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Abstract: Biochar produced during pyrolysis of biomass has the potential to reduce greenhouse gas (GHG) emissions from soils. In order to evaluate the effect of four different biochar additions on the emission of the greenhouse gases CO2 and N2O, two incubation experiments were established in a temperate sandy loam soil. Digestate, a waste-product of the wet fermentation of swine manure, and willow wood was slowly pyrolyzed at 350 °C and 700 °C, yielding four biochar types (DS350, DS700, WS350 and WS700). In the first incubation experiment (117 days), C mineralization was monitored in soil amended with biochar at a quantity of 10 Mg ha−1 on an area-basis (biochar to soil ratio of 1:69 on a mass basis) at 50% water filled pore space (WFPS). CO2 emissions from the 350 °C biochar treatments were significantly higher than the control (no biochar) treatment, while we observed no significantly different net C mineralization in the treatments with the 700 °C biochars compared to the control. After fitting a combined zero- plus first-order model to the cumulative C mineralization data, the parameter for the easily mineralizable C pool (CAf) positively correlated with the volatile matter (VM) contents of the biochars. Microbial biomass carbon consistently increased due to all biochar additions, while the dehydrogenase activity increased in the 350 °C biochar treatments but decreased in the 700 °C biochar treatments. Principal component analysis (PCA) of the extracted phospholipid fatty acids (PLFAs) demonstrated that divergent microbial community structures established after the addition of all biochars. The markers for Gram-positive and Gram-negative bacteria were more abundant in the 350 °C biochar treatments compared to the control and to the other biochar treatments. Net N mineralization was higher in the digestate biochar treatments than in the willow biochar treatments and decreased with increasing pyrolysis temperatures and increasing C:N ratio. In a second incubation experiment (15 days) N2O emissions were measured at WFPS of 70% and the same biochars were added in the same quantity as for C mineralization, with the addition of 40 mg KNO3–N kg−1. The cumulative N2O emission after 15 days was positively correlated with the volatile matter content of the biochars and was significantly lower in the 700 °C biochar treatments compared to the control, while no significant differences were found for the 350 °C biochar treatments. This study suggests that volatile matter content could be an important property of biochars in explaining short-term CO2 and N2O emissions from biochar-amended soils. [Copyright &y& Elsevier]

Although the amendment of various forms of exogenous organic matter (EOM) is a common practice in cropland production, it is to date not clear if its mineralisation in soil depends on application rate. Previous research suggested that spatial concentration of EOM in soil positively impacts its degradability. Here, we seek to test these reports and furthermore to investigate if an interactive effect exists with native soil organic carbon (SOC) concentration. We hypothesised that at a low EOM dose, more EOM remains undecomposed in soil and that this effect becomes stronger at lower SOC level. Moreover, as a secondary goal, we explored if priming of native SOC mineralisation depends on EOM dose. Therefore, we set up seventy-day soil incubation experiments with six varying C4-maize residue (δ13C = −12.7‰) doses (0–6 g kg−1) in soil mesocosms of loamy sand subsoils (δ13C = −25.7‰) with three different native SOC levels (i.e. 0.1, 0.5, and 1.0%). Soil CO 2 emissions and isotopic signature measurements of CO 2 allowed to discern EOM and SOC mineralisation. We found that EOM-derived C mineralisation increased proportionally to EOM dose, refuting the hypothesised relative undecomposed EOM at low concentration. Volumes of larger pore neck size diameter classes (60–100 and >300 μm) almost doubled at high EOM dose, demonstrating formation of macropores. Yet, this apparently did not impact EOM mineralisation, probably because O 2 supply was always sufficient to allow unlimited activity of EOM degraders, even at higher EOM doses, as indicated by generally higher measured redox potential. With EOM dose, fungal marker PLFA abundances increased in the 1% SOC soil and protozoan abundances increased in all three soils, but apparently these shifts did not result in an enhanced relative degradation of the EOM. Increasing EOM doses induced negative priming, e.g. EOM ≥1 g kg−1 reduced SOC mineralisation by >43% and >24% compared to the control in the 0.1 and 0.5% SOC soils, respectively; whereas no priming occurred in soil with 1% SOC. These results were largely explained by the amount of added C relative to microbial biomass carbon, and the theorised switch of slow decomposers (so called K-strategists and involved in recalcitrant compound decomposition) from SOM to preferentially decompose EOM at higher doses. We also postulate that at low SOC %, the obvious increased O 2 consumption with higher EOM dose more readily results in local anaerobic conditions in finer pores, i.e. where SOC is located and mineralised. We conclude that on the short term, agricultural management for SOM thus does not need to consider EOM doses but only the total amount of EOM. • We hypothesised relative OM decomposition to increase at higher concentration in soil. • Dose of maize straw however did not impact its relative degradability in soil. • High doses did enhance soil macroporosity and generally increased soil Eh. • Increasing maize straw dose induced stronger negative priming of SOC at low SOC. [ABSTRACT FROM AUTHOR]

We have studied the possibilities of manipulating N mineralization from high N vegetable crop residues by the addition of organic materials, with the aim of initially immobilizing the mineralized residue N with a view to stimulating remineralization at a later stage. Residues of leek (Allium porrum) were incubated with soil, alone and in combination with straw, two types of green waste compost (with contrasting C:N ratios) and tannic acid. Evolution of mineral N was monitored by destructive sampling. After 15 weeks, molasses was added to part of the samples in each treatment, and incubation continued for another 12 weeks. All materials added during the first incubation stage, except the low C:N compost, resulted in significant immobilization of the residue N. The immobilization with the high C:N compost (41.4 mg N kg−1 soil) was significantly larger than with tannic acid and straw (both immobilized about 26 mg N kg−1 soil). In the straw treatment, remineralization started in the first stage of incubation from day 50 onwards. The addition of molasses caused a strong and significant remineralization in the second stage (equivalent to 73% of the N initially immobilized) in the treatment with the high C:N ratio compost. In the case of tannic acid, there was no consistent effect on mineralization from addition of molasses. This was attributed to the fact that the immobilization observed was due to chemical rather than biological fixation of the residue N. A number of non-toxic organic wastes could be considered for use in mediating release of immobilized N from high N crop residue materials in an attempt to synchronize residue N availability with crop N demand. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Soil nematodes are the most abundant soil fauna, with a potential great impact on soil N mineralization via interaction with soil microorganisms. As a consequence, nematodes likely also influence soil N 2 O production and emission but the very few studies on this matter were carried out in simplified setups with single nematode species and in (highly) disturbed soil conditions. Here we measured soil N 2 O emissions in a 74-day incubation experiment in the presence or absence of the entire soil nematode community with minimal disturbance of the soil microbial community and soil nutrients. This was e.g. evidenced by readily recovery of nitrifiers after the mild and selective sterilization and soil powder inoculation. N 2 O emissions increased in the presence of nematodes, varying between soils +747.7 % in a loamy sand, +55.8 % in a loam, and +51.9 % in a silt loam cropland topsoil, in line with nematode abundance in these soils. In particular, the loamy sand soil showed an atypical N 2 O emission peak at the time of high nematode abundance. Soil nematodes also increased net N mineralization by +8.4, +6.8 and +4.75 %, in these respective soils and to a smaller extent C mineralization as well. The extra soil nitrate buildup and the overall net stimulation of N mineralization by nematodes could not or just slightly explain the observed increased N 2 O emission. This research revealed the important role of soil nematodes in regulating N 2 O emission, and further stresses the need to consider the change in community composition and activity of denitrifiers, and connectivity of soil pores, rather than the stimulation of N mineralization as potential explanations for this role of nematodes. • The effect of soil nematode community on N 2 O emission was tested in realistic soil microcosms. • Nematodes increased N 2 O emissions with organic amendment in loamy sand, loam, and silt loam soil. • Loamy sand soil showed an atypical peak of N 2 O emission with high nematode density present. • Soil nematodes increased N mineralization and to a smaller extent C mineralization. • The enhanced N 2 O emission was not mainly mediated by increased N mineralization. [ABSTRACT FROM AUTHOR]

Both herbivorous and bacterivorous microfauna have been shown to influence root development, soil nitrogen (N) mineralization, and plant productivity. However, our knowledge of these effects is limited as multitrophic interactions remain largely unexplored. We investigated whether and how herbivorous nematodes (Pratylenchus zeae) and bacterivorous nematodes (Poikilolaimus oxycercus), alone and in combination, affect plant biomass (Lolium multiflorum) through root traits and/or soil N mineralization. Bacterivorous nematodes increased, whereas herbivorous nematodes decreased, plant productivity. We found that root trait coordination in response to soil microfauna was consistent with the concept of root economics space. The negative interaction between herbivorous and bacterivorous nematodes on plant productivity at high herbivorous nematode infestation could be explained by reduced N mineralization and variation in the root nitrogen concentration-root tissue density (RNC-RTD) axis aligned with increased herbivory intensity. This study revealed that herbivorous and bacterivorous nematodes moderated each other's effect on plant productivity via root trait coordination and N mineralization, and suggests, for the first time, the value of the root economics space concept for interpreting phenotypic root plasticity and functioning in response to local biotic factors. • We investigated whether and how microfauna interactively affect plant productivity. • Herbivorous and bacterivorous nematodes had a negative interaction on plant biomass. • The negative interaction was due to variation in N mineralization and root traits. • Root traits influenced by microfauna were in line with the root economics space. [ABSTRACT FROM AUTHOR]

Soil texture determines the location of moisture in the soil matrix. At lower moisture content, the contact between soil water and particulate organic matter (POM) might be critical for POM mineralization, because such areas of contact could function as a 'bridge' for transport of moisture, substrates, enzymes and microorganisms. Additionally, water absorption by porous POM itself likely also depends on soil texture and this may thus bring about another indirect textural control on POM mineralization. However, the relevance of such indirect soil textural controls on POM mineralization has not been investigated. Here we compared mineralization of 13C-labeled maize-straw particles (C maize -min) in a loamy sand and silt loam soil at three levels of water-filled pore space (20%, 40% and 60%WFPS). The distribution of discrete maize-straw particles and moist soil volume was assessed at the microscale using X-ray μCT and contrast-enhancing agents to visualize soil moisture. We hypothesized that a lower contact between POM and moist soil minerals in the loamy sand soil would lead to a reduced POM mineralization compared with the silt loam soil. On the other hand, we expected that POM itself could attract more moisture from the surrounding loamy sand soil due to a higher water potential than in the silt loam soil at equal WFPS level. Contrary to our expectation, there was less contact between zones of moist soil and the added POM in the silt loam soil. But as expected, POM was able to attract more water from the loamy sand soil, and it preserved a moist state even when soil was at a low moisture content, which may explain the surprisingly limited inhibition of C maize -min in drier loamy sand soil. This finding suggests that such a 'sponge-effect' of POM may be a relevant yet largely overlooked element in studying the relationship between soil moisture and POM mineralization. Contrary to our expectation, C maize -min was higher in the silt loam soil texture across all three moisture levels (P < 0.05). At low moisture content particularly, fungal and actinobacteria biomarker abundances were elevated in the silt loam soil. A shift towards a fungi-oriented microbial community could explain the higher C maize -min rates in the silt loam soil under dry than moist conditions. Overall, our results showed that mineralization of added POM was surprisingly independent of soil moisture, but explanatory pathways were probably very much soil texture dependent. • POM mineralization was higher in finer-textured soil regardless of moisture level. • This textural control did not relate to variable POM-moist soil contact. • Added POM drew in soil moisture and such sponge effect was stronger in loamy sand. [ABSTRACT FROM AUTHOR]

Most studies looking into the effect of amendment of biochar on soil microbial functioning employ short-term laboratory studies and probably describe relatively transient phenomena. Multi-year experiments, spanning beyond initial degradation of biologically labile biochar constituents, on the other hand are more scarce, although these are much needed to establish the medium-term effect of biochar on soil organisms. In the present study, soil was sampled from biochar-amended and control plots of four biochar field trials at Lincoln (UK), Rivignano, Rocca Bernarda and Beano in Italy. Air-dried pre-incubated soil samples were incubated at 15 °C for 8-9 weeks to follow-up carbon dioxide (CO2) emissions. We then determined soil β-glucosidase and dehydrogenase enzyme activity, and used PLFA analysis to quantify the total soil microbial biomass and community structure. The analysis indicated that soil microbial activity was either not affected or inhibited to different extents in the biochar-amended plots. At Lincoln, with the highest application rate (49 t ha−1), an overall inhibition of all investigated measures of microbial activity, a lower sum of extracted PLFAs and lower fungal abundance were observed. On the other end at Beano, depth dispersion of biochar by deep tillage and a lower application rate (20 t ha−1) probably explain the absence of any significant effect on microbial activity in that experiment. At Rivignano and Rocca Bernarda, dehydrogenase activity was lower in the biochar amended soil and C-mineralization was lower as well for Rivignano. Interestingly, however, β-glucosidase activity and the sum of extracted PLFAs was not affected by biochar treatment. Several mechanisms could reconcile the different effect of biochar application on overall microbial activity on the one hand and microbial abundance and rate of cellulose degradation on the other. Biochar amendment led to a lowered or equal soil microbial activity and abundance in most field sites. In contrast to many short-term laboratory studies, it therefore seems unlikely that biochar would still function as a substrate 1-4 years after incorporation in the field. [ABSTRACT FROM AUTHOR]

Abstract: 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 NO3 −–N g−1 dry soil in undisturbed and disturbed cores respectively, while NH4 +–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. [Copyright &y& Elsevier]

Abstract: 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 NO3 − and NH4 +), microbial biomass C (Cmic), and phospholipid fatty acid (PLFA) concentration and composition were determined over a 4 week incubation period. An increase in the concentration of NO3 − 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 NH4 + 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Soil texture is well known to directly affect bioavailability of organic matter to heterotrophs, but it also steers their activity by moderating soil moisture fluctuation. Disentangling these direct and indirect textural controls is, however, not trivial and attempts to do so are very scarce. Most attention has just gone to the stimulation of soil carbon (C) mineralization by soil moisture fluctuation per se. To quantify the indirect moisture-mediation control of soil texture on C mineralization, we monitored maize straw degradation in various soil texture/moisture regime combinations. Moisture levels were firstly kept fixed at 32% WFPS (experiment Fixed 32) in a sand, sandy loam and silt loam soil or allowed to fluctuate between 20% and 50% water-filled pore space (WFPS, Dry-wet 20-50). Total maize-C (C maize) mineralized was highly similar between these three textures and thus the direct textural control was minor. On the contrary with the fluctuating moisture level, around threefold more added C maize was mineralized (P < 0.01) in the sand (86%) than in the silt loam (25%) soil. We owe this boost in C maize mineralization to the rewetting of larger pores in the sandy soil which should contain most of the ground maize residue. This determining control of texture on distribution of moisture as well as the maize substrate clearly exceeded the direct impact of texture on organic matter stability. In a third and fourth experiment, timing and dose to remoisten a silt loam soil back to 50% (experiment Equal 20-50) or 35% WFPS (experiment Equal 20-35) when dried out to 20% WFPS were additionally mimicked in the sand and sandy loam soils. These scenarios correspond more closely to a field situation in which nearby differently textured soils all receive the same precipitation input. After several rewetting cycles, the silt loam soil eventually had a 13.9% higher WFPS than the sandy soils. By then moisture stress clearly limited C maize mineralization in the sandy soil (0.13 (Equal 20-50) and 0.05 (Equal 20-35) mg kg−1 h−1) as it proceeded at only half of the rate as in the silt loam soil (0.21 (Equal 20-50) and 0.12 (Equal 20-35) mg kg−1 h−1). The amount of C maize mineralized after 120 days in the silt loam soil was simulated to surpass that in the sandy soil. We conclude that the effect of soil texture on decomposition of a fresh substrate is largely indirect, i.e. through mediation of soil water content and its distribution in the soil matrix. Moreover, our data suggests that in the event of prolonged drought C maize mineralization will be less limited in finer textured soil, contradicting the widespread idea that organic matter would degrade more rapidly in coarser textured soils. • At fixed moisture level OM decomposition was not strongly impacted by soil texture. • With moisture fluctuation textural control enlarged by mediating presence of water. • With equal moisture supply, drying overridingly inhibited C mineralization. • This indirect textural impact depended strongly on moisture regime. [ABSTRACT FROM AUTHOR]