Characterising macropores and preferential flow of mountainous forest soils with contrasting human disturbances

Abstract

Preferential flow can develop in soil macropores, and macropores are sensitive to human disturbances. This study investigated soil macropore features and the main factors controlling preferential flow at four sites with different levels of human disturbance in a mountainous area in Central China. The level of human disturbance decreased with increasing elevation, with the lowest elevation areas covered with coniferous trees (LF) > middle mountain areas covered with tea gardens (TG) > middle mountain areas covered with deciduous trees and mixed shrubs (MF) > subalpine areas covered with evergreen coniferous trees (HF). At each site, the soil macropore structure at 0–20 cm soil depth was analysed using computed tomography scans (0.6 mm resolution) and Image J software. Preferential flow was determined by analysing the breakthrough curve (BTC) of nitrate. The macroporosity, surface area density, mean macropore size, macropore number density, length density and node density were all ranked in the order of HF ≥ MF ≥ TG = LF. Less disturbed sites had stronger evidence of preferential flow as shown by faster breakthrough, longer tails and greater asymmetry of the BTCs. There were significant (P < 0.05) positive influences of soil macropore properties on pore water velocity and the solute dispersion coefficient. The dispersivity parameter was mainly affected by the macropore equivalent hydraulic radius. This study showed that human disturbance in the mountain forest areas significantly decreased soil macropores by changing soil physical properties (e.g. bulk density, texture and soil organic matter content) and root distribution, thus increasing the risk of surface runoff and nutrient losses.