Seasonal irrigation affects the partitioning of new photosynthate carbon in soil

Abstract

Long-term irrigation of temperate pastures has been reported to either increase or decrease soil organic carbon (SOC) stocks when compared with dryland systems. Understanding the short-term effects of irrigation on the fixation and partitioning of carbon (C) to plant and soil components may be important to explaining the observed differences. Continuous ¹³CO₂ pulse labelling of ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) planted mesocosms was used to quantify the net accumulation and partitioning of new photosynthate C to above- and below-ground components of the plant-soil system, including soil particle size fractions: > 250 μm, 53–250 μm, 20–53 μm, 5–20 μm and <5 μm, under simulated irrigation and dryland conditions. After the ¹³CO₂ labelling, irrigation increased the quantity of ¹³C partitioned into herbage by 16%, while reducing the quantity partitioned into roots in the 15–25 cm soil depth by 35%. However, less new photosynthate C was observed in rhizosphere soil (0–15 cm depth), while more new photosynthate C was partitioned into the 53–250 μm and <5 μm soil fractions under irrigation. Despite these differences, the net amount of new photosynthate C in the whole soil (0–25 cm depth) was similar between treatments (2511 kg new C ha⁻¹ dryland and 2509 kg new C ha⁻¹ irrigated). Therefore, irrigation did not increase the net amount of new photosynthate C in the soil despite increased above-ground pasture productivity. Based on our results, we hypothesise that the recently reported losses of SOC from irrigated pastures may be driven by faster turnover of root-derived C, which may explain the increase in photosynthate C in the fine POM soil size fraction (53–250 μm), rather than a reduction in photosynthate C inputs to the soil.