[Phosphorus characteristics in leachate from soils irrigated with livestock wastewater]

Phosphorus fractions could enter water body from surface runoff and leachate due to excessive irrigation of centralized farm wastewater. Organic P is more mobile than inorganic P in the soil profile and represents a significant proportion of P present in leachate from irrigated farm soils. A set of parallel experiments were conducted to compare the characteristics of organic phosphorus in leachate. The experiment was established in a complete randomized block design with nine replicates. The plots received different combinations of P fertilizer and different rates of pig slurry, i. e. 100, 200,300, 400 t x (hm2 x a)(-1), accordingly, the phosphorus was added to the plots was 6.2, 12.4, 19.2, 24.8 kg x (hm2 x a)(-1), respectively. Leachate was collected over a year period and analyzed for different P fractions. Physico-chemical fractionation of P in leachate indicated that the majority of the P loss from the irrigated soil occurred in unreactive particulate (77% - 90%) P forms. 31P nuclear magnetic resonance analysis of eight leachate samples indicated that unreactive P was mainly comprised of monoester and diester forms of organic P. The presence of phosphomonoesterase and phosphodiesterase activity in leachate resulted in hydrolysis of 9% - 29% of total unreactive P (TUP), indicating that some of the monoesters and diesters can be eventually hydrolyzed into inorganic P forms during P transport. To the treatment P45 + F200, enzyme hydrolysis showed that 33% of the TUP was present as labile monoester P (LMP), followed by 17% as inositol hexakisphosphate (IHP) and 9% as diesters (phospholipids and nucleic acids). The results suggest that LMP, IHP and diesters are an important component of organic P leaching from the irrigated soil. The identification of these organic P forms will improve our understanding of the mechanisms responsible for their release from soils, so that specific mitigation strategies can be implemented at the P source.