Irrigation and grazing management affect leaching losses and soil nitrogen balance of lucerne

Intensification of agricultural management practices, including irrigation and addition of nitrogen (N) fertilizers, can lead to enhanced N leaching and loss of soil fertility. In New Zealand, expansion of the dairy industry has rapidly increased irrigated land area, particularly on shallow, stony soils of the Canterbury region that are prone to leaching, leading to degradation of surface- and ground-water quality and losses of soil N and carbon (C). In this study, we measure components of N balance for two adjacent fields of lucerne (Medicago sativa L., alfalfa) harvested for cut-and-carry feed and grazed in situ. One field was non-irrigated and one irrigated with both water and dairy effluent. Inputs from N fixation associated with the legume crop were quantified using a natural abundance isotopic approach. Drainage from the root zone and leaching were measured with 6 large lysimeters in each field. Leaching losses from non-irrigated lucerne were 7–30 kg N ha-1 y-1 with the largest losses occurring in a year with primarily grazing management. Losses from irrigated lucerne were 39–102 kg N ha-1 y-1, with the largest losses resulting from summer drainage events exacerbated by irrigation. Fixation of N was the largest input to both systems, contributing 192–257 kg N ha-1 y-1 for non-irrigated lucerne. Under irrigation, biomass production increased, but N uptake from effluent and soil stocks contributed to biomass N to a greater extent and fixation was 262–286 kg N ha-1 y-1. Management influenced N balance through inputs from animal excreta and effluent additions and exports through harvest and grazing removals. Management practices which reduce N losses from the soil are needed to minimize environmental impacts and protect soil fertility.