Assessment of phosphorus fertilizer practices in altered wetland soils using uncertainty analysis

Intensive P fertilizer application is widely practised in East Mediterranean altered wetland soils, even when results from the Olsen-P extract indicate sufficiency (>10mg P/kg). To reduce or even eliminate this practice we conducted a large field study to test the feasibility of a major reduction in the use of P fertilizers. A total of 90 sample sites were evenly spaced with respect to their parent material (deep peat, shallow peat, marl). Soils were analysed for Olsen-P, bulk density, pH, electrical conductivity, and nitrate, chloride and sulphate in saturated paste extracts. We determined variograms and estimated the spatial structure of Olsen-P data by ordinary kriging. We then used a sequential Gaussian model to produce 100 alternative simulations to measure the joint spatial uncertainty of P concentrations in the study area (ca.1500ha). Finally, we post-processed the simulation results to compute the probability of exceeding the threshold Olsen-P value of 10mg/kg. The simulation results indicate a high probability of exceeding the 10mg/kg threshold in most peat soils supporting an earlier recommendation to reduce or even eliminate P fertilizer application in these fields. On the other hand, a very low probability of exceeding this threshold was observed in most marl soils justifying continued fertilizer application. However, the added P is likely to affect adjacent waterways and increase potential downstream eutrophication due to high conductance and flow continuity between the marl soils and adjacent waterways.