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Pseudomonas isolates from tropical environments have been underexplored and may form an untapped reservoir of interesting secondary metabolites. In this study, we compared Pseudomonas and cyclic lipopeptide (CLP) diversity in the rhizosphere of a cocoyam root rot disease (CRRD) suppressive soil in Boteva, Cameroon with those from four conducive soils in Cameroon and Nigeria. Compared with other soils, Boteva andosols were characterized by high silt, organic matter, nitrogen and calcium. Besides, the cocoyam rhizosphere at Boteva was characterized by strains belonging mainly to the P. koreensis and P. putida (sub)groups, with representations in the P. fluorescens, P. chlororaphis, P. jessenii and P. asplenii (sub)groups. In contrast, P. putida isolates were prominent in conducive soils. Regarding CLP diversity, Boteva was characterized by strains producing 11 different CLP types with cocoyamide A producers, belonging to the P. koreensis group, being the most abundant. However, putisolvin III-V producers were the most dominant in the rhizosphere of conducive soils in both Cameroon and Nigeria. Furthermore, we elucidated the chemical structure of putisolvin derivatives-putisolvin III-V, and described its biosynthetic gene cluster. We show that high Pseudomonas and metabolic diversity may be driven by microbial competition, which likely contributes to soil suppressiveness to CRRD.

Purpose Keystone taxa play an important role in soil nutrient cycling and crop growth and can be influenced by soil tillage. We investigated the composition of keystone taxa and their relationships with soil properties under different long-term tillage practices. Methods Four tillage treatments were applied (i.e., CT, conventional tillage; NT, no tillage with mulch; RT, reduced tillage; and SS, subsoiling with mulch), maintained for 21 years. Co-occurrence network (CoNet) was constructed to identify the keystone taxa, and redundancy analysis (RDA) was carried out to explore the relationships between keystone taxa and soil properties under four tillage practices at two growth stages (elongation and grain filling stages) of winter wheat. Results Compared with CT, RT had no significant effect on the microbial community and the keystone microbiome, while NT and SS remarkably altered the microbial community structure and the keystone microbiome at both crop stages. Massilia was the keystone genus under CT and RT, while Sphingomonas , Asanoa and Hoeflea were the keystone genera under NT and SS. RDA results showed that keystone genera were significantly correlated with soil organic carbon (SOC), dissolved organic carbon (DOC) and microbial biomass nitrogen (MBN) at both stages, but especially at the elongation stage. Our results further revealed that the effects of NT and SS on crop growth might be related to the changes in keystone microbiome. Conclusion Our study suggests that NT and SS were suitable conservation regimes and may contribute to the development of sustainable agricultural production in the Chinese Loess Plateau.

Inherently low soil fertility and costly chemical fertilizers contribute to low maize yield and food insecurity among rural smallholder farmers in sub-Saharan Africa. The use of fertilizer trees such as Faidherbia albida (Faidherbia) to improve the soil fertility and yield of maize is recognized by many as a good practice. This study examined the litterfall pattern, quantity, and nutrient deposition from 8-, 15- and 22-year old Faidherbia trees at two locations in Zambia before and during the 2014/15 and 2015/16 rainy seasons. Litterfall including its nutrient content was estimated using nylon mesh litter traps erected under three randomly selected representative Faidherbia trees of each age. Further, we assessed the yield levels of maize grown under 8-, 15- and 22-year old Faidherbia trees. All trees started dropping litter before the onset of the rains in both seasons. The leaf litterfall averages of the two seasons were 1.6, 1.7 and 3.8 t DM ha−1 from 8-, 15- and 22-year-old trees, respectively. These litterfall quantities translate to potential carbon and nutrient deposition of 0.7–1.6 t C ha−1, 34–83 kg N ha−1, 1.8–4.3 kg P ha−1 and 10–26 kg K ha−1 per year. The yield levels of maize for the two growing seasons under Faidherbia tree canopy were 7–12 times higher than from outside the canopy, and these yield differences were much more pronounced in the driest year (2015/16 growing season). The large nutrient deposition through litterfall and increase in maize yield under Faidherbia tree canopies show the potential of these trees to contribute to food security and mitigate the risk of crop failure on resource-poor smallholder farmers’ fields, especially in drought years.

Despite massive efforts over the past twenty-five years to reduce nitrogen (N) losses, too high nitrate (NO3−) concentrations in surface and groundwater from agricultural sources remain a major environmental concern, especially in field grown vegetable production areas. A strict restriction of the N fertiliser application rates is accepted to be the best N management strategy to minimise NO3− leaching losses to surface and groundwater. We analysed Flemish field experiments on lettuce, spinach, leek, carrots, cauliflower and Brussels sprouts with various N fertiliser application rates (2009–2016) with a view to fine-tune the N fertiliser application rates where possible. To find an economic and ecological optimum we considered both yield quantity, quality (leaf colour and uniformity for all vegetables, and nitrate concentration for leafy vegetables) and residual soil mineral N (RSMN) to rooting depth at harvest in function of crop available N. Even at relatively high crop available N, NO3− concentrations in lettuce and spinach remained below the legal maximum concentration. Contrary to some farmers’ perception, low RSMN can be obtained without a low score for quality parameters leaf colour and uniformity, except for the un- and underfertilised plots. When crop available N exceeds the optimum, RSMN increased steeply (i.e. a breakpoint) for all considered vegetables, except carrots, implying an increased risk of NO3− leaching. The results indicate that N fertilisation advices and maximum allowed N fertiliser application rates can be reduced, at least for some vegetables, without a risk of decreasing the marketable yield quantity and quality. As N uptake continues to increase, to a variable extent, after the maximum marketable yield has been obtained, N fertiliser application rates obviously should be based on marketable yield rather than total yield.

The separate and collective role of soil nematodes and protists at community level in soil biogeochemical processes remains poorly quantified due to the lack of appropriate methodologies to study them independently from other soil biota under conditions emulating natural soil. Here we present a methodological exploration for the selective removal of nematodes and protists using gamma irradiation and the subsequent reinoculation with metazoan-free soil powder inoculum that allows reconstructing a complex microbial food web comprising the native microflora and soil protist communities, without other fauna. Nematodes, and culturable flagellates and ciliates were removed from 6 kGy onwards, while a small fraction of the amoebae survived (8% of the entire protist population) at 16 kGy but were totally removed at 27 kGy. Reinoculation with pulverized soil powder successfully established a protist community of similar size and composition as the control soil as assessed by both amplicon sequencing-based detection and microscopic observation after cultivation. A viable native microflora community remained from 6 to 16 kGy doses, but microbial activity was completely inhibited at 27 kGy. We suggest that, in ecological relevant experiments where selective removal of nematodes or other fauna is the purpose and protist community is meant to be kept intact, gamma irradiation needs to be followed routinely by soil powder inoculation in order to restore the suppressed protist community. This experiment also opens up new perspectives for studying the ecological roles of the entire protist community or specific groups thereof (flagellates and ciliates) in real soil and in the presence of a viable native microflora community.

Roller-crimping of cover crops grown during winter may facilitate reduced tillage efforts in organic vegetable production. A one-year field experiment was conducted twice (autumn 2015–2017) in sandy loam soil in Denmark testing two cover crop termination systems, roller-crimping (RC) without tillage before crop planting, and full incorporation by tillage (FI) of mowed cover crops in organic white cabbage (Brassica oleracea L. convar. capitata var. capitata f. alba) production. Three legume species were investigated: winter faba bean (Vicia faba L.), winter pea (Pisum sativum L.), hairy vetch (Vicia villosa Roth), in pure stands or in a mixture with winter rye (Secale cereale L.). Roller-crimping reduced total white cabbage biomass by 31% (2016) and 19% (2017). Marketable white cabbage yield was 100% (2016) or 24% (2017) lower under RC than FI, likely caused by delayed N release from cover crops and reduced soil dehydrogenase activity. White cabbage root growth was reduced by RC following pea/rye, where N availability was low. Despite reduced root growth and yield, RC still had the advantage of reducing weed growth by 63% compared with FI three weeks after cover crop termination in 2017 and of decreasing N leaching risk, indicated by reduced soil mineral N content in 0.5–1.5 m depth in autumn. Marketable yield was 35% higher following legumes compared with legume/rye mixtures in both termination systems in 2017, due to 105 kg N ha−1 higher mineral soil N content in 0–2.5 m depth in the spring and faster N mineralisation from plant material with a lower C/N ratio. This yield increase corresponded with increased root growth following RC legumes. In contrast, legume/rye mixtures had the advantage of decreasing weed growth by 50–68% and N leaching risk by 22 kg ha−1 in 0.5−2 m depth. Although RC demonstrated ecological benefits such as weed suppression and indicated reduced N leaching risk, the trade-off with yield losses could create a barrier to adoption. This trade-off could be mitigated by using pure legume cover crops and by improving management of supplemental N fertility.

In Cote d’Ivoire, smallholder farmers cultivate cassava on poor, highly weathered soils without improved varieties or fertilizer inputs. Land pressure combined with high demand result in premature harvests and low yields. Furthermore, subsistence agriculture limits the adoption of input-intensive technologies. This study aimed at identifying integrated soil fertility management systems for increasing cassava productivity and profitability in two locations in southern Cote d’Ivoire. The integrated technical interventions included improved cassava genotypes and modified spacing, legume intercropping, and application of fertilizer at moderate doses. Overall, an improved cassava variety (Yavo) generally out-yielded the other varieties at both sites. Legumes performed better at the higher soil fertility site and gave higher grain yield and biological nitrogen fixation in a 2 × 0.5 m cassava spacing compared to 1 × 1 m. The response of cassava to nutrient amendments varied between sites, suggesting the ...

SUMMARYIn order to intensify cassava production in southern Côte d'Ivoire, multiple farmers’ field trials were carried out in two important cassava-producing areas (Dabou and Bingerville). The effectiveness of an Integrated Soil Fertility Management (ISFM) approach as intensification pathway was evaluated for poor sandy soils. Overall, ISFM-based systems outperformed the local cultivation system. The average cassava yield was 14.5 and 6.5 t ha−1 and 12.6 and 5.7 t ha−1 for the ISFM and traditional systems, in Dabou and Bingerville, respectively. The average profitability of ISFM systems was ca. 2500 Euro ha−1, whereby 66% was generated by cowpea intercrop, which yielded on average 3.1 t ha−1. Taking into account the limited farmer's resource endowment and poor soil fertility, ISFM is a crucial component for sustainable intensification of cassava production and poverty alleviate in southern Côte d'Ivoire.

To restore species-rich terrestrial ecosystems on ex-agricultural land, establishing nutrient limitation for dominant plant growth is essential because in nutrient-rich soils, fast-growing species often exclude target species. However, N-limitation is easier to achieve than P-limitation (because of a difference in biogeochemical behavior), biodiversity is generally highest under P-limitation. Commonly used restoration methods to achieve low soil P-concentrations are either very expensive or take a very long time. A promising restoration technique is P-mining, an adjusted agricultural technique that aims at depleting soil-P. High biomass production and hence high P-removal with biomass are obtained by fertilizing with nutrients other than P. A pot experiment was set up to study P-mining with Lolium perenne L. on sandy soils with varying P-concentrations: from an intensively used agricultural soil to a soil near the soil P-target for species-rich Nardus grassland. All pots received N- and K-fertilization. The effects of biostimulants on P-uptake were also assessed by the addition of arbuscular mycorrhiza (Glomus spp.), humic substances or phosphate-solubilizing bacteria (Bacillus sp. and Pseudomonas spp.). In our P-rich soil (111 µg POlsen/g), P-removal rate was high but bioavailable soil-P did not decrease. At lower soil P-concentrations (64 and 36 µg POlsen/g), bioavailable soil-P had decreased but the P-removal rate had by then dropped 60% despite N- and K-fertilization and despite that the target (

Vegetable crop residues take a particular position relative to arable crops due to often large amounts of biomass with a N content up to 200 kg N ha−1 left behind on the field. An important amount of vegetable crops are harvested during late autumn and despite decreasing soil temperatures during autumn, high rates of N mineralization and nitrification still occur. Vegetable crop residues may lead to considerable N losses through leaching during winter and pose a threat to meeting water quality objectives. However, at the same time vegetable crop residues are a vital link in closing the nutrient and organic matter cycle of soils. Appropriate and sustainable management is needed to harness the full potential of vegetable crop residues. Two fundamentally different crop residue management strategies to reduce N losses during winter in intensive vegetable rotations are reviewed, namely (i) on-field management options and modifications to crop rotations and (ii) removal of crop residues, followed by a useful and profitable application.

The present study examines the effects of different fertilization treatments (chemical fertilization, farmyard manure, plant compost, and mycorrhiza-inoculated compost) on the soil fungi under a crop rotation of wheat (Triticum aestivum L.) and corn (Zea mays L.) in a long-term field experiment established in Mediterranean Turkey in 1996. Soil samples were collected in May, August, and October 2009. Soil pH, organic carbon, plant-available nitrogen and phosphorus, mycorrhizal colonization, and a series of biochemical markers (phospholipid and neutral lipid fatty acid [PLFA and NLFA] profiles, soil ergosterol content, and glomalin related soil protein [GRSP] as indicators of abundance of bacteria, saprotrophic, and arbuscular mycorrhizal [AM] fungi) were assessed. No significant difference was observed in soil organic C and plant available N in relation to long-term fertilization treatments, but plant available P in soil changed significantly in relation to the fertilization treatment used and the sampling season (between 11.5–33.8 mg · kg-1in spring, 10.4–28.6 mg · kg-1in summer, and 10.5–33.2 mg · kg-1in autumn). Mycorrhizal colonization patterns were similar for both plants. However, mycorrhiza-inoculated compost treatment exhibited higher root colonization (77.3%) over control (16.3%), chemical fertilization (10.0%), farmyard manure (19.3%), and plant compost (20.0%). No statistically significant change was observed in ergosterol content. The effect of long-term organic treatments on soil PLFA structure was statistically prominent; whereas seasonality only affected bacterial PLFAs. Organic fertilization increased GRSP (mean annual ranging from 0.91 to 2.46 mg · g-1total GRSP) but long-term annual mycorrhizal inoculation had no significant effect on the soil GRSP pool. © 2015, Taylor & Francis Group, LLC.

This study, aiming to unravel whether topsoil conditions under tree species with nutrient-poor leaf litter can be altered by admixing a shrub layer, was performed in 12 pine (mainly Pinus sylvestris) and 12 oak (Quercus robur) stands on sandy podzolic soils in north-east Belgium. We examined the effects of presence of a shrub layer on forest floor mass and topsoil chemical properties related to soil acidification. The shrub species included were European rowan (Sorbus aucuparia), alder buckthorn (Rhamnus frangula) and black cherry (Prunus serotina). For each tree species, 60–90 years old stands were selected containing shrubs present in varying cover classes: sparse ( 70%). The oak stands were characterized by less but ‘nutrient-richer’ litterfall, compared to the pine stands. This was reflected in less humus build-up (FH-horizon) and higher pH, CEC, BS and lower C/N in the topsoil in the oak stands compared to the pine stands. However, despite the fact that the shrubs produced litter with significantly higher base cation and N concentrations than that of the studied tree species, we did not find any significant changes in topsoil conditions in the pine and oak stands under study, even under dense shrub layers (87–91% cover).

Several ecosystem processes such as nitrogen mineralization are mainly controlled by complex multitrophic interactions in which nematodes play major roles. Despite the abundance and diversity of these nematodes, studies on their contribution to N mineralization have been limited to a few selected species and often in completely sterilized media. Such simplified experiments are unrealistic and usually give inaccurate results as part of the interaction is missed. Therefore the contributions of nematodes to N mineralization need to be quantified more accurately in realistic conditions where native microbes, nematodes and plants interact. To do this, we set up a microcosm incubation experiment in which the whole nematode communities were extracted from fresh soil and reinoculated into the soil defaunated by 5 kGy dose gamma irradiation that leaves the microbial community largely intact. Three treatments namely control, +Nem (with nematodes) and −Nem (without nematodes), with or without plants were incubated for 86 days. In bare microcosms, +Nem cores increased total mineral nitrogen with 32% compared to −Nem. However, no significant ( p > 0.05) differences were observed on total mineral nitrogen and plant N uptake between +Nem and −Nem treatments in planted microcosms. Nematode abundances and community structures were significantly changed over time and differed in bare and planted microcosms. Bacterivores dominated in bare, while herbivores dominated in planted soils throughout the incubation period. This could explain the contrasting effects of nematodes in N mineralization in bare and planted microcosms. Optimization of gamma irradiation technique is required to further reduce its side effects on nutrient flush and non-targeted organisms.

Soil fauna, particularly nematodes, are considered to strongly contribute to nitrogen mineralisation through grazing on microflora during decomposition. Demonstration of this effect has mostly relied on calculation-based soil food web analyses or experiments involving simplified and artificially constructed food webs. We carried out an incubation experiment in which defaunated soil cores were reinoculated with entire soil nematode populations extracted from bulk soil and during which nitrogen mineralisation was measured. Both undisturbed and disturbed cores were prepared to investigate whether a representative pore structure influences the effect of entire free-living nematode populations on nitrogen mineralisation. Cores were subjected to a 5 kGy gamma irradiation dose sufficient to eliminate all soil fauna while leaving the microbial biomass largely intact. Half of the irradiated cores were reinoculated with nematodes extracted from a corresponding volume of bulk soil and incubated for 82 days. The microbial biomass was not strongly affected by gamma irradiation or nematode addition but declined strongly in all treatments during incubation. Reinoculation of nematodes was successful in establishing populations of a similar size and composition as in the control samples. Net nitrogen mineralisation from indigenous soil organic matter was observed in all treatments throughout the incubation, but was always more pronounced in irradiated cores. Total mineral nitrogen concentrations did not differ significantly between simply irradiated and irradiated then reinoculated cores. However by the end of the incubation period nematode addition resulted in 87% and 23% more NO 3 − –N g −1 dry soil in undisturbed and disturbed cores respectively, while NH 4 + –N g −1 dry soil decreased by 50% in both core types. We found no convincing evidence for a contribution of free-living nematodes on total nitrogen mineralisation, but the activity of nitrifying organisms was clearly stimulated by nematode grazing.

The microbial community (fungi and bacteria) is the main decomposer of organic matter during composting. Its composition is sometimes used as a proxy to assess compost maturity. Although nematodes are probably its most important grazers, only one previous study has highlighted clear shifts in nematode species composition during composting, and the assumption that nematodes reflect changes in the microbial community in compost has not yet been formally tested. Here, the microbial and nematode communities of a single composting process are analyzed together for the first time. Although both displayed broadly similar patterns, the abundance of fungal-feeding nematodes showed a distinct delay as compared to the increase in fungal PLFA. We argue that the nematode community may be a more promising tool to use in assessing compost maturity because it allows to discriminate between the three composting phases. First, during the thermophilic phase, bacterial-feeding nematodes dominate; during cooling, the bacterial-feeding/predators bloom; and during maturation, the abundance of fungal-feeding nematodes increases. Based solely on the microbial community, it was only possible to discriminate between the thermophilic phase and the rest of the process. Bacteria dominated during the thermophilic phase, while bacterial and fungal PLFA had more-or-less equal shares during cooling and maturation.

The nitrogen status of most Zambian soils is inherently low. Nitrogen-fixing trees such as Faidherbia albida (F. albida) could have the potential to restore soil fertility. We conducted a study to examine the role of mature F. albida trees on the soil microbial communities and overall N fertility status in Zambia. Soil samples were collected under and outside the canopies of F. albida trees in representative fields from two sites namely; Chongwe (loamy sand) and Monze (sandy loam). To assess the long term canopy effects; total N, mineral N and soil organic carbon (Corg) content were directly measured from soils collected under and outside the canopy. Short term litter effects were assessed by subtracting concentrations of biochemical properties of non-amended controls from amended soils with F. albida litter during an 8 week incubation experiment. We also determined N mineralization rates, microbial community structure—Phospholipid fatty acids, microbial biomass carbon, and labile organic carbon ( $${\text{C}}_{{{\text{org[K}}_{ 2} {\text{SO}}_{ 4} ]}}$$ ) during incubation. For the long term canopy effect, average N mineralization rate, Corg, total N and mineral N content of non-amended soils under the canopy were (all significant at p

The role of soil fauna in crucial ecosystem services such as nutrient cycling remains poorly quantified, mainly because of the overly reductionistic approach adopted in most experimental studies. Given that increasing nitrogen inputs in various ecosystems influence the structure and functioning of soil microbes and the activity of fauna, we aimed to quantify the role of the entire soil nematode community in nutrient mineralization in an experimental set-up emulating nutrient-rich field conditions and accounting for crucial interactions amongst the soil microbial communities and plants. To this end, we reconstructed a complex soil foodweb in mesocosms that comprised largely undisturbed native microflora and the entire nematode community added into defaunated soil, planted with Lolium perenne as a model plant, and amended with fresh grass-clover residues. We determined N and P availability and plant uptake, plant biomass and abundance and structure of the microbial and nematode communities during a three-month incubation. The presence of nematodes significantly increased plant biomass production (+9%), net N (+25%) and net P (+23%) availability compared to their absence, demonstrating that nematodes link below- and above-ground processes, primarily through increasing nutrient availability. The experimental set-up presented allows to realistically quantify the crucial ecosystem services provided by the soil biota.

A way to bring phosphate-saturated soils back to an environmentally safe P level is by P mining through plants. Phosphate-solubilizing bacteria (PSB) could be very useful for increasing mining efficiency over time. The goal of this research was to investigate the adaptation and performance of PSB in conditions of high total P content in soil. In the first experiment, the P-solubilizing capacity of five PSB species (three Bacillus spp. and two Pseudomonas spp.) were tested under fully controlled conditions on several growth media with different forms of insoluble phosphate (FePO4, AlPO4, or (Ca)3(PO4)2) added at different rates. The colony growth after 14 days of inoculation demonstrated that all five bacteria were able to proliferate and solubilize P on each of the tested growth media, in contradiction with the normally used technique of halo determination. In the second experiment, the same bacterial species were inoculated in pure quartz sand amended with a nutrient solution and P was added separately in an insoluble form, as Fe–P, Al–P, or Ca–P. The extractable ammonium lactate ranged from 3.2 to 6.9 and 29.0 to 40.7 mg kg−1 sand for the insoluble Al–P and Fe–P treatments, respectively. Pseudomonas putida and Bacillus brevis performed best as PSB at high P concentration where the P is fixed with Al or Fe. In the third experiment, P. putida and B. brevis were inoculated in an acidic sandy, P-saturated soil for 4 weeks. The inoculation of the PSB gave promising results in solubilizing P.

We explored the value of ergosterol, phospholipid fatty acid (PLFA) profiles and neutral lipid fatty acid (NLFA) 16:1ω5c as soil quality indicators for the intensive systems of vegetable production in the humid tropical climate of West Java, by comparing organic and conventional management. Additionally, we measured dehydrogenase and β-glucosidase activity. A secondary forest was included to obtain reference values under undisturbed conditions. Organic and conventional agriculture, and secondary forest each differed in the composition of the microbial community measured by PLFA profiles. Ergosterol appeared not to be universally applicable as a fungal biomarker and in this respect seemed to be inferior compared to PLFA fungal markers. NLFA 16:1ω5c may provide additional information as an indicator of arbuscular mycorrhizal fungi, but its high variability complicated the interpretation of data. The ratio of cy17:0 to 16:1ω7c was effectively applied as an indicator of physiological stress experienced by the bacterial community. Conventional vegetable production resulted in higher cy17:0/16:1ω7c ratios. Finally, a soil quality index, developed by stepwise canonical discriminant analysis and based on the absolute amount of PLFA 16:0, the relative amount of PLFAs 10Me16:0 and 10Me18:0, and dehydrogenase activity, was successfully validated.

The techniques available for sterilisation or defaunation of soil in ecological experiments mostly have strongly unwanted effects on soil structure and the dynamics of major nutrients such as nitrogen. The potential for using gamma irradiation to prepare defaunated soil microcosms was investigated by subjecting undisturbed soil cores to a range of irradiation doses (0, 5, 10, 20 and 40 kGy). The absence of living nematodes at the lowest irradiation dose was confirmed by microscopic observation. The effects of irradiation dose on mineral nitrogen (as NO 3 − and NH 4 + ), microbial biomass C (C mic ), and phospholipid fatty acid (PLFA) concentration and composition were determined over a 4 week incubation period. An increase in the concentration of NO 3 − occurred during the incubation period after exposure to 0, 5 and 10 kGy but was barely detectable at 20 and 40 kGy. The effect of irradiation dose on NH 4 + release was complex and highly variable within treatments, with the 10 kGy dose resulting in the highest concentrations. Microbial biomass carbon was significantly reduced following a 20 kGy irradiation dose and below detection at 40 kGy. Most remarkably, the sum of all measured PLFAs did not differ significantly between most treatments and was not correlated with microbial biomass. In most cases the concentration of signature fatty acids only differed significantly between the control and the highest irradiation dose treatments. To ascertain the sensitivity of microbial taxa to acute gamma irradiation with accuracy, measures of microbial community structure other than PLFA analysis are needed.

SUMMARYNitrogen (N) nutrition is a key factor for vegetable growth and yield. However, different rates of nitrogen fertilization may trigger different responses to vegetables. A survey was conducted to investigate the effect of soil fertility management practices on nitrate concentration in vegetables. The survey results were used to plan experiments on the effect of chicken and cattle manures on nitrate levels in Chinese cabbage (Brassica rapa) and amaranthus (Amaranthus cruentus) grown in Tanzania and the patterns of mineral nitrogen in soils under open field conditions. Chicken or cattle manure at 200, 300 kg N ha−1and 170 250 kg N ha−1for Chinese cabbage and amaranthus respectively, and control were compared in a randomized complete block design. We observed a higher nitrate content in fertilized Chinese cabbage at day 30 than at day 44 after sowing, ranging from 3243 to 4993 mg kg−1fresh matter regardless of the N source and rates. Only application of manures at high levels (250 kg N ha−1) induced significantly (p< 0.05) higher nitrate contents in amaranthus at day 28 after sowing, although there was a clear indication of nitrate accumulation even at 170 kg N ha−1application. Soil NH4+-N + NO3−-N in both Chinese cabbage and amaranthus plots were increased with increasing N application rates and differences between control and amended soils were significant (p< 0.01). There was a positive relationship between NO3−concentration in vegetables and NO3−-N in the rooted top soil layer (0–15 cm). However, higher NH4+concentrations depressed NO3−build up in crops and a significant negative relationship between soil (NH4+-N)/(NO3−-N) ratio and crop NO3−content was found. It is concluded that low manure application rates result in similar yields to high rates but reduces nitrate accumulation in vegetables and excess mineral nitrogen in soils.

Rhizoctonia solani causes worldwide losses in numerous crops. Sclerotia of R. solani remain viable for several years in soil and are an important source of primary infection. In this study the effect of soil incorporation of Kraft pine lignin, a side product of the paper industry, on viability of R. solani AG1-1B sclerotia was investigated. The efficacy of lignin was assessed in a sandy loam (Oppuurs) and a silt loam soil (Leest) collected from commercial fields in Belgium. Evaluating sclerotial viability after 4 weeks incubation in the two soils amended with 1% (w/w) Kraft pine lignin demonstrated a soil-dependent effect. In Leest soil the addition of lignin resulted in a significantly reduced sclerotial viability, together with an increased mycoparasitism by Trichoderma spp.; in Oppuurs soil, on the other hand, only a slight and insignificant reduction in sclerotial viability was observed. Based on phospholipid fatty acid analysis, different changes in microbial community structure upon lignin amendment were detected in the two soils. Both amended soils showed a significant increase in Gram negative bacteria. In Leest soil this increase was accompanied with a significantly higher increase in fungi and actinomycetes compared with Oppuurs soil. In addition, Kraft pine lignin resulted in both soils in a small but significant increase in manganese peroxidase activity and this increase tended to be higher in Leest soil. Manganese peroxidase produced by lignin-degrading basidiomycetes has previously been shown to degrade melanin, which protects the sclerotia against biotic and abiotic stress. We hypothesize that lignin-degrading fungi increased the susceptibility of the sclerotia to sclerotial antagonists such as Trichoderma, Gram negative bacteria and actinomycetes. Clearly, the effect observed here did not rely on the stimulation of one microbial group, but is the result of an interaction of different groups.

To evaluate the effect of tillage intensity on the N mineralization pattern of winter wheat residues, sugar beet residues, Italian ryegrass and maize residues undisturbed soil samples were taken from six sites under different tillage management. Site NTK had been managed for 10 years under reduced tillage (RT), whereby the last 4 years the crops were sown using direct seeding (NT). Site RTCSE had been managed for 20 years under reduced tillage (RT) and site RTH for 3 years. For each site under RT a nearby site under conventional tillage (CT) was selected (CTK, CTCSE and CTH). On site NTK and site RTCSE a significantly higher amount of SOC in the 0–10 cm was accumulated compared to the respective CT sites. Between site RTH and site CTH no such significant difference was found. However, the content of microbial biomass C (MB-C) and the β-glucosidase and urease activities were higher on all RT sites compared to the respective CT sites. This indicates that these microbiological and biochemical parameters seem to be very sensitive for alterations in management intensity. After 98 days, more N was immobilized under NTK than under CTK by adding winter wheat residues (expressed as kg ha−1 and as % of total added N). This higher immobilization potential can be explained by a higher microbial activity and a change in microbial population. Under RTCSE and RTH net N immobilization of the winter wheat residues was found, but the pattern was less pronounced than for NTK. However, when expressed as % of total N added, N immobilization of winter wheat residues was higher under CT than under RT, which indicates that high C:N residues when incorporated, decompose more slowly under RT than under CT. Similar results were found comparing the N mineralization pattern of maize residues under RTH and CTH. The residues of sugar beet and Italian ryegrass at site CTH released N more rapidly and to a higher extent, 74.1% and 66.2%, respectively (expressed as % of total N added) than under RTH at the end of the incubation. The slower mineralization of N rich crop residues under RT compared to CT means that there is less potential risk for nitrate leaching to occur, which may result in a higher N efficiency in RT compared to CT.

On a field experiment, started in spring 2005, we examined the influence of different types or qualities of exogenous organic matter (farmyard manure, cattle slurry and three different types of compost, compared with mineral fertilization, and two unfertilized treatments, cropped and uncropped) on the earthworm density and biomass. Earthworms were sampled with mustard powder and subsequent handsorting at the start of the experiment and one year (i.e. two organic matter applications) later. The organic matter applications had a significant effect on the earthworm populations. Earthworm abundance was highest on all organically amended plots, while the lowest number and biomass was found on plots without fertilization. On the latter earthworm number and biomass were equal to or lower than the initial situation. The highest earthworm biomass was recorded on plots amended with farmyard manure. There were indications too that the presence of a crop and the crop performance have an effect on the earthworm populations.

In Flanders, vegetable, fruit and garden (VFG) wastes are collected selectively and composted. We studied the effects of the combined application of three different doses of VFG compost and cattle slurry during 7 years on maize dry matter yields and three soil faunal groups: nematodes, micro-arthropods (springtails and mites) and earthworms. Combined application of VFG compost and slurry resulted in the highest yields. Initially, there was a clear yield depression on plots amended with compost in contrast to an upward trend in the last application years, proving a significant residual N effect from cumulative compost applications. The plant parasitic nematodes Pratylenchus sp. and the Tylenchidae were less abundant on plots receiving organic amendments, while the population of the bacteriophagous Rhabditidae was higher on these plots. Adding organic amendments resulted in increased numbers of micro-arthropods, springtails as well as mites. Earthworms were significantly more abundant when organic amendments were applied. The VFG compost had a larger overall positive effect on the three soil faunal groups than cattle slurry.

To understand the roles of nematodes in organic matter (OM) decomposition, experimental setups should include the entire nematode community, the native soil microflora, and their food sources. Yet, published studies are often based on either simplified experimental setups, using only a few selected species of nematode and their respective prey, despite the multitude of species present in natural soil, or on indirect estimation of the mineralization process using O-2 consumption and the fresh weight of nematodes. We set up a six-month incubation experiment to quantify the contribution of the entire free living nematode community to carbon (C) mineralization under realistic conditions. The following treatments were compared with and without grass-clover amendment: defaunated soil reinoculated with the entire free living nematode communities (+Nem) and defaunated soil that was not reinoculated (-Nem). We also included untreated fresh soil as a control (CTR). Nematode abundances and diversity in +Nem was comparable to the CTR showing the success of the reinoculation. No significant differences in C mineralization were found between + Nem and -Nem treatments of the amended and unamended samples at the end of incubation. Other related parameters such as microbial biomass C and enzymatic activities did not show significant differences between + Nem and -Nem treatments in both amended and unamended samples. These findings show that the collective contribution of the entire nematode community to C mineralization is small. Previous reports in literature based on simplified experimental setups and indirect estimations are contrasting with the findings of the current study and further investigations are needed to elucidate the extent and the mechanisms of nematode involvement in C mineralization.

Little is known concerning the N mineralization or N immobilization of (vegetable) root residues in soil. The (bio)chemical composition of a range of vegetable root residues (large and fine roots of red cabbage, white cabbage, Brussels sprouts, savoy cabbage, and leek), and plant parts of two green manures (ryegrass and white mustard) was determined by standard chemical analysis (total C, total N, and C:N ratio) and by a modified Stevenson fractionation method (water-soluble, hemicellulose, cellulose, and lignin fraction). Fresh chopped crop residues, homogenously mixed with a sandy loam soil, were incubated during 4 months at constant temperature (21 °C) and constant moisture content (14% w/w). The net N mineralization of each residue was determined by destructive sampling. All fine roots, except these of Brussels sprouts, showed a net N release throughout the incubation. All large roots showed a slight N immobilization at the start of the incubation, but at the end of the incubation a small net N release was observed, except for Brussels sprouts. The roots of Brussels sprouts immobilized N throughout the entire incubation. The leaves of the green manures released more N than both stems (only for white mustard) and roots. It was possible to fit a first-order kinetics model to 8 of the 14 N mineralization patterns: AN(t)=AN(1−exp(−kt)). For the six other residues where the first-order model could not be fitted, the amount of N mineralized at the end of the incubation was taken as an estimate of the AN parameter. Both the mineralization parameters AN and k were correlated to a large number of (bio)chemical parameters. The amount of mineralized N, AN, was best correlated with the C:N ratio (R=−0.86), and the rate constant k was best correlated with the lignin:N ratio (R=−0.94). The predictive relationship between the N mineralization of vegetable root residues and green manures and their (bio)chemical composition has the advantage that is independent of the length of the incubation time. The critical C:N ratio, i.e. the break point between net N mineralization and net N immobilization (AN=0) was found to be 36.6.

Trees play an important role in the biogeochemical cycling of metals, although the influence of different tree species on the mobilization of metals is not yet clear. This study examined effects of six tree species on fluxes of Cd, Zn, DOC, H(+) and base cations in forest floor leachates on a metal polluted site in Belgium. Forest floor leachates were sampled with zero-tension lysimeters in a 12-year-old post-agricultural forest on a sandy soil. The tree species included were silver birch (Betula pendula), oak (Quercus robur and Q. petraea), black locust (Robinia pseudoacacia), aspen (Populus tremula), Scots pine (Pinus sylvestris) and Douglas fir (Pseudotsuga menziesii). We show that total Cd fluxes in forest floor leachate under aspen were slightly higher than those in the other species' leachates, yet the relative differences between the species were considerably smaller when looking at dissolved Cd fluxes. The latter was probably caused by extremely low H(+) amounts leaching from aspen's forest floor. No tree species effect was found for Zn leachate fluxes. We expected higher metal leachate fluxes under aspen as its leaf litter was significantly contaminated with Cd and Zn. We propose that the low amounts of Cd and Zn leaching under aspen's forest floor were possibly caused by high activity of soil biota, for example burrowing earthworms. Furthermore, our results reveal that Scots pine and oak were characterized by high H(+) and DOC fluxes as well as low base cation fluxes in their forest floor leachates, implying that those species might enhance metal mobilization in the soil profile and thus bear a potential risk for belowground metal dispersion.

Phytostabilization of metals using trees is often promoted, although the influence of different tree species on the mobilization of metals is not yet clear. This study examined effects of six tree species on the soil characteristics pH, organic carbon (OC) content and cation exchange capacity (CEC) and on the redistribution of cadmium (Cd) and zinc (Zn) on a polluted sandy soil. Soil and biomass were sampled in 10-year-old stands growing on former agricultural land. The tree species included were silver birch ( Betula pendula ), oak ( Quercus robur and Quercus petraea ), black locust ( Robinia pseudoacacia ), aspen ( Populus tremula ), Scots pine ( Pinus sylvestris ) and Douglas fir ( Pseudotsuga menziesii ). In the short period of 10 years, only aspen caused significant changes in the soil characteristics. Due to accumulation of Cd and Zn in its leaf litter, aspen increased the total as well as the NH 4 OAc–EDTA-extractable Cd and Zn concentrations in the topsoil compared to deeper soil layers and to other tree species. Also, topsoil pH, OC content and CEC were significantly higher than under most of the other species. This caused rather low ‘bioavailable’ CaCl 2 -extractable concentrations under aspen. Nevertheless, given the risks of aboveground metal dispersion and topsoil accumulation, it is recommended that aspen should be avoided when afforesting Cd and Zn contaminated lands.

International audience; Fertilisation of agricultural land causes an accumulation of nutrients in the top soil layer, among which phosphorus (P) is particularly persistent. Changing land use from farmland to forest affects soil properties, but changes in P pools have rarely been studied despite their importance to forest ecosystem development. Here, we describe the redistributions of the P pools in a four-decadal chronosequence of post-agricultural common oak (Quercus robur L.) forests in Belgium and Denmark. The aim was to assess whether forest age causes a repartitioning of P throughout the various soil P pools (labile P, slowly cycling P and occluded P); in particular, we addressed the time-related alterations in the inorganic versus organic P fractions. In less than 40 years of oak forest development, significant redistributions have occurred between different P fractions. While both the labile and the slowly cycling inorganic P fractions significantly decreased with forest age, the organic fractions significantly increased. The labile P pool (inorganic + organic), which is considered to be the pool of P most likely to contribute to plant-available P, significantly decreased with forest age (from >20 to

Acidification of forest soils in Europe and North America has been an important concern over the last decades. The last area-covering survey of forest soil acidification in Flanders (North Belgium) goes back to 1985 [Ronse A, De Temmerman L, Guns M, De Borger R. Evolution of acidity, organic matter content, and CEC in uncultivated soils of North Belgium during the past 25 years. Soil Sci; 146, (1988), 453-460] and highlighted a significant acidification of the upper layer (0.3-0.4 m) of forested podzols during the period 1950-1985. The present study aimed to assess (1) to what extent further acidification of forested podzols occurred during the period 1985-2000 at different depths and (2) whether the average annual acidification rate accelerated or slowed down between 1985 and 2000 compared to the period 1950-1985. Average soil pH-KCl values of podzols in northern Belgium dropped during the period 1985-2000. This decline extends to a depth of about 50 cm but was most pronounced and significant in the A horizon. In the A0, A1 and A2 horizons, average pH dropped with 0.2, 0.3 and 0.1 units, and in the Bir and C horizons with 0.1 units. No change in average pH value occurred in the Bh horizon. Average annual acidification rate of the A1 horizon was significantly higher in the period 1985-2000 than in the period 1950-1985. Changes in pH occurred in the entire soil profile during the period 1950/67-1985 likely because sulphate was the major form of acid deposition before 1985. After 1985, acid sulphur deposition decreased with more than 50% in North Belgium. In contrast, ammonium deposition almost doubled between 1950 and 1980, which may explain why soil acidification between 1985 and 2000 has been restricted to the upper soil horizons.