Moss biocrusts regulate surface soil thermal properties and generate buffering effects on soil temperature dynamics in dryland ecosystem

Biocrusts are promising ecosystem engineers in dryland ecosystems, but their effects on soil temperature, which is the most important environmental factor of soil biological and biochemical processes, have not yet been well understood. In a semiarid ecosystem on the Chinese Loess Plateau, the thermal properties of moss-dominated biocrust layer and bare soil (upper 2 cm) were measured, and their correlations with the biocrust characteristics as well as soil properties (especially soil water content, theta) were analyzed. Afterwards, the soil temperature dynamics of the biocrust covered soil and bare soil were continuously recorded at 2, 6, and 10 cm depths during a year. From in-situ measurements in wet season, we found that the biocrusts increased surface soil heat capacity (C) by 10.3% (1.42 vs. 1.28 MJ m(-3) K-1), thermal conductivity (lambda) by 27.7% (0.69 vs. 0.54 W m(-1) K-1), and thermal diffusivity (alpha) by 27.9% (5.01 vs. 3.92 x 10(-7) m(2) s(-1)) as compared with the bare soil, through holding more soil water (theta of the biocrusts vs. bare soil = 0.06 vs. 0.03 cm(3) cm(-3)). However, in dry season (theta < 0.05 cm(3) cm(-3)) they decreased 33.4% of surface soil C (0.78 vs. 1.17 MJ m(-3) K-1), 54.9% of lambda (0.17 vs. 0.37 W m(-1) K-1), and 22.9% of a (2.32 vs. 3.01 x 10(-7) m(2) s(-1)) mostly by decreasing surface soil bulk density. Accordingly, the biocrusts decreased soil temperature by as much as 6.3-11.1 degrees C at 0-10 cm depth in wet season (summer), and they increased soil temperature by up to 1.3-3.7 degrees C in dry season (winter). More importantly, the diurnal range of soil temperature of the biocrust covered soil was sometimes as much as 6.8-9.4 degrees C lower than that of the bare soil at 0-10 cm depth. The decreasing or increasing effects of the biocrusts on soil temperature were exactly explained by the increased thermal properties of the biocrust layer in wet season or reduced thermal properties of the biocrust layer in dry season. We concluded that the biocrusts regulated surface soil thermal properties through increasing soil water holding capacity and decreasing soil bulk density; thus, they generated considerable buffering effects on soil temperature dynamics. In dryland ecosystem, such buffering effects of the biocrusts on soil temperature should be highly considered in various soil biological and biochemical processes.