THE EFFECT OF DAIRY MANURE APPLICATIONS ON NITROGEN FIXATION BY ALFALFA UNDER MEDITERRANEAN CLIMATE CONDITIONS

Alfalfa is a legume with a perennial growth habit and deep root system. It is thought to protect surface and groundwater quality by taking up residual soil nitrogen in fields that have received excessive rates of fertilizers. The natural abundance stable isotope method was used to estimate biological nitrogen fixation by alfalfa growing in fields having a history of dairy manure/wastewater applications. This approach exploits the 15N content of manure and manure-enriched soils, which typically is elevated above that of the atmosphere. Nitrogen content was determined on all samples by combustion and 15N /14N was determined by IRMS. We estimated the proportion of alfalfa N derived from the atmosphere (%Ndfa) from soil in manured and nonmanured fields. The manured/wastewater fields have an elevated 15N content ranging from 1 to 5‰. The %Ndfa is lower in manured/wastewater fields ranging from 20 to 75%, while in the non-manured fields the %Ndfa is significantly higher at about 95%.