NITROGEN AND PHOSPHORUS TRANSPORT IN SOIL USING SIMULATED WATERLOGGED CONDITIONS

Abstract

The diffusive and convective transport of ammonium (NH4-N) and phosphate (PO4-P) was investigated under laboratory conditions using two soils which had been waterlogged for a period of 21 days. Ammonium and PO4-P diffusion studies were conducted using a krasnozem soil and the organic surface material from a Melaleuca wetland. Approximately 30 g of each material were placed into a 60 mL polypropylene tube, and mixed with 45 mL of either deionized water, di-ammonium phosphate or potassium nitrate solution. The concentrations of water-soluble NH4-N and PO4-P were determined on day 0 and day 21. The diffusive flux of these nutrients into or out of the soil materials over the 21 day period was calculated using Fick's First Law. Ammonium-N diffusive fluxes for the krasnozem ranged from 147 to 808 mg m−2 d−1, and from 7 to 621 mg m−2 d−1 for the wetland material.Waterlogging increased porewater NH4-N concentrations in the krasnozem, and this was attributed to displacement of resident NH4-N by iron (Fe) and manganese (Mn) solubilized under reducing conditions. In contrast, waterlogging decreased porewater NH4-N concentrations in the wetland material, and this was attributed to sorption by newly created exchange sites. Phosphate diffusive fluxes for the krasnozem ranged from 3 to 1,050 mg m−2 d−1, and from 1 to 1,175 mg m−2 d−1 for the wetland material. Waterlogging increased porewater PO4-P concentrations in both soils, which was attributed to loss of sorption sites with solubilization of Fe and Mn hydous oxides under reducing conditions. Column (10 cm i.d.× 25 cm long) leaching studies were undertaken using samples of wetland material that had been previously waterlogged for 21 days, and samples which had not been waterlogged. Retention of surface-applied NH4-N and PO4-P was greater for waterlogged samples compared with retention by non-waterlogged material. Increased NH4-N retention was attributed to an increase in cation exchange capacity generated under waterlogged conditions, while increased PO4-P retention may be due to sorption and/ or precipitation reactions with solubilised Fe. Much of the sorbed NH4-N was readily desorbed by 0.01 M CaCl2, suggesting this cation was primarily retained by weak electrostatic forces, while the limited desorption of PO4-P suggests this anion was retained by much stronger forces. Results from this study demonstrate the importance of fundamental soil properties such as cation exchange capacity and competition for sorption sites on NH4-N and PO4-P transport in waterlogged sediments.