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Purpose: This study investigated the C and N mineralisation potential of solid fractions (SFs) from co-digestated pig manure after P-stripping (P-POOR SF) in comparison with P-rich SFs, as a means to estimate their organic matter stability in soil. Compost (COMP) and biochar (BCHR) (made from P-POOR SF) were also included in the study as reference biosolids. Methods: The SFs were incubated in a sandy-loam soil under moist conditions to determine production of CO2 and mineral N. At specified intervals, CO2 evolution in the mixtures was measured via the alkali trap method and titration over a period of 81 days, while mineral N was measured using a flow analyser after KCl extraction over a period of 112 days. Results: The various SFs showed similar patterns of C mineralisation (15–26% of added total C in 81 days) that were clearly higher than for COMP and BCHR (6% and 7%, respectively). Temporary N immobilisation was observed in biosolids with a high C/N ratio. The effective organic matter (EOM) of the SFs was calculated based on the C mineralisation data and varied between 130 and 369 kg Mg−1. Conclusions: The SF with a reduced P content had a high EOM/P ratio which is beneficial in areas where P status of the soil is already high. Moreover, the N mineralisation patterns confirm that a high C/N ratio may also reduce risks for N leaching due to temporary N immobilisation. [ABSTRACT FROM AUTHOR]

Evidence is accumulating that belowground crop residues contribute more efficiently to the build‐up and maintenance of native soil organic carbon (SOC) than aboveground plant parts. We corroborated previous testing of the hypothesis that root‐C would preferentially accumulate in soil microaggregates, where it is physically protected against microbially mediated decomposition. In three European field trials with C3 to C4 crop transitions, we compared the content of maize‐C (Zea mays L.) in soil from rotations with grain maize (MG) or silage maize (MS) (i.e. with incorporation of roots and shoots or roots only). After decades of maize cultivation, SOC content did not differ within three out of four MG–MS pairs, although obviously larger amounts of shoot biomass were added to soil in the case of MG. We found that relative contribution of roots was on average 3.5 times more than shoots to the build‐up of SOC per equivalent mass of residue C added to soils. Preferential occlusion of root‐C as silt‐sized intra‐microaggregate particulate organic matter (iPOM) was not observed. There were much larger effects from shoot incorporation on maize‐C in the >53‐μm fraction and free silt and clay. Storage of root‐C as sand‐sized iPOM was not quantified here, but first estimates suggested that physical entrapment at this level could only partly explain the longevity of root‐C in soil. We reconfirm the relative stability of root‐C in soil, but do not conclude that this stems from preferential physical entrapment over shoot‐C. Future work should investigate the cause of preferential root‐C association with the clay‐sized fraction and if this occurs before or after microbial processing. Highlights: The hypothesized preferential physical stabilization of root‐C was assessed.Aboveground biomass had a minor effect on SOC storage in European long‐term trials.Contribution of maize root‐C to maintenance of SOC was three‐fold that of shoot‐C inputs.There was no preferential occlusion of root‐C in silt‐sized microaggregates. [ABSTRACT FROM AUTHOR]

• Land management affect the weathering of minerals. • Decrease of net acidification through agricultural practice. • The change of soil chemical properties under agricultural practice is considered to be an expression of soil phenoforms. • Increase of organic carbon stock after conversion to cultivated land. • Formation of SRO aluminum promotes retention of organic carbon. We investigated changes in geochemical soil properties in response to deposition age and land use management over 30 – 50 years on tropical volcanic soils. Our purpose was to find out how weathering stage and land use interactively affect soil properties and organic carbon, and to check if phenoforms (management-related soil subtypes) exist within the genoforms (genetic soil types). Soil samples were taken at land uses that have been converted (pine forest and agricultural land) and a natural forest as the original land use. The results showed that pine forest soil displayed more intense weathering as indicated by higher values of three weathering indices. Intensive agricultural practices also improved soil chemical properties such as pH, exchangeable base cations, base saturation, and organic carbon stock leading to WRB-qualifier of " eutric" in cultivated soils, whereas the average of bulk density was relatively similar between forests and cultivated soils. Positive correlations were found between amorphous materials and Al o , specific surface area, and micropore volume. Correlations between the content of short-range order Al (hydr-) oxides (indicated by Al o) and organic carbon were found in pine forest and agricultural soils, particularly in subsoils. Our results clearly indicate the increase of base cations retention due to less acidification and an increase of organic carbon stock under agricultural land use, likely due to stabilization with non-crystalline materials. [ABSTRACT FROM AUTHOR]

Unusually high SOC levels have been reported for sandy cropland soils in North-Western Europe. A potential link with their general heathland land-use history was investigated by comparing two soil pairs of relict heathland and cultivated former heathland in the Belgian sandy region. A sequential chemical fractionation yielded similar sizes in corresponding SOM fractions between the heathland and cropland soils (i.e. NaOCl resistant: 12.3–15.0 g C kg−1 and NaOCl + HF resistant: 2.6–5.3 g C kg−1). Higher amounts of clay sized N in the cropland plots can be attributed to N additions from mineral fertilizers and animal manure. Temperature resolved Pyrolysis Field Ionization Mass Spectroscopy analysis showed that the composition of both relict heathland and cultivated soils was surprisingly similar, in spite of over 60 years of intense cropland management. The mass spectra of SOM in both heathland-cropland soil pairs investigated was dominated by signals from lipids, alkylaromatics and sterols. The accumulation of this SOM rich in aliphatics was logically linked to the high input of lipids, long-chain aliphatics and sterols from heathland vegetation and the low soil pH and microbial activity. Based on the relatively high OC surface loadings of HF-extractable OM (13–44 mg C m−2 Fe and 1.2–2.3 mg C m−2 clay), direct organo-mineral bonds between OM and Fe-oxides or clay minerals seem to be only partly involved as a stabilization mechanism in these soils. The distinct bimodal shape of the thermograms indicates that OM-crosslinking could furthermore contribute substantially to SOM stabilization in these soils. This study therefore corroborates the previously proposed view that lipids may be bound in networks of alkylaromatics, the structural building blocks of OM macromolecules. We hypothesize that such binding is able to explain the measured retention of these OM components, even under several decades of cropland management. [ABSTRACT FROM AUTHOR]

Renewed interest in temporal soil organic carbon (SOC) stock changes has stressed the importance of reliable methods for quantitative assessment of organic compound (OC) content. Particularly with the establishment of modern dry-combustion analyzers, which are replacing the traditional wet-oxidation methods, the need for correct relationships between both is of crucial importance for comparison of past and current SOC data in long-term SOC stock change studies. Dry combustion with a Variomax CNS-analyzer was the standard to evaluate three other methods for Belgian agricultural soils. Excellent linear relationships were found with the Walkey and Black method and the Springer and Klee method, whereas a Shimadzu TOC-analyzer slightly underestimated the OC content. Precision of the investigated methods was comparable and tended to be dependent on the sample size used for measurement. The OC oxidation efficiency of the most widely applied method of Walkey and Black for the soils in this study was very close to the generally accepted 75%. Mass loss on ignition at 800°C could be very well related to the soil OC content and the clay content. The traditional factor of 1.724 used to convert OC measurements to organic matter percentages is not valid for the investigated soils, which demonstrates that rather regional-specific factors (in this study 1.911) should be determined and adopted. [ABSTRACT FROM AUTHOR]

Sterilization (partial or full) is one of the most fundamental tools for studying the ecological roles of soil biota and notably soil fauna, but there is a lack of knowledge on how these techniques affect other soil- inhabiting micro-organisms (non-target), and on their recovery after sterilization. We investigated the effect of a partial soil sterilization with gamma (γ) irradiation (target dose of 6 kGy, proven to selectively remove the entire nematode community and higher soil fauna) on the soil microbial community and its recovery in a microcosm experiment during 12 weeks. We specifically studied the effects, following partial sterilization, of recovery time (4 sampling events after 1, 4, 8 and 12 weeks) on the microbial activity (activities of dehydrogenase and β-glucosidase, and nitrification), microbial biomass and microbial community composition (using PLFA analysis), in two soils of contrasting texture (silt loam and loamy sand). We also studied whether inoculation of finely sieved and homogenized soil powder (SP) from these same soils (containing prokaryotes, fungi and protists, but no metazoan organisms) would stimulate the microbial recovery process. Dehydrogenase activity was strongly reduced by γ-irradiation in the silt loam (and SP did not stimulate its recovery) but not in the loamy sand, whereas there was no effect of γ-irradiation on β-glucosidase activity in either soil. Nitrification was strongly affected in the loamy sand and did not yet recover by the end of the incubation (week12), but the SP inoculation brought nitrification to the level of the unirradiated control by week4. Surprisingly, nitrification rate was not affected by γ-irradiation in the silt loam. The factors "recovery time", "irradiation" and "SP inoculation" affected the microbial community composition (based on PLFA), but the direction of these changes did not seem to follow a clear pattern. These results urge to carefully consider the re-establishment of microbial activity in studies on the roles of micro- and mesofauna following selective sterilization, even after several months. Also, SP inoculation can be a useful way to stimulate the recovery of the nitrification process, which would be crucial in studies of the role of soil fauna in N dynamics. [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]

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]

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. • Gamma irradiation was applied to selectively eliminate soil nematodes and/or protists in soil microcosms. • Nematodes, and culturable flagellates and ciliates were removed from 6 kGy onwards, • Soil amoebae showed higher resistance to gamma irradiation than flagellates and ciliates. • A viable native microflora community remained after gamma irradiation up to 16 kGy. • Soil powder reinoculation successfully established a representative protist community in (partially) sterilized soil. [ABSTRACT FROM AUTHOR]

• Gross NH 4 + fixation and defixation co-occur and in part depend on redox status. • Resulting net NH 4 + fixation was lower in paddy soil at higher redox potential. • Redox status also seems to indirectly affect NH 4 + (de)fixation via other processes. • (De)fixation occurs as exchangeable and fixed NH 4 + evolve to the soil equilibrium. Abiotic fixation and defixation of ammonium (NH 4 +) in silicate interlayers are common processes in paddy soils, owing to their often high levels of 2:1 type clay minerals. Fixed NH 4 + hence forms a buffer during the supply and loss of plant-available N. The soil redox potential (E h) is suspected to influence NH 4 + (de)fixation by its impact on the negative charge of Fe3+-containing clay minerals, yet experimental confirmations are scarce. We examined the effect of a fluctuating or generally increased E h on dynamics of NH 4 + (de)fixation in a Bangladeshi paddy soil (total N: 1.9 g kg−1 | CEC: 43 cmol kg−1) during two incubation experiments. In those, we either 15N-labelled the fixed NH 4 + pool (with initially 87 mg kg−1 exchangeable versus 393 mg kg−1 fixed NH 4 +-N) or the exchangeable pool (with initially 141 mg kg−1 exchangeable versus 249 mg kg−1 fixed NH 4 +-N), and this under three E h -controlling treatments: continuous flooding (CF), CF with extra MnO 2 added (3.5 g kg−1 Mn4+) (CF-MnO 2), and alternate wetting and drying (AWD). The CF treatment overall led to somewhat higher net NH 4 + fixation than AWD and, less clearly, also compared to the CF-MnO 2 treatment, resulting from higher gross NH 4 + fixation as well as lower gross defixation. These fluxes always led to a dynamic equilibrium between the exchangeable and fixed NH 4 + pool within a few days of incubation, and this equilibrium was the same for both experiments but depended on the E h treatment. However, by altering other NH 4 + transformation processes, E h treatments also indirectly impacted NH 4 + (de)fixation fluxes, for which the equilibrium ratio between exchangeable versus fixed NH 4 + turns out to be the determining factor. [ABSTRACT FROM AUTHOR]