ITALIAN RYEGRASS AND NITROGEN-SOURCE FERTILIZATION IN WESTERN OREGON IN 2 CONTRASTING CLIMATIC YEARS .2. PLANT NITROGEN ACCUMULATION AND SOIL-NITROGEN STATUS

To develop optimum nitrogen (N) fertilization practices with the least impact on environmental quality and with the greatest economic return , it is imperative that a greater understanding of crop and soil N dyn amics be sought. This paper reports on research conducted with these o bjectives: (i) to determine the relationship between plant N and dry m atter accumulation and soil N status as affected by N-source fertiliza tion as a function of accumulated growing degree days (GDD), and (ii) to determine if western Oregon soil conditions favor ammonium (NH4) ov er nitrate (NO3) nutrition during the period of grass seed crop growth . In a companion paper, plant growth and seed yield component data wer e discussed in relation to N-source treatments and climatic year effec ts. Western Oregon field plots of Italian ryegrass (Lolium multiflorum Lam.) were fertilized with calcium nitrate, ammonium nitrate, ammoniu m sulfate, ammonium chloride, and urea-dicyandiamide (DCD) to manipula te soil NH4 and NO3 ratios. Italian ryegrass accumulated the greatest portion of plant N and dry mass between tiller elongation and mid-head ing. Reduced growth and seed yield in 1991, compared to 1992, were ass ociated with lodging and low soil pH. Higher soil NH4 levels in 1991 w as most likely responsible for a greater reduction in soil pH for that year. Declines in soil pH due to elevated NH4 levels during climate y ears normal to western Oregon, wet and cool, may have an additive effe ct to other factors limiting seed yield. When cool wet soil conditions exist NH4 was the predominate mineral N-form. Information reported he re and in the companion paper is valuable to farm managers and consult ants in the context of N fertilization of ryegrass grown for seed in w estern Oregon. It begins to establish criteria for the future developm ent of site specific nutrient management plans and adds knowledge that will aid in improving N-use efficiency through improving N fertilizer timing and N source use.