The dynamics of reactive nitrogen in clean till and no till organic agroecosystems
Organic agroecological systems are intended to “produce products using methods that preserve the environment” but can be a substantial source of greenhouse gases (GHG)s (nitrous oxide (N2O) and carbon dioxide (CO2)) if not managed properly. The objective of this experiment is to monitor several transformations within the nitrogen (N) cycle i.e. ammonification, nitrification and denitrification by measuring shifts in indicators of soil health. Soil samples (0-30 cm) were collected from plots in the Long Term Organic Tillage Study established in 2010 at North Dakota State University Dickinson Research and Extension Center. A 149 day laboratory incubation was conducted to determine the effects of previous tillage management (clean-till/no-till) and simulated winter freeze thaw events on reactive forms of N, CO2 and gene copies of microorganisms that control nitrogen cycling. Field moist sieved soil samples to be incubated were either amended with 5% 15N labelled urea, 1% 15N labelled sugarbeet residues or left unamended (control) and maintained at 40, 60 and 80% of water filled pore space (WFPS). Keeling plots with atom % 15N-N2O on the y-axis and 1/ N2O on the x-axis were used to determine the source of N2O. PROC MIXED ANOVA (SAS 9.2) was used for statistical analysis.
No significant amount of reactive N was lost to ammonia volatilization. Statistical analyses of N2O and CO2 emissions verified a significant time*amendment* tillage*WFPS and time*amendment* tillage interaction that affected N2O (P<0.06) and CO2 (P<0.0001) concentrations respectively. No-till plots released more cumulative CO2-C (15.9-185.2 mg mm-2) vs clean-till plots (18.2-166.6 mg mm-2). Unamended soil taken from no-till plots released 3.5 times more N2O relative to soils taken from clean-till plots. Cumulative N2O-N in amended clean-till (3.95 mg mm-2) treatments was significantly higher than in amended no-till treatments (3.14 mg mm-2). Results suggest that even at low temperatures the presence or addition of organic residues or inorganic urea results in substantial loss of N as N2O. Keeling plots (R2=0.7-0.9) indicated that N2O emitted from soils after the freeze thaw event in both amended tillage managements was primarily from heterotrophic denitrification. Preliminary data indicate that denitrifier nosZ gene copies remain constant unless incubated soils are at 80% WFPS and amended with sugar beet residues. Further research is ongoing and will relate gene copies of soil nitrifier and denitrifier communities to reactive N and N2O.