Effect of long-term no-tillage and nitrogen fertilization on phosphorus distribution in bulk soil and aggregates of a Vertisol

Much remains unknown regarding the behaviour of P in cropping soils. We have examined a low-input long-tern cropping system not receiving P fertilizers in subtropical Queensland (Australia) in order to determine how, after 50 y, P concentration and distribution in bulk soils and in soil aggregates is affected by tillage practices (no tillage [NT] and conventional tillage [CT]) and by N fertilization. For the bulk soil, neither the tillage practice nor N fertilization altered concentrations of total P, inorganic P (P-i), or organic P (P-o) despite soil organic C concentrations differing significantly depending upon both tillage and N fertilization. Although bulk concentrations of P did not differ significantly, we observed significant changes in NaHCO3-P-i, M HCl-P-i, NaOH-P-i, NaOH-P-o determined using sequential extraction, seemingly due to changes in pH associated with N fertilization. We also used sequential fractionation to examine the forms of P in the various water-stable aggregate sizes, finding that the tillage practices and N fertilization did not cause significant changes in either the distribution of the various aggregate sizes nor did it cause marked changes in the forms of P in these various aggregates. Thus, our data illustrate that even though the long-term usage of NT in low-input soils may cause a comparative increase in TOC, there was not a concomitant increase in P-o, with this being important for soil fertility and functioning. This information assists in understanding how management practices alter P-o, behaviour in long-term, low-input subtropical cropping soils, as is required to manage these soils more appropriately.