Several reviews have analyzed the factors that affect the change in soil organic C (SOC) when forest is converted to agricultural land; however, the effects of forest type and cultivation stage on these changes have generally been overlooked. We collated observations from 453 paired or chronosequential sites where forests have been converted to agricultural land and then assessed the effects of forest type, cultivation stage, climate factors, and soil properties on the change in the SOC stock and the SOC turnover rate constant (k). The percent decrease in SOC stocks and the turnover rate constants both varied significantly according to forest type and cultivation stage. The largest decrease in SOC stocks was observed in temperate regions (52% decrease), followed by tropical regions (41% decrease) and boreal regions (31% decrease). Climate and soil factors affected the decrease in SOC stocks. The SOC turnover rate constant after the conversion of forests to agricultural land increased with the mean annual precipitation and temperature. To our knowledge, this is the first time that original forest type was considered when evaluating changes in SOC after being converted to agricultural land. The differences between forest types should be considered when calculating global changes in SOC stocks.
It is hypothesized that particulate organic matter (POM) contributes to aggregate stability. However, little is known about the dynamics of the POM fraction or its role in aggregate formation. A simulated no‐till study was conducted to examine changes in free and aggregate‐associated POM during the decomposition of in situ 14C‐labeled roots during a 1‐yr incubation in a loess‐derived silt loam. Two water pretreatments (capillary‐wetted and slaked) were applied to soil samples collected during the incubation, and the samples were then wet sieved to obtain five aggregate size fractions. Densiometric separations were used to isolate free and released POM (frPOM) and intraaggregate POM (iPOM). Small macroaggregates (250–2000 μm) were enriched in iPOM‐14C on Day 0 which suggested that many of these aggregates formed around cores of new, root‐derived POM during the growth and senescence of the oat plants. Slaking resulted in the disruption of many of the small macroaggregates (250–2000 μm) and a large increase in frPOM‐14C on Day 0. The amount of 14C released into the frPOM pool with slaking declined with time. In contrast, there was a significant linear increase in the amount of new, root‐derived iPOM‐14C in large microaggregates (53–250 μm) that were released when unstable macroaggregates (>250 μm) slaked. These data support the hypothesis that new microaggregates are formed within existing macroaggregates and provide strong evidence that, in no‐till, aggregate formation and stabilization processes are directly related to the decomposition of root‐residue and the dynamics of POM C in the soil.
A review of the transient liquid phase (TLP) bonding process is presented in this paper. This review concentrates on the mechanisms of the TLP process, including both microstructural development and wettability aspects. However, design related issues and engineering applications are also considered. The paper explains how the TLP bonding process offers the potential for producing joints with microstructures and hence mechanical properties that are similar to those of the parent materials. The process of isothermal solidification is discussed at some length. The paper considers the ways in which specific microstructural features of the substrate material-interlayer material combination influence microstructural development. A distinction is drawn between cases where the progression of isothermal solidification dominates the final microstructure and those where events occurring after the completion of isothermal solidification are paramount. The paper describes the practical limitations on the production of parent-metal-like microstructures and the complexity of microstructural development in real materials, especially where dissimilar substrates are involved. Modification of the TLP bonding process to accommodate cases of limited solid solubility and/or low diffusion coefficients is discussed. STWJ/447
No‐till practices have the potential to increase soil organic C, but little is known about the relative contribution of surface residue and roots to soil organic C accumulation. In a simulated no‐till experiment, we studied the fate of 14C‐labeled surface residue and in situ roots during a 1‐yr incubation. Soil samples collected during the incubation were chemically dispersed and separated into five particle size and density fractions. The organic C, 14C, and total N content of each fraction was determined. Alkali traps were used to measure 14C losses due to respiration. After 360 d, 66% of the 14C contained in the surface residue on Day 0 had been respired as 14CO2, 11% remained in residue on the soil surface, and 16% was in the soil. In comparison, 56% of the root‐derived 14C in the soil was evolved as 14CO2 and 42% remained in the soil. The large (500–2000 μm) and small (53–500 μm) particulate organic matter (POM) fractions together contained 11 to 16% of the initial root‐derived 14C in the soil. In contrast, POM contained only 1 to 3% of the inital surface residue–derived 14C. These data show clear differences in the partitioning of surface residue– and root‐derived C during decomposition and imply that the beneficial effects of no‐till on soil organic C accrual are primarily due to the increased retention of root‐derived C in the soil.
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