Organic wastes as alternative sources of phosphorus for plant nutrition in a calcareous soil.

Recycled organic wastes (OW) can be a valuable P source; however, their P-fertilising capacity is still poorly known. In this study, we selected three anaerobic digestates [wastewater sludge (D1), winery sludge (D2), and bovine-slurry/energy crops (BD)] and two animal effluents [bovine slurry (BS) and swine slurry (SS)] to test their P-release and P-fertilising capacities via sequential chemical extraction (SCE), X-ray diffraction (XRD), and 31P-nuclear magnetic resonance (31P NMR). Subsequently, the three digestates (30 mg P kg-1 of soil) were compared for the release of Olsen-P during a soil incubation and for plant-P apparent recovery (ARF) in a pot experiment using ryegrass (112 days) in a soil with poorly available-P (Olsen-P < 5 mg kg-1), under a non-limiting N environment. The amount of labile-P (H2O + NaHCO3), as determined from SCE, related well to the Olsen-P following OW addition to the soil. It was shown via 31P NMR spectroscopy that orthophosphate was the leading P-form in highly P-releasing OW. The amount of labile-P, however, was affected by soil adsorption, thereby reducing plant-P uptake. The plant-P ARF (%) showed that the recycled P-sources were clustered in highly (BD and SS: ≈20%), intermediately (D1 and BS: ≈15%), and poorly performing OWs (D2: ≈10%) vs. chemical P-source (P-chem: 20%). Therefore, only BD and SS were effective alternatives to P-chem; however, the other OW can be efficient P-sources in soils with higher Olsen-P. Thus, crop fertilisation can be tailored on a P-basis by SCE as a function of soil adsorption capacity and on an N-basis according to the demand.

Phosphorous extractability and ryegrass availability from bio-waste composts in a calcareous soil.

In this work four stable bio-waste composts (C1, C2, C3, C4) were selected on the basis of their increasing water soluble P (H2O-P). The P speciation was assessed via sequential chemical extraction (SCE) on the same products. Moreover, the plant-available P was assessed via apparent recovery fraction approach (ARF) in a pot test on ryegrass over 21 weeks at 15 mg P kg-1 of soil. An inorganic P source (P-chem) was added as a reference at the same P rate in addition to a non-fertilized control (Control). SCE showed that the sparingly soluble P (HCl-P) was the most important fraction in all composts: C1 (HCl 65% > NaHCO3 17% = NaOH 17% > H2O 1%); C2: (HCl 51% > NaOH 23% > NaHCO3 18% > H2O 7%); C3: (HCl 58% > NaOH 21% > NaHCO3 12% > H2O 9%); C4: (HCl 39% > NaOH 23% > NaHCO3 22% > H2O 16%). The plant test showed that the different treatments had a different ARF (%) at the first harvest: P-chem (14.7)> C4 (14.4)> C3 (14.1)> C2 (3.4)>C1 (3.1), compared to the cumulated ARF (%) of the six harvests: C4 (50.1)> C3 (35.0)> C1 (21.1)> C2 (18.3)> P-chem (17.4). Data showed a good correlation of H2O-P vs. plant ARF at the first harvest and a good correlation of labile P (H2O-P + NaHCO3-P) vs. total plant ARF over 21 weeks. The free and labile P forms from SCE can be a valuable tool in the assessment of fast and middle term plant-available P from stable bio-waste composts in calcareous soils.

Conversion of C5 into C6 cyclic species through the formation of C7 intermediates.

We have recently proposed that the addition of C2H2 to the cyclopentadienyl radical can lead to the rapid formation of the cycloheptatrienyl radical and, in succession, of the indenyl radical. These reactions represent an interesting and unexplored route for the enlargement of gas-phase cyclic species. In this work we report ab initio calculations we performed with the aim of investigating in detail the gas-phase reactivity of cycloheptatrienyl and indenyl radicals. We found that the reaction of the cycloheptatrienyl radical with atomic hydrogen can lead to its fast conversion into the more stable benzyl radical. This reaction pathway involves the intermediate formation of heptatriene, norcaradiene, and toluene. Successively we investigated whether this reaction mechanism can be extended to polycyclic aromatic hydrocarbons (PAHs). For this purpose we studied the reaction of C2H2 with the indenyl radical, which can be considered as a superior homologue of the cyclopentadienyl radical. This reaction proceeds through a pathway similar to that proposed for C5H5 but with a reaction rate about an order of magnitude smaller. The present calculations extend thus the previously proposed C5-C7-C9 mechanism to bicyclic PAH and suggest a fast route for the conversion of C5 into C6 cyclic radicals, mediated by the formation of C7 cyclic species.