Seasonal irrigation affects the partitioning of new photosynthate carbon in soil

Citation

Carmona CR, Clough TJ, McNally SR, Beare MH, Tregurtha CS & Hunt JE (2020) Seasonal irrigation affects the partitioning of new photosynthate carbon in soil. Soil Biology and Biochemistry, 143, Art. No.: 107751. https://doi.org/10.1016/j.soilbio.2020.107751

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.

Keywords
Irrigation; Pasture; Photosynthate carbon; Soil carbon

Journal
Soil Biology and Biochemistry: Volume 143