Soil carbon in regenerating high country grassland agroecosystems in New Zealand

Abstract

Pastoral grasslands in New Zealand's mountainous landscapes contribute substantially to a primarily agricultural national economy. This landscape supports mosaics of regenerating indigenous biodiversity among more productive naturalized exotic herbage, each raising fundamental ecological, agronomic, and environmental concerns. Our objective was to investigate whether increased or lesser attention to native plant assemblages in these vegetation mosaics significantly influences soil carbon (C) stocks. At mid-altitudes below the original tree line, we compared paired plots of regenerating successional endemic myrtaceous woody shrub communities (Kunzea ericoides, kānuka) with exotic pasture at comparable slopes and aspects. We also investigated native snow tussock grass communities, the dominant vegetation at higher altitudes bordering the original tree line. Soils beneath kānuka had significantly higher C concentrations, C stocks, and carbon:nitrogen (N) ratios than adjacent areas of pasture. Soil C stocks were 15.43% higher under kānuka than under adjacent pasture. The bases of snow tussocks were frequently raised 10–20 cm above the surrounding inter-tussock land surface with a sparse vegetation cover that provides pathways for stock, making them more susceptible to soil erosion. Soil directly beneath the snow tussocks had significantly higher C, nitrogen, and phosphorus (P) in comparison with adjacent inter-tussock spaces. Soil C stocks were 38.7% higher under snow tussock than in the adjacent inter-tussock spaces. Our findings indicate that maintaining and enhancing endemic woody shrub and snow tussock assemblages is beneficial to productive and sustainable pasturage, while also playing a significant role in biodiversity conservation and climate change mitigation.