Net soil carbon balance in afforested peatlands and separating autotrophic and heterotrophic soil CO<sub>2</sub> effluxes

Abstract

Abstract. Peatlands are a significant global carbon (C) store, which can be compromised by drainage and afforestation. Quantifying the rate of C loss from peat soils under forestry is challenging, as soil CO2 efflux includes both CO2 produced from heterotrophic peat decomposition and CO2 produced by tree roots and associated fungal networks (autotrophic respiration). We experimentally terminated autotrophic below-ground respiration in replicated forest plots by cutting through all living tree roots (trenching) and measured soil surface CO2 flux, litter input, litter decay rate, and soil temperature and moisture over 2 years. Decomposition of cut roots was measured and CO2 fluxes were corrected for this, which resulted in a large change in the fraction heterotrophic : autotrophic flux, suggesting that even 2 years after trenching decaying root biomass makes significant contributions to the CO2 flux. Annual peat decomposition (heterotrophic CO2 flux) was 115 ± 16 g C m−2 yr−1, representing ca. 40 % of total soil respiration. Decomposition of needle litter is accelerated in the presence of an active rhizosphere, indicating a priming effect by labile C inputs from roots. This suggests that our estimates of peat mineralization in our trenched plots are conservative and underestimate overall rates of peat C loss. Considering also input of litter from trees, our results indicate that the soils in these 30-year-old drained and afforested peatlands are a net sink for C, since substantially more C enters the soil organic matter than is decomposed heterotrophically. This study does not account for fluvial C fluxes, which represent a small flux compared to the CO2 soil efflux; further, root litter and exudate deposition could be a significant C source that is only partially sampled by our approach, adding to these plantations being a potential carbon sink. However, the C balance for these soils should be taken over the lifespan of the trees, in order to determine if the soils under these drained and afforested peatlands are a sustained sink of C or become a net source over longer periods of forestry.

A globally robust relationship between water table decline, subsidence rate, and carbon release from peatlands

“…1d, e), reflecting the similar dynamics of these two effluxes in response to WT decline. Recent annual measurements also revealed strong linear correlation between SR and HR from temperate to boreal pristine and drained peatlands 40 . These results collectively indicated that increases in SR from global pristine peatlands under WT decline were mainly due to HR rather than AR.…”

Impact of Lulucf Accounting Rules for Climate Change Mitigation Goals: Winning or Losing?

“…However, peatlands can be now optionally accounted under EU rules until 2026, afterwards accounting of this category becomes mandatory (EU 2018/841). Furthermore, recent study in Scotland (Hermans et al, 2022) shows that afforestation of previously drained peatlands and further forest management activities can even increase soil carbon stocks due to litter input which exceed heterotrophic peat decomposition, while default reporting guidelines (Tier 1 level, IPCC 2006) still request reporting of GHG emissions due to drainage of organic soils. Hence, increasing reporting accuracy by setting reliable emission factors on the national levels and moving to higher Tier level, as well as aligning reporting and accounting rules under different mechanisms would be beneficial.…”

Temporal Variation and Hysteresis of Soil Respiration and Sap Flow of Pinus densiflora in a Cool Temperate Forest, Japan

“…Using the trenching method, Tomotsune et al [31] found that the soil temperature response of Rh is higher than that of Ra in a broad-leaved secondary forest. The trenching method allows Ra and Rh to be physically separated [3,32,33], and some studies have…”

“…Using the trenching method, Tomotsune et al [31] found that the soil temperature response of R h is higher than that of R a in a broad-leaved secondary forest. The trenching method allows R a and R h to be physically separated [3,32,33], and some studies have separated not only R h but also soil organic matter respiration [12] and mycorrhizal respira-tion [12,34]. The contribution of R a is 45.8% among forest sites around the world [10,11].…”