Compost legacy effects in active roadside restoration: Medium‐term shifts in plant succession

Abstract Soil amendments such as compost are increasingly used in ecological restoration to accelerate vegetation recovery and rebuild soil fertility. However, they may also introduce exotic propagules and alter community trajectories. The construction of National Route 40 in Argentinean Patagonia and subsequent tephra deposition from the 2011 Puyehue–Cordón Caulle eruption created severely degraded roadside substrates lacking topsoil and vegetation—an ideal setting to test compost effects on early ecosystem development. We evaluated the medium‐term effects of two contrasting composts—municipal waste compost (MC) and biosolids compost (BC)—applied once in 2012 on plant community dynamics and substrate properties. From 2013 to 2019, vegetation cover and species composition were monitored across treatments and compared with two reference vegetation types (roadside and native forest). Substrate samples were analysed 6 years post‐treatment to assess changes in physicochemical properties. Compost application markedly enhanced substrate fertility, increasing organic carbon, total nitrogen and available phosphorus, and promoted substantial vegetation cover (40%–100%), while control plots remained mostly unvegetated (<15%). Over time, both compost treatments shifted from dominance by diverse exotic annuals to distinct communities of exotic perennials, revealing strong and persistent ‘compost legacy effects’. These results demonstrate that compost amendments can rapidly stabilize and enrich highly degraded volcanic substrates, but they also reshape plant community composition in ways that may diverge from native restoration targets. Restoration practitioners should therefore select compost types according to project goals, considering not only their nutrient content but also their biological and environmental origins. Practical implications . Compost use can be an effective strategy for restoring fertility and stabilizing primary substrates where natural recovery is slow, such as post‐volcanic or mining landscapes. Yet, careful compost selection and monitoring are essential to minimize exotic species introduction and guide successional trajectories toward desired native outcomes.