Nitrogen and water uptake of lucerne and cocksfoot-perennial lupin pastures under dryland conditions.

Abstract

A field experiment was conducted to quantify the water use and nitrogen uptake of lucerne and cocksfoot-lupin pastures under dryland conditions. To do this, the study made use of established pastures located at Lincoln University, Lincoln and Sawdon Station, Tekapo. Lucerne and cocksfoot-lupin pastures were sown in December 2013 at Lincoln University. At Sawdon Station, lucerne was sown in 2009 and grass-lupin pasture sown in 2003.

From July 2014-June 2015 at Lincoln University, lucerne had a greater yield (3,100 kg DM/ha more) and water use efficiency (WUE) (4.4 kg DM/ha/mm water used more) than cocksfoot-lupin pastures. Nitrogen (N) content of lucerne was consistently above 4.0% which ensured higher rates of leaf photosynthesis were obtained per unit of water used compared to 3.43% for cocksfoot-lupin pasture. Greater yield and WUE for lucerne was also due to a greater rooting depth and ability to extract water from further down the soil profile (53 mm more water extracted). Lucerne had a greater rooting depth than cocksfoot-lupin (2.0 v 1.7 m) which allowed lucerne to grow for a further two months during summer when cocksfoot-lupin pasture growth noticeably declined at mid-November. Soil moisture was consistently below 27% soil volumetric water content during the season indicating cocksfoot was moisture limited throughout the season.

N application did not affect yield of lucerne at Lincoln University regardless of application date. Cocksfoot-lupin growth was improved by 1,261 kg DM/ha when 600 kg N/ha was applied in July. This was primarily due to N stimulating cocksfoot growth which constituted 90-100% of the sward. N content of cocksfoot only exceeded 5.2% (below which plants do not photosynthesis at their maximum) during this increase in growth suggesting cocksfoot was moderately N stressed for the majority of the season which lowered potential growth.