PS 49-142
The effect of dairy manure applications on nitrogen fixation by alfalfa under Mediterranean climate conditions

Wednesday, August 7, 2013
Exhibit Hall B, Minneapolis Convention Center
Bridget G. Guiza, Environmental Systems Program, University of California, San Diego, San Diego, CA

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 15N 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 15N is pri­marily due to selective loss of 14N from the manure during ammonia volatilization following ex­cretion. We measured the 15N /14N  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 (and one wastewater) 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. Nitrogen content was de­termined on all samples by combustion and 15N /14N  was deter­mined by isotope ratio mass spec­trometry (IRMS). We used these data to estimate the proportion of alfalfa N de­rived from the atmosphere (%Ndfa) from soil in both manured/wastewater and non manured fields.


The fields with a history of manure/wastewater applications have an elevated 15N content ranging from 1 to 5‰. The %Ndfa (nitrogen derived from the atmosphere) is lower in manured/wastewater fields ranging from 20 to 75%, while in the nonmanured fields the %Ndfa is significantly higher at about 95%.  Alfalfa can take up a substantial portion of Nitrogen derived from the soil in manured/wastewater fields having the potential to prevent excessive nitrate leaching and cleaning water sources. Plant root depth and growth patterns of alfalfa and reference plants will be considered when measuring ¹⁵N in the future. Studies on alfalfa nitrogen fixation under non-Mediterranean climates will be compared to alfalfa fields in California. The age of the alfalfa stands and the manure/wastewater applications will be considered when estimating the %Ndfa.