A variety of physical and chemical techniques are used to fractionate soil organic matter, but detailed comparisons of the different approaches and tests of how separation methods influence the properties of isolated organic matter pools are lacking. In this case study based on A horizon samples of 2 California coniferous forests soils, we 1) evaluate the effects of root removal and ultrasonic dispersion on the properties of the <2 g cm3 light fraction and 2) compare the properties of fractions obtained by sequential density separations of ultrasonically treated soil with those obtained by density followed by acid/base hydrolysis (Trumbore et al. 1996). A root-removal effort based on hand-picking visible roots reduced the radiocarbon content and increased the estimated turnover time of the light fraction by roughly 12%. Root-removal protocols that vary between investigators thus can potentially confound variability in carbon cycling for this fraction caused by environmental factors, such as climate. Ultrasonic dispersion did not have a clear effect on the light fraction C and N content or isotopic signature, but led to a decrease in the % C and C/N of the recovered heavy fractions, and losses of 1219% of the total soil C to the sodium metatungstate density solution. Sequentially isolated density fractions clearly differed in mineralogy and organic matter chemistry, but natural-abundance 14C analyses indicated that distinct mineral phases did not correspond to unique C-turnover pools. Density fractions containing kaolinite group minerals alone and in combination with hydroxy-interlayered vermiculite were found to harbor both fast and slow cycling carbon. In contrast, severe chemical treatment isolated a carbon pool with the lowest overall 14C content and longest inferred mean turnover time. Overall, our results show that care must be taken when relying on physical (density) separation to isolate soil fractions with different dynamics, as the details of treatment will influence the results.

Rates of input, accumulation, and turnover of C differ markedly within soil profiles and in soils with different drainage in the BOREAS northern study area. Soil C storage increases from ∼3 kg C m⁻² in well-drained, sandy soils to greater than 100 kg C m⁻² in wetlands. Two modes of C accumulation were observed in upland soil profiles. Large annual C inputs (0.06–0.1 kg C m⁻² yr⁻¹) and slow decomposition (turnover times of 6–250 years) lead to rapid C accumulation in regrowing surface moss and detrital layers following fire. Deep organic layers that have accumulated over the millennia since the initiation of soil development, and are located below the most recent charred horizon, show slower rates of input (0.015–0.03 kg C m⁻² yr⁻¹) and turnover (100–1600 years) and accumulate C about 10 times slower than surface detrital layers. Rates of C input to soils derived from C and ¹⁴C data were in accord with net primary production estimates, with highest rates of input (0.14–0.6 kg C m⁻² yr⁻¹) in wetlands. Turnover times for C in surface detrital layers were 6–15 years for well-drained sand soils that showed highest soil temperatures in summer, 30–40 years for wetlands, and 36–250 years for uplands with thick moss cover and black spruce trees. Long (>100 years) turnover times in upland black spruce/clay soils most likely reflect the influence of woody debris incorporated into detrital layers. Turnover times for deep organic and mineral layer C were controlled by drainage, with fastest turnover (80–130 years) in well-drained sand soils and slowest turnover (>3000 years) in wetlands. Total C accumulation rates, which account for C losses from both deep organic and surface detrital layers, are close to zero for sand/jack pine soils, 0.003–0.01 kg C m⁻² yr⁻¹ for moderately to poorly drained sites in mature forest stands, and 0.03 kg C m⁻² yr⁻¹ for a productive fen. Decomposition of organic matter more than several decades old accounts for 9–22% of total heterotrophic respiration at these sites. The rates of C accumulation derived here are decadal averages for specific stands and will vary as stands age or undergo disturbance. Extrapolation to larger regions and longer timescales, where burning offsets C gains in moss layers, will yield smaller rates of C storage.

High count rates of lithium ions in an AMS measurement system for 14C were traced to the lithium content 

in the quartz tubes used in the graphitzation of the samples. The lithium contamination was nearly eliminated by the use of borosilcate glas reaction tubes at a lower reaction temperature. The ion beam curent and the measurement precison of the isotope ratio were not afected. 

We report measurements of the ¹⁵N and ¹⁸O signature of N₂O emitted from tropical rain forest soils at the La Selva Biological station in Costa Rica and in the Fazenda Vitoria in the State of Pará, Brazil. The δ¹⁵N values ranged from −34 to 2‰ with respect to atmospheric N₂, while δ¹⁸O values had a smaller range, from −4 to 18‰ with respect to atmospheric O₂. We attribute these large variations to differences in microbial production, consumption, and transport of N₂O. In general the δ¹⁵N of N₂O emissions from an Oxisol soil in Brazil were consistently enriched by ∼20‰ in ¹⁵N compared to those from Ultisol and Inceptisol soils in Costa Rica. Denitrification is the most likely source of N₂O in both locations during the rainy season, and the ¹⁵N of nitrate was similar in both locations. We attribute the overall variability in emitted ¹⁵N to differences in the ratio of N₂O:N₂ escaping from the soil to the atmosphere, with a larger fraction of the N₂O reduced to N₂ at the Brazilian sites. We found light δ¹⁵N-N₂O values associated with high N₂O emissions in a fertilized agricultural site in Costa Rica and in a “hot spot” of high emissions in the forest site in Brazil. This result suggests that the increase of substrate availability might increase the fractionation associated with N₂O production. Overall, the Brazilian Oxisol soils had the most enriched δ¹⁵N-N₂O emissions yet measured from soils. If these are more representative of tropical soil emissions than the Costa Rica emissions, then the globally averaged δ¹⁵N-N₂O tropical rain forest soil source is more enriched than previously estimated. The large variations in isotopic signature for N₂O emissions demonstrate the potential utility of stable isotopes as tools for understanding the processes of N₂O production and consumption in soils.

Measurements of the emission of ²²²Rn from Amazon forest soils, and profiles of ²²²Rn in air were used to study the ventilation of the soil atmosphere and the nocturnal forest canopy. The emission of ²²²Rn from the yellow clay soils dominant in the study area averaged 0.38±0.07 atom cm⁻² s⁻¹. Nearby sand soils had similar fluxes, averaging 0.30 ± 0.07 atom cm⁻² s⁻¹. The effective diffusivity in the clay soil (0.008±0.004 cm² s⁻¹), was lower than that for the sand soil (0.033±0.030 cm² s⁻¹). Profiles of ²²²Rn and CO₂ in air showed steepest concentration gradients in the layer between 0 and 3 m above the soil surface. Aerodynamic resistances calculated for this layer from ²²²Rn and CO₂ varied from 1.6 to 18 s cm⁻¹, with greater resistance during the afternoon than at night. Time averaged profiles of²²²Rn in the forest canopy measured during the evening and night were combined with the soil flux measurements to compute the resistance of the subcanopy to exchange with overlying air. The integrated nocturnal rate of gas exchange between the canopy layer (0 to 41 m) and overlying atmosphere based on ²²²Rn averaged 0.33±0.15 cm s⁻¹. An independent estimate of gas exchange, based on 13 nights of CO₂ profiles, averaged 0.21±0.40 cm s⁻¹. These exchange rates correspond to flushing times for the 41 m canopy layer of 3.4 and 5.5 hours, respectively. Comparison of ²²²Rn and CO₂ profiles show that the nocturnal production of CO₂ by above-ground vegetation was about 20% of the soil emission source, consistent with data from eddy-correlation experiments (Fan et al., this issue).