Soil organic carbon stocks in hill country pastures under contrasting phosphorus fertiliser and sheep stocking regimes, and topographical features

Temporal and spatial measurements of soil organic carbon (C) under grazed pastures are needed to quantify the effects of different grazing management regimes on C stocks. We examined soil organic C stocks under permanent pastures at the Ballantrae Hill Country Research Station in southern Hawke's Bay, New Zealand. Soils were sampled to three depths (0–75, 75–150, 150–300 mm) in 2003 and to the two upper depths in 2014, in three farmlets under different annual phosphorus (P) fertiliser inputs and stocked with sheep to maintain similar grazing pressure (i.e., stock units per unit of pasture production) across farmlets since 1975. The farmlets examined were NF = no annual P applied, LF = 125 kg single superphosphate (SSP) ha−1, and HF = 375 kg SSP ha−1, on an annual basis since 1980. The permanent sites included three slope classes [low slope (LS; 1–12○), medium slope (MS; 13–25○), high slope (HS; >25○)], on three different aspect locations grouped relative to the true north [east (E; 35–155○), southwest (SW; 275–35○), northwest (NW: 155–275○)]. A year-by-farmlet interaction trend on soil C stocks in the upper depth (0–75 mm) was associated with linear numerical differences in soil C stocks in 2003 (30.9, 32.5 and 35.1 Mg C ha−1 on the NF, LF, and HF farmlets, respectively) but not in 2014. This trend was not seen in the deeper soil layers (75–150 and 150–300 mm). In contrast, slope and aspect had major effects on soil C stocks. Overall, soil samples collected on the steepest slope class (>25○) resulted in higher soil bulk densities (BD) and carbon-to‑nitrogen (C:N) ratios, and lower N and C concentration, and soil C stocks at all soil depths, compared with samples collected at the other two slope classes. Soil samples collected on the NW-facing slopes resulted in higher BD, and lower N and C concentration, and soil C stocks at all soil depths, compared with samples collected at the other two aspect locations. Both of these topographic features need to be considered in soil sampling regimes of hill grazing lands to obtain an accurate estimate of organic C stocks. Data from this long-term study provide science, policy and industry with invaluable insights on soil organic C stocks in grazing hill-country soils and highlight the value of long-term structured experiments for monitoring soil C stocks.