Search

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.

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.

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.

Crop residues of field vegetables are often characterized by large amounts of biomass with a high N-content. Despite low soil temperatures during autumn, high rates of N mineralization and nitrification still occur causing important N-losses through leaching1. Crop residues pose a possible threat to maintaining water quality objectives, while at the same time being a vital link in closing the nutrient and organic matter cycle of soils. Appropriate and sustainable management is needed to obtain the full potential of crop residues. In this research some novel strategies for vegetable crop residue management are being investigated. The outcome of this work will help in defining the best practices in vegetable production in order to better meet the requirements set by the nitrate directive, whilst making best use of a valuable organic material. Vegetable crop residues constitute an important link in the soil nutrient and organic carbon cycle and aid in maintaining soil quality and fertility. However during autumn large amounts of N are released by vegetable crop residues causing important N-losses through leaching. Appropriate and sustainable management is needed to obtain the full potential of crop residues while meeting water quality objectives. In this research the potential of removal and valorisation of vegetable crop residues by means of ensilage or composting was evaluated.

Due to an increasing demand of leaf vegetables, and hence their economic importance in the tropics, it is very common that excessive fertilizer N rates are applied to vegetable gardens and fields to attain high yield. This calls for more information on their nutrient requirements. In this study, we designed experiments to explore the effect of organic N levels on the yield and agronomic N use efficiency (ANE) by chinese cabbage (Brassica rapa) and amaranthus (Amaranthus cruentus). The experimental design was a randomized complete block design consisting of chinese cabbage (CC) and amaranthus (AM) with three replicates. Chicken manure (CHM) and cattle manure (CAM) were the source of N. The treatments were 0, 200, 300 kg N ha(-1) and 0, 170, 250 kg N ha(-1) for CC and AM, respectively. Chicken manure resulted in increased fresh and dry matter yield of CC and AM compared to CAM. All treatments at first harvest induced higher marketable yield of vegetables than controls except with low levels of CAM N. At second harvest, only 300 kg CHM N ha(-1) resulted in significantly (P

Soil fauna are estimated to contribute to approximately 30 % of nitrogen mineralization (Verhoef ∧ Brussaard, 1990). Soil nematodes are important contributors to this process through their key trophic positions as microbial grazers. Quantification of this contribution has mostly relied on theoretical food web analyses (Hunt et al., 1987) or laboratory incubations with simplified and artificially constructed ecosystems (Ferris et al., 1998). Incubations are often performed on homogenized soil, though soil biota is known to be responsive to physical disturbance. Furthermore, sterilization typically relies on methods disruptive of soil structure (e.g. autoclaving, freezing). The aim of this experiment was to quantify the contribution of nematodes to nitrogen mineralization during incubation. Intact cores with a representative pore structure and entire nematode populations instead of single species were used. Gamma irradiation was selected as a sterilization method to remove only soil fauna, leaving the microflora and soil structure largely intact (McNamara et al., 2003).

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.

Rhizodeposits consist of over 200 organic compounds, mainly low-molecular-weight organic substances (LMWOS) such as amino acids (AA), carbohydrates (CH) and carboxylic acids (CA), lipids and phenols. Those LMWOS influence nutrient turnover, particularly N turnover. However, the exact influence of these organic substances on nitrogen mineralization is yet unknown. Therefore, the stimulatory effects of low molecular weight organic substances on nitrogen mineralization in the rhizosphere of a silty loam soil from Bangladesh were studied. A two week incubation experiment was set up consisting of four different treatments (in three replicates) with addition of artificial root exudates, namely (1) low molecular weight organic acids (a mixture of malic, tartaric, succinic, citric and lactic acid at a C-ratio of 80:9:5:4:2), (2) carbohydrates (glucose), (3) a mixture of amino acids (a mixture of histidine, valine, alanine and glycine at a C-ratio of 43:35:9:14) along with a (4) control. In this experiment a novel method was used for the simulation of rhizospheric environment for the localized application of the exudates and sampling of an artificial rhizosphere. For this purpose small perforated plastic straws were inserted into the soil columns and the soils in these straws, representing the rhizosphere was sampled separately. After 7 days of pre incubation at 25°C inside the incubator under saturated condition, root exudates were artificially injected inside the perforated straws at a rate of 103, 93 and 10 µg C g-1 soil week-1 for low molecular weight organic acids, carbohydrates and amino acids, respectively. N mineralization was assessed by extracting and measuring NO3- and NH4+ concentrations at regular time intervals. All LMWOS stimulated N mineralization in soil, but to a different degree. The highest N mineralization rate was obtained from amino acid amendment (5.69 mg N kg-1 soil day-1) followed by organic acids and carbohydrate (4.30 mg N kg-1 soil day-1) and the lowest from the control (3.31 g N kg-1 soil day-1). Our findings show that locally high concentrations of (artificial) root exudates indeed stimulate N mineralization.

Soil organic matter (SOM) quality and iits decomposibility is influenced by cropping pattern. However, little is known on this matter, particularly so far rice monoculture or comination of vegetables with rice culture. Therefore, four pairs of fields having vegetable-rice and rice-rice cropping pattern were selected from four different locations in Bangladesh, covering floodplain and terrace soils. Soils were first physically fractionated into particulate organic matter (POM) and silt and clay sized OM. The silt and clay sized OM was further chemically fractionated by oxidation with 6% NaOCl to isolate an oxidation-resistant OM fraction, followed by extraction of mineral bound OM with 10% HF (HF-res OM). The results show that there is a small increase in POM in vegetable compared with rice soils. Among the chemical treatments, NaOCl oxidized the largest amount of OC and N from the silt and clay fractions of vegetable soils. The silt and clay N in vegetable soil was found to be more susceptible to NaOCl treatment compared with OC. Both the HF-extracted (HF-ex) and HF-res OM fractions were found to be lower in vegetable soils compared with rice soils. Results from both physical and chemical fraction of SOM suggest that SOM accumulated in vegetable-rice cropping pattern are more labile than the solely rice based cropping pattern and prone to decompose quickly in any change of land use.