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The addition of exogenous organic matter (EOM) generally stimulates microbial activity and shapes soil structure, likely with stronger effects at a higher application dose. It is not known, however, if and how the application dose of EOM would impact its own mineralisation and that of native soil organic carbon (SOC) by such mediation of soil pore network structure. In 90-day soil incubation experiments, we compared mineralisation of ± 2 cm pieces of 13C-labelled ryegrass as EOM reference at two doses (0.5 g kg−1 vs. 5 g kg−1) and mineralisation of SOC in sandy loam and silt loam soils that were initially subjected to different degrees of disturbance, namely soil sieving at < 2 mm and < 10 mm. We found that the high EOM dose-induced mesoporosity (i.e. vol% increase of pore neck class > 300 μm class by 130% vs. unamended control) and macroporosity (i.e. EOM doses 0.5 g kg−1 and 5 g kg−1 increased the vol% of the 60–100 μm class by 80 and 120% vs. unamended control) formation in the sandy loam soil, whereas the porosity in the silt loam soil remained unaffected. The % of EOM mineralised was independent of EOM dose and therefore did not relate to the changes in soil pore network structure. However, the observed development of meso- and macropores in the sandy loam soil appeared to be associated with stronger stimulation of SOC mineralisation at increasing EOM dose (30% and 71% more SOC mineralisation at low and high dose vs. unamended control). In the silt loam soil, only the high EOM dose-stimulated SOC mineralisation vs. unamended control (28%) but soil pore network structure remained unresponsive to EOM dose. This suggests that stimulation of SOC mineralisation following EOM addition might also be in part linked to the formation of larger pores in the sandy loam soil, and that this stimulation is not always proportional to EOM dose. [ABSTRACT FROM AUTHOR]

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 O2 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. [ABSTRACT FROM AUTHOR]

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]

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]

Abstract: 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. [Copyright &y& Elsevier]

Focuses on the quantification of effect of soil fumigation on N mineralization and on nitrification in different soils. Relationships between the effect of fumigation on processes and soil properties; Number of soils representing a range in soil texture and organic matter contents; Differences in average short-term N mineralization rates between fumigated and unfumigated soils.

Judicious management of organic matter supply and tillage practice is crucial for sustaining soil quality in horticulture. We have studied whether N fertilization of vegetable crops should be adapted in the short term when soil management changes to include compost application and reduced tillage as soil quality improving factors. The experimental setup was a multiyear field trial on a sandy loam soil with a vegetable crop rotation. Soil tillage in spring was either conventional (moldboard plough) or reduced, non-inversion tillage (with a specially designed chisel plough). Starting in 2008, farm compost was applied each autumn at three rates: 0, 15 and 45 Mg per hectare. The three-year crop rotation (2009–2011) consisted of broccoli ( Brassica oleracea , var. Italica Group), carrots ( Daucus carota ) and leek ( Allium porrum ). Small but significant differences in N dynamics were found between treatments. Broccoli had a higher N uptake and biomass production under reduced compared to conventional tillage. In accordance with this, the apparent net N mineralization tended to be higher for reduced tillage compared to conventional tillage. In contrast, for leek, N uptake did not differ between tillage practices despite a higher apparent net N mineralization under conventional tillage. A tendency for a higher biomass production under conventional tillage was in line with a tendency for a higher amount of residual soil mineral N that might have increased the risk of N losses. In the carrot crop, a higher amount of mineral N under conventional tillage did not increase root yield. Compost application maintained soil organic matter content and did not result in a higher amount of residual soil mineral N. Only in the carrot growing season, compost application increased soil mineral N content to a limited extent. The small short-term changes in overall N availability observed between treatments do not necessitate changes in N fertilization strategy or additional precautions regarding residual soil mineral N. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]