The measurement of symbiotic nitrogen fixation in established pastures and some aspects of the nitrogen cycle in a sequence of pastures of increasing age

Abstract

The acetylene reduction (A.R.) assay was investigated for quantitative measurement of symbiotic nitrogen fixation in established legume-based pastures, under field conditions.

It was found that the rate of ethylene production was relatively constant for approximately 6 hours, for the system employed.

For an accurate estimation of the nitrogen fixing activity, many field samples are required to overcome the errors due to the inherent spatial distribution of white clover (Trifolium repens) within the pasture. Also, it is necessary to physically integrate the day to day and diurnal variations in the nitrogen fixing activity of the legume to obtain a reliable estimate of the rate of ethylene production and hence symbiotic nitrogen fixation.

The A.R. assay and an N-15 technique were compared for measuring symbiotic nitrogen fixation in established pastures.

The A.R. assay, with a 3-hour incubation gave the best estimate of symbiotic nitrogen fixation, relative to the N-15 technique. For the A.R. assay, the 1-hour incubation overestimated and the 6-hour incubation under-estimated the rate of symbiotic nitrogen fixation, relative to the N-15 technique.

Some aspects of the nitrogen cycle in a sequence of pastures of increasing age were investigated in a field trial, for a period of 1 year.

The N-15 technique was used to measure symbiotic nitrogen fixation and tension plate lysimeters were installed to measure leaching losses of nitrogen.

The chemical properties of the soils could be related to their physical properties, especially soil texture, and were not apparently a function of the age of the pasture.

Generally, the dry matter production and the percentage of clover in the pasture decreased with increasing age of the pasture.

The annual amounts of symbiotically fixed nitrogen harvested in the clover tops, decreased with increasing age of the pasture from 142 kg N/ha in the youngest to 45 kg N/ha in the oldest pastures.

The efficiency of symbiotic1 nitrogen fixation, expressed in terms of an enrichment ratio, was similar for all pastures. The clover components of the two oldest pastures were slightly more efficient than those of the two younger pastures.

Leaching losses of nitrogen occurred mainly as nitrate and the results indicate the possibility that leaching losses of nitrogen under pastures may not be negligible.