DOC in soil solution and stream water
The mean DOC concentration in the soil solution was 25.56 (± 12.06) mg.L-1 at 50 cm depth and 21.76 (± 7.71) mg.L-1 at 100 cm depth for the studied hillslope. There was significant spatial variation so that SMT showed higher values for both depths compared to VBT, and that difference was less significant for 100 cm (P = 0.021) compared to 50 cm depth (P < 0.001) (table 5).
Soil solution DOC concentrations tended to decrease with increasing depth, and this reduction was smaller (P = 0.0356) at the SMT topsoil (0-50 cm) in comparison to VBT. However, there is an additional behavior associated with increased precipitation intensity which results in a reduction in this trend and causes the concentration to increase in depth sometimes.
Such behavior occurred in the richest OM-incorporated compartments: soil top (0-50 cm) in the SMT; and at higher depth (50-100 cm) in the VBT, where the typical local colluvial dynamics of relief evolution usually bury OM. In the VBT case, this behavior was observed in five of the eight events analyzed, resulting in no significant difference (P > 0.05) between their two depths about the mean DOC concentrations (eight-events mean). In contrast, that difference was quite significant in SMT (P < 0.001).
In this context, the regression analyses showed significant linear models inversely proportional to the precipitation intensity and: the difference between the DOC concentrations at 0 (UL data) - 50 cm in the SMT; the difference between DOC concentrations at 50-100 cm in the VBT (table 6). In general, our results indicate that the decrease tendency of DOC concentrations in the deep soil solution is reduced in events of higher precipitation intensity, so that part of the incorporated rich mineral OM compartment may behave as a DOC source (desorption) rather than an adsorption trend.
The SW showed similar DOC concentrations (P > 0.05) in post-event baseflow conditions. The SW DOC was lower (P = 0.0117) than that found in the BP, which demonstrates the soil efficiency as OM sorption and mineralization environment. Even with sandy and relatively shallow soils, it can efficiently retain the OM originated by the high primary productivity of the area.
A significant inversely proportional trend was observed between the SW DOC concentrations and the rainfall intensity. This result indicates that the OM desorption behavior in higher rainfall intensity occasions does not materialize in an increase in the SW DOC concentration when it returns to the base level. On the contrary, there was a tendency for a slight reduction in concentrations.