Contribution of advection to nighttime ecosystem respiration at a mountain grassland in complex terrain

Abstract

Net ecosystem carbon dioxide (CO2) exchange (NEE) at FLUXNET sites is typically evaluated by means of the eddy covariance technique using a set of instruments on a single tower. However, in complex terrain, such as mountain areas, and during nighttime atmospheric conditions, with low turbulent mixing and stable stratification, this approach is known to underestimate the nighttime NEE and thus bias longer-term carbon balances. This study reports on the quantification of advection at a subalpine grassland site in Northern Italy (2160 m asl) situated in complex mountainous terrain. We show that different published methods for indirectly or directly accounting for advection resulted in a large divergence in the annual carbon balance. Advection, and in particular the horizontal term, reached non negligible values during nighttime and its inclusion in the CO2 conservation equation increased NEE by a factor of two. NEE calculated by taking into account all terms (NEEacmb), i.e. turbulent exchange, change in storage and advection of CO2, matched the approach based on the after-sunset maximum in the vertical turbulent flux and change in the storage of CO2 and ecosystem respiration measured by automated chambers. Accounting for advection led to a 169% change in the annual carbon budget, turning the ecosystem from a sink (−108 gCm−2 y−1) to a source (75 gCm−2 y−1) of CO2

Comparing ecosystem and soil respiration: Review and key challenges of tower-based and soil measurements

“…During calm and stable night conditions, advection may be predominant (Cooper et al, 2006;Gu et al, 2005;Horst and Doran, 1986;Massman and Lee, 2002;Van Gorsel et al, 2007); thus, CO 2 produced near the ground can be transported laterally, and not measured by the EC tower (Aubinet et al, 2003;Baldocchi et al, 2000;Feigenwinter et al, 2008;Finnigan, 1999;Lee, 1998;Roland et al, 2015;Speckman et al, 2015). In contrast, during daytime, convective mixing often minimizes advection (Galvagno et al, 2017), creating appropriate micrometeorological conditions to apply the EC technique. Finally, since NEE is the difference between GPP and R eco there are two general ways to estimate R eco from EC (Desai et al, 2008): 1) estimating GPP using light-response curves fitted to daytime NEE (NEE Day ) to estimate daytime R eco as the difference of GPP and NEE (Lasslop et al, 2010); and 2) estimating R eco using nighttime NEE (NEE Night ) to fit an exponential relationship with air or soil temperature (Arrhenius, 1889) and extrapolating to daytime (thus, assuming that temperature functional relationship is the same for night and daytime); consequently, GPP is derived by adding NEE and R eco (Reichstein et al, 2005).…”

Evidence for a non-linear carbon accumulation pattern along an Alpine glacier retreat chronosequence in Northern Italy

“…The gas exchange measurements were done with the use of an automated closed transparent chamber (LI 8100-104C) attached to the analyzer LI 8100 (Li-Cor Biosciences, Lincoln, NE, USA). The measurements (see Varolo et al, 2016; Galvagno et al, 2017; Pavelka et al, 2018; Zhao et al, 2018 for comparison and evaluation of the technique) were conducted on 15 numbered iron collars, 20 cm diameter, divided into groups of five plots (C1; …C5) in each of the three transects. To understand the response of the selected small ecosystem to solar radiation, Nylon filters having the same dimension of the collar were inserted in increasing number inside the chamber, in order to progressively intercept the solar radiation.…”

Evapotranspiration of an Abandoned Grassland in the Italian Alps: Influence of Local Topography, Intra- and Inter-Annual Variability and Environmental Drivers

“…There is one site on a steep abandoned slope but it has no EC setup (AT-Stp, Stubai abandoned meadow, Austria ( [32], Appendix A, Table A1). Only one long-term EC site is located on an abandoned grassland in the Alps for eco-hydrological purposes, the Torgnon (IT-Tor) site [33] in Valle d'Aosta Region, in the Italian Western Alps. In that Region, grasslands cover up to 38% of the land surface area (1238 km 2 over 3260 km 2 , with the second highest portion of grassland cover between 1700 and 2200 m a.s.l., Figure A2 in Appendix A), and the topography can be very challenging.…”