Soil composition—its abiotic physical and chemical properties—changes under an aggrading forest. Through "space for time studies" in the Piedmont of North Carolina, researchers have examined changes in soil composition and processes among three successional stages of old fields. Differences in nitrification rates, nutrient concentration, and acidity indicate that soil nutrient availability decreases during the course of forest succession.
Soil Change Through Time
The characteristics of forest change are highly interconnected with changes in soil conditions through time. A positive relationship exists between species richness and soil nutrient availability. Organisms can change the chemical composition of the soil, which in turn influences the forest composition. For example, some species of trees and other plants decrease soil pH while others increase nitrogen availability. Much study and speculation has focused on the processes and direction of soil changes. Classical, Clementsian succession theory suggested that soil conditions should improve with forest stand age given increases in soil organic matter and soil nutrients. Subsequent research has shown the loss of essential soil nutrients through leaching, chemical fixation in unusable forms, or sequestration in dead organic matter as a more typical case for soil alteration. Peet notes that if soil nutrients maintain a constant supply rate, their availability should be inversely related to the rate at which they are sequestered within biomass as it accumulates. Eventually a steady state evolves where nutrient loss from both living and dead biomass equals uptake.
Soil Studies in the Piedmont
In North Carolina, Piedmont soils suffered significant losses of fertility and high levels of erosion due to exhaustive agricultural practices throughout the 19th century. By the early 20th century, most farms—and their depleted soils—had been abandoned. Secondary succession upon these old fields proceeds from an initial cover of herbs through pine stands to mixed hardwood stands. During the past 50 years, researchers have examined plots in old field, pine, and hardwood stages as a means of studying secondary succession and forest soil change.
Concepts of Organized Change
Coile studied soil changes in the Duke Forest shortly after its establishment in 1938, finding that carbon:nitrogen (C:N) levels were higher in forested stands than in old fields. Coile suggested that nitrification proceeded more slowly in forest soils than in fields, highlighting the idea that nitrification decreases with increasing successional age. Since less nitrification leads to less plant-available nitrogen, this "could result in increased ecosystem nutrient retention consistent with the general 'strategy of ecosystem development'".
Nitrification Re-visited in the Duke Forest
In a study of old field, pine, and oak-hickory hardwood plots located on Ultisols of the White Store series, Montes and Christensen found that the two forested ecosystems had higher concentrations of ammonium and lower concentrations of nitrate, as well as less nitrifying bacteria, than old field plots. Nitrate production rates were highest in soils of old fields, intermediate in hardwood soils, and lowest in pine soils, in accordance with Coile's findings. Soils within the pine stands were sandier as well as lower in pH, organic matter (probably from coarse texture), and cation exchange capacity (CEC) than either the hardwood or old-field soils. Old field soils contained more clay. Rather than the result of succession, the study concluded that soil differences derived from initial site conditions and histories of cultivation, liming, and erosion.
Christensen and MacAller revisited the limited nitrification in mature ecosystems found in the 1979 study. They noted the proposed mechanisms which have been offered for this successional process: allelopathy; variations in soil inorganic chemical characteristics (pH, cation, and phosphate availability); and competitive relations between heterotrophic microbes, plant roots, and nitrifying bacteria due to C:N and availability of reduced carbon (C). The only clear trends found in the data were higher pH as well as concentrations of calcium (Ca), magnesium (Mg), potassium (K), and phosphate- phosphorus (PO4-P) in old field soils. (These findings coincide with those of Richter et al., which demonstrated increasing acidity and decreasing concentrations of Ca and Mg during succession.) Christensen and MacAller asserted that variation of the factors limiting ammonification and nitrification among sites within successional ages should be assessed in order to form generalizations about soil change during succession.
A general principle of soil succession still eludes researchers due to the lack of causal mechanisms. Yet, changes in soil composition can be measured in the Piedmont: old field soil conditions do not match those of hardwood stands, reflecting biological and edaphic alterations through time.
- Christensen, N.L. and T. MacAller, 1985. Soil mineral nitrogen transformations during succession in the piedmont of North Carolina. Soil Biology & Biochemistry, 17:675-681.
- Coile, T.S., 1940. Soil changes associated with loblolly pine succession on abandoned agricultural land of the piedmont plateau. Duke University School of Forestry Bulletin 5.
- Montes, R.A. and N.L. Christensen, 1979. Nitrification and succession in the piedmont of North Carolina. Forest Science, 25:287-297.
- Peet, R.K., 1992. Community structure and ecosystem function. In: Plant Succession. (ed: D.C. Glenn-Lewin, R.K. Peet, and T.T. Veblen), Chapman & Hall: New York: 103-151. ISBN: 0412269007
- Richter, D.D., Markewitz, D., Wells, C.G., Allen, H.L., April, R., Heine, P.R. and Urrego, B., 1994. Soil chemical change during three decades in an old-field loblolly pine (Pinus Taeda L.) ecosystem. Ecology, 75:1463-1473.