Comparing deep soil organic carbon stocks under kiwifruit and pasture land uses in New Zealand

Soil organic carbon (SOC) is important natural capital for agricultural production, as it affects soil physical, chemical and biological functions and the provision of ecosystem services. Measures of land-use effects on SOC stocks generally focus on the top 0.3 m of soil, as the topsoil has the highest SOC concentration. However, while subsoil horizons have low SOC concentrations, they contain a greater absolute amount of SOC with longer mean residence times than topsoil layers. Perennial horticultural crops offer potential to store SOC deep in the soil profile because of their long-lived and deep rooting systems. To investigate the hypothesis that kiwifruit (Actinidia chinensis Planch.) can increase subsoil SOC stocks, we sampled soils from 19 paired kiwifruit and pasture sites in New Zealand in 2018. Pasture was selected for comparison as it was the antecedent land use before establishment of the kiwifruit orchards. Paired land uses were located within 100 m of each other on the same soil type. Kiwifruit vines were at least 15 years old and the pasture was not cultivated during that time. Total SOC and nitrogen (N), and labile soil SOC stocks were assessed to a depth of 2 m. Kiwifruit production resulted in a modest increase in SOC and N stocks at a depth of 1.5-2.0 m (1.6 Mg C ha(-1) and 0.52 Mg N ha(-1)), averaging to increases of 0.06 Mg C ha(-1) y(-1) and 0.02 Mg N ha(-1) y(-1). However, cumulative SOC and N stocks to 2-m depth were not different between land uses. The labile water extractable pools of SOC were lower under kiwifruit in the topsoil (0-0.1 m) and corresponded with lower N stocks and a higher soil C:N ratio at this depth. Further work on the dynamics of subsoil SOC pools is needed to understand the contribution of perennial horticulture crops to subsoil SOC storage.