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 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.