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Capillary forces in soil allow counter-gravity moisture transport from groundwater to unsaturated layers. This matric suction driven process is often disregarded in biogeochemical studies because the impact on topsoil is largely unknown (capillary rise is not included in soil C models & difficult if not impossible to control in empirical research). As climate change is expected to intensify the occurrence and severity of droughts, the control of groundwater depth and capillary rise on topsoil heterotrophic activity requires better understanding. In this study, a laboratory set-up is employed to (1) examine topsoil moisture supply by capillary rise and (2) consequently the effect on mineralization of added plant material. Undisturbed two-meter-long soil columns of three different textures (loamy sand, (sandy) loam and silt loam) were subjected to two artificial groundwater table depths (–115 cm and –165 cm) while moisture transport was followed by volumetric water sensors. Additionally, carbon mineralization of added plant material was monitored over a period of ten weeks. We observed an upward moisture flux of capillary rise towards the topsoil for all textures and both groundwater depths. However, higher moisture contents were observed for the shallow groundwater level treatment (–115 cm) as a result of more rapid fluxes, with the largest effect for silt loam soils. Contrary to our expectations, this had no effect on the mineralization of added plant material. In summary, capillary rise succeeded to reach topsoil at groundwater depths to –165 cm for all textures, but no differences were found in mineralization of added plant material when groundwater increased by 50 cm. Field studies with native soil C instead of a plant-biomass substrate are ongoing.

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.

To cope with climate change, the use of water-saving irrigation practices in rice production is increasingly promoted, instead of the traditional practice of continuous flooding. Yet, it is unclear how the periodic introduction of oxygen during drying events impacts some fundamental biogeochemical processes in paddy soils because of the difficulty to quantify them. This knowledge gap actually complicates the development of mechanistic biogeochemical models, which are key for the prediction of paddy soil nutrient dynamics over a range of edaphic, agronomic and climatic settings. However, stable isotopes may help to shed some light on dynamics of important soil nutrients, i.e., by exploiting their natural or artificial (by labelling) variations over different biogeochemical transformations. In an effort to improve their implementation in soil models, we experimentally investigated the effect of water-saving irrigation management on three prominent but understudied carbon and nitrogen transformations in paddy soils from Italy and Bangladesh by means of stable isotopes. In particular, we assessed the oxidation of methane (CH4) in situ, the decomposition of soil organic carbon (SOC) after the incorporation of organic residues, and the fixation and defixation of ammonium (NH4+) in clay interlayers, always as a function of irrigation management. As it turns out, the adoption of water-saving irrigation practices had the potential to relatively stimulate in situ CH4 oxidation, in spite of some methodological difficulties, which implies that the commonly observed lower CH4 emissions in paddy fields under water-saving irrigation probably do not only stem from lower CH4 production rates under less anaerobic conditions, but also from this mitigation mechanism. Secondly, the stimulation of native SOC dissolution after the incorporation of organic residues was restricted under water-saving irrigation, and redox-active iron oxides with sorbed SOC potentially play a role in this. However, this effect on SOC dissolution was not proportionally reflected in SOC-derived emissions of CH4 and carbon dioxide (CO2), so that irrigation management probably does not influence these emissions and the long-term SOC balance through the proposed mechanism. Thirdly, the effect of irrigation management on abiotic fixation and defixation dynamics of NH4+ into or from interlayers of clay minerals was rather limited, but these processes – which determine the bioavailability of NH4+ in paddy soil – were instead predominantly dictated by an equilibrium between bioavailable and clay-fixed NH4+ phases. Overall, with a well-considered experimental setup in which untargeted isotope contrasts can be quantified and the appropriate pools sampled, stable isotopes are apparently a powerful tool to quantify gross nutrient transformations in paddy soil.

The drumbeat to reach net-zero emissions by 2050 and foster a European circular economy has arguably thrust anaerobic digestion front and centre due to its ability to produce renewable energy and fertilisers from recycled nutrients in the form of digestate. In this context, it is of paramount importance to better understand the fate of the nitrogen (N) and carbon (C) from digestate when applied to soil. In this study, the C and N mineralisation rates of five digestates from various feedstocks, undigested pig slurry and compost were studied in back-to-back incubation experiments in which predetermined amounts of these products were applied to soil. The treated soil mixtures were placed in PVC tubes, packed to a bulk density of 1.4 g cm-3, and their water content adjusted to 50% water-filled pore space. For soil incubations aimed at assessing N mineralisation patterns, destructive sampling was carried out every 20 days (over 4 months), during which ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) were measured. For C soil incubations aimed at monitoring organic carbon (Corg) mineralisation, each PVC tube was placed inside an airtight glass jar with an NaOH 0.5 M vial to trap CO2 from the treated soils. The treatments were incubated over 149 days, during which time the NaOH traps were periodically removed (17 sampling moments) and back titrated with 0.5 M HCl to measure the evolved CO2. Table 1 summarises the main results of the C and N incubations. Net N mineralisation (Nmin) was determined by subtracting the mineral N content of each treatment on day 0 from the mineral N at all subsequent measurements and expressed as percentage of applied organic N (Norg). As shown in Table 1, a positive Nmin (%Norg added) was observed for all products at the end of the experiment (day 127). Regarding C incubations, the amount of mineralised C (Cmin) from the treatments was calculated by subtracting the cumulative CO2 evolved from the untreated soil and was expressed as a percentage of added Corg from each product. A second-order kinetic model was fitted to the Cmin pattern of each of the tested products (Sleutel et al.,2005), from which the humification coefficient (Hc) of each treatment, defined as the amount of C remaining after one year, could be extrapolated. Results show that the Hc ranged from 50 to 81% for raw digestates (Table 1). The initial TC:TN ratio of the products was significantly negatively correlated (p < 0.01) to the amount of Cmin on the last day of C incubations. Also on the final day of N incubations, Nmin (p < 0.05) was significantly positively correlated with the initial NH4+-N:TN ratio of the products and negatively correlated with TC:TN (p < 0.05), as highlighted previously (Tambone & Adani, 2017). Thus, the TC:TN ratio of products proved a reliable predictor of C and N mineralisation rates in the incubated soils. Moreover, the Nmin rates (%Norg) of the studied digestates on the last day of the experiment ranged from 20 to 39% (Table 1). In other words, this suggests that anywhere between 60 to 80% of Norg was still present in the fertilised soil treatments after 127 days. Therefore, more attention might be given to the Norg fraction that still remains after application. In this respect, field trials would be warranted to assess the long-term fate of mineralised N when digestates are applied to a plant-soil system. Under this scenario, the synchrony, or possible asynchrony, between the mineralisation rates of the remaining Norg pools from the digestates and plant N uptake could be better monitored. Also of note, while digestates all exhibited an initial N immobilisation phase (not shown), the dynamics were very different from one product to the other, with some showcasing a comparatively faster remineralisation (from day 20 onwards) than others (from day 40 onwards). Thus, an understanding of the immobilisation/remineralisation pattern of a specific digestate could constitute a useful tool, among others, to reach an informed decision in the field regarding appropriate timing of N fertilisation, thereby reducing potentially counterproductive effects on crop growth. The amounts of Hc after one year, from the C incubation experiment, ranged from 50 to 81% for the digestates (Table 1). This result suggests that, without reaching thelevels of compost (99%), these products had an equally interesting C sequestration potential in the incubated soil, aside from their fertilising value.

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