Browsing by Author "Richter, Daniel"

This study seeks to investigate the dynamics of dead plant carbon over fifty years of old-field forest development at the Calhoun Long Term Soil-Ecosystem Experiment (LTSE) in South Carolina, USA. Emphasis is on the transition phase of the forest, which is less well studied than the establishment and early thinning phase or the steady state phase. At the Calhoun LTSE, the biogeochemical and ecosystem changes associated with old field forest development have been documented through repeated tree measurements and deep soil sampling, and archiving of those soils, which now allow us to examine changes that have occurred over the course of forest development to date.

In this dissertation, I first quantify the accumulation of woody detritus on the surface of the soil as well as in the soil profile over fifty years, and estimate the mean residence times of that detrital carbon storage. Knowing that large accumulations of C-rich organic matter have piled onto the soil surface, the latter chapters of my dissertation investigate how that forest-derived organic carbon has been incorporated into mineral soils. I do this first by examining concentrations of dissolved organic carbon and other constituents in soil solutions throughout the ecosystem profile and then by quantifying changes in solid state soil carbon quantity and quality, both in bulk soils and in soil fractions that are thought to have different C sources, stabilities, and residence times. To conclude this dissertation, I present the 50-year C budget of the Calhoun LTSE, including live and dead plant carbon pools, to quantify the increasing importance of detrital C to the ecosystem over time.

This exceptional long term soil ecosystem study shows that 50 years of pine forest development on a former cotton field have not increased mineral soil carbon storage. Tree biomass accumulated rapidly from the time seedlings were planted through the establishment phase, followed by accumulations of leaf litter and woody detritus. Large quantities of dissolved organic carbon leached from the O-horizons into mineral soils. The response of mineral soil C stocks to this flood of C inputs varied by depth. The most surficial soil (0-7.5cm), saw a large, but lagged, increase in soil organic carbon (SOC) concentration over time, an accumulation almost entirely due to an increase of light fraction, particulate organic matter. Yet in the deepest soils sampled, soil carbon content declined over time, and in fact the loss of SOC in deep soils was sufficient to negate all of the C gains in shallower soils. This deep soil organic matter was apparently lost from a poorly understood, exchangeable pool of SOM. This loss of deep SOC, and lack of change in total SOC, flies in the face of the general understanding of field to forest conversions resulting in net increases in soil carbon. These long term observations provide evidence that the loss of soil carbon was due to priming of SOM decomposition by enhanced transpiration, C inputs, and N demand by the growing trees. These results suggest that large accumulations of carbon aboveground do not guarantee similar changes below.

Land use histories affect the rate and pattern of soil nutrients at regional and global scale. However, former studies have rarely focused on soil trace elements (B, Mn, Zn, Cu and Fe). In this study, we aimed at the long-term biogeochemical cycling pattern and spatial heterogeneity of soil trace elements in response to land use changes. We conducted experiment at Calhoun experiment forest in SC and surrounded relic hardwood forest, cultivated land and secondary pine forests with contrasting land use histories in a statistically rigorous and spatially explicit design. Our first study indicated that spatial heterogeneity is greatly reduced in many soil properties by agricultural practices, but that successional forest growth on previously cultivated soils re-structures heterogeneity of soil properties within a few decades. We document cases in which land use alters both the soil property's central tendencies and their heterogeneity (C, N, CN, Ca, K), and cases in which changes are apparent in central tendency but much less so in their heterogeneity (Db). In our second study, samples of the upper 0.6-m mineral soil archived in 1962 and 1997 revealed three cycling patterns: 1) Extractable B and Mn were significantly depleted because tree uptake of B and Mn from mineral-soil greatly outpaced resupplies from atmospheric deposition, mineral weathering, and deep-root uptake. 2) Extractable Zn and Cu changed little during forest growth, indicating that nutrient resupplies kept pace with accumulations by the aggrading forest. 3) Oxalate-extractable Fe increased substantially during forest growth, by about 10-fold more than accumulations in tree biomass. This study indicated that forest Fe cycling is qualitatively different from that of other macro- and micro-nutrients. Thirdly, our results revealed that long-term cultivation substantially diminished the activity of soil iron oxides relative to forest growth. Forest Fe cycling is derived from mineral soil weathering, which suggests a need to explore the underlying mechanisms by which bioturbation (e.g.earthworms) mediates transformations of iron bioavailability and oxidation of organic matter in soils. Overall, the wide range of responses to land use changes among the ecosystem's trace elements and other biogeochemical features illustrates the great dynamics of the soil system over time scales of decades to centuries.

Invasive species are rapidly expanding in riparian wetlands while concurrently anthropogenic causes are increasing nitrogen (N) into these ecosystems. Microstegium vimineum (Microstegium) is a particularly abundant invasive grass in the Southeast United States. To evaluate impacts of Microstegium on both plant diversity and N cycling in a riparian floodplain, paired plots of Microstegium hand-weeded and unweeded were established for three years. Plots without Microstegium increased from 4 to 15 species m-2 and 90% of the newly establishing species were native. The Microstegium community accumulated approximately half the annual N in biomass of the diverse community, 5.04 versus 9.36 g-N m-2 year-1, respectively (p=0.05). Decomposition and release of N from Microstegium detritus was much less than in the diverse community, 1.19 versus 5.24 g-N m-2 year-1. Rates of soil N mineralization estimated by in-situ incubations were relatively similar in all plots. While Microstegium invasion appears to greatly diminish within-ecosystem circulation of N through the under-story plants, it might increase ecosystem N losses through enhanced denitrification (due to lower redox potentials under Microstegium plots). Microstegium removal ceased in the fourth growing season and formerly weeded plots increased to 59% (± 11% SE) Microstegium cover and species richness decreased to <8 species m-2.

To learn how Microstegium responds to increased N, we conducted a greenhouse competition experiment between Microstegium and four native plants across an N gradient. There was a unique competition outcome in each species combination, yet Microstegium was most dominant in the high levels of N.

Last, we disturbed a floodplain similar to wetland restoration disturbance and tracked available N. We also established a native community of plants with and without Microstegium in three levels of N. Disturbance to the floodplain dramatically increased inorganic N, especially in the form of NO₃ which was five times higher in the disturbed floodplain than the undisturbed floodplain. N levels remained elevated for over a year. Microstegium was N responsive, but did not show negative effects to the planted vegetation until the second year. Ironically, restoration activities are increasing available N, and favoring invasive species which in turn detracts from restoration success.