Theme 1-1: Rangeland/Grassland Ecology--Oral Sessions

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A temporal and spatial assessment is required to quantify the effects of nutrient inputs and varying grazing management regimes on soil organic carbon (SOC) stocks under grazed pastures in complex landscapes. We examined SOC stocks under permanent pastures in three farmlets under a range of different annual phosphorus (P) fertiliser and associated sheep stocking regimes. The farmlets examined had either no annual P applied (NF), 125 kg single superphosphate (SSP) ha-1 (LF), or 375 kg SSP ha-1 (HF) on an annual basis since 1980. Soils were sampled to three depths (0-75, 75-150, 150-300 mm) in 2003 and 2020, and to the two upper depths in 2014. Each farmlet included three slope classes [low slope (LS), medium slope (MS), high slope (HS)], on three different aspect locations [east (E), southwest (SW), northwest (NW)]. Although a trend (P = 0.07) was observed for greater SOC stocks in the upper depth of the HF farmlet (34.0 Mg C ha-1) compared with the other two farmlets (31.6 Mg C ha-1), this trend was discontinued in deeper layers. Accumulated SOC stocks (0-300 mm) were 111.1 (NF), 109.8 (LF) and 111.5 (HF) Mg C ha-1. Soil samples collected on HS resulted in higher soil bulk densities (BD) and carbon-to-nitrogen (C:N) ratios, and lower C concentration and SOC stocks, compared with samples collected on the other two slope classes. Soil samples collected on the NW-facing slopes resulted in higher BD, and lower C concentration and SOC stocks, compared with samples collected on the other two aspect locations. Under the current conditions, contrasting P fertiliser and sheep stocking regimes had minimal effects on SOC stocks. In contrast, topographic features had major effects on SOC stocks, and need to be considered in soil sampling protocols that monitor soil organic carbon stocks over space and time.

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Soil Carbon Stocks Are Stable under New Zealand Hill Country Pastures with Contrasting Phosphorus and Sheep Stocking Regimes

A temporal and spatial assessment is required to quantify the effects of nutrient inputs and varying grazing management regimes on soil organic carbon (SOC) stocks under grazed pastures in complex landscapes. We examined SOC stocks under permanent pastures in three farmlets under a range of different annual phosphorus (P) fertiliser and associated sheep stocking regimes. The farmlets examined had either no annual P applied (NF), 125 kg single superphosphate (SSP) ha-1 (LF), or 375 kg SSP ha-1 (HF) on an annual basis since 1980. Soils were sampled to three depths (0-75, 75-150, 150-300 mm) in 2003 and 2020, and to the two upper depths in 2014. Each farmlet included three slope classes [low slope (LS), medium slope (MS), high slope (HS)], on three different aspect locations [east (E), southwest (SW), northwest (NW)]. Although a trend (P = 0.07) was observed for greater SOC stocks in the upper depth of the HF farmlet (34.0 Mg C ha-1) compared with the other two farmlets (31.6 Mg C ha-1), this trend was discontinued in deeper layers. Accumulated SOC stocks (0-300 mm) were 111.1 (NF), 109.8 (LF) and 111.5 (HF) Mg C ha-1. Soil samples collected on HS resulted in higher soil bulk densities (BD) and carbon-to-nitrogen (C:N) ratios, and lower C concentration and SOC stocks, compared with samples collected on the other two slope classes. Soil samples collected on the NW-facing slopes resulted in higher BD, and lower C concentration and SOC stocks, compared with samples collected on the other two aspect locations. Under the current conditions, contrasting P fertiliser and sheep stocking regimes had minimal effects on SOC stocks. In contrast, topographic features had major effects on SOC stocks, and need to be considered in soil sampling protocols that monitor soil organic carbon stocks over space and time.