The effect of dairy manure applications on nitrogen fixation by alfalfa under Mediterranean climate conditions

Background/Question/Methods

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.

Results/Conclusions

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.