Application of phosphorus fertilizer can lead to changes in soil test P and increase both labile and non-labile phosphorus pools. Sequential fractionation showed that the labile (H2O-P and NaHCO3-P) fractions significantly (P < 0.05) increased with the addition of phosphorus fertilizer across all sites. Significant increase was observed in the non-labile (NaOH-P, HCl-P and Residual-P) fractions during the accumulation phase. During the depletion phase, when no further P was added, the accumulated P in the labile P fractions declined but not to the original level. The rates of P application, soil properties, soil test P methods used and time effect all had significant effects on soil test P changes in the extraction experiment. Mehlich-3 extracted the greatest amount of P from the two depths and Olsen-P was intermediate while the smallest amount was extracted with water. The pattern of the rate of change in extractable P with P addition for the 0 – 7.5 cm depth was not site dependent as the results obtained at the different sites were similar. / October 2016

A low-loaded constructed wetland was investigated with respect to phosphorus retention. Since the main long-term phosphorus retention mechanism is sedimentation and sediment accretion, the study focused on these processes. The purpose of the study was 1) to investigate how the calculated value of phosphorus retention (Pin – Pout), corresponded with the measured amount of phosphorus in the sediment, 2) to find out where in the wetland the phosphorus had accumulated, and in what form it was retained, and 3) to investigate the role of vegetation. The calculated value was 12 kg ha-1 and the measured value was 104 kg ha-1, which indicated the importance of internal phosphorus circulation, where plants probably take phosphorus from the underlying clay. Hence, vegetation could possibly increase the total phosphorus content in the wetland. The composition of phosphorus in the sediment was analysed using sequential fractionation. The dominating form of phosphorus in the sediment was iron-bound phosphorus (29 %). In total, 48 % of the phosphorus was stable, i.e. tightly bound in the sediment, and 35 % was relatively stable. The bioavailable fraction, which could cause eutrophication in downstream waters, was 17 % of the total phosphorus content, or 41 kg ha-1. The amount of total phosphorus was significantly higher near the inlet, compared to the outlet, which is explained by rapid sedimentation of particulate phosphorus entering the wetland. The phosphorus amount near the inlet represented 80 % of the total phosphorus load – which indicates the importance of internal circulation of phosphorus, both biological and geochemical.

<p> </p><div><table style="height: 340px;" border="0" cellspacing="0" cellpadding="0" width="639" align="left"><tbody><tr><td style="padding: 0cm 7.1pt;" height="289" align="left" valign="top"><div style="border: 1pt solid windowtext; padding: 1pt;"><p class="MsoNormal" style="border: medium none; padding: 0cm;"><span style="font-size: 10pt;" lang="EN-GB">A low-loaded constructed wetland was investigated with respect to phosphorus retention. Since the main long-term phosphorus retention mechanism is sedimentation and sediment accretion, the study focused on these processes. The purpose of the study was 1) to investigate how the calculated value of phosphorus retention (P_in – P_out), corresponded with the measured amount of phosphorus in the sediment, 2) to find out where in the wetland the phosphorus had accumulated, and in what form it was retained, and 3) to investigate the role of vegetation. The calculated value was 12 kg ha^-1 and the measured value was 104 kg ha^-1, which indicated the importance of internal phosphorus circulation, where plants probably take phosphorus from the underlying clay. Hence, vegetation could possibly increase the total phosphorus content in the wetland. The composition of phosphorus in the sediment was analysed using sequential fractionation. The dominating form of phosphorus in the sediment was iron-bound phosphorus (29 %). In total, 48 % of the phosphorus was stable, i.e. tightly bound in the sediment, and 35 % was relatively stable. The bioavailable fraction, which could cause eutrophication in downstream waters, was 17 % of the total phosphorus content, or 41 kg ha^-1. The amount of total phosphorus was significantly higher near the inlet, compared to the outlet, which is explained by rapid sedimentation of particulate phosphorus entering the wetland. The phosphorus amount near the inlet represented 80 % of the total phosphorus load – which indicates the importance of internal circulation of phosphorus, both biological and geochemical. </span></p></div></td></tr></tbody></table></div>