Preferential flow has an important role as it strongly influences solute transport in forest soil. The quick passage of water and solutes through preferential flow paths without soil absorption results in considerable water loss and groundwater pollution. However, preferential flow and solute transport under different infiltration volumes in southwestern China remain unclear. Three plots, named P20, P40 and P60, were subjected to precipitation amounts of 20, 40 and 60 mm, respectively, to investigate preferential flow and solute transport characteristics via field multiple-tracer experiments. Stained soils were collected to measure Br⁻ and NO₃⁻ concentrations. This study demonstrated that precipitation could promote dye tracer infiltration into deep soils. The dye tracer reached the maximum depth of 40 cm in P60. Dye coverage generally reduced with greater depth, and sharp reductions were observed at the boundary of matrix flow and preferential flow. Dye coverage peaked at the soil depth of 15 cm in P40. This result demonstrated that lateral infiltration was enhanced. The long and narrow dye coverage pattern observed in P60 indicated the occurrence of macropore flow. Br⁻ and NO₃⁻ were found at each soil depth where preferential flow had moved. Increasing precipitation amounts increased Br⁻ and NO₃⁻ concentration and promoted solute movement into deep soil layers. Solute concentration peaked at near the end of the preferential flow path and when preferential flow underwent lateral movement. These results indicated that the infiltration volume and transport capacity of preferential flow had important effects on the distribution of Br⁻ and NO₃⁻ concentrations. The results of this study could help expand our understanding of the effects of preferential flow on solute transport and provide some suggestions for protection forest management in southwestern China.

Understanding the response of preferential flow paths to water movement is an important topic in soil hydrology. However, quantification of the complicated distribution patterns of preferential flow paths remains poorly understood. Therefore, dye experiments were conducted to investigate preferential flow characteristics under three different precipitation amounts (20, 40 and 60 mm, numbered as the G20, G40 and G60, respectively) in Simian Mountain grassland, Chongqing province, China. O-ring statistics were used to analyze the spatial distribution characteristics and the spatial correlation of preferential flow paths. Results revealed that precipitation could promote dye tracer infiltration into deeper soils, reaching the maximum depth of 55 cm in G60. The number of preferential flow paths in G60 plots was 3.0 and 7.4 times greater than those of G40 and G20, respectively. Structural distribution of the preferential flow paths showed a gradually clumped pattern with the increase of precipitation, which was conducive to enhancing the correlation between preferential flow paths in each pore size range. These results could expand our understanding of the effects of precipitation on the characteristic of preferential flow paths in grassland, which is helpful to evaluate the water movement in the study area.

Within the context of global climate change, carbon sequestration by poplar is receiving increasing attention, both in China and globally. However, factors including limited sample sizes, multiple data sources, and inconsistent methodologies have limited accurate estimates of carbon sequestration by poplar to only that of aboveground biomass (AGB) and belowground biomass (BLB) carbon pools; whereas the estimations of forest understory (LGB) and dead wood (KSB) remain uncertain. This study conducted a field campaign involving 123,385 monitoring plots and 163 carbon sink-specific standard plots. The results showed that the total carbon pool of the poplar ecosystem was 413.64 Tg C, of which 98.39% and 1.61% were stored in biomass and dead organic matter, respectively. The carbon densities of different carbon pools exhibited strong correlations with climate, with the carbon sequestration capacities of AGB and BLB first decreasing sharply and then increasing gradually with increasing mean annual temperature (MAT) and mean annual precipitation (MAP), following which they gradually stabilized after exceeding a certain threshold value. The rank of climate variables in terms of their influence on the remaining carbon pools was mean relative humidity (MRH) > MAT > MAP. The results also showed strong correlations between different carbon pools and altitude, slope degree, slope aspect, and slope position. This study provided updated estimates of the carbon pools of poplar ecosystems based on direct field measurements and can provide a valuable reference for the validation and parameterization of carbon models in China and globally.