D. Andrew Scott

Rapid indices of potential nitrogen mineralization for intensively managed hardwood plantations

Abstract Short‐rotation hardwood plantations generally require repeated applications of nitrogen (N) fertilizer to maintain desired growth and are being installed on two previous land uses: agricultural fields and cutover forest lands. Because the soil organic matter chemistry is different between agricultural field and cutover soils, indices of N availability developed for one land use or the other may not work well across both land use types. The standard aerobic incubation index is time consuming and costly. Therefore, three rapid methods were tested for estimating it on six converted agricultural fields and seven cutover pine sites currently in intensively managed sweetgum plantations on the eastern coastal plain of the United States. Two procedures (anaerobic incubation and hot KCl extract) were not useful for estimating mineralizable N in either agricultural field soils or cutover pine soils. The third index, a 3‐day incubation of rewetted soils previously dried, was linearly correlated to mineralizable N (p<0.0001, R2=0.88). This method, which had not been tested in forest soils previously, worked best for the cutover soils (p<0.002, R2=0.82) and less so for the agricultural field soils (p<0.097, R2=0.66). However, none of the procedures estimated potentially mineralizable N better, across all sites, than total N (p<0.0001, R2=0.93). Total N was not effective, however, in estimating mineralizable N within the agricultural field sites (p<0.220, R2=0.44). Further work will be needed to assess if potentially mineralizable N, total N, or the drying‐rewetting index can be used to help predict fertilizer rates and timings in intensively managed hardwood plantations, especially on cutover forest lands.

Biogeochemical research priorities for sustainable biofuel and bioenergy feedstock production in the Americas

Rapid expansion in biomass production for biofuels and bioenergy in the Americas is increasing demand on the ecosystem resources required to sustain soil and site productivity. We review the current state of knowledge and highlight gaps in research on biogeochemical processes and ecosystem sustainability related to biomass production. Biomass production systems incrementally remove greater quantities of organic matter, which in turn affects soil organic matter and associated carbon and nutrient storage (and hence long-term soil productivity) and off-site impacts. While these consequences have been extensively studied for some crops and sites, the ongoing and impending impacts of biomass removal require management strategies for ensuring that soil properties and functions are sustained for all combinations of crops, soils, sites, climates, and management systems, and that impacts of biomass management (including off-site impacts) are environmentally acceptable. In a changing global environment, knowledge of cumulative impacts will also become increasingly important. Long-term experiments are essential for key crops, soils, and management systems because short-term results do not necessarily reflect long-term impacts, although improved modeling capability may help to predict these impacts. Identification and validation of soil sustainability indicators for both site prescriptions and spatial applications would better inform commercial and policy decisions. In an increasingly inter-related but constrained global context, researchers should engage across inter-disciplinary, inter-agency, and international lines to better ensure the long-term soil productivity across a range of scales, from site to landscape.

Biogeography and organic matter removal shape long-term effects of timber harvesting on forest soil microbial communities

The growing demand for renewable, carbon-neutral materials and energy is leading to intensified forest land-use. The long-term ecological challenges associated with maintaining soil fertility in managed forests are not yet known, in part due to the complexity of soil microbial communities and the heterogeneity of forest soils. This study determined the long-term effects of timber harvesting, accompanied by varied organic matter (OM) removal, on bacterial and fungal soil populations in 11- to 17-year-old reforested coniferous plantations at 18 sites across North America. Analysis of highly replicated 16 S rRNA gene and ITS region pyrotag libraries and shotgun metagenomes demonstrated consistent changes in microbial communities in harvested plots that included the expansion of desiccation- and heat-tolerant organisms and decline in diversity of ectomycorrhizal fungi. However, the majority of taxa, including the most abundant and cosmopolitan groups, were unaffected by harvesting. Shifts in microbial populations that corresponded to increased temperature and soil dryness were moderated by OM retention, which also selected for sub-populations of fungal decomposers. Biogeographical differences in the distribution of taxa as well as local edaphic and environmental conditions produced substantial variation in the effects of harvesting. This extensive molecular-based investigation of forest soil advances our understanding of forest disturbance and lays the foundation for monitoring long-term impacts of timber harvesting.

Wood Bioenergy and Soil Productivity Research

Timber harvesting can cause both short- and long-term changes in forest ecosystem functions, and scientists from USDA Forest Service (USDA FS) have been studying these processes for many years. Biomass and bioenergy markets alter the amount, type, and frequency at which material is harvested, which in turn has similar yet specific impacts on sustainable productivity. The nature of some biomass energy operations provides opportunities to ameliorate or amend forest soils to sustain or improve their productive capacity, and USDA FS scientists are leading the research into these applications. Research efforts to sustain productive soils need to be verified at regional, national, and international scope, and USDA FS scientists work to advance methods for soil quality monitoring and to inform international criteria and indicators. Current and future USDA FS research ranges from detailed soil process studies to regionally important applied research and to broad scale indicator monitoring and trend analysis, all of which will enable the USA to lead in the sustainable production of woody biomass for bioenergy.