the Ecuadorian Andes

Tree plantations in the high elevations of the tropics constitute a growing land use, but their effect on ecosystem processes and services is not well known. We examined changes in soil organic carbon (C) and water retention in a chronosequence of Pinus radiata stands planted in páramo grasslands in Cotopaxi province, Ecuador. Water retention at 10, 33, and 1,500 kPa declined with stand age, with soils in the oldest pine stands retaining 39%, 55%, and 63% less water than grassland soils at the three pressures tested. Soil organic C in the 0–10-cm depth also declined with stand age, from 5.0 kg m^–2 in grasslands to 3.5 kg m^–2 in 20–25-year-old pine stands (P < 0.001); at greater depth in the A horizon, C contents decreased from 2.8 to 1.2 kg m^–2 (P = 0.047). There were no significant differences among age classes in the AC and C horizons (P = 0.15 and P = 0.34, respectively), where little or no weathering of the primary material has occurred. Inputs of C may be affected by the significantly higher carbon–nitrogen (C:N) ratio of the litter under older pine stands (P = 0.005), whereas outputs are influenced by substrate quality as well as soil environmental factors. Soil ratios at the 0–10 cm depth were significantly higher in grasslands and young pine stands (P < 0.001), whereas carbon–phosphorous (C:P) ratios at 0–10-cm depth followed a similar but not significant trend. However, there was no significant difference in short-term decomposition rates (P = 0.60) when the soils were incubated under uniform temperature and moisture conditions. In páramo ecosystems, where high soil moisture plays an important role in retarding decomposition and driving high C storage, the loss of water retention after afforestation may be the dominant factor in C loss. These results suggest that soil C buildup and water retention respond rapidly to changes in biota and need to be assessed with regard to implications for C sequestration and watershed management.     

Glasshouse evaluation of the growth ofAlnus rubra andAlnus glutinosa on peat and acid brown earth soils when inoculated with four sources ofFrankia

The effects of soil type (an acid peat and 2 acid brown earths) andFrankia source (3 spore-positive crushed nodule inocula and spore-negative crushed nodules containing the singleFrankia ArI5) on nodulation, N content and growth ofAlnus glutinosa andA. rubra were determined in a glasshouse pot experiment of two years duration. Plants on all soils required additional P for growth. Growth of both species was very poor on peat withA. glutinosa superior toA. rubra. The former species was also superior toA. rubra on an acid brown earth with low pH and low P content. Some plant-inoculum combinations were of notable effectivity on particular soils but soil type was the major source of variation in plant weight. Inoculation with crushed nodules containingFrankia ArI5 only gave poor infection of the host plant, suggesting that inoculation with locally-collected crushed nodules can be a preferred alternative to inoculation withFrankia isolates of untested effectivity. Evidence of adaptation ofFrankia to particular soils was obtained. Thus, while the growth of all strains was stimulated by mineral soil extracts, inhibitory effects of peat extracts were more apparent with isolates from nodules from mineral soils than from peat, suggesting that survival ofFrankia on peat may be improved by strain selection.