Summary Phosphate‐solubilising microorganisms (PSM) are often reported to have positive effects on crop productivity through enhanced phosphorus (P) nutrition. Our aim was to evaluate the validity of this concept. Most studies that report ‘positive effects’ of PSM on plant growth have been conducted under controlled conditions, whereas field experiments more frequently fail to demonstrate a positive response. Many studies have indicated that the mechanisms seen in vitro do not translate into improved crop P nutrition in complex soil–plant systems. Furthermore, associated mechanisms are often not rigorously assessed. We suggest that PSM do not mobilise sufficient P to change the crops’ nutritional environment under field conditions. The current concept, in which PSM solubilise P ‘for the plant’ should thus be revised. Although PSM have the capacity to solubilise P to meet their own needs, it is the turnover of the microbial biomass that subsequently provides P to plants over a longer time. Therefore, the existing concept of PSM function is unlikely to deliver a reliable strategy for increasing crop P nutrition. A further mechanistic understanding is needed to determine how P mobilisation by PSM as a component of the whole soil community can be manipulated to become more effective for plant P nutrition.

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Background: Penicillium bilaii may enhance P availability to plants, since it has been shown to increase plant growth and P uptake. There is currently increasing interest in using microorganisms to promote P mobilisation from organic P sources. An investigation was conducted to determine the effects of P. bilaii on P uptake and growth of wheat in the presence and absence of sewage sludge. Two soils differing in P contents and pH were used, as it was hypothesised that these affect the efficiency of P mobilisation.Methods: A pot experiment, in which wheat was grown for 35 days in a moderately acidic soil of low P status and a calcareous soil of moderate P status, was conducted. A full factorial design was used with two non-sterilised soils, three amendments [control, sewage sludge and triple superphosphate (TSP)] and two P. bilaii treatments (with/ without). Shoot and root length, biomass and nutrient contents were analysed in plant, whereas soil samples were analysed for water-extractable P and soil pH. Results:The shoot length and root biomass of wheat were significantly higher when sewage sludge was applied in combination with P. bilaii seed inoculation, in the moderately acidic soil. In contrast, shoot length and biomass and root biomass were higher with P. bilaii compared to the control, but no synergistic effects of P. bilaii and the organic P source were observed in the calcareous soil. A systematic, but not significant increase in total P uptake was found for all treatments inoculated with P. bilaii and for both soils, with the control of the low fertility moderately acidic soil being a notable exception. Conclusions:Sewage sludge was seen to be an efficient P source, on par with TSP in the moderately acidic soil. In the calcareous soil, the P. bilaii treatments without added P fertilisers had the greatest effect, with both root and shoot biomass increasing significantly.

Olive groves are undergoing a marked change in the way that inter-row land is managed. The current regulation and recommendation encourages the implementation of plant cover, mainly to improve soil fertility and reduce erosion. However, there is no quantitative information on the dynamics and pools of soil organic carbon (SOC) fractions of different protection levels of the plant-residue-derived organic carbon (OC). This study was conducted to provide a range of annual OC inputs in commercial olive oil groves under natural plant cover, to assess the influence of the annual application of aboveground plant cover residues on unprotected and physically, chemically and biochemically protected SOC. In addition, we tested the carbon saturation hypothesis under plant cover. Ten olive oil orchards under plant cover management (PC), together with five comparable bare soil olive oil orchards (NPC) were selected and annual aboveground natural plant residues and SOC pools were sampled and quantified. Annual aboveground plant cover biomass and OC production in PC olive orchards averaged 1.48 t dry-weight (DW) ha-1 and 0.56 t C DW ha-1, respectively with a great variability among sites (coefficient of variation of about 100 %). SOC concentration in PC orchards was, on average, 2.8 (0 -5 cm soil) and 2.0 (5 -15 cm) times higher than in bare soils of NPC, and the pool of protected SOC in the top 15 cm was 2.1 times higher in the PC (17.9 mg C g-ndard deviation) compared to NPC (8.5 mg C golive orchards. Linear or saturation type relationships between each SOC fraction and total SOC content for the range of SOC of the commercial olive oil orchards were statistically indistinguishable, and thus linear models to predict SOC accumulation due to plant cover in olive orchards are suitable, at least for the studied range of SOC. Overall, at regional scale where olive oil groves represent a very high proportion of the agricultural land, the use of plant cover appears to be a promising practice that promotes protection of the SOC, thus improving SOC sequestration.

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