Soil fauna drives vertical redistribution of soil organic carbon in a long‐term irrigated dry pine forest

Abstract

Summer droughts strongly affect soil organic carbon (SOC) cycling, but net effects on SOC storage are unclear as drought affects both C inputs and outputs from soils. Here, we explored the overlooked role of soil fauna on SOC storage in forests, hypothesizing that soil faunal activity is particularly drought‐sensitive, thereby reducing litter incorporation into the mineral soil and, eventually, long‐term SOC storage. In a drought‐prone pine forest (Switzerland), we performed a large‐scale irrigation experiment for 17 years and assessed its impact on vertical SOC distribution and composition. We also examined litter mass loss of dominant tree species using different mesh‐size litterbags and determined soil fauna abundance and community composition. The 17‐year‐long irrigation resulted in a C loss in the organic layers (−1.0 kg C m−2) and a comparable C gain in the mineral soil (+0.8 kg C m−2) and thus did not affect total SOC stocks. Irrigation increased the mass loss of Quercus pubescens and Viburnum lantana leaf litter, with greater effect sizes when meso‐ and macrofauna were included (+215%) than when excluded (+44%). The enhanced faunal‐mediated litter mass loss was paralleled by a many‐fold increase in the abundance of meso‐ and macrofauna during irrigation. Moreover, Acari and Collembola community composition shifted, with a higher presence of drought‐sensitive species in irrigated soils. In comparison, microbial SOC mineralization was less sensitive to soil moisture. Our results suggest that the vertical redistribution of SOC with irrigation was mainly driven by faunal‐mediated litter incorporation, together with increased root C inputs. Our study shows that soil fauna is highly sensitive to natural drought, which leads to a reduced C transfer from organic layers to the mineral soil. In the longer term, this potentially affects SOC storage and, therefore, soil fauna plays a key but so far largely overlooked role in shaping SOC responses to drought.

Younger carbon dominates global soil carbon efflux

“…Indeed, our sensitivity analysis suggests that global average a efflux is increased by 72% (from 194 to 334 year) if vertical transport is not considered (Figure S5). Vertical transport of soil C through soil profile has been recognized as a critical process regulating whole‐profile C dynamics (Bruun et al, 2007; Guidi et al, 2022; Kaiser & Kalbitz, 2012; Rumpel & Kögel‐Knabner, 2010). Yet, few studies have explored the detailed role of vertically transported soil C in controlling whole‐profile soil C dynamics, especially across large extents (Luo et al, 2020).…”

“…Indeed, our sensitivity analysis suggests that global average a efflux is increased by 72% (from 194 to 334 year) if vertical transport is not considered (Figure S5). Vertical transport of soil C through soil profile has been recognized as a critical process regulating whole-profile C dynamics (Bruun et al, 2007;Guidi et al, 2022;Kaiser & Kalbitz, 2012;Rumpel & Kögel-Knabner, 2010).…”

Drought resistance and resilience of rhizosphere communities in forest soils from the cellular to ecosystem scale – insights from 13C pulse labeling

“…The observed community shifts in the pine forest could have resulted either from long‐term adaptation of the microbial communities to changes in soil moisture or from changed soil properties and substrate availability associated with the different water regimes. The latter explanation is supported by observation of an altered vertical distribution of soil organic matter (SOM) with long‐term irrigation in the pine forest, with SOM losses in the organic layer but gains in mineral soil horizons (Guidi et al ., 2022). Our finding of increased ratios of gram‐positive to gram‐negative bacteria under drought in the pine forest suggests a shift toward communities using more recalcitrant SOM compared with labile plant‐derived C (Fanin et al ., 2014).…”

“…The mature pine forest is a naturally regenerated forest, dominated by c. site of 1.2 ha was randomly divided into eight plots of 25 9 40 m (1000 m 2 ) each, separated by a 5-m-wide buffer area (Guidi et al, 2022; Fig. S2).…”

“…575 mm, respectively. Soils are shallow Pararendzina with a mull‐ and Xeromoder‐type organic layer developed on an alluvial fan (Guidi et al ., 2022). The experimental site of 1.2 ha was randomly divided into eight plots of 25 × 40 m (1000 m 2 ) each, separated by a 5‐m‐wide buffer area (Guidi et al ., 2022; Fig.…”