Long-term irrigation in a drought-prone pine forest leads to vertical redistribution of C stocks and accelerates C cycling in the soil

European forests are facing higher frequencies of extreme droughts, potentially impairing tree growth and ecosystem functioning. Drought limits the metabolic activity of plants and soil organisms, either directly or through reduced belowground carbon (C) allocation of recent assimilates, thus affecting C cycling in the plant-soil system. However, the net effect on belowground soil C storage is still unclear, as drought suppresses both C inputs from plants and outputs from soils. Moreover, understanding the underlying mechanisms is complicated due to long-term acclimation and adaptation of plant and soil organisms to water limitation.

We investigated the impact of repeated summer droughts in a Scots pine (Pinus sylvestris L.) forest on soil C storage and C cycling, taking advantage of a large-scale irrigation experiment  running since 2003 in a dry inner-Alpine valley in Switzerland (Pfynwald, Valais), which removed the “natural” water limitation. We assessed the responses of soil organic carbon (SOC) stocks and C fluxes by measuring litter fall and decomposition, fine root biomass and production, soil CO₂ effluxes, C-mineralization, and ¹³C-labelled glucose utilization by soil microorganisms. 

After 16 years of irrigation, the organic layers lost significant amounts of C (-1000 g m^-2), despite a 50% increase in litter fall. This C loss was almost compensated by a C gain in the mineral soil (+870 g m^-2) under irrigation. The decrease in C storage in the organic layers can be related to a three-fold increase in litter decomposition mainly through soil macrofauna as indicated by a litter-bag experiment. In parallel, the C gain in the mineral soil can be attributed mainly to increased incorporation of litter by soil fauna, together with greater C input from the rhizosphere (+70% fine root biomass for Scots pine in mineral soil). Furthermore, irrigation stimulated soil CO₂ efflux as well as microbial C-mineralization of organic and mineral soil, indicating enhanced soil C cycling. Addition of ¹³C-enriched glucose to mineral soils revealed a stronger utilization of this easily available C substrate in the drought than in the irrigated soils, together with a negative priming of soil organic matter (SOM) decomposition shortly after substrate addition. These results suggest that the altered quantity and quality of C inputs under irrigation has increased the availability of easily degradable C in soil.

This study reveals that long-term summer irrigation in a drought-prone pine forest has strong impacts on multiple interlinked processes of the soil C cycle. The removal of water limitation strongly altered vertical soil C distribution, accelerated soil C cycling and altered the substrate use by soil organisms, but had only a small net effect on the whole-profile SOC stocks.