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Effect of forest age on soil organic matter at Mount Vernon, VA.

ABSTRACT. After depletion by agriculture, soil organic matter (SOM) content often increases during forest succession. In January 2011, replicated soil samples (0-10 cm) were taken along a chronosequence of 14 forested plots of four age classes within Mount Vernon, Virginia. SOM was determined using loss on ignition and a significant difference in mean SOM was found between age classes (p=0.039) with a post hoc test confirming a difference between the two younger age classes (1901-2000 and 1861-1900). These results confirm an initial increase in SOM and suggest that SOM may equilibrate during the later stages of succession.

KEY WORDS: forest succession, soil organic matter, chronosequence

INTRODUCTION

Soil nutrient cycling, particularly the amount of organic matter held in the soil, is significantly impacted by the land use history of the forested area. Soil organic matter (SOM) may be lost when land is used for agriculture since tillage of soil may expose organic matter causing rapid rates of decomposition as the organic matter is now more readily accessible to decomposers (Compton and Boone, 2000). In temperate regions, agricultural practices typically lead to losses of 30% of soil carbon (Murty et al., 2002). Land cultivation causes reductions in SOM both by reducing inputs of plant debris and increasing decomposition rates through higher soil temperatures and mixing of topsoil by plowing (Richter and Markewitz, 2001). Additionally, agricultural practices increase bulk density in the upper 15 cm of the soil which results in a greater decrease in soil carbon on a percent by weight basis even more than a mass per volume basis (Richter and Markewitz, 2001). Soils can slowly replenish SOM when agricultural practices are abandoned and vegetation returns through succession (Post and Kwon, 2000). Although trees may quickly invade abandoned farmland, SOM may not recover as quickly since most of the carbon may be stored in standing biomass while little is invested in the soil (Compton and Boone, 2000).

This study attempts to determine the effect of forest age and past land use on the amount of organic matter contained in the soil using soil samples obtained from George Washington's Mount Vernon plantation. Mount Vernon serves as a useful case study since it has a documented land use history extending back to the colonial era with forest stands of various ages distributed across the plantation. Additionally, few, if any studies have addressed organic matter changes after land abandonment and forest succession in the coastal plain of Virginia. Characterizing the dynamics of SOM has importance not only to the carbon cycle (Post and Kwon, 2000), but also as a potential indicator of forest age. The hypothesis for this study is that SOM will increase with forest age until reaching an equilibrium between litter input and decomposition rates (see Schlesinger, 1997).

Mount Vernon, Virginia, is located on unconsolidated, coastal plain sediments (Fig. 1) and its Ultisol Udults soils possess a low base saturation, which requires farmers to abandon fields after a few years of production (Soil Survey Staff, 1999). The soils within the study portion of the Mansion Farm quarter at Mount Vernon range from silt loams to sandy loams in the upper portions of soil profiles (Soil Survey Staff, 2011). Soils common within the study area include Beltsville silt loam, Sassafras-Marumsco complex, Kingstowne-Beltsville complex, and other associated soil complexes (Soil Survey Staff, 2011).

Agricultural land use in Virginia greatly diminished soil quality beginning in the colonial period (Nelson, 2008; Wingo, 1949). The land that was to become Mount Vernon was initially patented by Washington's great grandfather in 1674 (Wilstach, 1916) and like many plantations in colonial Virginia, tobacco became the staple crop grown on its soil. Tobacco production involved clearing and burning of the forest, taking advantage of the pulse of nutrients released by the standing biomass to improve yield. As the yield diminished, new land was cleared and the cycle repeated (Wells and Brown, 2000; Stetson, 1956). During his tenure at Mount Vernon, Washington initially focused on tobacco production, but then diversified his farms growing less tobacco and more wheat; however, he still found wheat production detrimental to soil quality at Mount Vernon (Stetson, 1956). After George and then Martha Washington's death, Mount Vernon was transferred through a series of heirs until the efforts of the Mount Vernon Ladies' Association acquired the 'Mansion farm' quarter portion of the original plantation in 1860 (Dodge, 1932). By that time, the use of these lands for agriculture had greatly diminished and the appearance of the land was that of an early successional forest as "[c]edar and scrub pine possessed the neglected fields" (Wilstach, 1916). The Mount Vernon Ladies' Association maintained some land in agriculture but the majority was in forest described as "second-growth" (Dodge, 1932).

MATERIALS AND METHODS

Field Sampling

This study employed a chronosequence design, substituting space for time in the analysis of SOM. While this substitution is not a perfect substitute for following the same sites through time (Richter and Markewitz, 2001; Yanai et al., 2000), it has been successfully applied to study the response of forest soils to past agricultural land use (e.g. Switzer et al., 1971; Covington, 1981; Hamburg, 1984; Flinn and Marks, 2007). Soil samples were obtained from Mount Vernon in January 2011. Two to three replicate samples were taken from each of the 14 sample plots. Plots were interspersed (Hurlbert, 1984) and were placed at similar elevations (9 to 38 m), slopes (1 to 22[degrees]), and soil types as in Covington (1981). Plots were categorized based on maximum crossdated age of at least two canopy dominant or intermediate trees (sensu Oliver and Larson, 1996) present at that particular site (See Fig. 2). Forest age classes were defined with 2 plots from 1789-1800, four plots from 1801-1860, three plots from 1861-1900, and four plots from 1901-2000. These classes were defined based on historical events that likely had important impacts on agricultural land use at the plantation: 1900 (modern period), ownership by Mount Vernon Ladies' Association (1860), George Washington's death (after the 1799 growing season) and Washington's ownership (prior to 1800). Unfortunately, only two sites were located with trees in the oldest age category (extending back to Washington's ownership), which limited the replication of this age category. After removing the leaf litter component (L) of the organic horizon (0), samples were collected to a depth of 10 cm since that layer is thought to be most sensitive to disturbance. Thus these samples contain some organic matter from the forest floor, including the fermentation (F) and humus (H) layers in addition to the upper A horizon of the soil surface (Switzer et al., 1979).

Lab Analysis

Soil samples were sieved to 2 mm and were stored in a refrigerator for 4-5 weeks until needed for analyses. Storing the samples at lower temperatures ensured that microbial activity would be slowed and that there would be little or no decomposition of organic matter. The loss on ignition method was used to determine percent organic matter present in subsamples of soil. Approximately 10 g of soil from each of the samples obtained was placed in a labeled crucible and left overnight in a drying oven set at 105[degrees]C. This allowed for the removal of water from the samples before combustion. The dried soil samples were then removed from the drying oven and reweighed before being placed in a muffle furnace where they were left exposed to temperatures of 550[degrees]C for 4 hours (Sun et al., 2009). Subsamples were reweighed immediately after being removed from the furnace to avoid any re-absorption of water. The difference in mass between the dried and combusted soil was used to calculate the percent organic matter in each subsample and statistical analyses were conducted using SPSS (2009). The third replicate (21C) within one plot in age class 3 (1801-1860) was removed from all statistical analyses since it contained insufficient sample mass due to high root content.

RESULTS AND DISCUSSION

The ages of forest stands estimated by tree rings agreed well with a 1937 aerial photograph with the youngest stands often only in partial canopy cover at that time but the older three classes in compete canopy cover (Fig. 2B). A univariate ANOVA (Fig. 3) showed a significant difference in mean SOM between these four age classes (p=0.039). A Tukey posthoc test showed that only the means for the two youngest age classes were significantly different (p=0.026). Average percent SOM values were of similar magnitude to those reported by Hamburg (1984), but greater than a reference set of forest soils reported by Sun et al. (2009). The values reported in this study are likely higher because they also include the F and H layers of the O horizon and also the loss of some structural water during ignition (Sun et al., 2009).

Considering the first two age classes, these results support the hypothesis that SOM increases with forest succession following agricultural abandonment. These results are similar to those reported by Flinn and Marks (2007) who found that forests which had established 85 to 100 years ago on previously farmed lands had 15% less SOM than adjacent, potentially primary forests. In a chronosequence of shortleaf pine stands that had developed from abandoned farmland, Billings (1938) reported that the initially high rate of increase in SOM in the A2 horizon slowed through time. The results of this study also show an initial increase, but then SOM equilibrates in the two oldest age classes in amounts not significantly different from either of the first two age classes. This result supports the expected hypothesis of an increase in SOM with forest age, but it may also be possible that SOM peaks and then declines with forest age. Switzer et al. (1979) showed an increase in total organic matter followed by a slight decline along a secondary succession chronosequence. Switzer et al. (1979) attribute this decrease in forest floor organic matter to changes in leaf litter composition as early successional pines transitioned to deciduous hardwoods, altering the decomposition rate. Notably, this transition was largely driven by changes in organic matter on the forest floor (L and F layers); the soil surface only increased and then maintained an equilibrium value. Their results may explain why SOM in the oldest two age classes was not significantly different from the youngest, suggesting that these samples captured the response of both the forest floor and the soil surface to increasing forest age. If only the soil surface had been sampled, then the two oldest age classes may have also shown a significant increase in SOM from the earliest age class. In this study, remnant early successional pines were also observed in all but the oldest age class, although it may also be that an underlying gradient (such as soil moisture) also influenced the SOM values across the study area. In conclusion, this study finds that SOM initially increases with forest age and then equilibrates. These changes in SOM have important implications both for the storage of carbon within coastal plain soils subject to past agricultural land use as well as the potential of SOM to serve as an indicator of forest age in this region.

ACKNOWLEDGMENTS

Special thanks to the Mount Vernon Ladies' Association for permission to obtain soil samples for this project and to Bristol-Myers Squibb whose undergraduate summer fellowship supported this research. Hongbing Sun and Chuck Garten assisted in determining suitable methods for analyzing samples. Allison Ingram and Michael McCormack assisted in collecting samples. Dirk Vanderklein and an anonymous reviewer provided valuable suggestions for the manuscript. Thanks also to John Rutherford with the Fairfax Co. Park Authority for providing the 1937 aerial photograph of Mount Vernon.

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CHRISTINE SOOKHDEO AND DANIEL L. DRUCKENBROD

RIDER UNIVERSITY, GEOLOGICAL, ENVIRONMENTAL AND MARINE SCIENCES (GEMS), LAWRENCEVILLE, NEW JERSEY 08648
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Author:Sookhdeo, Christine; Druckenbrod, Daniel L.
Publication:Bulletin of the New Jersey Academy of Science
Article Type:Report
Geographic Code:1USA
Date:Jun 22, 2012
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