Biological invasions of plants and their associated microbiome can have profound effects on soil nutrient cycling, and these effects may remain even after the plant has been removed. Pine (Pinaceae) invasions are a major economic and ecological problem, especially in the southern hemisphere. Pines co-invade with their non-native ectomycorrhizal fungi and modify plant, fungal and bacterial communities, along with altering ecosystem functioning. Our aim was to determine whether the belowground legacies of pines persist following removal and if so, for how long. Understanding these legacies is critical to restoring ecosystems.We sampled soils across 23 sites on the South Island of New Zealand to test the following hypotheses: 1) legacies of pine invasion persist in the environment, and 2) legacies of pine alter soil ecosystem function (e.g. soil enzyme activity) through changes in soil biological communities. We extracted and amplified the DNA from each soil sample using fungal specific primers fITS7 and ITS4 and bacterial specific primers 515F and 806R. To relate these communities to soil ecosystem function we measured the soil nutrient (both macro- and micronutrients) and enzyme (cellulose degrading: cellobiohydrolase, β-glucosidase; chitin: N-acetyl-glucosaminidase; phosphorus: acid phosphatise; lignin: laccase and Mn-peroxidase) profile of each sample.
The proportion of ectomycorrhizal fungi decreased as time since removal increased. Ectomycorrhizal fungi closely associated with invasive pines (e.g. Amanita, Suillus, Sistotrema) remained in the environment up to three years following removal of pines. The community composition of bacteria did not differ significantly as a function of time since removal. Changes in enzymatic activity were driven by both pine legacies and abiotic (soil nutrients) and biotic (fungi and bacteria) soil properties. However, different enzymes showed distinct patterns. The activity of enzymes involved in cellulose and chitin degradation increased as time since removal of pines increased but the activity of enzymes involved in lignin and phosphorus breakdown decreased. Enzymes involved in the breakdown of lignin were most sensitive to fungal communities: laccase activity was positively correlated with the proportion of ectomycorrhizal fungi and Mn-peroxidase was the only enzyme driven solely by fungal communities.
In conclusion, the legacies of pines and their associated ectomycorrhizal fungi can persist in the environment for years which has subsequent impacts on soil nutrient cycles and ecosystem function. The cascading effects of these changes suggest potential implications for the success of future plant establishment and restoration of co-invaded ecosystems.