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.Key PointsSuppressed heterotrophic respiration by drought and fertilization amplified carbon sequestration responses compared to those in productivityCarbon sequestration efficiency increased up to threefold under both drought and fertilizationTrade-off between biomass production and carbon sequestration has implications for forest management for climate mitigation purposesPlain Language SummaryTwo metrics of ecosystem productivity—vegetation biomass accumulation (net primary production, NPP) and net ecosystem carbon gain (NEP)—are often used interchangeably, as their difference, heterotrophic respiration, is assumed a relatively constant fraction of the former. Here, we show that soil heterotrophic respiration responds to water availability (a major climate change variable) and nutrient availability (a major management factor) differently than does NPP, leading to the divergence of NPP and NEP responses to water and nutrient availability.