NITROGEN DYNAMICS UNDER GROWTH CHAMBER CONDITIONS AS INFLUENCED BY METHOD OF ALFALFA TERMINATION 2 - PLANT-AVAILABLE N RELEASE

Herbicide application has been proposed as an alternative to tillage f or termination of established alfalfa (Medicago sativa L.) stands but it may alter the pattern and amount of N released from alfalfa residue s. A controlled environment study was conducted to investigate the eff ect of termination technique on the availability of N to four barley ( Hordeum vulgare L.) crops. Four treatments consisting of a factorial c ombination of two termination methods (chemical, mechanical) and two m ethods of residue placement (surface, incorporated) were established. Nitrogen uptake by the four consecutive crops of barley was measured d uring a 125-d period after termination. Termination method, particular ly residue placement, strongly affected N release from alfalfa residue s. Nitrogen accumulation by the initial barley crop accounted for >60% of cumulative N uptake in incorporated treatments compared with 39% a nd 24% for herbicide and tillage treatments in which alfalfa residue w as surface applied. Herbicide application also slightly increased N up take by the initial barley crop. Nitrogen uptake by subsequent barley crops was not affected by termination method; however, cumulative N up take remained substantially greater for incorporated treatments throug hout the 125 d experiment. Effects of residue particle size on N relea se from alfalfa residues were small. These results suggest that herbic ide termination in which residue is retained on the soil surface may r educe the short-term plant-available N supply. Provided that mineraliz ation is sufficient to meet the N needs of subsequent crops, maintaini ng a smaller reservoir of soil inorganic N may be beneficial in reduci ng the potential for leaching or denitrification losses.