Seasonal greenhouse gas flux and microbial community dynamics along a landslide soil chronosequence at the west side of New Zealand's Southern Alps

Landslides are an important erosion mechanism in mountainous terrain. They strip large quantities of organic material from ecosystems and expose bedrock to weathering. At the west side of New Zealand’s Southern Alps, super-humid climate and slope instabilities create ideal conditions for frequent landslides, allowing studies of soil formation processes in short-term chronosequences. Here, we investigated soils from three landslides that occurred in 2019 (“young”), 1997 (“intermediate”), and 1965±5y (“old”), respectively. Additionally, reference forest soils located outside of landslide areas were sampled at each location. Closed PVC chambers were installed at different positions on landslide surfaces and reference soils, and gas samples were collected for flux analysis. Soil samples were collected at the same positions from the surface and 20 cm depth and investigated for physico-chemical parameters and microbial community composition.

Net CO₂ emissions reached 830 mg h^-1m^-2 at the “old” site in summer and remained below 356 mg h^-1m^-2 in winter. Net N₂O emissions showed a patchy spatial pattern, reaching rates of 0.2 mg h^-1m^-2 at the “old” site in summer. At most locations, N₂O flux was below the detection limit during winter.

Soils were dominated by bacterial phyla Acidobacteria, Bacteroidota, Chloroflexi, Gemmatimonadota, Planctomycetota, Proteobacteria and Verrucomicrobiota. Dominant archaeal phyla comprised Thermoplasmatota and Crenarchaeota. Beta diversity analysis revealed distinct community composition patterns with the “young” site forming a separate cluster from the older landslides and reference soils. Surface- and depth-associated microbial communities showed high similarity at the “young” site, but they became increasingly distinct at the “intermediate”, “old” and reference soil sites. Community composition at the “old” site showed the least difference to reference sites, indicating that ecosystem development rapidly reached a state similar to older mature forests.

In general, the three landslide sites showed a gradient in the development of soil chemical parameters, microbial community composition and soil respiration rates, with the “old” site being closest to reference sites. Soil respiration rates showed strong seasonal dependence and soil temperature sensitivity.

Our results indicate that respiration rates and microbial community composition of landslide soils reach those of older mature forest soils within a few decades after mass wasting events, if no reactivation occurs and soil development can proceed without disturbance.