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.

Soil organic matter (SOM) in agricultural soils comprises a significant part of the global terrestrial C pool. It has often been characterized by utilizing a combination of chemical dispersion of the soil followed by physical separation. We fractionated soil samples under continuous corn (Zea mays L.) rotations at four long‐term sites in the Corn Belt to determine the concentration of C and N associated with soil fractions (light fraction [LF], particulate organic matter [POM], silt size, clay size, and Bradford reactive soil protein [BRSP]) and to identify the change in C concentration and δ13C signal of each fraction using laboratory incubations. Light fractions comprised 3 to 5% of the soil organic carbon (SOC), with no significant difference between conventional tillage (CT) and no‐till (NT) treatments. The POM fraction accounted for 5 to 11% of the SOC in the soils with >30% clay and 17 to 23% for the soils with <20% clay. The clay‐size fraction contained the highest proportion of SOC. Measurement of 13C during long‐term incubation showed that the average mean residence time (MRT) of corn‐derived C in the LF was 3.5 yr, whereas the POM fractions ranged from 6 to 12 yr. The 13C changes during incubation show that both fractions consist of a mixture of active and resistant materials, with movement between fractions. The BRSP has long MRTs except in the NT Hoytville soil. Measurement of the dyna mics of these fractions provides a basis for C models to test the impacts of land use and management on C sequestration.

ABSTRACTelevated CO 2 (Allen et al., 2000) but no statistically significant differences in fine root production, microbial Soil C sequestration in predicted, future elevated CO 2 environbiomass, or plant chemistry. Growth in elevated CO 2 ments will be important to atmospheric CO 2 levels, soil tilth, and fertility. An elevated CO 2 study with ponderosa pines (Pinus poncan lead to changed plant species composition and litter derosa Laws) grown in chambers produced above ground vegetation quality and quantity (Oene et al., 1999). This together

Intensive agriculture and increased N fertilizer use have contributed to elevated emissions of the greenhouse gases carbon dioxide (CO(2)), methane (CH(4)), and nitrous oxide (N(2)O). In this study, the exchange of CO(2), N(2)O, and CH(4) between a Quincy fine sand (mixed, mesic Xeric Torripsamments) soil and atmosphere was measured in a sweet corn (Zea mays L.)-sweet corn-potato (Solanum tuberosum L.) rotation during the 2005 and 2006 growing seasons under irrigation in eastern Washington. Gas samples were collected using static chambers installed in the second-year sweet corn and potato plots under conventional tillage or reduced tillage. Total emissions of CO(2)-C from sweet corn integrated over the season were 2071 and 1684 kg CO(2)-C ha(-1) for the 2005 and 2006 growing seasons, respectively. For the same period, CO(2) emissions from potato plots were 1571 and 1256 kg of CO(2)-C ha(-1). Cumulative CO(2) fluxes from sweet corn and potato fields were 17 and 13 times higher, respectively, than adjacent non-irrigated, native shrub steppe vegetation (NV). Nitrous oxide losses accounted for 0.5% (0.55 kg N ha(-1)) of the applied fertilizer (112 kg N ha(-1)) in corn and 0.3% (0.59 kg N ha(-1)) of the 224 kg N ha(-1) applied fertilizer. Sweet corn and potato plots, on average, absorbed 1.7 g CH(4)-C ha(-1) d(-1) and 2.3 g CH(4)-C ha(-1) d(-1), respectively. The global warming potential contributions from NV, corn, and potato fields were 459, 7843, and 6028 kg CO(2)-equivalents ha(-1), respectively, for the 2005 growing season and were 14% lower in 2006.

This study was conducted to determine the effect of natural drainage on sand‐, silt‐, and clay‐size minerals in two sandy hydrosequences of Spodosols in northern Michigan. Depth to water tables were monitored using piezometers. Minerals in the dominant sand fraction were identified and quantified using a petrographic microscope. Minerals in the silt and clay fractions were identified and semiquantified using x‐ray diffraction. Sand mineralogy was dominated by quartz with small amounts of feldspars and sericite (altered muscovite). Silt fractions were composed of quartz with lesser amounts of feldspars. Weathering of minerals in the sand and silt fractions appeared to be hindered by high water tables, especially in the poorly drained soils. Clay mineral distribution in the hydrosequences suggests that muscovite has transformed to vermiculite in B horizons and through vermiculite to smectite in E horizons. Weathering intensity of this sequence was apparently impeded by water saturation in these two hydrosequences. Most trioctahedral chlorite appeared to have completely dissolved or decomposed to soluble products in E and B horizons.