Summer Irrigation increases organic carbon turnover in managed pastures

Summer irrigation in temperate managed pastures enhances aboveground productivity during soil water-limited periods, but its effects on soil organic carbon (SOC) dynamics remain insufficiently understood, with reported effects often contradictory. Our objective was to quantify the effects of summer irrigation on the short-term fate of photo-assimilated carbon (C) in the entire pasture and soil system.

Using mesocosms containing ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) that were maintained to simulate spring conditions in New Zealand, a ¹³CO₂ labelling experiment was conducted. After the labelling, mesocosms underwent an irrigation period during summer, followed by a post-irrigation period. The ¹³C-labelled mesocosms (n = 48) were sampled in sets over five sampling times: 1, 15, 140, 225 and 334 days after the last labelling event.

Over the irrigation period (15 and 140 days after the last labelling event), irrigation increased carbon losses through leaf harvest (threefold higher than non-irrigated systems) and reduced root biomass by 2000 kg dry matter ha⁻¹. At the end of both, the irrigation and post-irrigation periods, the quantity of ¹³C remaining in roots in the irrigated treatment was lower by 70% and 60%, respectively, compared to non-irrigated conditions. Non-irrigated conditions favoured the retention of photo-assimilated ¹³C in roots and in the mineral-associated organic matter size fraction (<5 µm), while irrigation promoted fine particulate organic matter formation (53-250 µm).

These findings highlight that summer irrigation accelerates carbon turnover in roots and mineral-associated fractions, potentially reducing long-term SOC storage under intensified pastoral systems.