Legacy Effects of Long-term Manure Applications on Soil-derived Nitrous Oxide Emissions

Termination of the manure application treatments at the Dixon long-term manure research site in Humboldt, Saskatchewan provided a unique opportunity to explore how a change in management regime to annual urea applications would affect nitrous oxide (N2O) emissions.My hypotheses were that long-term manure applications would produce a legacy (or priming) effect that would result in enhanced N2O emissions following the changeover to a more readily available nitrogen source and that this effect would be relatively short-lived.The impacts of long-term manure application and change in fertility management in the sub-humid prairies of Saskatchewan has not been investigated in great depth, this work provided an opportunity for greater insight into changes in N transformation and gaseous N loss from a manured agroecosystem.Nitrous oxide fluxes associated with the long-term manure and fertilizer application from the Dixon site were measured during a 37-month period (i.e., from May 2011 to June 2014).In addition, denitrification enzyme activity (DEA) was measured in a subset of the plots starting in June 2011 and continuing three times per year (i.e., prior to and after the spring fertilizer application and again in early fall).Treatment-induced N2O emissions for the various historical amendment treatments indicate that past management can result in considerable N being lost from the system as N2O.Indeed, summed over the three-year post-manure period (i.e., from fertilizer application in 2011 through the 2014 spring thaw), N2O-N losses accounted for 2% to 6% of the total applied fertilizer-N.Moreover, under environmental conditions that optimize denitrification, N2O-N losses can be even greater.For example, high DEAs coupled with warm moist soil conditions resulted in large N2O emission events following the spring 2013 fertilizer application and during the 2014 spring thaw.As a result, cumulative annual N2O-N losses in 2013/14 were much greater than those in previous yearswith emissions from the liquid swine manure (LSM)amended plots ranging from 3% to 15% of applied N.These data support my earlier hypothesis that long-term applications of manure-N canespecially at high application rates and following frequent application -produce a 'priming' effect that exacerbates N2O emission when a more available form of N (e.g., urea fertilizer) is applied to the soil.Moreover, this priming effect appears to be relatively long-livedpersisting in the soil more than four and a half years after the last manure application.In any given year, however, the impact of the priming effect on iv cumulative N2O emissions depends on environmental conditionsbeing greatest during years with above average precipitation and temperature during the spring thaw period and a seeding/fertilizer application.Overall, my data demonstrate that management history can have a significant impact on soil N turnover in agricultural soils, and that long-term annual application of manure-N at high rates can produce a N2O 'priming' effect thatunder appropriate environmental conditionscan significantly intensify N2O emissions long after the manure applications have ceased.

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