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An ideal and efficient nitrogen (N) recommendation for precision fertilization (PF) should account for potential soil mineralizable N. This study aimed at estimating management zone (MZ) specific soil N mineralization rate (SNMR) of unamended soils. A total of 76 soil samples were collected from 21 MZs across 5 fields. An aerobic laboratory incubation was conducted under controlled conditions for two months with seven sub-sampling events. N mineralization was assessed as net increase in soil mineral N over time. Results indicated a considerable variation in mineralized soil N (9.12–41.93 mg kg−1 soil) across fields. Highest and lowest net SNMRs were 0.50 and 0.0004 mg kg−1 soil day−1, respectively. SNMRs significantly differed across MZs in three fields, while in the other two fields no significant differences were observed. In turn, 3 of 34 MZ-pairs differed (marginally) significantly (padj = 0.02–0.09) from one another, namely in MZ pairs with high variation in soil particle sizes. MZ-specific SNMRs were mostly positively correlated to pH (0.20–1.00), total N (0.12–0.99), soil mineral N (0.11–1.00) and sand (0.34–0.99), negatively correlated with clay (− 1.00 to − 0.11) and correlations with SOC were mixed (0.41 to − 0.62). This along with the support from regression analysis corroborated the existing knowledge that TN is a better predictor of mineralization than SOC. While there were only few statistically significant differences in SNMR amongst MZ per field despite mathematical differences, still incorporating MZ-specific SNMR in management decisions will be crucial in optimizing the N use efficiency in precision farming, and along with other management actions, lead to more environmentally friendly PF schemes. [ABSTRACT FROM AUTHOR]

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 (i.e., CT, conventional tillage; NT, no tillage with mulch; RT, reduced tillage; and SS, subsoiling with mulch) were maintained for twenty-one 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 the 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 growth 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, 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 may contribute to the development of sustainable agricultural production in the Chinese Loess Plateau. [ABSTRACT FROM AUTHOR]

Summary: 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. [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]

• Vis-NIRS was evaluated for predicting soil nitrogen mineralisation rate (SNMR). • Vis-NIR spectra predicted SNMR with moderate to high accuracy (0.44 ≤ R2 ≥ 0.90). • The highest model accuracy was obtained for sandy-loam texture group. • Spectral bands of 2150–2260 & 2470–2480 nm contributed most significantly in SNMR prediction. Soil nitrogen mineralization rate (SNMR) influences crop N uptake and nitrate leaching leading to environmental pollution. This study aims at (i) examining whether visible and near-infrared reflectance spectroscopy (vis-NIRS) can predict SNMR and (ii) investigating if incubation periods and ancillary soil attributes can improve the prediction accuracy. Total 133 soil samples collected from seven fields were incubated under aerobic conditions for 60 days with seven batches of sub-samples. Mineral N was measured at regular time intervals and soil samples were scanned using a vis-NIRS sensor (Tec5 Technology, Germany) parallelly. SNMR was determined by fitting a zero-order kinetic to the net mineralized N as a function of the incubation time. Soil total nitrogen (TN), total carbon (TC) and electrical conductivity (EC) were determined once. Partial least squares regression (PLSR) models were calibrated individually for each field both for vis-NIR spectra and its combinations with TN, TC and EC. Six out of seven batches of sub-samples were used for calibrating PLSR when remaining one batch was used for model validation, and it rotated across all seven batches. Vis-NIRS alone predicted SNMR with moderate accuracy in five of seven fields (coefficient of determination, 0.53 ≤ R2 ≥ 0.66, ratio of prediction to deviation, 1.51 ≤ RPD ≥ 1.76), while models were poor in two fields (R2 = 0.23–0.26, RPD = 1.18 – 1.20). Inclusion of soil TC, TN and/or EC was expected to improve accuracy, but improvements varied across fields (R2 = 0.23–0.79, RPD = 1.18 – 2.26). Similarly, the incubation period increased vis-NIRS prediction accuracy, but frequently occurred among 2nd to 6th batches (R2 = 0.35–0.82, RPD = 1.28 – 2.44). Even incorporating secondary properties and increasing incubation duration hardly improved predictions, improvement can be compromised since it is not significant mostly and often underperformed or remained unchanged. Considering the time and effort required to incubate and analyze soil properties, this study suggests using a vis-NIRS sensor to estimate SNMR in fresh soil conditions i.e., without incubation and incorporation of secondary properties. [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]

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 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]