Reduced soil respiration beneath invasive Rhododendron ponticum persists after cutting and is related to substrate quality rather than microbial community

Abstract

Invasive plants depositing recalcitrant, polyphenol-rich leaf litter may alter decomposition dynamics, leading to an accumulation of soil organic matter. Removing invasives is critical in restoring native habitats, but our understanding of its impacts upon soil processes remains limited. Here, we test the hypothesis that clearing of Rhododendron ponticum leads to increased soil respiration, at a site within Snowdonia National Park, Wales, UK. Soil samples were collected, and soil respiration was monitored over 32 weeks on plots cleared of R. ponticum, plots infested with R. ponticum which were left uncleared, and uninvaded plots of native vegetation. Soil respiration was significantly higher in native vegetation plots, relative to uncleared plots. Clearing R. ponticum led to a short-term (<four weeks) increase in soil respiration relative to uncleared plots and was related to elevated soil temperature post-clearance. However, this respiration response was transient, with no significant difference between cleared and uncleared plots over the whole growing season (32 weeks). Declining soil respiration responses to soil warming have been attributed to altered soil microbial communities and substrate limitation. Analysis of microbial phospholipid fatty acids (PLFAs) detected no differences among cleared, native and uncleared plots post-clearance. However, Fourier-transform mid-infrared spectroscopy detected a decline in organic matter aromaticity over the growing season in the native and uncleared plots, whilst there was no change in the cleared plots. The findings show that despite a pulse in soil respiration during the initial four weeks post-clearance, R. ponticum litter and associated soil organic matter in cleared plots continued to decompose at a similar rate to uncleared plots over the whole growing season. This was likely a result of substrate limitation and altered soil organic matter composition following R. ponticum clearing, with labile carbon becoming depleted and an enrichment of more recalcitrant aromatic structures.