Effects of perennial (‘Russell’) lupins on soil nitrogen and carbon in acid high-country soils

Abstract

Many high-country soils in the South Island have low soil pH and high exchangeable Al concentrations, limiting establishment and persistence of pasture and forage legumes. Perennial lupin (Lupinus polyphyllus) is able to grow in acidic soil (pH<5.6) with high levels of exchangeable Al (Al>3ppm, 0.01 CaCl2) toxic to most other legumes. This study examined the effects of perennial lupin stands of varying ages on soil nitrogen (N) and carbon (C) concentrations (0-15 cm). Eight lupin sites, varying in age, and neighbouring long-term pasture sites were soil sampled and analysed for N and C concentrations. Sites planted with perennial lupin had significantly (P<0.001) higher total soil N (%) and soil mineralisable N (kg/ha) compared to adjacent pasture soils at the eight sites. Soil N status also declined with increasing soil depth in both lupin and pasture soils linked with plant residue accumulation in the topsoil. Soil N level generally increased with increasing lupin stand age (P<0.001), though the oldest (30 years) site did not fit this trend. This study provides strong evidence that lupins substantially increase soil total and labile soil N. Results indicate that perennial lupins may be a suitable and important species to develop poor quality soils on South Island high-country farms.