TEMPORAL AND SPATIAL PATTERNS OF DEEP PERCOLATION AND PEDOGENESIS IN SANDY SOILS OF THE NORTHERN GREAT LAKES REGION

Precipitation falling onto the surface may either evaporate, run off or infiltrate into soils. In this paper, we focus on water that infiltrates into and through soils, as estimated by a water-balance model and as captured/verified by in situ lysimeters. Rarely are soils so completely instrumented as we have done at our sites in the northern hardwood and pine forests of northern Michigan. Here, we have installed zero-tension lysimeters below the O, E and B horizons of six different Spodosol pedons. Three sites are beneath red pine plantations, while three others are in mixed hardwoods. At these six sites, we captured soil water after 36 major precipitation and snowmelt events, resulting in 505 water samples over a 2-year timespan. After the soil water volumes were determined, we analyzed it for dissolved organic carbon (DOC) content. Our data suggest that deep percolation, and hence, pedogenesis and groundwater recharge, in this environment is mainly limited to two periods: fall rains and spring snowmelt. These two periods represent the primary times when podzolization is occurring, and when deep percolation is most likely. Translocation of DOC and its associated metals, i.e., podzolization, is most pronounced during snowmelt, when percolation is deep, fairly steady, and may last for weeks. Percolation from fall rains may be more intermittent, but fresh litter on the forest floor at this time make large amounts of C available, thereby promoting this secondary period of podzolization. Little pedogenesis, and almost no groundwater recharge, occurs during summer or winter.

Related research on treethrow microtopography in this area has shown that, because they have such high permeabilities, pits and mounds formed in these sandy soils are only slowly eroded. Thus, they have great longevities, becoming almost semi-permanent features of the forest floor. Percolation and pedogenesis are promoted within pits, whereas snowmelt and rain are shed off mounds. Pit sites, therefore, may remain as percolation foci for thousands of years, and soils within them are often exceptionally developed. As a result, this landscape shows how microtopography can provide foci for deep percolation and groundwater recharge, even at times when the climate or soil-landscape at large may not otherwise be wet enough for it.