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.

据报道，与旱地系统相比，长期温带牧草灌溉会增加或减少土壤有机碳（SOC）储量。了解灌溉对碳（C）固定和分配到植物和土壤成分的短期影响可能对解释观察到的差异很重要。连续¹³ CO ₂黑麦草（的脉冲标记黑麦草大号。）和白三叶草（白三叶L.）种植的介观膜用于量化新光合产物C在植物-土壤系统的地上和地下成分中的净积累和分配，包括土壤粒径级分：> 250μm，53-250μm，20-53μm，在模拟灌溉和旱地条件下为5–20μm和<5μm。

在标记¹³ CO ₂之后，灌溉使分配到草丛中的¹³ C的量增加了16％，而减少了15–25 cm土壤深度中分配到根部的¹³ C的量了35％。但是，在根际土壤（0-15厘米深）中观察到较少的新光合产物C，而在灌溉条件下，更多的新光合产物C被分成53-250μm和<5μm的土壤部分。尽管存在这些差异，但不同处理之间（2511千克新的Cha ^-1旱地和2509千克新的Cha ^-1灌溉）之间，整个土壤（0-25厘米深）中新光合产物C的净含量相似。因此，尽管提高了地上牧场的生产力，但灌溉并没有增加土壤中新光合产物C的净含量。

根据我们的结果，我们假设最近报道的灌溉牧场的SOC损失可能是由根系C的更快周转引起的，这可能解释了POM细土壤粒径（53–250μm）中光合产物C的增加。 ，而不是减少光合作用C向土壤的投入。