Effects of organic acid composition in the decomposed liquid of green manure crops on the activation level of AlPO₄ and FePO₄·2H₂O

Objectives The organic acid composition of decomposed green manures was studied to understand the different effects of green manures on improving soil phosphorus availability. The study aims to provide a reference for selecting suitable green manure crops for an efficient rotational system.

Methods Six types of green manure crops were selected for the experiment: milkvetch (Astragalus sinicus L.), radish (Raphanus sativus var. Longipinnatus), Orychophragmus violaceus, double-low rapeseed (Brassica napus L.), double-high rapeseed (Brassica napus L.), and mustard (Brassica juncea). The green manure plants were cut and loaded into centrifugal tubes, and 1 mL of soil extract solution was added and isolated before incubation for 45 days under natural shading conditions. The decomposed liquids of the green manure crops were collected by centrifuge to determine the contents of 10 organic acids (propane diacid, malic acid, citric acid, oxalic acid, lactic acid, tartaric acid, acetic acid, succinic acid, maleic acid and fumaric acid). A simulation experiment was conducted by eluting AlPO₄ (Al-P) and FePO₄·2H₂O (Fe-P) from the decomposed liquids, the water-soluble P contents in the dilution were then analyzed.

Results The total organic acid content in the decomposed solution of Orychophragmus violaceus was the highest, followed by radish, double-low rapeseed, mustard, double-high rapeseed, and milkvetch. The descending order of the 10 organic acids by concentration was tartaric acid, propane diacid, citric acid, succinic acid, lactic acid, malic acid, acetic acid, oxalic acid, maleic acid and fumaric acid in all decomposed liquids of the green manure crops. tartaric acid and propane diacid accounted for more than 15% of the total organic acid, while maleic acid and fumaric acid accounted for less than 1%. The decomposed liquid of radish had the highest capacity to activate insoluble P, corresponding to a theriotical activation of 0.6–1.2 kg insoluble P per 1000 kg of radish plant. This was followed by milkvetch (0.6–1.0 kg per 1000 kg) and mustard green manure (0.6–1.0 kg per 1000 kg), and Orychophragmus violaceus recorded the lowest activation capacity of 0.2–0.6 kg per 1000 kg. Correlation analysis showed that the activation level of Al-P was positively related to the content of citric, malic, and oxalic acids. The activation level of Fe-P was positively related to tartaric acid content.

Conclusions The proportion of citric, malic, and oxalic acids in the total organic acids of the green manure crops is closely related to the activation level of Al-P, and that of tartaric acid is related to the activation level of Fe-P. The decomposed liquid of radish has the highest tartaric acid and a relatively higher proportion of citric and succinic acid than other green manures. This implies that radish has the highest ability to activate Fe-P. Milkvetch and mustard rape have higher proportions of oxalic, citric and succinic acids in the total organic acids, indicating that they have a strong ability to activate Al-P. Therefore, radish, milkvetch, and mustard are recommended as green manures in rotational systems to increase the biological availability of Al-P and Fe-P in soils.