Cutting regime affects the amount and allocation of symbiotically fixed N in green manure leys

Cutting strategy effect on N(2) fixation and distribution of fixed N above and below ground in red clover (Trifolium pratense L.) and mixed red clover/perennial ryegrass (Lolium perenne L.) green manure leys was quantified in field experiments including in situ mezotrons and microplots. Symbiotically fixed N in clover, transfer of fixed N to grass in the mixed stands and the fate of (15)N contained in mulch were estimated by isotope dilution. Below ground clover-derived N was estimated by leaf labelling. Total N(2) fixation was estimated by correcting fixed N in plant shoots with plant-derived N below ground and recycled N from mulch. The total N(2) fixation was larger in harvested and mulched stands (average 45 g m(-2)) than in the intact stands (32 g m(-2)). Of the fixed N, 53% (intact), 46% (harvested) and 60% (mulched) was found below ground. The average recycling of N in mulch was 21% and contributed 13.7% (pure clover) and 2.2% (mixed) of the clover N in the regrowth. Recycling of N did not decrease N(2) fixation in the mulched compared with harvested stands. The results indicate that cutting regime should be considered when estimating total amounts of N fixed by green manure leys.