Purpose <p>The purpose of the study was to clarify the relevance of colloidal phosphorus (P_coll) to total P (TP) leaching and its link to dissolved organic P (DOP) leaching in forest topsoils under field conditions. Effects of heavy rainfall after a drought and the influence of P and N inputs to forest soils on P_coll leaching from the topsoil were assessed to better understand the causes of P_coll leaching.</p> Methods <p>We carried out an irrigation experiment with zero-tension lysimeters collecting leachate from the organic forest floor layers and mineral topsoils of a sandy, low-P and a loamy, high-P soil.</p> Results <p>The contribution of P_coll to TP concentrations was on average around 10±14% irrespective of sampling date, season and soil. The highest P_coll concentrations were recorded upon irrigation after a summer dry period with the increase in concentrations being most pronounced for the 20–70&#xa0;nm size fraction with low median C:P ratios (29 to 31) typical for microbial residues. Significant relationships between P_coll and DOP concentrations suggested that 27% (low P soil) and 17% (high P soil) of DOP were of colloidal nature. Independent of the soil P stock, the addition of P and P + N affected neither P_coll concentrations nor their share of TP significantly.</p> Conclusion <p>While P_coll hardly affected the overall estimates of beech forest P budgets, it provides important further insights into processes governing the mobilization, transport, and cycling of P in forest ecosystems.</p>