Pasture type and fertilization effects on N2 fixation, N budgets and external energy inputs in western Canada

A grazing experiment was conducted in Brandon, Manitoba, Canada. The objectives were to examine the effects of including alfalfa and fertilizer management on N2 fixation by alfalfa and plant N dynamics, and to compare N budgets in the four contrasting pasture systems and external energy inputs between fertilizer-N-based and legume-based pasture systems. Estimates of annual amounts of N2 fixed, based on shoot herbage production in grazed mixed alfalfa/grass pastures, ranged from 40 to 118 kg N ha−1 y−1. The amounts would be in the range of 52–153 kg N ha−1 y−1, if the amounts of fixed N stored in the roots, were included. Compared to grass-only pastures, total amounts of N2 fixed in the mixed pastures should be sufficient to improve total external N inputs, replace N fertilizer and sustain plant protein for grazing. The reliance of alfalfa (Medicago sativa L.) on N2 fixation for growth was high (70–95%), and %N derived from the atmosphere by alfalfa (%Ndfa) was not affected by P fertilizer management. Thus, the amounts of N2 fixed were predominantly regulated by alfalfa dry matter productivity. The data also indicated that alfalfa fixed 27 kg N t−1 dry matter produced. In mixed alfalfa/grass pastures, high soil mineral N uptake by companion grasses, was essential to effectively utilize N that was fixed by alfalfa and returned to soils through the decomposition of alfalfa litter and roots. Compared to grass-only pastures with or without N fertilizer, alfalfa-based pastures could supply sufficient plant protein for grazing animals through N2 fixation, and at same time, sustain animal productivity with only 28% of the external energy input of the grass-only pasture with N fertilizer.