Philip Schoeneberger

Directional saturated hydraulic conductivity and macropore morphology of a soil-saprolite sequence

Abstract The soils in the Piedmont and Mountain regions of the southeastern United States are characterized by the presence of saprolite at or near the soil surface. This study was conducted to describe the macropore network of a representative soil-saprolite sequence and to relate the directional saturated hydraulic conductivity of the soil profile to water flow through macropores. Large observation pits were constructed at three geomorphic positions (ridge top, shoulder and ridge nose). Soil-saprolite macropores and their morphology were described in the field. Triplicate intact soil cores were collected in five orientations: one vertical, two horizontals (perpendicular to one another) and two diagonals (perpendicular to one another) from the Bt, B/C and C (massive saprolite) horizons at each of the geomorphic positions. The saturated hydraulic conductivity (Ka) of each intact core sample was determined in the laboratory. In situ Ka of each horizon was also determined near the three observation pits. Mean Ka values for the five orientations, three horizons and three geomorphic positions ranged from 8.20×10−8 m s−1 to 2.75×10−6 m s−1. Mean in situ Ka values ranged from 7.50×10−8 m s−1 to 2.66×10−6 m s−1. The results indicated that no significant differences existed among the Ka values in different orientations for each horizon-landscape combination, with no exceptionally high conductivity value. The in situ Ka results generally agree with the results for the core samples.

Simulation of the Potential Impacts of Acidic Deposition on Forest Soils in the South

Various criteria have been used to evaluate the sensitivity of soils and waters to acidic desposition (Chap. 5), but these criteria remain largely unvalidated. Two approaches to test the relevance of these criteria are computer simulation modeling and long-term field studies. Simulation models have the advantage of rapid evaluation, but the key processes depicted in models need to be tested against field data to develop confidence in model predictions. Long-term field studies can provide definitive evaluations of sensitivity criteria; unfortunately, long-term historical data are currently limited and cannot be used for regional assessments. In the absence of regionally representative data from long-term projects, we used the MAGIC model (Modeling Acidification of Groundwater in Catchments, Cosby et al. 1985a) to evaluate the sensitivity of a representative set of southern forest soils. We chose this model over others currently available because it depicts critical chemical processes that are thought to control soil water acidification and because its limited data requirements allowed us to use information available in the South (unlike more complicated, data-intensive models such as the Integrated Lake-Watershed Acidification Study [ILWAS]) (Goldstein et al. 1985). Sensitivity of soils was gauged by (1) depletion of exchangeable basic cations, (2) increases in soil-solution concentrations of aluminum, and (3) the alkalinity of drainage waters.

Forest Soils of the South

The impacts of acidic deposition depend strongly on the specific chemistry of each soil and the nutrient cycles within each ecosystem. To provide perspective on the general characteristics, patterns, and diversity of forest soils in the South, the general geomorphology of the region and the major features of the most important and extensive soil groups are described herein.

Nutrient Cycles and Nutrient Limitations in the South

The impact of acidic deposition on forest soils depends largely on the nutrient cycles of the forest, and management treatments (such as fertilization) may also be important. As noted in chapter 2, if a forest utilizes the nitrate or sulfate from atmospheric deposition, the associated acidity is neutralized. The acidity of a soil with an abundant supply of nutrient cations may be less affected than a similar soil with a lower supply, and soil changes may not be proportional to changes in forest productivity. In this chapter, we review the cycles of nitrogen, sulfur, and cations and we discuss the degree to which these nutrients limit forest productivity in the South.

Previous Evaluations of Sensitivity of Southern Soils

Several studies have provided information on changes in soil chemistry for specific sites in the South, and others have evaluated the regional sensitivity of soils to strong acid inputs. In this chapter, we discuss the site-specific studies and then examine the previous investigations that have assessed the regional sensitivity of soils in the South to acidic deposition. We conclude by considering the criteria that have been used to define sensitive soils, and we develop our own definitions for use in the computer simulation runs in chapter 7.

Magnitudes and Patterns of Nitrogen and Sulfur Deposition in the South

Precipitation in the South currently has a pH of about 4.4 to 4.6, and about 75% to 80% of the free acidity is associated with sulfuric acid, whereas the remainder is derived from nitric acid. Carbonic acid is present in the rain, but in this low pH range, it is essentially fully protonated. Dry deposition adds a largely unquantified amount of chemicals to forests; dry deposition probably exceeds wet deposition in some areas. In this chapter, the sources of sulfur and nitrogen compounds in the atmosphere and the pathways of deposition and the state of knowledge about historic and current deposition rates are summarized.

The Nature of Soil Acidity and H+ Budgets

Many processes affect the acidity of soils and soil solutions. It turn, the acidity of soil solutions affects many processes. In this chapter, we review the nature of soil acidity, and synthesize the major processes involved in the net generation and consumption of acidity in forest. Two case studies of loblolly pine (Pinus taeda L.) forests provide examples of how H+ budgets are linked with rates of soil acidification.

Synthesis and Recommendations

A variety of studies from Europe and North America have shown that soil chemistry is fairly dynamic, with substantial changes occurring over a period of decades. The causes behind such dynamics are less clear. Acidic precipitation certainly plays a role at some locations, but ecosystem dynamics are also likely to be important. Forest growth is sensitive to soil chemistry, as evidenced by response to fertilization. To date, forests in the South have responded most strongly to additions of phosphorus and nitrogen, but some evidence of limitation by other nutrients is available. Our conclusions about possible effects of acidic deposition are applicable to more general issues of forest nutrient cycling and productivity, and our suggested research directions would have value regardless of the degree of impact of acidic deposition. The recommended monitoring research, in particular, is needed for a wide range of forest health and productivity concerns.