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Soil organic carbon (SOC) levels generally increase with increasing clay and silt content under similar climatic conditions because of increased physicochemical protection. Surprisingly though, many silt loam soils in Western Europe have low top SOC levels compared to coarser textured soils. Soil texture also strongly controls soil moisture with consequent indirect impact on heterotrophic activity. Especially during periods of prolonged drought, which seem to be occurring more frequently throughout Europe, it could be expected that this soil textural control of moisture retention decisively affects added organic matter (OM) decomposition. We hypothesized that, contrary to expected textural effects on physicochemical protection of OM, the higher moisture availability in silty soils strongly favors added OM mineralization during periodic drought than sandy soils. Moreover, as a secondary objective, we investigated if during prolonged drought, capillary moisture supply could significantly alleviate the water stress on added OM decomposition in sand with rather shallow groundwater table. Therefore, we established a one-year field trial in which the upper 0.5 m soil layer was replaced by sand, sandy loam and silt loam soil with low SOC. In a fourth treatment, a gravel layer was added beneath the sand layer to exclude capillary rise. Maize residues were mixed homogenously with soil in top 25 cm in all plots. Changes in soil moisture and maize-C mineralization (C-maize-min) were measured. We found that soil texture did not affect C-maize-min until after about five months and thereafter C-maize-min rates were higher in the silt loam than in the sandy soils. Moisture content correlated positively with the C-maize-min rate for the sand-textured soils only. Capillary rise did not result in a significant increase in C-maize-min in the sandy soil. After one year, a larger share of added C-maize was mineralized in the silt loam soil (81 +/-& nbsp; 6%) than in the sandy soil (56 +/- 7%). These results clearly highlight that soil texture controlled C-maize-min indirectly through regulating moisture under the field conditions when the study area faced a period of unusual drought. Moreover, our results imply that, under future climate scenarios, more frequent droughts might lead to a lesser SOC depletion in sandy soils compared to in finer textured soils under similar management.

The few available studies on maize biomass in soil of field experiments have shown that belowground maize biomass resides more than twice as long as aboveground residues. Our principal objective was to investigate the effect of maize crop residue incorporation on the long-term dynamics of soil organic carbon (SOC) for a selection of croplands with sandy to sandy loam texture in Flanders, Belgium. After a period of more than 20 years, we resampled 31 and 29 parcels that had either silage maize (MS, only belowground biomass input) or grain maize (MG, both above- and belowground biomass input) dominated crop rotations. Changes in maize-derived SOC (Cmaize) of MS and MG systems between 1993 and 2018 were evaluated by detecting SOC content and its δ13C value. In addition, a 140-day incubation experiment (at 20.5 °C) was carried out to compare the stability of Cmaize and the related nitrogen (N) mineralization between the MG and MS fields. After two decades, δ13C raised 2.4 and 2.6‰, while Cmaize increased 1.6 and 2.2 g kg−1 in MS and MG systems, respectively. The estimated average humification coefficient over this period of maize-C was 0.11 for the MS system, about twice as that of the MG system (0.06). This reconfirms findings of stronger contribution of belowground biomass versus aboveground residues to stabilized SOC pools over the medium to long term. By the end of the incubation, on average 10.4% and 11.6% of Cmaize had been decomposed in the MS and MG soils, respectively. These Cmaize mineralization rates were statistically equal for the two systems, which indicates that mainly belowground biomass remained in the soils. Also, net N mineralization was indifferent to the crop rotations (MS: 58.8 mg N kg−1 and MG: 52.6 mg N kg−1). To reconcile the two main observations in this study, we hypothesize that the lower preservation of shoot-C could be manifested on a shorter term. Detailed research is needed to determine the time scale of increased stabilization of root-C versus shoot-C under field conditions.

Biochar addition to soils is heralded to reduce N2O emissions, but still, the explanatory mechanisms have not been resolved. Moreover, it is uncertain whether N2O emission reductions would persist after prolonged biochar incorporation in the field. In this study, we incorporated four biochar types in a loam textured cropland field and intact soil cores were sampled to investigate the physical control of biochar on denitrification after 7 months. During a first incubation experiment, we measured N2O emissions from undisturbed and disturbed (i.e. sieved (2 mm) and grounded) soil cores. Both in the disturbed and undisturbed soil cores biochar at water filled pore space (WFPS) of 80% reduced the N2O emissions by 50–90%, refuting the hypothesis that biochar exerts an indirect physical control over soil denitrification several months after incorporation. Secondly, we hypothesized that biochar creates denitrification ‘hotspots’ in soil, where complete reduction of N2O to N2 is promoted compared to non-amended soil. In these hotspots biochar particles could act as microlocations with local anaerobic conditions and local higher pH, stimulating in this way complete denitrification. Via the acetylene inhibition method we did not observe a reduction in the N2O/(N2O + N2) ratio, which could suggest that biochar did not promote the reduction of N2O to N2. Manipulations likely to promote labile C bioavailability, here either by glucose addition or by soil particulate OM disclosure after disruption of soil aggregates, resulted in the most prominent biochar-induced N2O emission reductions.

The majority of microbial mediated soil processes depend on availability of organic matter (OM), water and air. Because of its ability to visualize the 3D architecture of soil non-destructively, X-ray computed tomography (CT) is becoming a widespread tool for studying soil pore network structure. However, phase determination of pore space, soil OM, soil mineral matter (MM) and water is often limited even with the latest technological and software advances, allowing high resolution and better quality imaging. Contrast agents commonly used in histology enable enhancement of X-ray attenuation of targeted structures or compounds. Here we report on the first systematic investigation of the use of such X-ray contrast agents for soil research. An evaluation procedure as well as a method to apply the agents to soil samples was developed and applied on reference soil samples. The effectiveness and selectivity of the contrast agents was evaluated for soil organic matter (SOM), MM and water. Several products were found to selectively increase the attenuation of water or SOM. The four agents with the best OM-staining capabilities (phosphomolybdenic acid (PMA), silver nitrate, lead nitrate and lead acetate) were further tested on an OM-MM mixture and all showed an increased of the SOM attenuation coefficient above the MM values.

A renewed interest in chemical fractionation of soil organic matter (SOM) originates from the premise that it enables to isolate labile SOM from SOM protected through mineral binding and recalcitrant SOM. Both selective removal of labile non-bound SOM through oxidation or hydrolysis as well as selective removal of minerals and attached SOM are often applied. Molecular-level SOM characterization by means of temperature resolved Pyrolysis-Field Ionization Mass Spectroscopy analysis (Py-FIMS) was used here as an approach to obtain insight into the fate of SOM upon wet chemical treatment with regard to composition and thermal stability. The applied sequential chemical treatment with 6% NaOCl and 10% HF yielded similar sizes in stable SOM fractions between sandy semi-native heathland and cultivated cropland soil pairs (i.e. NaOCl resistant OC: 12.3–15.0 g C kg − 1 and NaOCl + HF resistant OC: 2.6–5.3 g C kg − 1 ). Py-FIMS spectra of bulk SOM in both heathland–cropland soil pairs were dominated by signals assigned to lipids, alkylaromatics and sterols. Difference spectra and thermograms showed selective loss of signals from sterols, lignin dimers and thermolabile lipids. This matches advancing SOM decomposition as derived from previously reported gradients in SOM composition as decomposition proceeds from plant material over particulate organic matter (OM) to SOM in silt and clay particle sizes. However, increased ion intensity attributed to carbohydrates, peptides and short-chained lipids after NaOCl treatment indicates that biologically labile SOM components were also enriched, and they may possibly have been protected through mineral binding or encapsulation in macromolecular OM structures. Subsequent HF treatment yielded increased volatilization in the thermostable region for mass signals tentatively assigned to phenols and lignin monomers and of heterocyclic N-containing compounds and thermostable alkylaromatics. The resistance to chemical treatment of the latter two components matches with their hypothesized structural function in macro OM molecules. However, even for the sites investigated here, with a very similar soil texture, climate, land-use, drainage and contents of pedogenic oxides, contrasting and not readily explainable results were found for other SOM constituents. Therefore, chemical fractionations seem to yield very site specific and less distinct patterns and this study demonstrated that derivation of useful information regarding SOM stabilization mechanisms from such experiments is by no means straightforward.

The concentration of dissolved organic matter (DOM) in the soil solution is predominantly determined by sorption processes to mineral surfaces. We compared four models describing DOM sorption isotherms to mineral soil material using the results of batch experiments for dissolved organic carbon (DOC) and nitrogen (DON) sorption to different horizons of a podsol. The Initial Mass and modified Langmuir isotherms could be fit to the data points in all cases. The three curvilinear models (Langmuir, Freundlich and exponential models) were found to result in similar goodness-of-fit and parameter estimates. We found that the most widely used, linear model (the Initial Mass isotherm) could be used for horizons with very small sorption capacity and horizons almost saturated with DOM, but failed to describe sorption in the deeper, DOM-unsaturated horizons characterized by a curvilinear shape of the sorption isotherm. The modified Langmuir isotherm was found to describe DOM sorption to mineral soil best and gave realistic parameter estimates, including a measure for maximum sorption capacity of the soil. Comparing the results of the sorption experiments with DOM concentrations in soil solution in the field indicated that the sorption isotherms can serve as a basis for larger DOM leaching models.