[A-GE28] 地質媒体における流体移動、物質移行及び環境評価

[AGE28-P02] Soil respiration in an apple orchard in Tsugaru region under high air temperature and high atmospheric CO₂ conditions

*加藤 千尋¹、遠藤 明¹、伊藤 大雄¹ (1.弘前大学農学生命科学部)

キーワード：土壌呼吸、地球温暖化、リンゴ樹園地、土壌有機物

Soil CO₂ dynamics under global warming conditions have been studied mainly in natural vegetation such as forests and grasslands, rice paddies, and upland fields, but few studies have been conducted in field-managed orchard areas (Kato et al., 2021). In apple orchards, organic carbon is supplied to the soil through cultivation management such as undergrowth mowing, pruning of shoots, and fruit picking. The objective of this study is to understand the effects of elevated air temperature and atmospheric CO₂ concentration on soil respiration, i.e., decomposition of soil organic matter (SOM) and respiration of plant roots, in apple orchards in Tsugaru region of northeastern Japan. This study was conducted in the apple orchards in three plastic greenhouses at Fujisaki Farm of Hirosaki University. Apples were cultivated under (A) control, (B) high air temperature (+3°C), and (C) high air temperature and high atmospheric CO₂ concentration (+3°C, +0.02%) conditions. Environmental controls were implemented from April 2019 to December 2022, and monitoring volumetric water content, soil temperature, and soil CO₂ concentration at 15 and 40 cm depth under each condition was conducted (Kato et al., 2021). CO₂ fluxes from the ground surface were also measured in November 2020 and May 2021. The aboveground of all apple trees were cut in April 2023, and soil samples were collected in June of the same year. Then soil ignition loss and C/N ratio were measured in the laboratory. Dissolved organic carbon (DOC) (mg/kg) was determined by extracting the soil with water and measuring TOC in the solution filtered through a 0.22-μm filter (Wagai and Phillip, 2002). A small amount of the soil was added to the extracted solution and incubated for approximately one month, and the percentage of DOC degraded during that time was calculated as biodegradable dissolved organic carbon (BDOC).
The CO₂ concentration in the soil was higher in the order of (C) > (B) > (A) throughout the experimental period (Kato et al., 2021). The CO₂ flux was also (C) > (B) > (A) in November 2020 (Kato et al., 2021), but (B) > (A) > (C) in May 2021. This may be because soil moisture in the surface layer of (C) was near saturation during the May CO₂ flux measurement, which limited CO₂ release to the atmosphere. The ignition loss and total soil carbon content at the depth of 0-10 cm were significantly higher under the high temperature and CO₂ conditions. The C/N ratio at 0-10 cm depth showed little difference among conditions, while at 20-30 cm depth, the C/N ratio was lower under high temperature and CO₂ conditions, indicating that the difference was not clear in the surface layer due to the high supply of organic carbon. Since soil DOC was about twice higher in the 0-10 cm depth compared to the 20-30 cm depth, and BDOC was also higher in the 0-10 cm depth (about 57-84%) than in the 20-30 cm depth (about 33-66%), easily decomposed organic matter is distributed closer to the soil surface. Both DOC and BDOC at 0-10 cm depth were lower in (B) and (C) than in (A). In this apple orchard, the organic matter (OM) supplied to the soil on a steady basis was C > B > A (Kato et al., 2021), and as mentioned above, ignition loss and total carbon content was higher in C. Therefore, while OM decomposition proceeded rapidly under high temperature and high CO₂ conditions, the proportion of readily decomposable OM may have been low compared to the increased total OM supplied. Meanwhile, Ito et al. (unpublished) showed that under high temperature and high CO₂ conditions in this apple orchard, root mass tended to increase, especially at depths deeper than 30 cm from the soil surface. These results suggest that high temperature and high CO₂ conditions increased the overall soil respiration rate by increasing soil CO₂ production in the surface layer through OM decomposition and respiration by roots of undergrowth and apples, and in the lower layer mainly through respiration by apple tree roots.