Bruce A. Caldwell

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.

Soil solution chemistry of ectomycorrhizal mats in forest soil

Abstract Survival and productivity of Douglas-fir [ Pseudotsuga menziesii (Mirb.) Franco] depend on close association between host trees and ectomycorrhizal fungi. Two of these fungi, Hysterangium setchellii (Fischer) and Gautieria monticola (Harkness), form extensive hyphal mats with the roots of Douglas-fir and other conifers in the surface of the ‘A’ horizon, often at the interface between mineral oil and litter. The fungal mat alters the chemistry and mineral nutrition of the soil microenvironment within the rhizosphere, producing conditions that favor increased tree growth by increasing nutrient availability. Forest soils with or without obvious ectomycorrhizal mats were sampled at two locations in the Pacific Northwest. Cation and anion chemistry, dissolved organic carbon (DOC) and oxalate anions were analyzed. Mean concentrations of DOC, oxalate, PO 4 , SO 4 , H, Al, Fe, Cu, Mn and Zn were significantly higher in mat than in non-mat soil solutions in both mat types and locations and on both sampling dates. Significant statistical correlations between DOC or oxalate and PO 4 indicate that organic acids influence weathering and solubility of PO 4 in the mat soils. Mean oxalate concentrations were significantly lower in soil solutions from Hysterangium mat soils than in those from Gautieria mat soils. Organic acids released to the rhizosphere by G. monilcola and H. setchellii may provide a local weathering environment that increases availability of PO 4 , SO 4 and trace nutrients.

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.

Utilization of major detrital substrates by dark-septate, root endophytes'

Utilization of major forms of carbon, nitrogen and phosphorus commonly present in plant litter and detritus was determined for cultures of Phialophora finlandia, Phialocephala fortinii and five dar...

Chronic nitrogen additions suppress decomposition and sequester soil carbon in temperate forests

The terrestrial biosphere sequesters up to a third of annual anthropogenic carbon dioxide emissions, offsetting a substantial portion of greenhouse gas forcing of the climate system. Although a number of factors are responsible for this terrestrial carbon sink, atmospheric nitrogen deposition contributes by enhancing tree productivity and promoting carbon storage in tree biomass. Forest soils also represent an important, but understudied carbon sink. Here, we examine the contribution of trees versus soil to total ecosystem carbon storage in a temperate forest and investigate the mechanisms by which soils accumulate carbon in response to two decades of elevated nitrogen inputs. We find that nitrogen-induced soil carbon accumulation is of equal or greater magnitude to carbon stored in trees, with the degree of response being dependent on stand type (hardwood versus pine) and level of N addition. Nitrogen enrichment resulted in a shift in organic matter chemistry and the microbial community such that unfertilized soils had a higher relative abundance of fungi and lipid, phenolic, and N-bearing compounds; whereas, N-amended plots were associated with reduced fungal biomass and activity and higher rates of lignin accumulation. We conclude that soil carbon accumulation in response to N enrichment was largely due to a suppression of organic matter decomposition rather than enhanced carbon inputs to soil via litter fall and root production.

Soil enzyme response to vegetation disturbance in two lowland Costa Rican soils

Abstract Conversion of forests to intensive agriculture often leads to degradation of weathered soils. The effects of two intensities of vegetation management on soil β-glucosidase (β-GLC) and phosphomonoesterase (PME) activities were studied on two river-terrace soils of differing fertility in Costa Rica. After approximately four years of annual harvest or continuous vegetation removal to bare soil, soil organic matter carbon (SOM-C), microbial biomass carbon (Mb-C), β-GLC and PME activity were reduced. Effects of continuous cropping to bare soil on Mb-C, β-GLC and PME were greater in the more weathered, acidic, Al-rich, P-limited upper-terrace soil than in the more neutral, base cation-rich lower-terrace soil. In contrast, more SOM-C was lost in the lower terrace. The annual harvest treatment produced intermediate decreases in SOM-C, Mb-C, β-GLC and PME on upper-terrace soils, intermediate reduction in lower terrace β-GLC, and no significant effect on lower terrace Mb-C or PME. β-GLC activity was the most sensitive indicator of treatment effect and may be a suitable alternative to Mb-C or SOM-C as a measure of change in soil health.

Chemistry and Dynamics of Dissolved Organic Matter in a Temperate Coniferous Forest on Andic Soils: Effects of Litter Quality

Dissolved organic matter (DOM) plays an important role in transporting carbon and nitrogen from forest floor to mineral soils in temperate forest ecosystems. Thus, the retention of DOM via sorption or microbial assimilation is one of the critical steps for soil organic matter formation in mineral soils. The chemical properties of DOM are assumed to control these processes, yet we lack fundamental information that links litter quality, DOM chemistry, and DOM retention. Here, we studied whether differences in litter quality affect solution chemistry and whether changes in litter inputs affect DOM quality and removal in the field. The effects of litter quality on solution chemistry were evaluated using chemical fractionation methods for laboratory extracts and for soil water collected from a temperate coniferous forest where litter inputs had been altered. In a laboratory extraction, litter type (needle, wood, root) and the degree of decomposition strongly influenced solution chemistry. Root litter produced more than 10 times more water-extractable dissolved organic N (DON) than any other litter type, suggesting that root litter may be most responsible for DON production in this forest ecosystem. The chemical composition of the O-horizon leachate was similar under all field treatments (doubled needle, doubled wood, and normal litter inputs). O-horizon leachate most resembled laboratory extracts of well-decomposed litter (that is, a high proportion of hydrophobic acids), in spite of the significant amount of litter C added to the forest floor and a tendency toward higher mean DOM under doubled-Litter treatments. A lag in DOM production from added litter or microbial modification might have obscured chemical differences in DOM under the different treatments. Net DOM removal in this forest soil was strong; DOM concentration in the water deep in the mineral soil was always low regardless of concentrations in water that entered the mineral soil and of litter input manipulation. High net removal of DOM from O-horizon leachate, in spite of extremely low initial hydrophilic neutral content (labile DOM), coupled with the lack of influence by season or soil depth, suggests that DOM retention in the soil was mostly by abiotic sorption.

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.

Hyphal mats formed by two ectomycorrhizal fungi and their association with Douglas-fir seedlings: A case study

The ectomycorrhizal fungi Gautieria monticola and Hysterangium setchellii both form dense hyphal mats in coniferous forest soils of the Pacific Northwest. We recently observed that all Douglas-fir seedlings found under the canopy of a maturing 60–75 year stand were associated with mats formed by ectomycorrhizal fungi. The significance of these mat communities in relation to seedling establishment and survival is discussed.

Microbial characteristics of ectomycorrhizal mat communities in Oregon and California

SummarySpecialized ectomycorrhizal fungi form dense mats in forest soils that have different enzyme levels, higher respiration rates, more biomass, different soil fauna, and different soil chemistry compared with adjacent soils not obviously colonized by these mats. In this study, mats formed by two genera of fungi collected in three locations were compared with a wide range of measurements. Per cent moisture, pH, chloroform fumigation-flush C, anaerobic N mineralization, exchangeable ammonium, and respiration, N2 fixation, and denitrification rates were compared between soils or litter colonized by ectomycorrhizal mat-forming fungi and adjacent non-mat material. Significant differences were observed between the two genera of mat-forming fungi and also between mats formed primarily in mineral soil and those formed in litter. These differences suggest that different mat-forming fungi perform different functions in forest soils and that these fungi function differently in mineral soil compared with litter.