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Temporal nitrogen (N) availability in fertilized rice paddies is the result of a balance of processes, mainly the gross rates of N mineralization, microbial and abiotic immobilization and N losses. Water and crop residue management practices often confound these established relationships making N the most difficult nutrient to manage in rice cropping systems. To investigate and quantify the interactive effects of soil redox conditions and straw incorporation on temporal fertilizer-N availability we treated a paddy soil with enriched ammonium-15 N and incubated for 160 days under flooded or non-flooded conditions, with or without the addition of rice straw. Changes in total available N as well as available and immobilized fertilizer-derived N (FDN) during incubation were evaluated. Under both oxic and waterlogged soils about 45-53% of applied N was rapidly immobilized. Whereas in the former most of this FDN was released contributing to the available N pool, flooded soils experienced significant losses from the soil/water system (≈ 67% of applied N). Addition of rice straw enhanced N immobilization, particularly under flooded conditions, that also contributed to limiting losses. Moreover, turnover of this labile organic matter pool supplied significant amounts of available N towards the later stages of the incubation, partly compensating for the immobilization of fertilizer-N.

Aims The response of rhizosphere and bulk soil indigenous microbial communities focusing on nitrifiers was evaluated after the application of different biological control agents (BCAs; Bacillus, Trichoderma, Pseudomonas) and compost in controlling lettuce Fusarium wilt. Methods and Results Experiments were conducted ‘in situ’ over two lettuce cropping seasons. Total fungal, bacterial and archaeal populations and the nitrifiers were analysed using quantitative polymerase chain reaction method. The pathogen, Fusarium oxysporum forma specialis lactucae (FOL), Bacillus, Trichoderma and Pseudomonas and three antifungal genes (chiA, 2,4‐diacetylphloroglucinol ‐ phlD and HCN synthase ‐ hcnAB genes) were also assessed. Quantitative data were corroborated with disease severity (DS), potential nitrification activity and soil chemical parameters. The application of BCAs and compost resulted in the disease reduction by as much as 69%, confirmed by significant negative correlations between Bacillus subtilis, Trichoderma and Pseudomonas sp. abundances and DS. The FOL presence in the untreated control resulted in the nitrifiers niche differentiation. Conclusions The used treatments were efficient against Fusarium wilt and did not influence negatively the nontarget microbial communities. Significance and Impact of the Study The use of BCAs and compost appears as an effective and safe strategy to implement sustainable agricultural practices.

Research paperAn assessment of the modulation of the population dynamics of pathogenic Fusarium oxysporum f. sp. lycopersici in the tomato rhizosphere by means of the application of Bacillus subtilis QST713, Trichoderma sp. TW2 and two other composts

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