Soil carbon and nutrients change with pine invasion into eucalypt forests

The co-invasion of pine trees (Pinus spp.) and their associated mycorrhizal fungi into the southern hemisphere is one of the most widespread invasions of non-native species on Earth. However, little is known about how pine invasions affect soil carbon (C) and nutrient content, particularly when invading into eucalypt forests. Pine invasions may impact the soil C, nitrogen (N), and phosphorus (P) of native eucalypt forests in several ways: pine root symbionts may mine increased amounts of N and P from the soil, soil C may be lost via soil disturbance, and/or C, N, and P could become locked in a growing needle litter layer that decomposes slower than eucalypt litter. Thus, we hypothesized that pine invasions would lower total soil C, N, and P relative to uninvaded eucalypt forests. To test this hypothesis, we compared soil C, N, and P concentrations and soil fungal communities across pine invasion fronts in six Australian states. We measured total C, total N, and bulk P on soils from sites containing three forest types: uninvaded native Eucalyptus forest, pine-invaded Eucalyptus forest, and non-native pine plantation. We then tested for a relationship between soil C, N, P, C:N, and C:P ratios and forest type. In support of our hypothesis, soils from pine-invaded eucalypt forests had significantly lower total N and P content than uninvaded eucalypt forests and non-native pine plantations. Additionally, soils from pine-invaded eucalypt forests and uninvaded eucalypt forests had significantly lower total C than non-native pine plantations. Soil C:N and C:P were significantly higher in invaded eucalypt forests relative to uninvaded forests. These findings indicate that either 1) pine invasions are changing soil systems to reduce the total N and P of native eucalypt forest soils as the invasion progresses, or 2) pines are preferentially invading eucalypt forests with low soil nutrient content. Lower soil N and P relative to soil C under pine invasions suggests that pines and their associated root microbiome may be able to mine soil N and P, and/or that N and P may become locked in a slow-decomposing needle litter layer. To further understand the role that soil fungal communities play in these soil nutrient dynamics, we are currently analyzing how the soil fungal communities are related to forest type and soil C, N, and P.