Depletion of soil carbon and aggregation after strong warming of a subarctic Andosol under forest and grassland cover

Abstract

Abstract. The net loss of soil organic carbon (SOC) from terrestrial ecosystems is a likely consequence of global warming and may affect key soil functions. The strongest changes in temperature are expected to occur at high northern latitudes, with forest and tundra as prevailing land cover types. However, specific soil responses to warming in different ecosystems are currently understudied. In this study, we used a natural geothermal soil warming gradient (0–17.5 ∘C warming intensity) in an Icelandic spruce forest on Andosol to assess changes in the SOC content between 0 and 10 cm (topsoil) and between 20 and 30 cm (subsoil) after 10 years of soil warming. Five different SOC fractions were isolated, and their redistribution and the amount of stable aggregates were assessed to link SOC to changes in the soil structure. The results were compared to an adjacent, previously investigated warmed grassland. Soil warming depleted the SOC content in the forest soil by −2.7 g kg−1 ∘C−1 (−3.6 % ∘C−1) in the topsoil and −1.6 g kg−1 ∘C−1 (−4.5 % ∘C−1) in the subsoil. The distribution of SOC in different fractions was significantly altered, with particulate organic matter and SOC in sand and stable aggregates being relatively depleted and SOC attached to silt and clay being relatively enriched in warmed soils. The major reason for this shift was aggregate breakdown: the topsoil aggregate mass proportion was reduced from 60.7±2.2 % in the unwarmed reference to 28.9±4.6 % in the most warmed soil. Across both depths, the loss of one unit of SOC caused a depletion of 4.5 units of aggregated soil, which strongly affected the bulk density (an R2 value of 0.91 and p<0.001 when correlated with SOC, and an R2 value of 0.51 and p<0.001 when correlated with soil mass in stable aggregates). The proportion of water-extractable carbon increased with decreasing aggregation, which might indicate an indirect protective effect of aggregates larger than 63 µm on SOC. Topsoil changes in the total SOC content and fraction distribution were more pronounced in the forest than in the adjacent warmed grassland soils, due to higher and more labile initial SOC. However, no ecosystem effect was observed on the warming response of the subsoil SOC content and fraction distribution. Thus, whole profile differences across ecosystems might be small. Changes in the soil structure upon warming should be studied more deeply and taken into consideration when interpreting or modelling biotic responses to warming.

Long‐term geothermal warming reduced stocks of carbon but not nitrogen in a subarctic forest soil

“…Studies with a relatively long time span of observation (e.g. Poeplau et al, 2020, Melillo et al, 2002, 2017) reported results that are consistent with our findings. Melillo et al (2017) observed a long‐term pattern of rapid SOC loss at the start of soil warming but reaching a new equilibrium between SOC mineralization and build‐up later in the experiment.…”

“…that in the S+A, S+C and rSOC fractions) was the same in all soils along the warming gradient. This is in contrast to the findings reported in a similar geothermal warming experiment in Iceland, where SOC in the S+A fraction, as well as aggregates, declined strongly with warming; in this study, the silt and clay-associated SOC was lost at a similar rate as bulk SOC (Poeplau et al, 2016(Poeplau et al, , 2020. However, this does not imply that those fractions were not affected by warming in the present study.…”

“…As the rSOC fraction consisted of less than 1 g, it was not possible to distinguish inorganic C and SOC. The fractionation method developed by Zimmermann et al (2007) has been applied in other comparable geothermal warming studies (Poeplau et al, 2016(Poeplau et al, , 2020. Furthermore, in a large-scale method comparison with 20 different fractionation methods, this method was found to be among the most successful in isolating pools of distinct turnover rates (Poeplau et al, 2018).…”

“…A possible structural change of the soil such as a greater decay of stable aggregates (Poeplau et al, 2020) affect microbial community composition and metabolism (Manzoni, 2017;Takriti et al, 2018). Recently, Ofiti et al (2021) also found that SOM in warmed soil was altered in its chemical composition and comprised more microbial-derived compounds.…”

“…Apart from evaluating changes in SOM quality, fractionation can also be used to indicate soil structural changes or reveal prevailing destabilization mechanisms. For example, in the Icelandic geothermal warming experiment mentioned earlier, Poeplau et al (2016, 2020) detected the depletion of stable aggregates associated with the loss of SOM and concluded that either loss of SOM decreased aggregate stability or weakening of aggregate binding mechanisms led to the destabilization of SOM.…”