The Effect of Dairy Manure Applications on Nitrogen Fixation by Alfalfa under Mediterranean Climate Conditions

Alfalfa (Medicago sativa L.) 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 manure and nitrogen fertilizers.  This has not been documented under irrigated Mediterranean climate conditions such as California. We used a natural abundance stable isotope method to estimate biological nitrogen fixation (BNF) by alfalfa growing in fields having a history of dairy manure applications. This approach exploits the N-15 content of manure (and soils receiving manure), which typically is elevated a small but measurable amount above that of the atmosphere. This small enrichment in N-15 is primarily due to selective loss of N-14 from the manure during ammonia volatilization following excretion. We measured the N-15/N-14 content (atom %) of alfalfa and non-legume reference plant (weed) samples collected in the Central Valley of California from commercial farm fields having a history of manure applications and others with no history of manure applications. Reference plants and alfalfa were collected in triplicate in each field and within a few meters of each other. N content was determined on all samples by combustion and N-15/N-14 was determined by mass spectrometry (IRMS). We will use these data to estimate the proportion of alfalfa N derived from the atmosphere and from soil in both manured and non-manured fields.