Changes in organic carbon and physical properties of soil aggregates under fiber farming

Interest in intensive forestry systems to produce woody fiber or "fiber farming" (FF) is increasing because of high energy costs and the risks of global warming. The FF-induced changes in soil structural properties at the microscale or aggregate level important to soil organic carbon (SOC) sequestration and soil-water dynamics are, however, not well understood. Thus, we compared differences in soil physical properties and SOC concentrations in aggregates among tree species after 7 years of plantation establishment on an Onaway fine sandy loam (fine loamy, mixed, active, frigid Inceptic Hapludalfs) at the Upper Peninsula Tree Improvement Center near Escanaba, Mich. Soil samples from 16.8 x 16.8-m plots under six tree taxa and three rates of wood ash application (0, 9, and 18 Mg ha(-1)) were collected in May, 2005. The six tree taxa were European larch (Larix deciduas Mill.), hybrid aspen (Populus tremula x Populus tremuloides), and four poplar taxa: NE-222 (Populus deltoids x Populus nigra var. caudina), DN-5 (Populus euramericana, cv. "Gelrica"), DN-34 (P. euramericana, cv. "Eugenei"), and NM-6 (P. nigra x Populus maximowiczii). Tree species induced measurable changes in most soil aggregate properties. Larch and DN-34 increased aggregate mean weight diameter more than N-222 and NM-6. Macroaggregates contained 20% more SOC concentration than microaggregates. Ash application increased tensile: strength, soil water retention, and SOC concentration, indicating the benefits of applying ash in FF systems. Within both macroaggregates and microaggregates, NM-6 had the highest SOC concentration, whereas aspen had the lowest of all tree species. The SOC concentration increased with increase in biomass input. Results show that differences in aggregate physical properties and aggregate-associated SOC concentration among tree species were relatively small and depended on the soil property.