Heterotrophic Respiration and the Divergence of Productivity and Carbon Sequestration

Abstract

Net primary productivity (NPP) and net ecosystem production (NEP) are often used interchangeably, as their difference, heterotrophic respiration (soil heterotrophic CO2 efflux, RSH = NPP−NEP), is assumed a near‐fixed fraction of NPP. Here, we show, using a range‐wide replicated experimental study in loblolly pine (Pinus taeda) plantations that RSH responds differently than NPP to fertilization and drought treatments, leading to the divergent responses of NPP and NEP. Across the natural range of the species, the moderate responses of NPP (+11%) and RSH (−7%) to fertilization combined such that NEP increased nearly threefold in ambient control and 43% under drought treatment. A 13% decline in RSH under drought led to a 26% increase in NEP while NPP was unaltered. Such drought benefit for carbon sequestration was nearly twofold in control, but disappeared under fertilization. Carbon sequestration efficiency, NEP:NPP, varied twofold among sites, and increased up to threefold under both drought and fertilization.

Asynchrony of the seasonal dynamics of gross primary production and ecosystem respiration

“…Thus, only when aboveground (especially leaf and apical) growth slows, are assimilated carbohydrates transported preferentially downward, to stem, roots and rhizosymbionts. Such spatially and temporally uneven availability of carbon substrates to different tissues is also consistent with the observations of (i) tight diurnal coupling between respiration and GPP (Mitra et al 2019), as well as (ii) their semiindependence (Noormets et al 2021).…”

“…The threshold-based phenology extraction method was used to extract the seasonality metrics for GPP and RE from FLUXNET and BEPS. We first removed the outliers based on a robust outlier exclusion method used in Yang and Noormets (2021) (t−t2) ) c2…”