The biological effects of Lupinus polyphyllus on soil nitrogen in acid high country soils

Abstract

Many high country soils in South Island have low soil pH and possibly high exchangeable aluminium (Al) concentrations, which limit establishment and persistence of pasture legumes for nitrogen (N) contribution. Lupinus polyphyllus (common name ‘Russell lupin’, also known as perennial lupin) is a species of flowering plants in the legume family, Fabaceae. Perennial lupin is able to grow in acidic soil (pH < 5.5) with high levels of exchangeable Al (Al>3ppm, 0.01 CaCl2) toxic to most other legumes, and low soil P (Olsen P <10). The growth ability of perennial lupin indicates that it is better adapted to low fertility environment than other conventional pasture legumes. The high tolerance of lupin to low fertility and acid soil shows that it occupies a unique edaphic niche among forage legumes. As such, this species may be suitable to develop poor quality soils on south island high country farms in South Island, New Zealand. This study focused on the biological effects of perennial lupin stands of varying ages on soil N and carbon (C) concentrations in acid high country soils. There were two experiments in this study: a field experiment and a glasshouse experiment. For the field experiment, soils were collected and analysed for total soil N, total soil C, soil mineral N, mineralizable N, soil pH, soil Olsen P and sulphate sulphur (S) from eight lupin stands of different ages and adjacent established pasture sites. There are eight sites across four farms: Sawdon Station, Glenmore Station, Omarama Station, Dasher Station and one from Lincoln University Campus. To quantify soil labile N status more directly, the glasshouse experiment also was conducted in soil from five sites from the field study. Annual ryegrass (Lolium multilforum) was grown in soil collected from established lupin stands and grass-dominant pasture, and exhaustively harvested to extract the labile soil N. Ryegrass dry matter yield and shoot N were analysed for calculating soil N status. Shoot N was analysed as a measure of labile soil N, in an effort to quantify the effects of perennial lupin biological N inputs to this suite of field soils. Adjacent long-term pasture soils were also included as comparative baseline controls. In the field experiment, perennial lupin significantly increased total soil N and soil mineralizable N in the plant rooting zone of high country soils and lupin stand soils showed higher soil N status than pasture soils, resulting from N fixation. The highest value of soil mineralizable N among eight sites occurred in Glenmore Station and Lake Tekapo (site 2) soils with 213.8 kg/ha. Soil N status also declined with increasing soil depth in both lupin soils and pasture soils caused by plant residue accumulation in the topsoil. Soil N level generally increased with increasing lupin stand age. The glasshouse experiment showed a similar trend as the field experiment, that lupin soils exhibited higher soil N status than pasture soils at each site. Higher soil mineralizable N lead to higher ryegrass dry matter yield and higher N uptake. Soil N level also decreased with increasing soil profile depth in both lupin stand soils and pasture soils. Ryegrass dry matter yield and N uptake by ryegrass generally increased with lupin stand age. The significant differences between each site in both experiments were caused by annual rainfall and soil conditions. This study provides strong evidence that lupin substantially increases soil total and labile soil N, which increases with lupin stand age.