Improved Chamber Systems for Rapid, Real-Time Nitrous Oxide Emissions from Manure and Soil

Abstract

Abstract. Nitrous oxide (N 2 O) emission rates have traditionally been measured using non-flow-through (NFT), non-steady-state (NSS) chambers, which rely on measuring the increase in N 2 O concentration in the sealed chamber headspace over time. These flux measurements are very labor- and time-intensive, requiring three to four gas samples collected over a 30 to 60 min period, followed by laboratory N 2 O measurement with a gas chromatograph (GC) and subsequent flux rate calculation. The objective of this research was to develop and evaluate improved, real-time flux chamber designs that rapidly quantify N 2 O emissions from manure and soil. The first chamber system consisted of six square 0.95 m 2 chamber pans. The chamber pans were mounted on a rail system to facilitate controlled indoor/outdoor laboratory research at a pilot scale. An aluminum lid was moved among the chamber pans. A second portable chamber system with a circular footprint (0.49 m internal dia.) was designed for use in field measurements. With both systems, N 2 O concentrations were measured each second with 0.1 ppb resolution by recirculating sample air through a real-time continuous N 2 O analyzer with return flow into the recirculating-flow-through (RFT-NSS) chamber. Performance and observational data are presented for different chamber vent designs, sealing mechanisms between the chamber pan and lid, recirculation pumps, and presence/absence of an internal fan that mixes headspace air within the sealed chamber. As examples of the repeatability and precision of the methodology, ten consecutive flux measurements were obtained using moist manure (32.6% wet basis water content, WC WB ) within a 15 min period in which chamber pans were fitted with lids for 60 s and removed for 30 s. The mean calculated N 2 O flux was 43.08 ±0.89 mg N 2 O m -2 h -1 . Using dry manure (WC WB = 10.8%), five consecutive flux measurements showed a very low, but consistent, flux that averaged 0.025 ±0.0016 mg N 2 O m -2 h -1 . Five case study experiments demonstrate the usefulness of these chamber systems and highlight discoveries and lessons learned to enhance future research efforts. Major discoveries and observations include: (1) installation of a small internal fan within the chamber lids decreased N 2 O fluctuation over small time periods, allowing precise measurement of manure N 2 O fluxes as low as 0.0073 mg N 2 O m -2 h -1 during a 60 s measurement period; (2) two distinct N 2 O peaks were observed at 1 and 21 d following the addition of water to manure (initial WC WB = 32.6%), with the second peak accounting for 83% of the total N 2 O emitted over 45 d; and (3) there was notable diurnal variation in N 2 O fluxes due to temperature variation, even when the manure was dry (WC WB = 10.8%). These flux chamber systems proved to be more rapid, precise, and repeatable than traditional flux chamber methods and offer promise for future greenhouse gas emissions research on manure and soil. Keywords: Cattle, Chamber, Diurnal, Fan, Feedlot, Greenhouse gas, Manure, Precision.