Observations on the population dynamics and vertical distribution of trichodorid nematodes in a Scottish forest nursery

The population dynamics of Trichodorus primitivus, T. velatus and Paratrichodorus pachydermus was followed between April 1974 and March 1977 in a forest nursery growing young Sitka spruce (Picea sitchensis) trees. Soil samples taken at 10 cm intervals between 0–50 cm indicated that nematode and root distribution were not correlated and numbers of T. velatus were greatest between 0–29 cm and T. primitivus between 30–49 cm. After 3 yr trichodorid numbers between 0–19 cm had increased and those between 40–49 cm had decreased. The sex ratio of T. primitivus and T. velatus males to females was 1:1.6 and 1:1.5 respectively while for P. pachydermus it was 1:1.0. The ratios of adults to larvae varied with depth, the proportion of T. primitivus and T. velatus larvae increasing with depth but for P. pachydermus the greatest larval to adult ratio was in the top 10 cm. No consistent seasonal fluctuations were observed in the populations of all three species although larval numbers were proportionally greater than adult numbers in the summer and autumn of 1975 and 1976.     

Distribution of ectomycorrhizal and pathogenic fungi in soil along a vegetational change from Japanese black pine (Pinus thunbergii) to black locust (Robinia pseudoacacia)

The nitrogen-fixing tree black locust (Robinia pseudoacacia L.) seems to affect ectomycorrhizal (ECM) colonization and disease severity of Japanese black pine (Pinus thunbergii Parl.) seedlings. We examined the effect of black locust on the distribution of ECM and pathogenic fungi in soil. DNA was extracted from soil at depths of 0–5 and 5–10 cm, collected from the border between a Japanese black pine- and a black locust-dominated forest, and the distribution of these fungi was investigated by denaturing gradient gel electrophoresis. The effect of soil nutrition and pH on fungal distribution was also examined. Tomentella sp. 1 and Tomentella sp. 2 were not detected from some subplots in the Japanese black pine-dominated forest. Ectomycorrhizas formed by Tomentella spp. were dominant in black locust-dominated subplots and very little in the Japanese black pine-dominated forest. Therefore, the distribution may be influenced by the distribution of inoculum potential, although we could not detect significant relationships between the distribution of Tomentella spp. on pine seedlings and in soils. The other ECM fungi were detected in soils in subplots where the ECM fungi was not detected on pine seedlings, and there was no significant correlation between the distribution of the ECM fungi on pine seedlings and in soils. Therefore, inoculum potential seemed to not always influence the ECM community on roots. The distribution of Lactarius quieticolor and Tomentella sp. 2 in soil at a depth of 0–5 cm positively correlated with soil phosphate (soil P) and that of Tomentella sp. 2 also positively correlated with soil nitrogen (soil N). These results suggest the possibility that the distribution of inoculum potential of the ECM fungi was affected by soil N and soil P. Although the mortality of the pine seedlings was higher in the black locust-dominated area than in the Japanese black pine-dominated area, a pathogenic fungus of pine seedlings, Cylindrocladium pacificum, was detected in soil at depths of 0–5 and 5–10 cm from both these areas. This indicates that the disease severity of pine seedlings in this study was influenced by environmental conditions rather than the distribution of inoculum potential.     

Endomycorrhizal fungal species mediate 15N transfer from soybean to maize in non-fumigated soil

The effect of mycorrhizal inoculation on ¹⁵N transfer from soybean to maize was studied in fumigated and non-fumigated soil. Three Glomus species and a non-inoculated control were compared.In spite of higher levels of root colonization and more abundant hyphae associated with plants growing in fumigated soil, mycorrhizae-enhanced ¹⁵N transfer to maize was significant only in non-fumigated plots. High ¹⁵N transfer was not only associated with high mycelium density in soil but also with low soil microbial carbon, suggesting that the effect of mycorrhizal fungi on soil microbial populations may be an important factor affecting N transfer between mycorrhizal plants.     

Effects of preplant inoculation of apple (Malus domestica Borkh.) with arbuscular mycorrhizal fungi on population growth of the root-lesion nematode,Pratylenchus penetrans

Six species of arbuscular mycorrhizal (AM) fungi (Glomus aggregatum, G. clarum, G. etunicatum, G. intraradices, G. mosseae and G. versiforme) were evaluated, in three greenhouse experiments, for their effects on reproduction of the root-lesion nematode, Pratylenchus penetrans, and growth of Ottawa 3 apple rootstock. Glomus mosseae increased total dry weights of nematode-inoculated and non-inoculated rootstock in all three greenhouse experiments, and G. intraradices increased dry weights in two of three greenhouse experiments. Plants inoculated with G. mosseae generally supported fewer P. penetrans per gram of root than plants inoculated with other AM fungi, but did not differ significantly from the controls in any greenhouse experiment. Colonization of roots by AM fungi was reduced by P. penetrans at initial inoculum densities greater than 250 nematodes/L soil. In field trials, preplant inoculation with either G. intraradices or G. mosseae increased rootstock growth and leaf concentrations of P, Mg, Zn and Cu in fumigated plots but not in non-fumigated plots, indicating that colonization by native AM fungi in non-fumigated plots may have been sufficient for adequate nutrient acquisition. The abundance of vesicles and arbuscules was greater in roots of plants inoculated with AM fungi before planting than in roots of non-inoculated plants, in both fumigated and non-fumigated plots. P. penetrans per gram of root and per 50 ml soil were significantly lower for G. mosseae- inoculated plants than for non-inoculated plants in fumigated soil but not in non-fumigated soil.     

Evidence for biological nature of the grape replant problem in California

A bioassay was developed to investigate causes of grape replant problems under controlled conditions. Soils were collected from methyl bromide-fumigated and non-fumigated plots at a site cleared from a 65-year-old grape vineyard (Vitis vinifera cv. Thompson seedless) at Parlier, CA. Subsamples of the non-fumigated soil were either left non-treated, subjected to autoclaving (twice 45 min), or heating at 40, 50, 60, 70, 80 or 90 °C for 30 min. Subsequently, the samples were placed in 120-mL pots, planted with rooted hardwood grape cuttings (V. vinifera, cv. Carignane) and placed in a greenhouse or growth chamber. Three months after transplanting, vines from non-treated or 40 °C-treated soil had lower shoot weights and densities of healthy lateral roots than vines from the other treatments. Pythium spp. were isolated from 45 to 55% of the plated root segments from vines grown in non-treated, or soil that had been heated at 40 or 50 °C but were not detected in roots from soil given other treatments. Egg masses of root-knot nematode, Meloidogyne spp., were produced on roots from non-treated or heated at 40 °C soil, but no egg masses were detected on roots of the other treatments. In another test with the same soils, remnant roots from non-fumigated or pre-plant methyl bromide-fumigated soil were extracted and amended to non-fumigated soil, soil from fumigated field plots, soil fumigated in a small container, or autoclaved potting mix. The transfer of old vine roots from non-fumigated field soil resulted in incidence of Pythium spp. on grape assay roots, but there was no measurable effect of the transfer on growth and health of the bioassay plant roots. The results of the bioassays indicate that grape replant problem at the California site had biological causes. The bioassay approach may aid in future determinations of the etiology of grape replant problems.     

Dynamics of phosphorus fractions during fallow with natural vegetation and planted Pueraria phaseoloides in south-western Nigeria

A study was conducted in 1998 and 1999 on a long-term fallow management trial, established in 1989 at the International Institute of Tropical Agriculture (IITA), Ibadan, in the derived savanna of southwestern Nigeria, to quantify P fractions under natural fallow (NF) and a Pueraria cover crop fallow. Plots with previous 1:1, 1:2, and 1:3 crop/fallow ratios before reverting to fallow in either 1998 or 1999 were considered. Biomass accumulation under the two fallow types was comparable and increased linearly with fallow age, reaching slightly above 7 t dry matter ha^–1 after 8 months of fallow. Phosphorus accumulation in the fallow vegetation ranged from 2.1 – 9.1 kg ha^–1 for natural fallow (NF) and from 1.5 – 6.6 kg ha^–1 for Pueraria. Magnesium was also higher under NF (9.1 – 21 kg ha^–1) than under Pueraria (4.4 – 13 kg ha^–1), whereas N, Ca, and K contents were higher in Pueraria biomass than under NF at 1 year after fallow. Pueraria fallow tended to lower soil pH compared with NF. However, plots with less frequent cropping (1:3 crop/fallow ratio) did not have significantly different pH irrespective of the fallow vegetation type. Olsen extractable soil P increased as fallow length increased irrespective of the fallow system and previous crop/fallow ratio. For example, under NF (0–5 cm depth, 1:1 crop/fallow ratio in1998) Olsen P increased from 12 mg kg^–1 to 17 mg kg^–1 after 1 year of fallow and under Pueraria, it increased from 8 mg kg^–1 to 15 mg kg^–1. Fallow type and previous crop/fallow ratio had no significant and consistent effects on soil P fractions. However, NaOH- and concentrated HCl- extractable organic P fractions increased with fallow length. In 1998, under NF, NaOH- extractable organic P increased from 12 to 21 mg kg^–1 (1:1 crop/fallow ratio) and from 10 to 19 mg kg^–1 for both 1:2 and 1:3 crop/fallow ratio. HCl- extractable organic P increased from 11 to 30 mg kg^–1 (1:1 crop/fallow ratio), from 13 to 27 mg kg^–1 (1:2 crop/fallow ratio) and from 18 to 35 mg kg^–1 (1:3 crop/fallow ratio). Similar trend was observed under Pueraria fallow. These results suggest that P was reallocated to non-readily available organic P fractions irrespective of fallow type and previous land use. These organic P fractions, which are usually more stable, reflect the overall change in soil organic P levels when the soil was stressed by cultivation and then reverted to fallow. These pools may thus represent an active reservoir (source and sink) of P in shifting cultivation under tropical conditions without inorganic fertilizer application.     

The association between species of Trichodorus and Paratrichodorus vector nematodes and serotypes of tobacco rattle tobravirus

Virus transmission bait tests with single trichodorid nematodes from England, the Netherlands, Scotland or Sweden showed that a substantial degree of specificity occurs between trichodorid vector species and tobravirus serotypes. This specificity was more apparent with associations between Paratrichodorus vector species and tobravirus serotypes than with those between Trichodorus species and tobravirus serotypes. P. pachydermus transmitted PRN-serotype tobacco rattle virus (TRV) isolates, P. teres ORE-serotype isolates and P. anemones TRV isolates which did not react with any of the antisera used, but which could be distinguished from all other isolates by their symptomatology in Chenopodium test plants. T. viruliferus, T. primitivus and T. cylindricus transmitted RQ-serotype isolates and the latter species also transmitted TRV isolates reacting with TCB2 and pea early-browning SP5-antisera. Several TRV isolates transmitted by T. cylindricus failed to react with any of the antisera used.     

The development of the bean seed fly <Emphasis Type="Italic">Delia platura</Emphasis> Mg. (Diptera,Anthomyiidae) and its harmfulness in forest-steppe agrocenoses of Samara Province

Delia platura Mg. is a polyphagous and polyvoltinic Holarctic species feeding mainly on booting cereals and seedling roots. In Samara Province, three generations of the species develop annually; the species hibernates as puparia in soil under winter crops at a depth of 10–20 cm; adults emerge during the first ten days of May. Most of all, the fly infests barley and winter wheat, preferring crop rotation systems with green-manure fallow and causing 2.6–7.2% yield loss.     

The Effects of Crop Rotation and Nitrogen Fertilization on Soil Chemical and Microbial Properties in a Guinea Savanna Alfisol of Nigeria

The impacts of crop rotation and inorganic nitrogen fertilization on soil microbial biomass C (SMBC) and N (SMBN) and water-soluble organic C (WSOC) were studied in a Guinea savanna Alfisol of Nigeria. In 2001, fields of grain legumes (soybean and cowpea), herbaceous legume (Centrosema pascuorum) and a natural fallow were established. In 2002, maize was planted with N fertilizer rates of 0, 20, 40 and 60 kg N ha⁻¹ in a split-plot arrangement fitted to a randomized complete block design with legumes and fallow as main plots and N fertilizer levels as subplots. Surface soil samples were taken at 4 weeks after planting and tasselling stage of the maize. Inorganic N fertilization had no significant (P>0.05) effect on SMBC, SMBN and WSOC, while crop rotation significantly (P<0.0001) affected both SMBC and WSOC. These results demonstrate that crop rotation do not necessarily influence the gross soil microbial biomass, but may affect physiologically distinct subcomponent of the microbial biomass. The soils under the various rotations had a predominance of fungi community as indicated by their wide biomass C/N ratio ranging from 9.2 to 20.9 suggesting fungi to be mainly responsible for decomposition in these soils. Soil microbial biomass and WSOC showed significant (P<0.05) correlation with both soil pH and organic carbon but no relationship with total N. Based on these results, it appears that the soil pH and organic carbon determined the flux of the soil microbial biomass and amount of WSOC in these soils.     

Development of nitrogen mineralisation gradients through surface soil depth and their influence on surface soil pH

Following mixing of the surface soil to about 7.5 cm depth in the field, soil layers (0–2.5, 2.5–5, 5–10 and 10–15 cm) were separately incubated in the laboratory to determine the rate of development of net N mineralisation gradients through surface soil depth under fallow, wheat and subterranean clover plots. Gradients in net N mineralisation were compared with those observed in the field, and their contribution to the observed pH changes was investigated.Heterotrophic activity, and thus net N mineralisation, decreased only slightly with depth immediately after soil mixing. This pattern persisted over time in soil layers sampled from fallow plots. In contrast, within 1 growing season after soil mixing, heterotrophic activity and net N mineralisation decreased significantly with depth in soil sampled from wheat and clover plots. In 0–15 cm soil sampled from under senescing plants, 32–38% of CO₂-C produced and net N mineralised originated from the surface 2.5 cm, while 52–56% originated from the surface 5 cm of soil. This resulted from an increase of pH and organic substrate concentration within the surface 2.5 cm of soil following plant residue return. Limitations of the in situ measurement of net N mineralisation in fallow soil was identified.Laboratory incubation studies showed that since most net N mineralisation occurred within the surface 2.5 cm of soil under senescing plants, nitrification and acidification were also concentrated at this depth. Despite this, compared to fallow soil, high potential acidification rates of 0–2.5 cm soil under senescing plants were not realised in the field due to the exposure to prolonged dry periods and moist-dry cycles. As a consequence, in the field the large magnitude of surface soil pH gradient which resulted from the return of alkaline plant residues was maintained over summer and autumn.     

Lime and ammonium carbonate fumigation coupled with bio-organic fertilizer application steered banana rhizosphere to assemble a unique microbiome against Panama disease

Microbiome plays a key role in determining soil suppressiveness against invading pathogens. Our previous study revealed that microbial community of bulk soil could be manipulated by lime and ammonium bicarbonate fumigation followed by biofertilizer application. However, the assembly of microbial community suppressive to banana Panama disease in the rhizosphere is still unclear. In this study, we used high-throughput sequencing and quantitative PCR to explore the assembly of rhizosphere microbiome associated with banana Panama disease suppression in a two-seasonal pot experiment. We found biofertilizer applied to lime and ammonium bicarbonate fumigated soil significantly (P < 0.05) reduced the abundance of rhizosphere Fusarium oxysporum compared to biofertilizer applied to non-fumigated soil. Principal coordinate analysis revealed that biofertilizer applied to lime and ammonium bicarbonate fumigated soil re-shaped the rhizosphere bacterial community composition by increasing the phylogenetic relatedness, and stimulating indigenous microbes, for example, Gemmatimonas, Sphingomonas, Pseudomonas, Lysobacter and Bacillus. Co-occurrence analysis revealed that potential species involved in disease suppression were more interrelated in disease-suppressive soils. Taken together, lime and ammonium bicarbonate fumigation followed by biofertilizer application could induce banana rhizosphere to assemble beneficial microbes dominated consortia to suppress banana Panama disease.     

Effect of a Terminated Cover Crop and Aldicarb on Cotton Yield and Meloidogyne incognita Population Density

Terminated small grain cover crops are valuable in light textured soils to reduce wind and rain erosion and for protection of young cotton seedlings. A three-year study was conducted to determine the impact of terminated small grain winter cover crops, which are hosts for Meloidogyne incognita, on cotton yield, root galling and nematode midseason population density. The small plot test consisted of the cover treatment as the main plots (winter fallow, oats, rye and wheat) and rate of aldicarb applied in-furrow at-plant (0, 0.59 and 0.84 kg a.i./ha) as subplots in a split-plot design with eight replications, arranged in a randomized complete block design. Roots of 10 cotton plants per plot were examined at approximately 35 days after planting. Root galling was affected by aldicarb rate (9.1, 3.8 and 3.4 galls/root system for 0, 0.59 and 0.84 kg aldicarb/ha), but not by cover crop. Soil samples were collected in mid-July and assayed for nematodes. The winter fallow plots had a lower density of M. incognita second-stage juveniles (J2) (transformed to Log₁₀ (J2 + 1)/500 cm³ soil) than any of the cover crops (0.88, 1.58, 1.67 and 1.75 Log₁₀(J2 + 1)/500 cm³ soil for winter fallow, oats, rye and wheat, respectively). There were also fewer M. incognita eggs at midseason in the winter fallow (3,512, 7,953, 8,262 and 11,392 eggs/500 cm³ soil for winter fallow, oats, rye and wheat, respectively). Yield (kg lint per ha) was increased by application of aldicarb (1,544, 1,710 and 1,697 for 0, 0.59 and 0.84 kg aldicarb/ha), but not by any cover crop treatments. These results were consistent over three years. The soil temperature at 15 cm depth, from when soils reached 18°C to termination of the grass cover crop, averaged 9,588, 7,274 and 1,639 centigrade hours (with a minimum threshold of 10°C), in 2005, 2006 and 2007, respectively. Under these conditions, potential reproduction of M. incognita on the cover crop did not result in a yield penalty.     

Distribution of trichodorid nematodes in Great Britain

This survey describes the distribution of trichodorid nematodes in Great Britain. Twelve species were recorded, including the newly described species Trichodorus hooperi Loof, 1973 and the first British record of T. sparsus Szczygiel, 1968. In the survey 3614 samples and records were analysed using the Statistical Package for Social Sciences computer programme. Seven hundred and ninety-two positive sites, containing 971 trichodorid populations, were located. Outline maps show the geographical distribution of each species. T. primitivus de Man, 1880 was the most commonly found species and had a cosmopolitan distribution. Paratrichodorus pachy-dermus Seinhorst, 1954 was the next most prevalent and occurred mostly in eastern England and Scotland. Populations of the other ten species were more restricted in both number and distribution. The distribution of trichodorid spp. with respect to altitude, soil type, pH and primary vegetation was tested using chi-squared contingency tests. A significant relationship between nematodes and environmental factors was found in all cases examined. However, interactions were found between several environmental factors and further tests were applied to study the independence of the species distributions with respect to these interactions.     

Effect of five tree crops and a cover crop in multi-strata agroforestry at two fertilization levels on soil fertility and soil solution chemistry in central Amazonia

The spatio-temporal patterns of soil fertility and soil solution chemistry in a multi-strata agroforestry system with perennial crops were analysed as indicators for the effects of crop species and management measures on soil conditions under permanent agriculture in central Amazonia. The study was carried out in a plantation with locally important tree crop species and a leguminous cover crop at two fertilization levels on a xanthic Ferralsol. Soil fertility to 2 m soil depth was evaluated 3.5 years after the establishment of the plantation, and soil solution chemistry at 10, 60 and 200 cm soil depth was monitored over 2 years. Several soil fertility characteristics exhibited spatial patterns within the multi-strata plots which reflected the differing properties of the plant species and their management, including the fertilizer input. Significant differences between species could be detected to 150 cm depth, and between fertilization treatments to 200 cm depth. Favourable effects on nutrient availability in the soil were found for annatto (Bixa orellana) (P, K) and cupuaçu (Theobroma grandiflorum) (Ca, Mg) in comparison with peach palm (Bactris gasipaes) and Brazil nut (Bertholletia excelsa). Nutrient concentrations of the soil solution showed pronounced fluctuations in the topsoil, corresponding to fertilizer applications. Large nutrient concentrations in the soil solution were accompanied by increased concentrations of aluminium and low pH values, caused by exchange reactions between fertilizer and sorbed acidity and reinforced by the acidifying effect of nitrification. The soil solution under the leguminous cover crop Pueraria phaseoloides had relatively large N concentrations during periods when those under the tree crops were small, and this could partly explain why no yield responses to N fertilization were observed at this site.     

A thorough study of a Paratylenchus sp. in glasshouse‐grown lettuce: Characterisation,population dynamics,host plants and damage threshold as keys to its integrated management

In glasshouses practising monoculture of butterhead lettuce in Belgium, high densities of pin nematodes (Paratylenchus spp.) are frequently associated with reduced plant growth. Growers currently apply chemical soil disinfestation measures to manage this problem, although stricter phytosanitary regulations are forcing a shift towards integrated management. Efficient implementation of such management requires knowledge about the factors influencing nematode population dynamics, and the damage threshold for lettuce. The nematode populations in five Belgian glasshouses were monitored for at least 1 year by frequently soil sampling at 0–30 cm and 30–60 cm depth. An undescribed species of Paratylenchus was identified in all glasshouses based on morphological and molecular features. High nematode densities (>20,000 (100 ml soil)^?1) occurred in winter and spring. Chemical soil disinfestation lowered these populations greatly, although up to 14% survived in the deeper soil layer. After soil steaming under negative pressure, no pin nematodes were found. After 2 months of black fallow pin nematode densities were reduced by 50%–76%. Lamb's lettuce, parsley and wild rocket were found to be poor hosts in a pot experiment, while reproduction factors (P_f/P_i) on lettuce cultivars varied between 1 and 3. In three experiments with butterhead lettuce ‘Cosmopolia’ in pots with a series of 9 or 10 densities of Paratylenchus sp. [up to 35,000 (100 ml soil)^?1], no damage to lettuce heads was observed. However, root weight and root quality were reduced, and the corresponding damage thresholds were rather low [1,754 and 362 Paratylenchus sp. (100 ml soil)^?1, respectively]. Management strategies such as crop rotation, soil disinfestation or fallow are recommended to avoid pin nematode population build‐up.     

Role of the soil Seed Bank in the restoration of a native pinewood at Glen Garry,Inverness-shire

Summary

The potential recovery of ground vegetation in a pinewood at Glen Garry, from which introduced tree species had been removed, was studied by sampling the upper soil seed bank and comparing the species composition of germinating seedlings with extant vegetation. Mean numbers and species of seedlings emerging differed between a control site (not under-planted) and two formerly under-planted sites cleared at different times. Germinating seedling numbers also differed with depth (0–10 cm). Calluna vulgaris and Juncus spp. dominated fifteen species germinating from the seed bank, while some other pinewood species were not found. Although seedling pine occurred, their survival will be affected by competition from Betula spp. and Deschampsia flexuosa. Other elements of the vegetation will recover from the seed bank or by migration, their distribution being influenced by topographic variations and the nature of the developing tree stand. Timing of clearance of exotics in relation to pine seed production and soil scarification might accelerate recovery of the pinewood flora generally.     

Root competition for phosphorus between the tree and herbaceous components of silvopastoral systems in Kerala, India

Root competition in polyculture systems involving combinations of four tree species and four grass species was evaluated based on ³²P recovery by each species in mixed and sole crop situations. The tree species were: Leucaena leucocephala, Casuarina equisetifolia, Acacia auriculiformis and Ailanthus triphysa, and the grass species were: Pennisetum purpureum (hybrid napier), Brachiaria ruziziensis (congo signal), Panicum maximum (guinea grass) and Zea mexicana (teosinte). Four lateral distance (25 and 50 cm) and depth (15 and 50 cm) treatments were included in the study to characterize the relative fine root distribution of trees. Absorption of ³²P was monitored through radioassay of leaves. Regardless of the species, ³²P uptake from 50 cm soil depth was lower than that of 15 cm depth. Absorption of ³²P from 50 cm lateral distance was also less than that of 25 cm distance in Acacia and Casuarina. Grass species in sole crop situations absorbed more ³²P than in mixed systems. None of the grass species when grown in association with tree components affected the absorption of ³²P by trees. All grass species exerted a complementary effect on ³²P absorption by Casuarina. Leucaena also benefited in the same way when grown in association with congo signal and/or teosinte. Of the tree species, Acacia and Leucaena adversely affected the ³²P uptake by grass species.     

Soil properties and crop performance on a kaolinitic Alfisol after 15 years of fallow and continuous cultivation

A long-term field experiment was established on a kaolinitic Alfisol in Ibadan, Nigeria, in 1972. The land was cleared manually from secondary forest and used for (i) continuous no-till cropping with maize (Zea mays L.) and maize/cassava (Manihot esculenta Crantz) intercropping, (ii) planted fallow of guinea grass (Panicum maximum Jacq.), leucaena (Leucaena leucocephala de Wit), and pigeon pea (Cajanus cajan Millsp.), and (iii) natural bush regrowth in a randomized complete block design with three replications. At the end of 15 years, the fallow plots were cleared manually and cropped with maize for three years. The chemical and physical soil properties and crop performance of the newly-cleared plots were compared with those under 15 years of continuous cultivation. A total of 26 woody species were identified on the bush regrowth plots. Above-ground biomass accumulation of the bush plots was 157 Mg ha^-1 containing 1316 kg N ha^-1. Guinea grass, leucaena and natural bush regrowth plots had comparable organic C concentrations (approximately 20 g kg^-1) in the surface soil (0 to 10 cm) after 15 years. The organic C concentration in the surface soil under pigeon pea was the lowest (9.5 g kg^-1) among the four fallow treatments. Soil under 15 years of continuous no-till maize with and without residue mulch, respectively, contained approximately half (10 g kg^-1) and a quarter (5.7 g kg^-1) of the organic C under natural bush or guinea grass fallow. The levels of exchangeable Ca, K, Mg and effective cation exchange capacity (ECEC) were lower in the soils under continuous cultivation than in those under natural bush and planted fallow. Soil acidification occurred in soils under continuous cropping as depicted by the lower pH values and greater exchangeable Al and Mn concentrations compared to the fallow plots. Grain yield of maize (3 to 5 Mg ha^-1) without fertilizer application in the plots newly cleared from natural bush, guinea grass and leucaena fallow was comparable with that of continuous no-till maize with residue mulch and chemical fertilizer (N, P, K, Mg, Zn) applications. Among the four fallow treatments, maize grain and stover yields were the lowest in plots cleared from pigeon pea fallow.     

Mediterranean pine and oak distribution in southern Spain: Is there a mismatch between regeneration and adult distribution?

Questions: What is the current distribution of pine and oak species along environmental gradients in southern Spain? Do pine and oak regeneration niches differ from the environmental niches of adults? Is oak species regeneration favoured under the canopy of pine forests? Location: Forest areas of Andalusia (～87 600 km², southern Spain). Methods: We compiled extensive forest inventory data to explore differences in abundance (basal area, m² ha^?1) patterns of adults (dbh >7.4 cm) and regeneration (dbh ≤7.4 cm) of five pine and five oak species. Canonical correspondence analysis (CCA) and generalized linear models were applied to explore species–environment relationships along climatic, edaphic, topographic and fire‐frequency gradients. Results: Both pines and oaks segregated along complex environmental gradients, with pines generally dominating in more severe (colder and drier) environments, while oaks dominated in milder, wetter winter areas. In 40‐55% of mature pine stands there was a lack of regeneration in the understorey, while in two oak species (Q. suber and Q. canariensis) 70% of stands did not show regeneration. Pine recruits were found at a higher frequency and abundance under the canopy of their congeners, whereas some oaks (Q. ilex) had greater regeneration under mixed pine–oak canopies. Conclusions: Climatic limitations and soil properties partly explained the regional distribution of pines and oaks. We found evidence for an upward shift of Q. ilex recruits towards areas with colder conditions in pine forests, which could be explained by a possible facilitative effect of the pine canopy on seedling establishment.     

科尔沁沙地典型固沙人工林地土壤水分时空特征及其对环境因子的响应

土壤水分时空动态特征对于干旱地区人工林的可持续经营与管理起着至关重要的作用。以位于科尔沁沙地南缘的樟子松和柠条固沙人工林为对象,于2018年11月-2019年11月连续观测了林地0-200 cm土壤剖面的含水量、温度及微气象因子,系统分析了土壤水分的时空变化特征及其对环境因子的响应。研究期内,两种林地土壤水分的季节变化可分为冻结期、补充期、消耗期和稳定期;依据土壤剖面的水分特征可分为易变层、活跃层和稳定层,但两种林地的分层深度有一定差异。在生长季内（5-10月）,土壤含水量对大气降雨的响应随着土层深度的增加而减弱;降雨对樟子松人工林0-20 cm层土壤水分的影响极显著（P<0.01）,对柠条人工林0-10 cm层的影响极显著（P<0.01）、20-60 cm层显著（P<0.05）。在土壤冻融周期内（2018年11月-2019年4月）,两种林地的土壤均表现为"单向冻结"和"双向融化"的特点;土壤温度是影响冻融期内土壤含水量的关键因素,两者呈极显著的指数函数关系;樟子松和柠条人工林土壤的最大冻结深度分别为170 cm和190 cm,前者10 cm土层解冻时间要比后者晚11 d,可能与乔木树冠的遮阴作用有关。潜在蒸散与柠条林0-60 cm层、樟子松林0-20 cm和200 cm层的土壤水分呈极显著相关（P<0.01）,而与樟子松林60 cm和160 cm层呈显著相关（P<0.05）,这与树木蒸腾和土壤蒸发等综合作用有关。研究表明,由于两种人工林的树种组成、树冠大小、郁闭程度和根系分布等结构特征不同会导致林地土壤水分时空特征的异质性及其对环境因素响应的差异。