Autor:innen:
Marcus Schiedung | Department of Geography, University of Zurich, Switzerland | Switzerland
Axel Don | Thünen Institut für Agrarklimaschutz
Michael Beare | New Zealand Institute for Plant and Food Research Limited | New Zealand
Samuel Abiven | Département de Géosciences, CNRS, École normale supérieure, PSL University | France
Soils have a large potential for the sequestration of soil organic carbon (SOC) to mitigate global climate change when introducing climate-smart practises, such as deep rooting systems. However, newly added root litter or exudates can enhance the mineralization of native SOC (positive priming effect), which may counterbalance the sequestration of new carbon (C). We used soils from a 20 years chronosequence of inverted grassland soils (>1 m deep melioration) and studied the root induced priming in newly formed topsoils, which accumulated C after inversion and in SOC enriched subsoils that consisted of former topsoils. These soils were incubated for six months after introducing labelled and fresh roots (>2 atom% 13C) by growing ryegrass under 13C-enriched atmosphere and removing aboveground biomass before incubation. By tracing the respired root-derived C and its recovery in soils and fractions, this chronosequence provided the unique opportunity to study the priming effect of roots in soils with different durations of C input under field conditions and consequently SOC levels. We found that newly added fresh root biomass causes positive priming of SOC in subsoils (331 ± 84% enhanced respiration) and newly formed topsoils (109 ± 27% enhanced respiration) within six months of incubation. This effect was a transitional phenomenon for new topsoils that accumulating SOC, which adapted to high C input within a few years, leading to no priming in long-term topsoils. Soils adapted to high C inputs showed a faster cycling of fresh roots and remaining roots were associated to the mineral soil fractions and a memory effect occurred if topsoils were buried below one meter depth. In recently buried topsoil the priming effect was low (50 ± 2% enhanced respiration) but substantially increased 20 years after burial (390 ± 32% enhanced respiration). Overall, C losses with priming never exceeded new root-derived C inputs and resulted in C gains even under highly favourable incubation conditions. We conclude that priming is a temporal reaction to additional C which attenuates when soils get adapted to high C inputs within a few years to decades.