Variation characteristics of soil water-soluble organic carbon in permafrost regions of the Shule River headwaters

Water-soluble organic carbon (WSOC) is the most active component of soil carbon pool, and its production, transport and transformation are of great significance to the evaluation of soil quality, fertility and carbon pool stability. However, little research has been conducted on the changes and influencing factors of soil WSOC in the active layer of permafrost regions of the Qinghai-Tibet Plateau. In this study, the alpine meadow, located at the permafrost regions of Shule River headwaters in the western part of the Qilian Mountains and the northeast margin of the Qinghai-Tibet Plateau was selected as the study area. The variation characteristics of WSOC and WSOC/SOC in the vertical profile and season, as well as their influencing factors were analyzed. We found that: 1) The WSOC seasonal variation was remarkable (P < 0.05) and showed a "V" shape pattern, i.e., high in winter and spring, but low in summer and autumn. 2) WSOC of different soil layers in the same season showed different trends with soil depth (P > 0.05); namely, WSOC decreased with depth in spring, increased with depth in summer, and showed a "V" form in autumn but an inverted "N" form in winter, and the average annual WSOC content generally showed a downward trend with soil depth. 3) The WSOC/SOC ratio showed significant seasonal and vertical changes (P < 0.05), among which WSOC/SOC in different soil layers was largely an "N" type, with high values in winter and summer, low values in spring and autumn, and a seasonal variation range of 0.14% ~ 0.29%. WSOC/SOC in different soil layers in the same season showed different distributions with increasing of soil depth; namely, an "M" type in spring, increasing trend in summer and autumn, and a "V" type in winter. 4) Soil temperature and soil water content were the controlling factors of WSOC seasonal and depth-related (vertical) changes, while pH and soil temperature were the main controlling factors of WSOC/SOC vertical and seasonal changes. Investigation of the seasonal and vertical variation characteristics of WSOC and SOC can provide scientific data for the assessment and prediction of soil carbon pools in the active layer of permafrost in the Qinghai-Tibet Plateau under the influence of global warming scenarios. The variation characteristics of WSOC and WSOC/SOC with seasons and profiles are apparent, and the main control factors vary somewhat.