Use of Biological Indicators of Soil Health to Estimate Reactive Nitrogen Dynamics in Long Term Organic Vegetable and Pasture Systems.

Diverse crop rotations, cover crops and the possibility of integrating livestock make organic systems potentially more sustainable than many other agroecosystems. Lower reactive nitrogen (N) in organic systems minimizes the potential for gaseous losses of N. However, addition of organic manures and residues containing mineralizable N and carbon (C) have the potential to enhance nitrous oxide (N₂O) emissions. Soil samples (0-10 cm) were collected from the Long-term Organic Vegetable Systems Experiment established in 2003 in Puyallup, Washington. Treatments included two cover crop managements: fall-seeded cereal rye-hairy vetch mix with 1) broiler litter or 2) mixed compost amendments (both received spader tillage) and 3) a 30-month pasture tilled, cropped and fertilized every 4 yr. A 39 day laboratory incubation was conducted to assess the key microbiological drivers controlling nitrification and denitrification in long term organic vegetable systems during simulated freeze thaw cycles. Soils were amended with ¹⁵N labelled sugar beet residue or left unamended. Soil samples were maintained at 40, 60 and 80% of water filled pore space (WFPS). Ammonium-N, nitrate-N and N₂O all varied significantly (P<0.001) due to a time*amendment*WFPS interaction that was determined for each variable by a completely randomized two-factor factorial ANOVA analysis. Gene copy numbers of the denitrifier gene, nosZ and the nitrifier genes, amoA in ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) were measured via qPCR. The 1^st freeze (-3°C), thaw (4°C) and 2^nd freeze (-2°C), thaw (4°C) events and addition of beet residue resulted in a significant decrease in AOA amoA gene copies in soil. The AOB gene copies were lowest in the compost amended soils irrespective of time or addition of beet residues. The AOB levels did not respond to the addition of beet residue but increased significantly (P<0.001) after the freeze thaw events. Principal components analysis confirmed that both nitrifiers and denitrifiers contributed to N₂O production. Our results indicate that nitrifier and denitrifier gene copy numbers are robust biological indicators of soil health that can be used to assess the impact of short and long-term management on biogeochemical processes that reduce or contribute to global climate change in long term organic vegetable and pasture systems.