Carol G. Wells

Rapid accumulation and turnover of soil carbon in a re-establishing forest

Present understanding of the global carbon cycle is limited by uncertainty over soil-carbon dynamics,,,,,. The clearing of the worlds forests, mainly for agricultural uses, releases large amounts of carbon to the atmosphere (up to 2× 1015 g yr−1), much of which arises from the cultivation driving an accelerated decomposition of soil organic matter,,,. Although the effects of cultivation on soil carbon are well studied, studies of soil-carbon recovery after cultivation are limited,,,,,,,. Here we present a four-decade-long field study of carbon accumulation by pine ecosystems established on previously cultivated soils in South Carolina, USA. Newly accumulated carbon is tracked by its distinctive 14C signature, acquired around the onset of forest growth from thermonuclear bomb testing that nearly doubled atmospheric 14CO2 in the 1960s. Field data combined with model simulations indicate that the young aggrading forest rapidly incorporated bomb radiocarbon into the forest floor and the upper 60 cm of underlying mineral soil. By the 1990s, however, carbon accumulated only in forest biomass, forest floor, and the upper 7.5 cm of the mineral soil. Although the forest was a strong carbon sink, trees accounted for about 80%, the forest floor 20%, and mineral soil <1%, of the carbon accretion. Despite high carbon inputs to the mineral soil, carbon sequestration was limited by rapid decomposition, facilitated by the coarse soil texture and low-activity clay mineralogy.

SOIL CHEMICAL CHANGE DURING THREE DECADES IN AN OLD-FIELD LOBLOLLY PINE (PINUS TAEDA L.) ECOSYSTEM'

The ability of soil to sustain its supply of nutrients to a growing forest is controlled by a complex of biogeochemical processes. Forest soil data are notably absent, however, that describe sustained nutrient supply or nutrient depletion. The objective of this study was to evaluate how exchangeable nutrient cations of a previously cultivated Ultisol responded to the first three decades of pine forest development. On six occasions during the three decades, the upper 0.6 m of soil was sampled from eight permanent plots and chemically analyzed with the same procedures. During this period, KCl-exchangeable acidity (as positive charges of adsorbed H and Al ions) increased by 37.3 kmol,/ha in the upper 0.6 m of soil and positive charges of exchangeable Ca and Mg were depleted by 34.8 and 8.9 kmolc/ha (by 696 and 108 kg/ha), whereas, exchangeable K was reduced by only 0.5 kmolc/ha (19 kg/ha). Depletion of soil exchangeable Ca was on the same order of magnitude as Ca removals (i.e., Ca accumulation in biomass and forest floor plus that lost in soil leaching). Removals of soil Mg also appeared to outpace resupply from recycling, atmospheric deposition, and mineral weathering, but not to the same degree as Ca. Over the three decades, soil leaching loss of these divalent cations (from 0.6 m depth) appeared equal to cation accumulation in biomass plus forest floor, with sulfate balancing about half these cations in leachates. In contrast to Ca and Mg, total K removals from the soil exceeded reductions in soil exchangeable K by nearly 20-fold. Exchangeable K was well buffered in surface mineral soils apparently due to a combination of biological recycling via leaching of canopies and forest floor plus mineral weathering release. These nutrient dynamics may be common to many nutrient-demanding forest ecosystems supported by soils with low activity kandic or oxic horizons. Such soils (Ultisols and Oxisols) occur on many hundreds of millions of hectares in temperate and tropical zones.

Estimates of nutrient removal, displacement and loss resulting from harvest and site preparation of a Pinus taeda plantation in the piedmont of north Carolina

Abstract Biomass and nutrient removal and redistribution were estimated for combinations of two levels of harvest utilization (stem-only vs. complete tree) and two methods of site preparation (chop/broadcast burn vs. shear-pile/disk) in the Piedmont of North Carolina. Stem-only harvest removed 57 kg ha −1 N, 5 kg ha −1 P, 35 kg ha −1 K, 52 kg ha −1 Ca and 14 kg ha −1 Mg. Complete-tree harvest increased N, P, K, Ca and Mg removal over stem-only harvest by 216, 304, 152, 254 and 151%, respectively, although biomass removal was increased by only 65%. Estimated displacement of N and P into windrows during site preparation depended on harvest utilization, but generally exceeded harvest removals by at least 200%. Nutrient losses resulting from broadcast burning were small, primarily due to ineffectual burns. Removals, displacements and/or losses of nutrients during harvest and intensive site preparation were equal for both harvest utilization levels. These amounted to 714 kg ha −1 of total N in biomass and soil, and 46, 154, 481 and 88 kg ha −1 of P, K, Ca and Mg, respectively, in biomass and Mehlich III soil extractions.

Nitrogen mineralization in high elevation forests of the Appalachians. I : Regional patterns in southern spruce-fir forests

Annual and seasonal rates of net nitrogen mineralization were determined for 19 sites in the spruce-fir forests of the Southern Appalachian Mountains. These sites included high and low elevation stands of red spruce (Picea rubens Sarg.) and Fraser fir (Abies fraseri (Pursh.) Poir.) on east and west exposures on Whitetop Mountain, Virginia; Mt. Mitchell, North Carolina; and Clingmans Dome in the Great Smoky Mountains National Park. Mineralization rates were determined using in situ soil incubations in PVC tubes with ion exchange resin bags placed in the bottom of the tubes to collect leachate. Throughfall was collected in resin bags placed in the top of the tubes. Average initial NH4-N + NO3-N ranged from 0.6 to 4.8 kg N/ha across all plots, and average mineralization rates ranged from 26 to 180 kg-N ha−1 yr−1. Throughfall ranged from 18 to 32 kg-N ha−1 yr−1 with NH4-N accounting for about two-thirds of the throughfall N across all sites. Throughfall and mineralization rates were not related to elevation or exposure. The high rates of N mineralization and relatively high nitrate concentrations indicate that leaching losses of nitrogen and associated cations could be substantial.

Determination of extractable sulfate and total sulfur from plant and soil material by an autoanalyzer

Abstract The method has been modified from Sinclair3,4 and tested for extractable sulfate and total sulfur on plant, soil, and water samples. Inorganic sulfur is extracted from plant and soil material by using dilute acidic extractants, and total sulfur is estimated from dry ashed or wet ashed material whereby various sulfur forms are oxidized to sulfate‐sulfur. The sulfate is precipitated in sample solutions as barium sulfate and determined turbidimetrically by AutoAnalyzer. The method is rapid, precise, and sensitive enough to be used on a routine basis.

Some possible tissue assay methods for n nutrition assessment of Pinus Taeda L.

Abstract Studies were conducted on loblolly pine to test the sensitivity of a number of N assay methods which would show the greatest increase of N in fertilized over unfertilized trees as the most indicative of the N status of stands. In November, total N in xylem and fresh needlefall was more sensitive than that in needles. Current needles and twigs collected in September were more sensitive than 1 year old tissue. Needles and twigs appeared to have similar sensitivity. Soluble‐ and arginine‐N appeared more sensitive than total N in needles and twigs; however, these N fractions were too variable to be statistically evaluated as well as their laboratory procedures not easily adapted for routine use.