Daniel D. Richter

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.

Atmospheric Deposition and Canopy Interactions of Major Ions in a Forest

Airborne particles and vapors contributed significantly to the nutrient requirements and the pollutant load of a mixed hardwood forest in the eastern United States. Dry deposition was an important mechanism of atmospheric input to the foliar canopy, occurring primarily by vapor uptake for sulfur, nitrogen, and free acidity and by particle deposition for calcium and potassium. The canopy retained 50 to 70 percent of the deposited free acidity and nitrogen, but released calcium and potassium. Atmospheric deposition supplied 40 and 100 percent of the nitrogen and sulfur requirements, respectively, for the annual woody increment. This contribution was underestimated significantly by standard bulk deposition collectors.

Simulating trends in soil organic carbon in long-term experiments using RothC-26.3

Abstract As part of a model evaluation exercise, RothC-26.3, a model for the turnover of organic carbon in non-waterlogged soils, was fitted to measurements of organic carbon from 18 different experimental treatments on 6 long-term experimental sites in Germany, England, the USA, the Czech Republic and Australia. In the fitting process, the model was first run with an annual return of plant C that had been selected iteratively to give the carbon content of the soil at the start of each experiment. This was done for the soil and climate of each site. If the radiocarbon content of the soil organic matter was known, the inert organic carbon (IOM) content of the soil was also calculated for the start of the experiment. Using these carbon and radiocarbon contents as a starting point, the model was then run for each of the experimental treatments to be fitted, using iteratively selected values for the annual return of plant materials to the soil. The value used for each treatment was selected to optimise the fit between modelled and measured data over the whole experimental period: fitting was done by eye. Thus fitted, RothC-26.3 gave an acceptable approximation to the measurements for 14 of the treatments, bearing in mind the experimental errors in measuring soil organic carbon on a per hectare basis. With four of the treatments (Highfield Bare Fallow, Park Grass plot 13d, Ruzyně farmyard manure plot and Tamworth rotation 5), the fit was less satisfactory.

How Deep Is Soil?Soil, the zone of the earth's crust that is biologically active, is much deeper than has been thought by many ecologists

arth is a most remarkable planet but not only because of its prodigious life, vast oceans, and oxygen-enriched atmosphere. Earth is remarkable because of its soil. Soil is the biologically excited layer of the earths crust. It is an organized mixture of organic and mineral matter. Soil is created by and responsive to organisms, climate, geologic processes, and the chemistry of the aboveground atmosphere. Soil is the rooting zone for terrestrial plants and the filtration medium that influences the quality and quantity of Earths waters. Soil supports the nearly unexplored communities of microorganisms that decompose organic matter and recirculate many of the biospheres chemical elements. Ecologists consider soil to be the central processing unit of the earths environment (Sanchez 1994). One of the most significant outcomes of biological evolution has been the coevolution of soil and terrestrial ecosystems. This coevolution was initiated during the Devonian era, approximately 350 million years ago. Plants spread across upland continental regions during the explosion of life that led directly

Simulating trends in soil organic carbon in long-term experiments using the century model

Abstract This paper describes Century Soil Organic Matter (SOM) Model simulations of seven long-term data sets that are the subject of this special issue. We found that Century successfully simulates SOM C across a variety of land use and climate types. Simulations of SOM were most successful in grass and crop systems. This exercise highlights a structural limitation of Century in simulating SOM in a forest with a developed litter layer. Simulations of tree biomass distributions, however, were generally successful. The model failed to capture extreme values of yield and N offtake, although annual averages were quite similar between observations and simulations, leading to reasonable estimates of SOM C. Average yields and SOM were generally higher in amended treatments or rotations including an N-fixing component such as alfalfa. The model successfully predicted SOM dynamics across climates, land use types, and treatments. This suggests that Century is a useful tool for ecosystem studies, particularly those focused on SOM dynamics.

Soil Diversity in the Tropics

Publisher Summary This chapter reviews the soil diversity in the tropics. Soil diversity in the tropics is relatively easy to demonstrate using maps with scales of 1.5 million. This chapter evaluates common misconceptions about “tropical soil” that are of ecological significance to consider reasons of misconceptions, and evaluates a hypothesis that soils in the tropics are noted for their marked taxonomic diversity. The process of paradigm change in soil taxonomy, especially among soil, and ecological scientists with interests in the tropics are discussed. Heterogeneity of soils throughout the tropics is quantified by estimating areal extents of soil taxa from recent soil maps of tropical America, Africa, and Asia. Soil diversity in the humid tropics is evaluated by examining the first comprehensive soil maps of the Brazilian Amazon River basin, where systematic soil surveys have been completed. Geographic information systems (GIS) are used to compare the FAO/UNESCO and the more recent Brazilian soil maps of Amazonia. The changing perspectives about soil taxonomy, the development of misconceptions, advances in soil taxonomy, and the creation of the world soil map are also discussed. The diversity of soils and taxonomic correlations among four major classification systems for soils are tabulated.

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.

EFFECTS OF FREE-AIR CO2 ENRICHMENT (FACE) ON BELOWGROUND PROCESSES IN A PINUS TAEDA FOREST

Terrestrial vegetation and soils may act as important carbon sinks if rising atmospheric CO2 stimulates plant production. We used free-air CO2 enrichment (FACE) technology to expose three 30 m diameter plots of a loblolly pine (Pinus taeda) forest to elevated CO2 at 200 FLL/L above ambient levels, while three control plots were outfitted with FACE apparatus but were fumigated with ambient air. We quantified litterfall mass and chemistry, fine root biomass increment and turnover, CO2 efflux from soils, 83C in soil C02, soil CO2, soil microbial biomass C and N, and potential net N mineralization. After two growing seasons, elevated CO2 caused significant increases in loblolly pine litterfall mass and fine root increment. Within the first year of FACE treatment, the con- centration of CO2 in soil had increased, and soil surface CO2 efflux was generally higher at elevated C02, but this difference was not statistically significant. Loblolly pine litter C:N ratio, fine root turnover, microbial biomass C and N, and potential net N mineralization were not significantly affected by elevated CO2. Our results suggest that elevated atmo- spheric CO2 may accelerate inputs of organic matter to soil C pools in loblolly pine forests, but it may also accelerate losses of C from belowground by stimulating soil respiration.

Prescribed fire: effects on water quality and forest nutrient cycling.

Prescribed fire, a practice applied annually to about 106 hectares of forests in the southeastern United States, had limited effects on soils, nutrient cycling, and hydrologic systems of a coastal plain pine forest. Hydrologic fluxes of nitrogen, phosphorus, sulfur, and basic cations, from burned pine litter to ground and stream waters, are not likely to have appreciable impacts on water quality in the Atlantic and Gulf Coastal Plain.

Geophysical imaging reveals topographic stress control of bedrock weathering.

Bedrock weathering runs to the hills Fractures in bedrock drive the breakdown of rock into soil. Soil makes observations of bedrock processes challenging. St. Clair et al. combined a three-dimensional stress model with geophysical measurements to show that bedrock erosion rates mirror changes in topography (see the Perspective by Anderson). Seismic reflection and electromagnetic profiles allowed mapping of the bedrock fracture density. The profiles mirror changes in surface elevation and thus provide a way to study the critical zone between rock and soil. Science, this issue p. 534; see also p. 506 Geophysical survey data and stress modeling connect surface topography to Earth’s critical zone. [Also see Perspective by Anderson] Bedrock fracture systems facilitate weathering, allowing fresh mineral surfaces to interact with corrosive waters and biota from Earth’s surface, while simultaneously promoting drainage of chemically equilibrated fluids. We show that topographic perturbations to regional stress fields explain bedrock fracture distributions, as revealed by seismic velocity and electrical resistivity surveys from three landscapes. The base of the fracture-rich zone mirrors surface topography where the ratio of horizontal compressive tectonic stresses to near-surface gravitational stresses is relatively large, and it parallels the surface topography where the ratio is relatively small. Three-dimensional stress calculations predict these results, suggesting that tectonic stresses interact with topography to influence bedrock disaggregation, groundwater flow, chemical weathering, and the depth of the “critical zone” in which many biogeochemical processes occur.