ABSTRACTor sink of atmospheric CO 2 depending on the rate of SOC formation and decomposition (Van Breemen andForested ecosystems have been identified as potential C sinks. Feijtel, 1990). It is therefore important to understand depleted (Johnson, 1992

Carbon (C) can be sequestered in the mineral soil after the conversion of intensively cropped agricultural fields to more extensive land uses such as afforested and natural succession ecosystems. Three land-use treatments from the long-term ecological research site at Kellogg biological station in Michigan were compared with a nearby deciduous forest. Treatments included a conventionally tilled cropland, a former cropland afforested with poplar for 10 years and an old field (10 years) succession. We used soil aggregate and soil organic matter fractionation techniques to isolate C pools that (1) have a high potential for C storage and (2) accumulate C at a fast rate during afforestation or succession. These fractions could serve as sensitive indicators for the total change in C content due to land-use changes. At the mineral soil surface (0-7 cm), afforesting significantly increased soil aggregation to levels similar to native forest. However, surface soil (0-7 cm) C did not follow this trend: soil C of the native forest site (22.9 t C ha À1 ) was still significantly greater than the afforested (12.6 t C ha À1) and succession (15.4 t C ha À1 ) treatments. However, when the 0-50 cm soil layer was considered, no differences in total soil C were observed between the cropland and the poplar afforested system, while the successional system increased total soil C (0-50 cm) at a rate of 0.786 t C ha À1 yr À1 . Afforested soils sequestered C mainly in the fine intraaggregate particulate organic matter (POM) (53-250 lm), whereas the successional soils sequestered C preferentially in the mineral-associated organic matter and fine intraaggregate POM C pools. NomenclaturePOM 5 particulate organic matter, i.e. organic matter of size 453 mm intra-POM 5 POM material contained within an aggregate fine intra-POM 5 intra-POM of size 53-250 mm coarse intra-POM 5 intra-POM of size 250-2000 mm total light POM 5 POM material having a density o1.85 g cm

The literature was reviewed and analyzed to determine the feasibility of using a combination of acid hydrolysis and CO 2 -C release during long-term incubation to determine soil organic carbon (SOC) pool sizes and mean residence times (MRTs). Analysis of 1100 data points showed the SOC remaining after hydrolysis with 6 M HCl ranged from 30 to 80% of the total SOC depending on soil type, depth, texture, and management. Nonhydrolyzable carbon (NHC) in conventional till soils represented 48% of SOC; no-till averaged 56%, forest 55%, and grassland 56%. Carbon dates showed an average of 1200 yr greater MRT for the NHC fraction than total SOC. Longterm incubation, involving measurement of CO 2 evolution and curve fitting, measured active and slow pools. Active-pool C comprised 2 to 8% of the SOC with MRTs of days to months; the slow pool comprised 45 to 65% of the SOC and had MRTs of 10 to 80 yr. Comparison of field 14 C and 13 C data with hydrolysis-incubation data showed a high correlation between independent techniques across soil types and experiments. There were large differences in MRTs depending on the length of the experiment. Insertion of hydrolysisincubation derived estimates of active (C a ), slow (C s ), and resistant pools (C r ) into the DAYCENT model provided estimates of daily field CO 2 evolution rates. These were well correlated with field CO 2 measurements. Although not without some interpretation problems, acid hydrolysis-laboratory incubation is useful for determining SOC pools and fluxes especially when used in combination with associated measurements. SOIL ORGANIC MATTER (SOM) is a complex mixture of plant-and microbiologically-derived compounds (Stevenson, 1994). Fractionation schemes have been employed to (i) isolate meaningful pools that provide information on C and nutrient cycling; (ii) test hypotheses on soil formation and ecosystem functioning; (iii) relate SOM characteristics to soil management and global change; and (iv) aid in developing models describing SOM dynamics. Observed differences in MRTs from months to centuries to millennia for different SOM fractions prompted the development of models with multiple soil C pools with differing MRTs (Jenkinson and Rayner, 1977;Van Veen and Paul, 1981;Parton et al., 1987). To date, no single biological, physical, or chemical fractionation technique has been developed that adequately describes the continuum of SOC that exists in nature. Mismatches between measurements of 14 C-labeled plant materials using three size-density fractions and model outputs have been identified (Magid et al., 1996). Motavalli et al. (1994) and Smith et al. (2002) were not successful in their attempts to equate the active pool to microbial biomass plus soluble C fraction or the light fraction. Similar problems were found when attempts were made to equate the size of the slow pool in models with the particulate organic matter fraction (Metherell et al., 1995). However, more recent work investigating free and aggregate-associated SOC has produced more promising ...

Afforestation of agricultural lands can provide economically and environmentally realistic C storage to mitigate for elevated CO 2 until other actions such as reduced fossil fuel use can be taken. Soil carbon sequestration following afforestation of agricultural land ranges from losses to substantial annual gains. The present understanding of the controlling factors is inadequate for understanding ecosystem dynamics, modeling global change and for policy decision-makers. Our study found that planting agricultural soils to deciduous forests resulted in ecosystem C accumulations of 2.4 Mg C ha À1 yr À1and soil accumulations of 0.35 Mg C ha À1 yr À1 . Planting to conifers showed an average ecosystem sequestration of 2.5 and 0.26 Mg C ha À1 yr À1 in the soils but showed greater field to field variability than when planted to deciduous forest. Path analysis showed that Ca was positively related to soil C accumulations for both conifers and deciduous afforested sites and played a significant role in soil C accumulations in these sites. Soil N increases were closely related to C accumulation and were two times greater than could be explained by system N inputs from atmospheric deposition and natural sources. Our results suggest that the addition of Ca to afforested sites, especially conifers, may be an economical means to enhance soil C sequestration even if it does not result in increasing C in aboveground pools. The mechanism of N accumulation in these aggrading stands needs further investigation.

The nature of any virus-specific T cells involved in controlling human cytomegalovirus (HCMV) infection in normal subjects harboring latent virus is unknown. As an approach to this problem, peripheral blood mononuclear cells (PBM) from normal seropositive subjects were cocultured with HCMV and responding T cells expanded in interleukin 2 (IL2)-dependent culture, determining in particular whether HCMV-specific cytotoxic T cells (Tc) were generated. Coculture of PBM with free HCMV resulted in the generation of short-term T cell lines of predominantly helper phenotype (Leu 3a+), expressing no cytotoxicity. However, when PBM were cocultured on HCMV-infected fibroblasts (autologous to the donor in these experiments) predominantly Leu 2a+ lines were generated, which lysed HCMV-infected cells. The cytotoxicity of these short-term IL 2-dependent lines was HCMV-specific and human HLA-restricted; HCMV-infected target cells expressing only early viral antigens were lysed. It is concluded that HCMV-specific Tc precursors are present in peripheral blood of latently infected individuals without preceding overt infection and that effector Tc may be capable of lysing infected cells prior to viral replication.

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ABSTRACTpart, be attributed to environmental impacts on the controls on SOM dynamics that exist on these sites. It is Interpretation of soil organic C (SOC) dynamics depends heavilyimpossible to sample the many sites, soil types, manage- (Campbell et al., 1967;Trumbore et al., 1996), provides

T emperature is an important factor controlling SOM turnover and understanding how temperature aff ects SOM decomposition will allow us to better predict how global climate change will aff ect SOM stocks. Understanding the temperature sensitivity of SOM decomposition is challenging because SOM is composed of many diff erent organic C compounds, with diff ering inherent kinetic properties (Davidson and Janssens, 2006). To simplify the process of modeling SOM decomposition, this range of compounds is usually classifi ed into a small number of discrete, kinetically defi ned pools with some portion of SOM being easily decomposable and the rest comprising one or more other pools decomposing more slowly.In most decomposition models, temperature eff ects are modeled as a decomposition rate multiplier for fi xed SOM pools (Lloyd and Taylor, 1994;Del Grosso et al., 2005), but some recent studies have hypothesized that temperature may actually alter the amount of substrate that would be considered easily decomposable (Zogg et al., 1997;Zak et al., 1999;Dalias et al., 2003;Rasmussen et al., 2006). Th e most broadly used terrestrial C models typically have a fi xed Q 10 of 1.5 to 2.0 or an Arrhenius-type function with the same respiration-temperature relationship to each of the diff erent SOM pools (Melillo et al., 1995;Burke et al., 2003;Friedlingstein et al., 2006 Th e uncertainty associated with how projected climate change will aff ect global C cycling could have a large impact on predictions of soil C stocks. Th e purpose of our study was to determine how various soil decomposition and chemistry characteristics relate to soil organic matter (SOM) temperature sensitivity. We accomplished this objective using long-term soil incubations at three temperatures (15, 25, and 35°C) and pyrolysis molecular beam mass spectrometry (py-MBMS) on 12 soils from 6 sites along a mean annual temperature (MAT) gradient (2-25.6°C). Th e Q 10 values calculated from the CO 2 respired during a long-term incubation using the Q 10-q method showed decomposition of the more resistant fraction to be more temperature sensitive with a Q 10-q of 1.95 ± 0.08 for the labile fraction and a Q 10-q of 3.33 ± 0.04 for the more resistant fraction. We compared the fi t of soil respiration data using a two-pool model (active and slow) with fi rst-order kinetics with a three-pool model and found that the two and three-pool models statistically fi t the data equally well. Th e three-pool model changed the size and rate constant for the more resistant pool. Th e size of the active pool in these soils, calculated using the two-pool model, increased with incubation temperature and ranged from 0.1 to 14.0% of initial soil organic C. Sites with an intermediate MAT and lowest C/N ratio had the largest active pool. Pyrolysis molecular beam mass spectrometry showed declines in carbohydrates with conversion from grassland to wheat cultivation and a greater amount of protected carbohydrates in allophanic soils which may have lead to diff erences found between the total...