Spatiotemporal Variation Of N2O Emissions In Diverse Perennial Cropping Systems.

In this study we compared seasonal and annual nitrous oxide (N₂O) emissions from five sites in the northern Central Valley of California; almond, walnut, wine grape, and processing tomatoes.  In these systems discrete application of irrigation water, nutrients, canopy and root structure together create high spatial variability in soil nutrient and water status.  Therefore N₂O measurements were made in at least two functional locations per system (vines/tree and row or berm, side, furrow) at major management and precipitation events throughout the year.  Across years and cropping systems, emissions averaged 1.6 ± 0.35 kg ha^-1 yr^-1.  Among perennial systems, growing season emissions averaged 0.57-0.67 kg ha^-1 season^-1, ranging from 0.13-1.96 kg ha^-1 season^-1 while fallow season emissions were much more variable, averaging 0.36-1.13 kg ha^-1 season^-1 and ranging from 0.13-5.3 kg ha^-1 season^-1 depending on functional location, year and crop.  Wine grape systems demonstrated the highest variability in emissions and were largely dependent on row cover crop management; at one site, fallow season emissions following a cover crop were 5.3 kg ha^-1 season^-1, while in the subsequent year when no cover crop was planted, emissions were 0.29 kg ha^-1 season^-1.  Following this pattern, vine emissions in the wine grape systems were roughly equal between seasons while in the row 68% of emissions occurred during the fallow season.  In the orchard systems, each functional location responded the same to season, with one third of emissions occurring during the fallow season and two thirds during the growing season.  Emission factors (EF) were uncorrected for background (zero N) fluxes, but fell well above and well below the IPCC default of 1.0%; 0.23%-0.28% in almonds and 1.5-10.4% in wine grapes.  Predictably, tree or vine emissions in the perennial systems peaked following fertigation while row emissions generally peaked during fallow season rain events.  Our results indicate that the use of leguminous cover crops contributed to higher N₂O emissions and emission factors in the wine grape systems, particularly when residue was incorporated during the growing season.  Over all we found that the use of organic fertilizer, fall fertilizer application, early season incorporation of leguminous cover crops, overhead sprinkler, and furrow irrigation may all contribute to increased N₂O emissions.  Our results also demonstrate the necessity of annual measurements and at multiple functional locations in regions with relatively mild winters and dry summers, particularly when cover crop or substantial organic amendments are involved.