Long-term addition of organic fertilizers has little effect on soil organic phosphorus as characterized by 31P NMR spectroscopy and enzyme additions

Abstract

Organic fertilizers are important in agroecosystems, but the fate of specific forms of added phosphorus (P) in the long-term is not well understood. We studied the effects of 62years of application of three organic fertilizers (dairy manure, compost and dried sewage sludge) on organic P forms in the topsoil as characterized by solution 31P NMR spectroscopy and enzyme additions in a field experiment on a slightly acidic Luvisol in Switzerland. Changes in inorganic P extracted with NaOH–EDTA were related to the P-input–output balances of the treatments, which ranged between −13kgPha−1yr−1 in the non-fertilized control and +93kgPha−1yr−1 with sewage sludge application. The majority of extractable P in the applied organic fertilizers (68–91%) was in the form of orthophosphate. The fertilizers differed with respect to specific organic P compounds identified by 31P NMR, with myo-inositol hexakisphosphate (myo-IP6) found in compost and dried sewage sludge, but absent from dairy manure. A small amount of scyllo-inositol hexakisphosphate (scyllo-IP6) was additionally found in compost. However, the specific forms of organic P in topsoil extracts remained unaffected, with similar concentrations of myo-IP6, scyllo-IP6, pyrophosphate and degradation products of phospholipids and nucleic acids present in all treatments. Using enzyme additions, a significant accumulation of non-hydrolyzed P in soil amended with dried sewage sludge was observed, but it accounted for only 9% of the total accumulation of P in this treatment compared to the treatment with manure addition which had an even input–output P balance. The similar composition of organic P in all treatments suggests that specific organic P compounds added with the three organic fertilizers were completely transformed in the topsoil or lost from it.