Evaluation of the potential of bryophytes to support seedlings development on waste rocks: a mesocosm study

Revegetating waste rocks is essential for re-establishing a functional ecosystem, but it poses significant challenges. Nutrient-poor soil and high exposure to elements cause high seedling mortality in the first year. Survival can be increased by amending soil with organic matter before plantation to improve water retention and nutrient availability. However, this method comes with high environmental and economic costs. Using mesocosms, we tested the use of bryophytes as an alternative to improve seedling survival due to their ability to create a beneficial microclimate with improved soil humidity and increased nutrient availability, especially through biological nitrogen fixation. Two bryophyte species ( Racomitrium canescens (Hedw.) Brid., an early succession, and Pleurozium schreberi (Brid.) Mitt., a late succession species) were evaluated with three higher plant species ( Pinus banksiana, Rhododendron groenlandicum, and Populus balsamifera) planted in waste rocks. We further evaluated two distinct techniques for the application of bryophytes on waste rocks: evenly spreading fragments on the soil surface and regrouping the fragments around the higher plant stem. Results showed that Racomitrium canescens fixed significantly more nitrogen than Pleurozium schreberi on waste rock. On the contrary, Pleurozium schreberi released more nitrogen into the soil than Racomitrium canescens, likely due to higher mortality and nutrient loss during drying/rewetting cycles. While the short-term impact of bryophytes on seedlings was limited in this 6-month experiment, results suggest that bryophytes have the potential to support higher plants in the challenging conditions of waste rock revegetation by increasing nitrogen supply to seedlings. Implications for practice Strategies for restoring waste rock piles using local plant species and limiting the use of exogenous material, such as organic matter amendment, are essential for responsible management of disturbed sites Including bryophytes in revegetation practices can improve seedlings survival after plantation on waste rocks by increasing the availability of essential nutrients such as nitrogen. Methods for applying bryophyte stems tested here in mesocosms provide a benchmark for future tests on real waste rocks.