Abstract: We measured total carbon content, organic carbon content, inorganic carbon content and water content of 0-5 m soil layers in five Pinus tabuliformis stands with densities of 1000, 1800, 2700, 3600 and 4500 plants·hm^-2 in the loess region of western Shanxi Province, to understand the responses of soil water and carbon at different soil layers to stand density. We used an improved coupling coordination degree model to quantify the intensity of the water-carbon trade-off/synergistic response to stand density. The results showed that soil organic carbon content exhibited surface aggregation (0-20 cm), and gradually decreased as soil depth increased. Soil inorganic carbon content initially increased and then decreased with soil depth, but there were no significant differences among soil depths. As stand density increased, soil organic carbon content increased gradually, while soil inorganic carbon increased initially but then decreased, and the density threshold was 2549 plants·hm^-2. Soil water content decreased initially and then increased with stand density, reaching a density threshold of 3268 plants·hm^-2. As stand density increased, the coupling coordinations of soil total carbon with water, and organic carbon with water, decreased initially and then increased. They showed synergistic developments among 1000-1800 plants·hm^-2, whereas transition harmonies among 1800-3600 plants·hm^-2, and then synergistic developments among 3600-4500 plants·hm^-2. That was a pattern of “synergy-trade-offs-synergy”. The coupling coordination of soil inorganic carbon and soil water decreased with the increase in stand density. There were synergistic developments among 1000-2700 plants·hm^-2, and transition harmonies among 2700-4500 plants·hm^-2, with a pattern of “synergy-trade-offs”. These results suggested that when the stand density of P. tabuliformis plantation was less than 1800 plants·hm^-2, it could reduce water consumption of forest and increase carbon sink, realizing the synergistic development of soil carbon sequestration and water conservation.

Key words: stand density, water-carbon coupling, trade-off and synergy, the loess region in western Shanxi