Estimating Alfalfa's Impact On Regional Nitrogen Budgets in the Central & Santa Clara Valley's of California Using the 15N Stable Isotope Natural Abundance Method.

Alfalfa (Medicago sativa L.) is a perennial legume with a deep root system.  It’s thought to protect surface groundwater quality by utilizing residual soil nitrogen in fields that have had excessive amounts of manure and nitrogen fertilizer applied.  The study used the natural abundance stable isotope method in order to estimate the biological nitrogen fixation of alfalfa growing in fields with and without a history of manure applications in the Santa Clara and Central Valley’s of California.  This experiment exploits the ¹⁵N content of manure that is applied to fields which is usually elevated a measurable but small amount above that of which is found in the atmosphere.  The enrichment of ¹⁵N is due to the greater loss of the lighter ¹⁴N isotope during ammonia (NH₃) volatilization.  Seventeen alfalfa fields were chosen with and without manure application histories.  Three plots of sudan grass were planted at random locations in each site to serve as a standard reference plant.  Alfalfa samples were collected in close proximity to reference plant samples two times during the growing season, the first sample occurring from June to August and the second from September to October.  The total nitrogen (N) content was determined by combustion and ¹⁵N/¹⁴N content was determined by isotope ratio mass spectrometry.  The data was then used to estimate the percent of N derived from the atmosphere (%Ndfa) for all fields.  The ¹⁵N content of alfalfa tissue from fields with a history of manure applications was significantly higher than the fields with no history of manure applications and ranged from -0.87 to 10.13⁰/₀₀.  The %Ndfa in fields with no manure application history ranged from 0 to 100% and was not significantly different than fields with a history of manure applications which ranged from 0 to 96%.