Phosphorus characterization and plant availability in soil profiles after long-term urban waste application

Recycling of waste products to agricultural fields is important for sustainable crop production, particularly for managing phosphorus (P). How different forms of waste affect the accumulation, transformation and plant availability of P in soil is therefore of major interest. A long-term experimental field that assesses conventional fertilizers with that of urban wastes was analyzed for P accumulation, P speciation characterized using XANES and NMR spectroscopy, as well as plant P availability with DGT in top and sub horizons. Treatments included unfertilized, NPK, manure, sludge and compost amended soils. We found that long-term sludge application showed higher soil P availability in the soil relative to P input and total P content than in manure and compost amended soils. In the sludge amended soil, P was found to be more associated to Fe as opposed to Ca in the compost amended soil. In the manure amended soil, unfertilized and NPK soils P was mainly associated to organic P. Based on characterization of the waste products using XANES spectroscopy our results indicate that the P species in the sludge amended soil do not directly relate to the sludge material applied to the soil, but rather to the processes occurring in the soil after application. A larger association of P to Fe was also found in sub horizons of the sludge amended soil. We did not detect substantial differences in classes of organic P between the sludge and compost amended soils by the P-31 NMR spectra. Our results indicate that sewage sludge is a useful fertilizer, as P applied with sludge remains highly plant available in these soils after long-term application, and it proves more plant available in the long-term than manure and compost fertilizers.