Long-term phosphorus supply in agricultural soils : size and dynamics of fast- and slow-desorbing phosphorus pools

Sammanfattning: To improve phosphorus (P) fertiliser management, a better understanding of inorganic P exchange between the soil solution and solid phase is needed. This thesis examined the dynamics of this exchange, distinguishing between pools of P that are fast- and slowdesorbing, and compared methods for quantification of these pools. This was done using soil and data from six locations in the Swedish long-term soil fertility trials. Treatments with four levels of P addition, from zero P to replacement + 30 kg P ha−1 , were sampled. Isotopic exchange kinetics (IEK) were used to study the dynamics and size of the isotopically exchangeable P pools. The size of the isotopically exchangeable pool at t=1 min (E1 min) was affected by exchangeable Ca2+, while that of the E1 day − E1 min and E3 months − E1 day pools was affected by soil pH and Al-ox + Fe-ox. The change in P-Olsen and P-ox over time was also related to exchangeable Ca2+. Phosphorus desorption kinetics were tested with iron(hydr)oxide-coated papers as an ‘unlimited’ sink. The P desorption values were fitted to the Lookman two-compartment model, giving information about the size and desorption rate of the fast- and slow-desorbing P pools (Q1 and Q2). The desorption experiments was shown to deliver similar information as the IEK, even if they assess different mechanisms.Two extraction methods (AL and Olsen) were tested for their ability to quantify the fast-desorbing P pool, by incubation of soil with radioactive 33P before extraction. This revealed that AL extraction dissolves more stable P forms, which was further proved by the relationship between P-AL and Q1 (r2 =0.63) being weaker than for P-Olsen and Q1 (r2 =0.96).A positive P balance increased the amount of ‘P bound to Al and Fe’, and/or CaP, (according to P K-edge XANES), the desorption rate from the slow-desorbing P pool and the fraction of total P present in the fast-desorbing pool. It did not increase wheat, barley, or oat grain yield. With no P fertiliser, the extractable and exchangeable P pools decreased, but about half locations had significantly lower grain yield. This shows that the P contribution from the slow-desorbing P pool is important for plant P uptake, and that this pool can supply P for a long period. When replacement P was added, yields was not affected but P-AL and P-Olsen decreased, making ‘P in balance’ a possibly useful strategy to lower soil P levels without grain yield loss.