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Vermicomposting converts organic wastes into high-quality nutrient-rich compost and has been considered as a suitable technology for developing countries, especially at the household and community level as it is a simple and natural technology that does not require sophisticated machinery and frequent process monitoring. In addition, attempts to address SOM and nutrient losses in a sustainable manner in sub Saharan Africa countries require careful optimization of use of organic amendments such as through vermicomposting. We performed a vermicomposting experiment using three-earthworm species, exotic (Eisenia fetida and Eisenia andrei) and indigenous (Eudrilus eugeniae), fed on six different substrates made from agricultural wastes (cow and donkey manure separately spiked with three types of equally mixed crop residues (maize with soybean (MS), maize with banana (MB) and soybean with banana (SB)) to assess the nutrient stabilization in the resulting compost; and performed an incubation experiment using a selection of these vermicomposts to study the net N mineralization of the composts, and how they affected soil microbial properties such as microbial biomass C, dehydrogenase and β-glucosidase activities, in Nitisols. Earthworm activities during vermicomposting of agricultural wastes accelerated the rapid decomposition of mixed substrates and resulted with a significant reduction of TOC (C mass loss > 65%) and C:N ratios (twofold compared to the control without earthworm) and an increase in N mineralization and the total concentration of P, K and S. The effect of vermicompost on N availability and soil microbial quality significantly varied depending on vermicompost sources. The vermicompost made from cow manure mixed with soybean and banana residues using Eudrilus eugeniae had the highest net N mineralization (32.4 µgNg-1 soil, 75% of N added). Regardless of vermicompost type, amended soils had significantly higher microbial and enzymatic activities than unamended control. Our results suggest that vermicompost sources tested could reliably use for long-term improvement of soil nutrient mineralization and microbial activity under Nitisols of Ethiopia due to their greatest microbial activities and nutrient dynamics during vermicomposting.

Soil texture determines microbial activity and organic matter (OM) decomposition in several ways. Next to direct physicochemical stabilization of OM, the location of moisture in the soil matrix also depends on soil texture. At lower moisture content, the contact between soil moisture and OM might be critical for OM decomposition, because such areas of contact could function as a ‘bridge’ for transport of moisture, substrates and microorganisms. Additionally, water absorption by porous OM itself likely also depends on soil texture and this may thus bring about another indirect textural control on OM mineralization. However, the relevance of such indirect soil textural controls on OM mineralization has not been investigated. We therefore compared the mineralization of 13C-labeled maize particles (Cmaize-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 maize particles and moist soil volume was assessed at the microscale using X-ray μCT and contrast-enhancing agents. We hypothesized that a lower contact between OM and soil in the loamy sand soil would lead to a reduced OM decomposition compared with the silt loam soil. On the other hand, we expected that OM 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, we found a lower contact between zones of moist soil and the added OM in the silt loam soil. But as expected, OM 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 Cmaize-min in drier loamy sand soil. This finding suggests that such a ‘sponge-effect’ of OM may be a relevant yet largely overlooked element in studying the relationship between soil moisture and OM decomposition. Contrary to our expectation, Cmaize-min was higher in the silt loam soil texture across all three moisture levels (P

Degradability of organic matter (OM) in soil depends on its spatial location in the soil matrix. A recent breakthrough in 3D-localization of OM combined dual-energy X-ray CT-scanning with OsO4 staining of OM. The necessity for synchrotron-based µCT and the use of highly toxic OsO4 severely limit applications in soil biological experiments. Here, we evaluated the potential of alternative staining agents (silver nitrate, phosphomolybdenic acid (PMA), lead nitrate, lead acetate) to selectively enhance X-ray attenuation and contrast of OM in CT volumes of soils containing specific mineral soil particle fractions, obtained via lab-based X-ray µCT. In comparison with OsO4, administration of Ag+ and Pb2+ resulted in insufficient contrast enhancement of OM versus fine silt (4. However, OM discrimination from other soil phases remained a challenge. Further development of segmentation algorithms accounting for grey value patterns and shape of stained particulate OM may enable its automated identification. If successful in undisturbed soils, PMA staining may form an alternative to OsO4 in non-synchrotron based POM detection.

The soil environment impacts carbon turnover in a multitude of ways. It has been shown that C emissions are higher when exogenous carbon is locally concentrated than when it is spread in the soil matrix. Hence, the spatial separation between decomposers and substrate may cause ineffective dispersion of the produced enzymes within the soil matrix. It is not clear, however, if and how lower doses of exogenous C impact its degradability and if an interaction exists with native soil organic matter (SOM) level. Therefore, a seventy-day incubation experiment with six varying exogenous OM doses (0-6 g C kg-1) in soil mesocosms of loamy sand subsoil (WFPS 50%) with three different native SOM contents (i.e. 0.1, 0.5, and 1.0 %) was carried out. C emission and isotopic signature measurements of 13CO2 by means of a CRDS-analyser were used to track the degradation of added maize shoots with a δ13C=-12.7‰ that contrasts to native SOM (δ13C=-25‰). Mineral N was added, and the redox potential (Eh) monitored. Eh suggested that O2 may have not become limiting for C mineralisation, even at higher doses, but it did not clearly relate to the added C dose. Cumulative CO2 emissions increased proportionally with C dose and so our hypothesis that a lower dose dispersion limits microbial activity was disproved. However, a higher native SOM level led to higher relative degradability of added C for the highest doses (i.e. 3 and 6 g kg-1 soil). Furthermore, a negative impact of OM doses above 1 g kg-1 soil on native SOM degradation was found. We conclude that positive priming of native SOM mineralisation did not occur. A disproportional response of microbial biomass carbon and soil C mineralisation at lower doses suggests that dose does impact the microbial metabolic quotient. Ongoing PLFA analyses will help us interpret these observations.

Mineral nitrogen (N) availability to heterotrophic micro-organisms is known to impact organic matter (OM) decomposition. Different pathways determining the N accessibility depend to a large extent on soil structure. Contact between soil mineral and OM substrate particles can facilitate N transport toward decomposition hot spots. However, the impact of soil structure on N availability to microbes and thus heterotrophic microbial activity and community structure is not yet fully understood. We hypothesized that carbon mineralization (Cmin) from low-N substrate would be stimulated by increased N availability caused by closer contact with soil particles or by a higher moisture level, enhancing potential for N-diffusion. Under opposite conditions retarded heterotrophic activity and a dominance of fungi were expected. A 128-days incubation experiment with CO2 emission monitoring from artificially reconstructed miniature soil cores with contrasting soil structures, viz. high or low degree of contact between soil particles, was conducted to study impacts on heterotrophic activity. The soil cores were subjected to different water filled pore space percentages (25 or 50% WFPS) and amended with either easily degradable OM high in N (grass) or more resistant OM low in N (sawdust). X-ray μCT image processing allowed to quantify the pore space in 350 μm around OM substrates, i.e., the microbial habitat of involved decomposers. A lower local porosity surrounding sawdust particles in soils with stonger contact was confirmed, at least at 25% WFPS. Mineral N addition to sawdust amended soils with small particle contact at 25% WFPS resulted in a stimulated respiration. Cmin in the latter soils was lower than in case of high particle contact. This was not observed for grass substrate particles or at 50% WFPS. The interactive effect of substrate type and soil structure suggests that the latter controls Cmin through mediation of N diffusion and in turn N availability. Phospholipid fatty acid did not reveal promotion of fungal over bacterial biomarkers in treatments with N-limited substrate decomposition. Combining X-ray μCT with tailoring soil structure allows for more reliable investigation of effects on the soil microbial community, because as also found here, the established soil pore network structure can strongly deviate from the intended one.

The role of soil structure in organic matter (OM) stabilization has been primarily investigated through physical fractionation studies operative at the scale of aggregates and smaller organo-mineral particles. By narrowing down soil structure to an arrangement of mineral and organic particles, the majority of studies did not explore the spatial organization of the soil pore network, the actual habitat of microorganisms. In a lab experiment we incubated a sandy loam soil (with application of ground grass or sawdust) in 18 small aluminum rings (Ø 1 cm, h 1 cm). Bulk density was adjusted to 1.1 or 1.3 Mg m-3 (compaction) and 6 rings were filled at a coarser Coarse Sand:Fine Sand:Silt+Clay ratio.

Reforestation of agricultural land in many European countries has progressed more slowly than anticipated. Many years’ fertilization has caused an accumulation of nutrients in the top soil layer. Phosphorus (P) has proven to be particularly persistent and elevated concentrations in soil and vegetation are found up to thousands of years after agricultural abandonment. This persistently elevated bioavailable P is a paradox for ecosystem recovery in post-agricultural forests: on one hand, higher P bioavailability stimulates stand productivity and tree growth but, on the other hand, it limits the development of typical forest plant communities (Baeten et al. in press) and associated organisms. Limitation of bioavailable P seems to be a necessity for the recovery of species-rich target communities and for the conservation of endangered species (Wassen et al. 2005). Reforestation of agricultural land results in a decrease in soil pH with increasing forest age, but the extent and rate of soil acidification differs highly between tree species. Gymnosperm tree species are often contrasted to angiosperms because of their acidifying impact on soils. We will present data showing that rapid (within 2 decades) changes in soil properties can also occur beneath different broadleaved tree species. Ca concentra¬tions in leaf litter seem to be the driving factor for differences between species: tree species with higher litter Ca concentrations such as Populus and Fraxinus support a significantly higher soil pH, base saturation degree and lower quantities of bioavailable aluminum compared to the Ca-poor Fagus and Quercus species. Tilia and Prunus trees are intermediate between these ‘soil preservers’ and ‘soil acidifiers’. Furthermore, tree species have a major influence on the short-term and long-term bioavailability of P. Below ‘soil acidifiers’ we found significantly higher bioavailable and slowly cycling P fractions compared to ‘soil preservers’ which mainly retained P in less soluble pools. To date, there are no integrated studies on the long-term effect of different broadleaved tree species on soil properties in general and the bioavailability of P through soil acidification in particular. Most studies evaluating the bioavailability of P use extraction methods that only provide short-term dose-response insights into the readily available P pool for plants and soil microbes. However, for studying the role of P in recovering (semi)natural ecosystems, it is much more relevant to gain insight into time-scales extending a single growing season (Richter et al. 2006) and to include more slowly cycling P fractions. We discuss six common garden experiments in which broadleaved tree species have been planted on loamy agricultural soil for between 5 and 35 years. We argue that vegetation choice can be an important driver of regional biogeochemistry and biodiversity.

Intensification of livestock production in combination with a decline in arable land has resulted in a discrepancy between excess load of nutrients from livestock production and the possibility to apply these nutrients in an environmental way on agricultural land. Consequently, the mineral input and output in livestock intensive regions is not balanced. The crop production systems rely on synthetic fertilizers for fertilization, whereas livestock production systems face problems in manure disposal. This dissertation aimed towards efficient use of resources by re-connecting crop and livestock production through animal manure processing. The processing results in animal manure derivatives such as liquid fraction (LF) of digestate, mineral concentrate and air scrubber water, that might have potential to be used as alternatives for synthetic nitrogen (N) fertilizers. The dissertation investigated N fertilizer performance of these materials with regard to crop yield and soil properties on laboratory, greenhouse and field scale. Results showed that LF of digestate, air scrubber water and mineral concentrate can be used as synthetic N fertilizer substitutes without adverse effects on crop yield. With regard to environmental aspects, it seems that these materials should not additionally increase the risk of nitrate leaching as compared to conventional fertilization practices. Some of their characteristics (e.g. form, nutrient concentration, stability) differ from the characteristics of synthetic N fertilizers, however their N performance indicates the potential of labelling LF of digestate, ASW and mineral concentrate as processed mineral fertilizers on European fertilizer market.

Elevated depositions of non-marine sulphate (SO42-) and inorganic nitrogen (N), as a consequence of air pollution, resulted in a progressive acidification and eutrophication of Flemish forests. Since the 1980s emission abatement reduced the acidifying and eutrophying emissions and depositions in Flanders. This thesis aimed to evaluate the impact of the evolution in depositions on soil solution chemistry, using long-term data collected in 5 plots of the ICP Forests monitoring (Level II) network in Flanders. The sharp decrease of SO42- and ammonium (NH4+) depositions made that abiotic N status started to improve and acidification slowed down during the past two decades. However, N depositions are still far above the critical loads for ectomycorrhiza and epiphytic lichens. Given the still very low soil pH (3.5‒4.5) unfavourable for microbial life, the generally observed tendency of increased dissolved organic carbon (DOC) and nitrogen (DON) mobility is likely a direct result of lowered ionic strength and partly rise in pH. Abiotic recovery is delayed by a simultaneous decrease in the deposition of base cations (Ca2+, K+ and Mg2+) and SO42- desorption. Biotic recovery is lagging behind on the changes in soil solution chemistry, as indicated by the stable but unbalanced tree mineral nutrition. Acidification and eutrophication will likely continue to produce after-effects for many decades. The results from this thesis indicate that the Programmatic Approach to Nitrogen (PAS) is partly missing its target for oxidized N compounds and that extra measures will be necessary to bring NOx emissions at an acceptable level.