Biological N-2 -fixation in grass-clover ley in response to N application in cattle slurry vs. mineral fertilizer

Background The nitrogen (N) response in grass-clover is influenced by quantity and timing of N-fertilizer, but less is known about the effect of fertilizer type (animal manure vs. mineral N) and N form (ammonium vs. nitrate). Method A field trial was established in a mixture of white clover (Trifolium repens L.), red clover (Trifolium pratense L.) and perennial ryegrass (Lolium. perenne L.) with mineral N and slurry. DM yield in grass and clover and N-2-fixation activity (%Ndfa) was determined in four cuts during the season. Results Fertilization resulted in a peak in soil mineral N within 1-2 weeks of application and decreased to background rate prior to plant harvest. No differences in white clover N-2-fixation activity were found between fertilizer types within each N level. Conclusions The experiment showed that mineral N form does not affect the regulation of N-2-fixation activity. The fertilizer N level was the only factor affecting white clover N-2-fixation activity. Further, the results demonstrated an N threshold below which annual %Ndfa was always high (similar to 90%). This was controlled by the companion grass' ability to deplete the soil mineral N pool. When N fertilization exceeded this threshold (above 180 kg N ha(-1) yr(-1) in our case) annual %Ndfa significantly decreased as did qBNE in the harvested biomass.

Automated summary

This study investigates the effects of different fertilizer types and nitrogen forms on grass-clover. The results show that the mineral nitrogen form does not affect nitrogen fixation activity in grass-clover, while the fertilizer nitrogen level is the main factor affecting nitrogen fixation activity. Furthermore, the study also identifies a nitrogen threshold, below which the nitrogen fixation activity in grass-clover remains high.