Peter S. Homann

Stabilization and destabilization of soil organic matter: mechanisms and controls

We present a conceptual model of the processes by which plant leaf and root litter is transformed to soil organic C and CO 2. Stabilization of a portion of the litter C yields material that resists further transformation; destabilization yields material that is more susceptible to microbial respiration. Stability of the organic C is viewed as resulting from three general sets of characteristics. Recalcitrance comprises, molecular-level characteristics of organic substances, including elemental composition, presence of functional groups, and molecular conformation, that influence their degradation by microbes and enzymes. Interactions refers to the inter-molecular interactions between organics and either inorganic substances or other organic substances that alter the rate of degradation of those organics or synthesis of new organics. Accessibility refers to the location of organic substances with respect to microbes and enzymes. Mechanisms by which these three characteristics change through time are reviewed along with controls on those mechanisms. This review suggests that the following changes in the study of soil organic matter dynamics would speed progress: (1) increased effort to incorporate results into budgets for whole soil (e.g., converting to a kg/ha basis) so that the relative importance of processes can be judged; (2) more attention to effects of inter-molecular interactions (especially Al complexation) on enzyme activity and substrate degradation; (3) increased effort to experimentally manipulate soils, preferably across a range of soil types; (4) study of stabilization and destabilization mechanisms under conditions that are well defined yet more relevant to soil environments than those used previously; and (5) experiments better designed to isolate mechanisms so results are not confounded by effects of other mechanisms operating simultaneously.

Carbon dynamics during a long-term incubation of separate and recombined density fractions from seven forest soils

Abstract Density fractions in soils differ in their turnover rates, but direct measurement of the C dynamics in the individual density fractions is limited. In 300-day incubations of mineral soils from forests in Washington and Oregon, USA, light fractions (LF), heavy fractions (HF), whole soils (WS), and physically recombined light and heavy fractions (RF), were measured for respiration and shifts in microbial biomass. A combined fraction was calculated from the incubation results of the light and heavy fractions, and called the summed fraction (SF). Carbon concentration followed the pattern: LF>RF>HF. In accordance with this pattern, when cumulative respiration was considered per gram of substrate, the physical fractions exhibited a predictable response: LF>RF>HF. However, when expressed per gram of initial C, the respiration of the LF was not significantly different from that of the HF. These findings suggest the recalcitrance of HF is similar to that of LF and, consequently, differences in their turnover rates in WS may be due to microbial accessibility or physical protection. Whether expressed per gram of substrate or per gram of initial C, the respiration of the SF was not different from that of the WS. Within the SF, the HF was responsible for 35% of the total respiration. Lower respiration in the RF compared with WS and SF might be explained by an antagonistic interaction between the varied microbial communities that degrade LF and HF; in the heterogeneous WS, these communities may be spatially separated to a greater extent than in the laboratory substrate. Unfortunately, the microbial data were highly variable and provided little evidence to either support or refute this idea. The density separation technique appears to be a viable method for isolating different soil organic matter fractions. However, the function of these fractions should be considered more carefully in the context of accessibility and C content.

Long-term effects of elevated nitrogen on forest soil organic matter stability

Nitrogen addition may alter the decomposition rate for different organic-matter pools in contrasting ways. Using a paired-plot design, we sought to determine the effects of long-term elevated N on the stability of five organic-matter pools: organic horizons (Oe+a), whole mineral soil (WS), mineral soil fractions including the light fraction (LF), heavy fraction (HF), and a physically recombined fraction (RF). These substrates were incubated for 300 days, and respiration, mineralized N, and active microbial biomass were measured. Samples with elevated N gave 15% lower cumulative respiration for all five substrates. Over the 300-day incubation, the Oe+a gave twice the cumulative respiration (g C kg−1 initial C) as the LF, which gave slightly higher respiration than the HF. Respiration was 35% higher for the WS than for the RF. Mineralized N was similar between N treatments and between the LF and HF. Net N mineralized by the LF over the course of the 300-day incubation decreased with higher C:N ratio, due presumably to N immobilization to meet metabolic demands. The pattern was opposite for HF, however, which could be explained by a release of N in excess of metabolic demands due to recalcitrance of the HF organic matter. Mineralized N increased with respiration for the HF but showed no pattern, or perhaps even decreased, for the LF. WS and RF showed decreasing active microbial biomass near the end of the incubation, which corresponded with decreasing respiration and increasing nitrate. Our results show that long-term elevated N stabilized organic matter in whole soil and soil fractions.

Changes in ecosystem carbon storage over 40 years on an old-field/forest landscape in east-central Minnesota

Organic carbon (C) storage in complex landscapes and its temporal change can be important in the global C budget. Change in C storage between 1938 and 1977 was estimated for a 2224 ha old-field/forest landscape in east-central Minnesota by coupling change in area of seven vegetation types (five forest and two non-forest) with vegetation-specific C densities (mg ha−1). Carbon densities were based on sampling carried out between 1974 and 1990. Areas of vegetation types in 1938 and 1977 were determined from aerial photographs. Carbon density was greatest in forest overstory (60–100 Mg ha−1) and organic and mineral soil (30–100 Mg ha−1 to 25 cm depth). Ecosystem C storage was approximately 212000 Mg in 1938 and 225000 Mg in 1977, an increase of ca. 13000 Mg across the study area. This was due largely to an increase in upland forest at the expense of non-forest area. The largest proportional increase in C storage was in trees (a 13% gain), while mineral soil gained 4% and herbs gained 6%. C storage in O horizon and shrubs remained constant. For the 20% of the landscape originally occupied by cultivated fields, an empirical model based on chronosequence studies indicated a 40% increase in C storage over 40 years; C increased in mineral soil, O horizon and trees as both herbaceous succession and forest encroachment occurred. Uncertainties of the estimates, based on propagation of standard errors, were 5% to 19% for C storage and 6% to 1000% for change in C storage. Uncertainty was due primarily to sample variability, but included uncertainty in biomass equations and GIS processing. This uncertainty demonstrates the difficulties associated with expanding from point sample data to landscape-scale estimates of C storage.

Nitrogen mineralization, groundwater dynamics, and forest growth on a Minnesota outwash landscape

We measured aboveground biomass and aboveground net primary productivity (ANPP), groundwater depth and fluctuation, andin situ nitrogen (N) mineralization in 13 upland and 4 wetland forest stands at Cedar Creek Natural History Area (CCNHA). The area, in east central Minnesota (45°25′ N, 93°10′ W), is on well-sorted glacial outwash of very uniform fine sand. Uplands are interspersed with peadands and the area has shallow groundwater. Stands were aggregated into six ecosystem types based on overstory composition: oak, pine-oak, mesic hardwoods, northern white-cedar, lowland hardwoods, and savanna. Aboveground overstory biomass ranged from 35 to 250 Mg ha−1; lowest in the savanna and highest in the pine-oak. The ANPP ranged from about 2 to 7.5 Mg ha−1; also lowest in the savanna but highest in the white-cedar. Over all types, the annual aboveground uptake of N was poorly related to available N measured byin situ mineralization (r2 = 0.01), but the relationship was better (r2 = 0.88) if N availability in the wetland stands was assumed to be a fixed proportion of N in the surface soil (1.5%). Over all types,in situ N mineralization was poorly related to ANPP (r2 = 0.05) and biomass (r2 = 0.38). Both ANPP and overstory biomass were more closely related to groundwater fluctuation (r2 = 0.87 and 0.28, respectively) than to depth (r2 = 0.01 and 0.21, respectively)). The strength of all relationships varied with the inclusion or exclusion of data from the wetland types or the savanna. Total soil N and rates of mineralization were inversely related (r2 = 0.42) because of data from wetland stands. Results demonstrate that the positive relationships between aboveground productivity and measuredin situ N mineralization observed in upland forests are not valid for the landscape that includes wetland forests either becausein situ measurements do not indicate N availability in wetlands or because of the presence of other limiting factors. The north temperate landscape includes an abundance of wetland forests with potentially strong linkages to uplands. This study suggests that the commonly-used measure of N availability provides inconsistent information about controls on ecosystems processes in this diverse landscape.

Belowground processes in forest-ecosystem biogeochemical simulation models

Abstract Numerical simulation models of forest ecosystems synthesize a broad array of concepts from tree physiology, community ecology, hydrology, soil physics, soil chemistry and soil microbiology. Most current models are directed toward assessing natural processes or existing conditions, nutrient losses influenced by atmospheric deposition, C and N dynamics related to climate variation, and impacts of management activities. They have been applied mostly at the stand or plot scale, but regional and global applications are expanding. Commonly included belowground processes are nutrient uptake by roots, root respiration, root growth and death, microbial respiration, microbial mineralization and immobilization of nutrients, nitrification, denitrification, water transport, solute transport, cation exchange, anion sorption, mineral weathering and solution equilibration. Models differ considerably with respect to which processes and associated chemical forms are included, and how environmental and other factors influence process rates. Recent models demonstrated substantial discrepancies between model output and observations for both model verification and validation. The normalized mean absolute error between model output and observations of soil solution solute concentrations, solid phase characteristics, and process rates ranged from 0 to >1000%. There were considerable differences among outputs from models applied to the same situation, with process rates differing by as much as a factor of 4, and changes in chemical masses differing in both direction and magnitude. These discrepancies are attributed to differences in model structure, specific equations relating process rates to environmental factors, calibration procedures, and uncertainty of observations. Substantial improvement in the capability of models to reproduce observed trends is required for models to be generally applicable in public-policy decisions. Approaches that may contribute to improvement include modularity to allow easy alteration and comparison of individual equations and process formulations; hierarchical structure to allow selection of level of detail, depending on availability of data for calibration and driving variables; enhanced documentation of all phases of model development, calibration, and evaluation; and continued coordination with experimental studies.

Denitrification enzyme activity of Douglas-Fir and red Alder forest soils of the Pacific Northwest

Abstract Nitrogen-fixing red alder ( Alnus rubra Bong.) increases soil organic matter and N content of forest soils. This study compared denitrification enzyme activity (DEA) to related N-cycling and microbial indicators in adjacent stands of alder and Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) in two Pacific Northwest U.S.A. research forests over 16 months. Laboratory denitrification rates were measured in non-amended soils and soils amended with combinations of water, NO 3 − , and glucose. The NO 3 − -and glucose-amended soils provided estimates of DEA. DEA in alder soils was greater than or equal to that in corresponding Douglas-fir soils. Denitrification in alder soils was occasionally limited by energy source (glucose) but not by NO 3 − , whereas in Douglas-fir soils, it was frequently limited by both NO 3 − and glucose. For a given soil, DEA was generally not well related to respiration potential, anaerobic mineralizable N, or exchangeable ammonium over time, but it was well related to nitrification potential across different soils and over time within two soils.

Forest Structure Affects Soil Mercury Losses in the Presence and Absence of Wildfire

Soil is an important, dynamic component of regional and global mercury (Hg) cycles. This study evaluated how changes in forest soil Hg masses caused by atmospheric deposition and wildfire are affected by forest structure. Pre and postfire soil Hg measurements were made over two decades on replicate experimental units of three prefire forest structures (mature unthinned, mature thinned, clear-cut) in Douglas-fir dominated forest of southwestern Oregon. In the absence of wildfire, O-horizon Hg decreased by 60% during the 14 years after clearcutting, possibly the result of decreased atmospheric deposition due to the smaller-stature vegetative canopy; in contrast, no change was observed in mature unthinned and thinned forest. Wildfire decreased O-horizon Hg by >88% across all forest structures and decreased mineral-soil (0 to 66 mm depth) Hg by 50% in thinned forest and clear-cut. The wildfire-associated soil Hg loss was positively related to the amount of surface fine wood that burned during the fire, the proportion of area that burned at >700 °C, fire severity as indicated by tree mortality, and soil C loss. Loss of soil Hg due to the 200,000 ha wildfire was more than four times the annual atmospheric Hg emissions from human activities in Oregon.