Occasional extreme rainfall events significantly influence colloid dynamics and nutrient cycling in the hyper-arid Atacama Desert. We hypothesized that the response of water-dispersible soil colloids (WDCs, <300 nm) and their associated phosphorus (WDC-P) dynamics to water addition is controlled by soil aggregation/disintegration and transport processes influenced by soil age. We conducted an irrigation experiment (44.8 mm h(-1) for 30 min) on an alluvial fan system at 1480 m a.s.L. in the Paposo that is characterized by two sections of different age. Soil aggregates and WDCs were analyzed before and after irrigation event using dry sieving and asymmetric flow field-flow fractionation (AF4). Although total WDCs and sub-fractions did not show statistically significant changes, full infiltration and lateral subsurface flow indicated that mobilization occurred. A negative correlation between WDCs and mean weight diameter (MWD) suggests that aggregate disintegration released colloids, which may have been offset by transport and implying a dynamic balance. This balance varied with sediment age and weathering. In the younger surface (0-1 cm), raindrop impact likely released fine colloids (FCs, 24-210 nm) by breaking weak aggregates, while similar to 32 % of deeper-layer WDCs (>1 cm) were mobilized or aggregated. In contrast, the older surface showed WDCs decline, likely due to higher clay content (7 %) and aggregate stability. Below 1 cm, swelling and clay dispersion contributed to WDCs accumulation. Soil age also affected WDC and nano-colloid (NCs, 0.6-24 nm) composition. Older soils were enriched in Al, Fe, and Si. Irrigation partially altered these patterns, increasing NC-Al in the younger fan. Notably, NC-P increased despite declining NCs, likely due to enhanced binding with Al (hydr)oxides, suggesting NC-P dynamics are governed by both content and composition of WDCs. These results highlight sediment age-dependent responses of WDCs and WDC-P to short-term water inputs and underscore their role in shaping nutrient fluxes and soil resilience in arid environments.