Response of phosphorus dynamics to sewage sludge application in an agroecosystem in northern France

Sewage sludge is a phosphorus (P) source alternative to P fertiliser derived from rock phosphate, but its impact on soil processes driving P cycling in agroecosystems requires further study. In order to optimise the use of sludge for sustainable P fertilisation, we need to elucidate the drivers of P dynamics. The present study aims at determining how different sludges (heated sludge, HS and composted sludge, CS) affect soil P pools and dynamics. A field experiment was established and soil was amended either with sludge or with inorganic P (triple superphosphate, TSP). Soil samples were collected five times during a vegetation period, and analysed for Hedley P fractions, microbial P and phosphatase activity. Phosphorus dynamics in soil was strongly influenced by P concentrations in sludge. About one year after application, sludge with the highest P concentration (HS) was as effective as TSP to improve soil P availability. The P source of TSP was immediately available for plant uptake, but the high phosphatase activity of the HS treatment evidenced that soil microorganisms released phosphatases which can hydrolyse HS-derived organic P compounds. In addition, the high content of microbial P in the HS treatment suggests that soil microorganisms assimilate P into their own biomass. By contrast, sludge with the lowest P concentration (CS) enriched primarily the weakly-soluble soil P fractions, resulting in lower P availability compared with that in the TSP treatment. Our findings suggest that both high P concentration and slow, but continuous microbial breakdown of organic P substrates derived from HS allow using this resource as an important source for plant mineral nutrition. This study stresses the need to both characterise P concentrations and P forms in sludge, prior to their application in the field.