COMPARISON OF BIOLOGICAL NITROGEN FIXATION IN FOUR PAIRS OF CONVENTIONAL AND ALTERNATIVE MIXED CROPPING FARMS WITH THREE ROTATIONAL STAGES OF PASTURE ESTABLISHMENT IN CANTERBURY, NEW ZEALAND

Abstract

It is generally recognized that in order for arable farming systems to remain sustainable in the long term, they need to be productive and their soil fertility must be maintained to continue to provide viable yields with minimum detrimental effects on soil and environmental quality. The use of biological nitrogen fixation (BNF) as the main source of nitrogen (N) for crops is an important sustainable management strategy. Amounts of BNF in four pairs of conventional and organic mixed cropping farms, each with three stages of pasture establishment in Canterbury, New Zealand were compared under farmers' field conditions over a period of one year using the 15N isotope dilution technique. Composite topsoil (0–75 mm) samples were taken from all sites at the beginning and end of the trials. These were analyzed for soil pH and available nutrients according to New Zealand “MAF Quick Test” methods. In addition, at the end of the trials, soil samples at four depths (0–50, 50–100, 100–150, 150–200 mm) were taken from inside and outside the experimental plots in pastures of three different ages in one of the pairs of conventional and organic farms and analyzed for total soil N. Results obtained showed that the amounts of biologically-fixed N varied from 19 to 68 kg N ha−1 year−1. There were no significant differences between paired conventional and organic farms or between pastures of different ages. This was attributed to the large variability present and the possibility that the farms studied were at a steady state condition in terms of BNF as supported by total soil N and available soil nutrient data. The BNF was significantly correlated with clover biomass and was also affected significantly by seasons.