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Soil erosion continuously redistributes soil and associated soil organic matter (SOM) on the Earth's surface, with important implications for biogeochemical cycling of essential elements and terrestrial carbon sequestration. Despite the importance of soil erosion, surprisingly few studies have evaluated the sources of eroded carbon (C). We used natural abundance levels of the stable and radioactive isotopes of C (13C and 14C) and stable isotope of nitrogen (15N) to elucidate the origins of SOM eroded from low-order catchments along the western slopes of the Sierra Nevada of California, USA. Our work was conducted in two relatively undisturbed catchments (low elevation = 1800 m, and high elevation = 2300 m) of the Kings River Experimental Watersheds (KREW) in the Sierra National Forest. Sediment captured in basins at the outlet of each gauged watershed were compared to possible source materials, which included: upland surficial organic horizons (i.e., forest floor) and mineral soils (0–0.6 m) from three landform positions (i.e., crest, backslope, and toeslope), stream bank soils (0–0.6 m), and stream-bed materials (0–0.05 m). We found that most of the organic matter (OM) in the captured sediments was composed of O-horizon material that had high C concentrations. Radiocarbon analyses also showed that the captured OM is composed of modern (post-1950) C, with fraction modern values at or above 1.0. Our results suggest that surface (sheet) erosion, as opposed to channeling through established streams and episodic mass wasting events, is likely the largest source of sediment exported out of these minimally disturbed, headwater catchments. The erosional export of sediment with a high concentration of C, especially in the form of relatively undecomposed litter from the O horizon, suggests that a large fraction of the exported C is likely to be decomposed during or after erosion; hence, it is unlikely that soil erosion acts as a significant net sink for atmospheric CO2 in these low-order, temperate forest catchments.

Shifts in microbial communities driven by anthropogenic nitrogen (N) addition have broad-scale ecological consequences. However, responses of microbial groups to exogenous N supply vary considerably across studies, hindering efforts to predict community changes. We used meta-analytical techniques to explore how amoA gene abundances of ammonia-oxidizing archaea (AOA) and bacteria (AOB) respond to N addition, and found that N addition increased AOA and AOB abundances by an average of 27% and 326%, respectively. Responses of AOB varied by study type, ecosystem, fertilizer type, and soil pH, and were strongest in unmanaged wildland soils and soils fertilized with inorganic N sources. Increases in nitrification potential with N addition significantly correlated with only AOB. Our analyses suggest that elevated N supply enhances soil nitrification potential by increasing AOB populations, and that this effect may be most pronounced in unmanaged wildland soils.

Establishment of native grasses is a primary objective of restoration in Pinus ponderosa var. scopulorum (P. & C. Lawson) forests in the southwestern United States. Interactions among native grasses and soil microorganisms generate feedbacks that influence the achievement of this objective. We examined soil chemical properties and communities of plants and soil microorganisms in clear-cuts and P. ponderosa stands thinned and maintained at low and medium tree densities for over 40 years along with high density (unthinned) stands. Phospholipid fatty acids (PLFA) in soils were analyzed to examine arbuscular mycorrhizal (AM) fungi and microbial communities in the three thinning treatments and the unthinned stands with and without a recent broadcast burn. Additionally, two native bunchgrasses, Festuca arizonica and Muhlenbergia wrightii were grown in containers filled with intact soil cores collected from each field plot to more thoroughly compare the abundance of AM fungi and microbial communities across different stand densities and burn treatments. Tree thinning decreased litter cover and increased the abundance and diversity and altered community composition of both herbaceous vegetation and AM fungi. In the mineral soil layer, the pH, total carbon, nitrogen, phosphorus and PLFA profiles did not differ significantly among the four stand density or burn treatments. Mycorrhizal colonization of the container grown grasses did not significantly differ with tree density or burn treatments; however, F. arizonica roots had a strong trend for decreased colonization when grown in soil from high density (unthinned) tree cover. Soil from the containers with F. arizonica had a greater abundance of AM fungal spores. Furthermore, bacterial community composition varied with grass species. Concentration of biomarkers for bacteria were higher in soil that supported F. arizonica compared to soil in which M. wrightii was grown. Our results indicate that the creation of clear-cut openings in forests may increase the abundance and richness of AM fungal propagules and soil bacterial communities were surprisingly resilient to tree thinning and low-intensity fire treatments. These results suggest managing forests to create clear-cut openings generate conditions that favor understory native grasses and AM fungi that are linked to soil bacterial communities.

In arid ecosystems, widely spaced vegetation and prolonged dry periods may enhance canopy capture of nutrients from dry deposition. Additionally, differences in precipitation type, plant canopy architecture, and soil nutrient limitation could affect canopy exchange of atmospherically derived nutrients. We collected bulk precipitation and throughfall underneath piñon pine (. Pinus edulis) and one-seed juniper (. Juniperus monosperma) along a substrate age gradient to determine if canopy interception or throughfall chemistry differed among tree species, season, or substrate age. The Substrate Age Gradient of Arizona consists of four sites with substrate ages ranging from 1ky to 3000ky-old, which exhibit classic variations in soil nitrogen (N) and phosphorus (P) availability with substrate age. Greater nutrient inputs below canopies than in intercanopy areas suggest throughfall contributes to the "islands of fertility" effect. Canopy interception of precipitation did not differ between tree species, but was greater in the summer/fall than winter/spring. We found that net canopy retention of atmospherically derived N was generally greater when N availability in the soil was low, but retention also occurred when N availability was relatively high. Taken together, our results were inconclusive in determining whether the degree of soil nutrient limitation alters canopy exchange of plant growth-limiting nutrients. © 2013 Elsevier Ltd.

Increases in stand-replacing wildfires in the western USA have widespread implications for ecosystem carbon (C) cycling, in part because the decomposition of trees killed by fire can be a long-term source of CO2 to the atmosphere. Knowledge of the composition and function of decay fungi communities may be important to understanding how wildfire alters C cycles. We assessed the effects of stand-replacing wildfires on the community structure of wood-inhabiting fungi along a 32-yr wildfire chronosequence. Fire was associated with low species richness for up to 4 yr and altered species composition relative to unburned forest for the length of the chronosequence. A laboratory incubation demonstrated that species varied in their capacity to decompose wood; Hypocrea lixii, an indicator of the most recent burn, caused the lowest decomposition rate. Our results show that stand-replacing wildfires have long-term effects on fungal communities, which may have

Soil organic matter (SOM) decomposition has the potential to radically affect carbon dioxide concentrations in the atmosphere. Priming, the increased decomposition of SOM after the addition of a labile carbon (C) source, may be an important regulator of SOM dynamics, yet little is known about the mechanisms of the priming effect. Two hypotheses generated in the last decade have suggested that priming is caused by either the nutrient conditions in soil or the response of the microbial community to labile C addition. We used a three million year substrate age gradient, with associated changes in nutrient availability and microbial communities, to test these two hypotheses. We added 13C labeled glucose to soil in quantities similar to increases in root exudation that can be expected in a high carbon dioxide environment, and traced the effect of C addition on soil C, the microbial community, and soil nutrient pools and fluxes. We observed positive priming, negative priming, and no net priming depending on substrate age. Priming was most positive at the youngest sites with the smallest nitrogen (N) pools, and most negative at the site with the most available N. In contrast, we found no significant relationships between priming and phosphorus availability. Though components of the microbial community size and structure (measured by phospholipid fatty acid analysis) changed as a result of C addition, soil N availability was a better explanatory mechanism of priming effects than microbial community dynamics. Our results suggest close linkages between C and N cycles regulate the magnitude and direction of priming. If general, the stability of SOM in temperate ecosystems is likely to be governed by soil N status.

Emerging research is challenging existing paradigms of nitrogen (N) cycling in arid and semiarid ecosystems that experience distinct seasonal patterns in precipitation. We measured equal or greater potential nitrification rates in the dry season than the wet season in chemically and physically distinct soils along a three million year substrate age gradient in Arizona. These surprising and counterintuitive results are supported by recently published work in California Mediterranean grasslands. Considered together, these studies call attention to the need to measure process rates during the dry, non-growing season of xeric ecosystems. Several mechanisms may be responsible for these patterns, but we highlight the importance of understanding the contribution of archaeal ammonia oxidizers to nitrification in these seasonally dry ecosystems.

Most organic carbon (C) in soils eventually turns into CO2 after passing through microbial metabolic pathways, while providing cells with energy and biosynthetic precursors. Therefore, detailed insight into these metabolic processes may help elucidate mechanisms of soil C cycling processes. Here, we describe a modeling approach to quantify the C flux through metabolic pathways by adding 1-13C and 2,3-13C pyruvate and 1-13C and U-13C glucose as metabolic tracers to intact soil microbial communities. The model calculates, assuming steady-state conditions and glucose as the only substrate, the reaction rates through glycolysis, Krebs cycle, pentose phosphate pathway, anaplerotic activity through pyruvate carboxylase, and various biosynthesis reactions. The model assumes a known and constant microbial proportional precursor demand, estimated from literature data. The model is parameterized with experimentally determined ratios of 13CO2 production from pyruvate and glucose isotopologue pairs. Model sensitivity analysis shows that metabolic flux patterns are especially responsive to changes in experimentally determined 13CO2 ratios from pyruvate and glucose. Calculated fluxes are far less sensitive to assumptions concerning microbial chemical and community composition. The calculated metabolic flux pattern for a young volcanic soil indicates significant pentose phosphate pathway activity in excess of pentose precursor demand and significant anaplerotic activity. These C flux patterns can be used to calculate C use efficiency, energy production and consumption for growth and maintenance purposes, substrate consumption, nitrogen demand, oxygen consumption, and microbial C isotope composition. The metabolic labeling and modeling methods may improve our ability to study the biochemistry and ecophysiology of intact and undisturbed soil microbial communities.

In order to study controls on metabolic processes in soils, we determined the dynamics of 13 CO2 production from two position-specific 13 C-labeled pyruvate isotopologues in the presence and absence of glucose, succinate, pine, and legume leaf litter, and under anaerobic conditions. We also compared 13 CO2 production in soils along a semiarid substrate age gradient in Arizona. We observed that the C from the carboxyl group (C1) of pyruvate was lost as CO2 much faster than its other C atoms (C2,3). Addition of glucose, pine and legume leaf litter reduced the ratio between 13 CO2 production from 1- 13 C pyruvate and 2,3- 13 C pyruvate (C1/C2,3 ratio), whereas anaerobic conditions increased this ratio. Young volcanic soils exhibited a lower C1/C2,3 ratio than older volcanic soils. We interpret a low C1/C2,3 ratio as an indication of increased Krebs cycle activity in response to carbon inputs, while the higher ratio implies a reduced Krebs cycle activity in response to anaerobic conditions. Succinate, a gluconeogenic substrate, reduced 13 CO2 production from pyruvate to near zero, likely reflecting increased carbohydrate biosynthesis from Krebs cycle intermediates. The difference in 13 CO2 production rate from pyruvate isotopologues disappeared 4e5 days after pyruvate addition, indicating that C positions were scrambled by ongoing soil microbial transformations. This work demonstrates that metabolic tracers such as pyruvate can be used to determine qualitative aspects of C flux patterns through metabolic pathways of soil microbial communities. Understanding the controls over metabolic processes in soil may improve our understanding of soil C cycling processes.

Soil microbial organisms are central to carbon (C) and nitrogen (N) transformations in soils, yet not much is known about the stable isotope composition of these essential regulators of element cycles. We investigated the relationship between C and N availability and stable C and N isotope composition of soil microbial biomass across a three million year old semiarid substrate age gradient in northern Arizona. The δ15N of soil microbial biomass was on average 7.2‰ higher than that of soil total N for all substrate ages and 1.6‰ higher than that of extractable N, but not significantly different for the youngest and oldest sites. Microbial 15N enrichment relative to soil extractable and total N was low at the youngest site, increased to a maximum after 55,000 years, and then decreased slightly with age. The degree of 15N enrichment of microbial biomass correlated negatively with the C:N mass ratio of the soil extractable pool. The δ13C signature of soil microbial biomass was 1.4‰ and 4.6‰ enriched relative to that of soil total and extractable pools respectively and showed significant differences between sites. However, microbial 13C enrichment was unrelated to measures of C and N availability. Our results confirm that 15N, but not 13C enrichment of soil microbial biomass reflects changes in C and N availability and N processing during long-term ecosystem development.

Concerns about the long-term sustainability of overstocked dry conifer forests in western North America have provided impetus for treatments designed to enhance their productivity and native biodiversity. Dense forests are increasingly prone to large stand-replacing fires; yet, thinning and burning treatments, especially combined with other disturbances such as drought and grazing, may enhance populations of colonizing species, including a number of non-native species. Our study quantifies plant standing crop of major herbaceous species across contrasting stand structural types representing a range in disturbance severity in northern Arizona. The least disturbed unmanaged ponderosa pine stands had no non-native species, while non-native grasses constituted 7–11% of the understory plant standing crop in thinned and burned stands. Severely disturbed wildfire stands had a higher proportion of colonizing native species as well as non-native species than other structural types, and areas protected from grazing produced greater standing crop of native forbs compared to grazed unmanaged stands. Standing crop of understory plants in low basal area thinned and burned plots was similar to levels on wildfire plots, but was comprised of fewer non-native graminoids and native colonizing plants. Our results also indicate that size of canopy openings had a stronger influence on standing crop in low basal area plots, whereas tree density more strongly constrained understory plant standing crop in dense stands. These results imply that treatments resulting in clumped tree distribution and basal areas

We compared energy fluxes between a site converted from ponderosa pine (Pinus ponderosa) forest to sparse grassland by a severe wildfire 10 years ago and a nearby, unburned forest. We used eddy covariance and associated instruments to measure total radiation, net radiation, albedo, and fluxes of energy into latent heat, sensible heat, and the soil. Total radiation, vapor pressure deficit, and air temperature were similar for each site. Compared to the unburned site, net radiation efficiency (net radiation/total radiation) was 30% lower and albedo 30% higher at the burned site. The magnitude of sensible and latent heats varied seasonally at both sites. Sensible heat was the major component of the energy balance in cold or dry seasons, whereas latent heat was the major component in the warm and wet season. Soil heat flux was the smallest in magnitude of the measured energy fluxes. Compared with the unburned forest, the burn-created grassland generally had lower sensible and latent heats, but greater soil heat flux for both soil cooling in winter and warming in summer. The grassland had similar maximum air temperature as the forest, and warmer surface soil temperature during the summer. Thus, the lower albedo and greater sensible heat of the forest did not produce a warmer site compared with the grassland, apparently because of the cooling effect of greater latent heat in the forest. Our results suggest only small changes in site air temperature, but larger changes in site surface soil temperature by shifts from forest to grassland caused by severe fire in northern Arizona ponderosa pine forests.

Forest soils are important components of the global carbon cycle because they both store and release carbon. Carbon dioxide is released from soil to the atmosphere as a result of plant root and microbial respiration. Additionally, soils in dry forests are often sinks of methane from the atmosphere. Both carbon dioxide and methane are greenhouse gases whose increasing concentration in the atmosphere contributes to climate warming. Thinning treatments are being implemented in ponderosa pine forests across the southwestern United States to restore historic forest structure and reduce the risk of severe wildfire. This study addresses how thinning alters fluxes of carbon dioxide and methane in ponderosa pine forest soils within one year of management and examines mechanisms of change. Carbon dioxide and methane fluxes, soil temperature, soil water content, forest floor mass, root mass, understory plant biomass, and soil microbial biomass carbon were measured before and after the implementation of a thinning and in an unthinned forest. Carbon dioxide efflux from soil decreased as a result of thinning in two of three summer months. Average summer carbon dioxide efflux declined by an average of 34 mg C m −2 hr −1 in the first year after thinning. Methane oxidation did not change in response to thinning. Thinning had no significant short-term effect on total forest floor mass, total root biomass, or microbial biomass carbon in the mineral soil. Understory plant biomass increased after thinning. Thinning increased carbon available for decomposition by killing tree roots, but our results suggest that thinning reduced carbon dioxide emissions from the soil because the reduction in belowground autotrophic respiration was larger than the stimulation of heterotrophic respiration. Methane oxidation was probably not affected by thinning because thinning did not alter the forest floor mass enough to affect methane diffusion from the atmosphere into the soil.

Changes in estimated standing stocks of carbon (C) in vegetation, forest floor, dead wood, and mineral soil for the fire and fire surrogate (FFS) network sites were evaluated in relation to the application of prescribed fire, mechanical treatments designed as surrogates for prescribed fire, and the combination of mechanical treatment and fire. Pre-treatment C stocks and changes in C stocks over two intervals (pre-treatment to first post-treatment year and first post-treatment to a 2nd, 3rd, or 4th post-treatment year, depending on site) were evaluated using meta-analytical methods. Total C storage across the network averaged 185 ± 8 (standard error) Mg C ha −1 , of which 45% was in vegetation, 38% in soil organic matter, 10% in the forest floor and 7% in dead wood. C stored in vegetation was not significantly affected by prescribed fire, but decreased ∼30 Mg ha −1 as the result of mechanical or mechanical + fire treatment; in contrast, forest floor C storage was reduced by ∼1–7 Mg ha −1 by fire or mechanical + fire treatment, but unaffected by mechanical treatment alone. Neither dead wood C nor soil organic C was significantly affected by the treatments. At the network scale, total ecosystem C was not significantly affected by fire, though four individual sites did exhibit significant C losses to fire. Mechanical treatment, with or without fire, produced significant reductions of 16–32 Mg ha −1 during the first post-treatment year, but this was partially balanced by enhanced net C uptake of ∼12 Mg ha −1 during the subsequent 1–3 years. In terms of C storage and uptake, western coniferous forests responded differently to the FFS treatments than did eastern deciduous, coniferous, and mixed forests, suggesting that optimal management for fire, harvesting, and C sequestration may differ between regions.

We report the first simultaneous measurements of δ 15 N and δ 13 C of DNA extracted from surface soils. The isotopic composition of DNA differed significantly among nine different soils. The δ 13 C and δ 15 N of DNA was correlated with δ 13 C and δ 15 N of soil, respectively, suggesting that the isotopic composition of DNA is strongly influenced by the isotopic composition of soil organic matter. However, in all samples DNA was enriched in 13 C relative to soil, indicating microorganisms fractionated C during assimilation or preferentially used 13 C enriched substrates. Enrichment of DNA in 15 N relative to soil was not consistently observed, but there were significant differences between δ 15 N of DNA and δ 15 N of soil for three different sites, suggesting microorganisms are fractionating N or preferentially using N substrates at different rates across these contrasting ecosystems. There was a strong linear correlation between δ 15 N of DNA and δ 15 N of the microbial biomass, which indicated DNA was depleted in 15 N relative to the microbial biomass by approximately 3.4‰. Our results show that accurate and precise isotopic measurements of C and N in DNA extracted from the soil are feasible, and that these analyses may provide powerful tools for elucidating C and N cycling processes through soil microorganisms.

Plants face a trade-off in distributing C and nutrients between above- and below-ground resource acquiring tissues. We compiled data on tree foliar and fine root nutrient concentrations from across a range of forest biomes and leaf forms to reveal general patterns in the covariance of nutrients in these tissues, as well as N and lignin concentrations in leaf litter and fine roots. Linear patterns between foliar and fine root tissues for N, P, Ca, and Mg clearly indicate covariance in nutrient concentrations between above- and below-ground resource acquiring tissues. However, concentrations of N and P in foliage were higher than in roots in nearly all cases. Slopes

Stable isotope analysis is a powerful tool in the study of soil organic matter formation. It is often observed that more decomposed soil organic matter is 13 C, and especially 15 N-enriched relative to fresh litter and recent organic matter. We investigated whether this shift in isotope composition relates to the isotope composition of the microbial biomass, an important source for soil organic matter. We developed a new approach to determine the natural abundance C and N isotope composition of the microbial biomass across a broad range of soil types, vegetation, and climates. We found consistently that the soil microbial biomass was 15 N-enriched relative to the total (3.2 %) and extractable N pools (3.7 %), and 13 C-enriched relative to the extractable C pool (2.5 %). The microbial biomass was also 13 C-enriched relative to total C for soils that exhibited a C3-plant signature (1.6 %), but 13 C-depleted for soils with a C4 signature (� 1.1 %). The latter was probably associated with an increase of annual C3 forbs in C4 grasslands after an extreme drought. These findings are in agreement with the proposed contribution of microbial products to the stabilized soil organic matter and may help explain the shift in isotope composition during soil organic matter formation. r 2006 Elsevier Ltd. All rights reserved.

Changes in nutrient inputs due to aboveground herbivory may influence the litter and soil microbial community responsible for processes such as decomposition. The mesophyll-feeding scale insect (Matsucoccus acalyptus) found near Sunset Crater National Monument in northern Arizona, USA significantly increases pinon (Pinus edulis) needle litter nitrogen (N) and phosphorus (P) concentrations by 50%, as well as litter inputs to soil by 21%. Because increases in needle litter quality and quantity of this magnitude should affect the microbial communities responsible for decomposition, we tested the hypothesis that insect herbivory causes a shift in soil microbial and litter microarthropod function. Four major findings result from this research: (1) Despite increases in needle inputs due to herbivory, soil carbon (C) was 56% lower beneath scale-susceptible trees than beneath resistant trees; however, soil moisture, N, and pH were similar among treatments. (2) Microbial biomass was 80% lower in soils beneath scale-susceptible trees when compared to resistant trees in the dry season, while microbial enzyme activities were lower beneath susceptible trees in the wet season. (3) Bacterial community-level physiological profiles differed significantly between susceptible and resistant trees during the dry season but not during the wet season. (4) There was a 40% increase in Oribatida and 23% increase in Prostigmata in susceptible needle litter relative to resistant litter. Despite these changes, the magnitude of microbial biomass, activity, and community structure response to herbivory was lower than expected and appears to take a long time to develop. These results suggest that herbivores impact soils in subtle, but important ways; we suggest that while litter chemistry may strongly mediate soil fertility and microbial communities in mesic ecosystems, the influence is lower than expected in this primary succession xeric ecosystem where season mediates differences in microbial populations. Understanding how insect herbivores alter the distribution of susceptible and resistant trees and their associated decomposer communities in arid environments may lead to better prediction of how these ecosystems respond to climatic change.

Soil microorganisms have numerous functional roles in forest ecosystems, including: serving as sources and sinks of key nutrients and catalysts of nutrient transformations; acting as engineers and maintainers of soil structure; and forming mutualistic relationships with roots that improve plant fitness. Although both prescribed and wildland fires are common in temperate forests of North America, few studies have addressed the long-term influence of such disturbances on the soil microflora in these ecosystems. Fire alters the soil microbial community structure in the short-term primarily through heat-induced microbial mortality. Over the long-term, fire may modify soil communities by altering plant community composition via plant-induced changes in the soil environment. In this review, we summarize and synthesize the various studies that have assessed the effects of fire on forest soil microorganisms, emphasizing the mechanisms by which fire impacts these vital ecosystem engineers. The examples used in this paper are derived primarily from studies of ponderosa pine-dominated forests of the Inland West of the USA; these forests have some of the shortest historical fire-return intervals of any forest type, and thus the evolutionary role of fire in shaping these forests is likely the strongest. We argue that the short-term effects of fire on soil microflora and the processes they catalyze are transient, and suggest that more research be devoted to linking long-term plant community responses with those of the mutually dependent soil microflora.

Nitrogen-fixing tree species have been shown to improve site fertility and increase N transformation rates, but the influence of N-fixing plants on the soil microbial community as a whole is largely unknown. We used patterns of individual carbon-source utilization and enzyme activities to assess the relative effects of N-fixing red alder on the soil microbial community in three adjacent stands (pure conifer, mixed alder-conifer, and pure alder) of a highly productive coastal Oregon forest where the density of red alder has been experimentally manipulated for over 65 years. Two major patterns were revealed: (1) bacterial and fungal carbon-source utilization patterns in soil from pure conifer stands were significantly different from both pure alder soils and mixed conifer-alder soils, while there was no difference in substrate utilization patterns between soils from the mixed alder-conifer and pure alder stands; and (2) the activities of nine extracellular enzymes involved in ligno-cellulose degradation and the mineralization of organic nitrogen, phosphate, and sulfate compounds were all significantly greater in pure alder soils compared to either pure conifer or mixed conifer-alder soils. Our results show that, in addition to an overall increase in soil fertility, microbial biomass, and microbial activity, the presence of N-fixing red alder significantly alters the physiological profile of the microbial community-even in an ecosystem already of high N status.

We investigated the influence of fumigation conditions of the chloroform fumigation‐ extraction method on the release of extractable carbon (C) and nitrogen (N) from organic and mineral soil horizons of 11 mature forests. Soil samples were fumigated with chloroform vapor for 1, 3, 5, or 10 d at two different water contents: field moist or field capacity (233 kPa matric potential). We found that for approximately half our soils, 0.5 M K2SO4-extractable N and C reached a maximum after 1 d of fumigation. The effect of soil wetting on C and N flushes from O horizons was variable but in mineral soils, increasing the soil water content generally resulted in more extractable C and N following fumigation. For the majority of soils assessed, increasing the soil water content did not change the fumigation time necessary to generate the maximum C or N flush. Observed changes in the C/N ratios of the fumigation flush with changes in fumigation time and the sensitivity of this C/N ratio to changes in soil water content suggest that different fumigation conditions may result in different organic pools being extracted. These effects appear to be extremely soil specific. We recommend that the effects of fumigation time and water content be evaluated before the C and N flushes from the fumigation extraction method are used to assess differences in microbial C and N among contrasting forest soils. q 2002 Elsevier Science Ltd. All rights reserved.

We conducted laboratory studies to determine the effects of red alder ( Alnus rubra Bong.) on soil N transformations and N availability indices at two conifer forest sites of contrasting productivity. The inclusion of red alder in conifer forests significantly increased gross rates of N mineralization, N immobilization, nitrification and NO 3 − immobilization, and the effects of alder were generally similar for soils from low- and high-productivity sites. However, the addition of alder to the conifer stand at the high productivity site increased gross N mineralization and immobilization processes more than at the low productivity site. At both sites, gross N and NO 3 − production were enhanced by alder more than gross N immobilization processes, leading to higher rates of net N mineralization and nitrification. At the fertile site, most microbial N assimilation occurred from the NO 3 − pool, compared with less than half at the infertile site (none as NO 3 − in the less productive pure conifer stand). Heterotrophic nitrification (as indicated by a lack of C 2 H 2 inhibition) accounted for 65–72% of the gross nitrification in all stands that exhibited nitrification (no nitrification was detected in the pure conifer stand at the infertile site). The inclusion of red alder had no effect on the proportion of total nitrification that was heterotrophic, despite the lower soil pH in mixed alder-conifer stands compared to conifer stands. Gross rates of N mineralization correlated well with both autotrophic and heterotrophic nitrification across all soils. Gross N mineralization may be a good index of NH 4 + availability to autotrophic nitrifiers, as well as the quality of organic N as a substrate for heterotrophic nitrification. Most estimates of microbial biomass and activity, N availability and N transformation rates were significantly correlated with each other. In general, gross N transformations were better correlated with other indices of N availability and microbial activity than estimates of net N transformations. Similar N cycling rates and microbial biomass N pool sizes in pure alder and adjacent alder-conifer stands at the fertile site suggest that continued inputs of N via symbiotic N-fixation by red alder in coniferous forest stands can lead to the elimination of N-limitation to forest ecosystem production.

A comparative study of N dynamics in an annual grassland and a young mixed-conifer forest, that occur on the same soil great group in California, was made by adding small amounts of 15NH4+ to mineral soils within field microplots and following changes in 15N recoveries in soil and plant pools over 12–16 months. In annual grassland microplots, the aboveground plant biomass contained 5–6% of the added 15N at the end of the growing season. One month after 15N addition almost 70% of the added 15N was recovered in the soil organic matter (SOM) pool, which included surface litter and roots but not microbial biomass. Laboratory incubation of sterilized soil cores indicated that as much as half of the 15N recovered in the SOM pool may have been fixed abiotically. Nevertheless, substantial and rapid incorporation of 15N in the SOM pool indicates a high potential for N immobilization and a rapid turnover of the microbial biomass-N pool in this soil. Recovery of 15N in microbial biomass ranged from 9 to 15%; recovery in the soil solution was lower than in the microbial biomass, but showed a similar seasonal trend. Total 15N recovery ranged from 72 to 85% over the study period. In forest microplots (which excluded plants), recovery of added 15N was generally lower than in the grassland for all soil pools examined. Recovery of 15N in SOM and microbial pools showed reciprocal trends seasonally, suggesting that there was considerable movement of N between these pools in the forest soil. Total 15N recovery was only about 54% 6 months after 15N addition, indicating a high potential for rapid N loss from the surface soil of this forest. Substantially lower recovery of 15N in the forest soil cannot be attributed entirely to the exclusion of plant uptake because plant 15N recovery in the grassland was relatively low. The low 15N recovery in the forest soil might be due to the low capacity of this soil to immobilize N and protect immobilized N from being remineralized and lost from the soil via leaching or denitrification. In both ecosystems estimates of N flow from the mineral soil to decomposing surface residues (presumably via fungal translocation) was significant (0.29 and 0.02 g-N m−2 yr−1 for the grassland and forest, respectively) relative to the net internal dynamics of N within the surface detritus. Comparison of net changes in litter N and accumulation of 15N during decomposition suggests that N was mineralized from litter concurrently as N was immobilized from the mineral soil in both sites.

Southwest United States ponderosa pine (Pinus ponderosa Dougl. ex Laws) ecosystems have received great attention due to fuel conditions that increase the likelihood of large-scale wildfires with severe fire behavior. The fire season of 2002 demonstrated these extreme fuel load conditions with the largest fires in southwest history. The Jemez District of the Santa Fe National Forest, New Mexico experienced a wildfire (Lake Fire Complex 08/26/02 to 09/01/02)) that consumed one complete replicate block that was part of the nationwide Fire/ Fire Surrogate (FFS) project. This event provided a unique opportunity to investigate wildfire effects on soil, and associated microbial populations. Pre-treatment sampling completed prior to the wildfire allowed resampling of the exact sites to investigate immediate post-fire effects. Soil microbial functional diversity and biomass were determined from the top 5 cm of mineral soil. Virgin Mesa and Tusa Tank FFS control and burn only blocks were used as controls since they had not been treated. We predicted that due to the direct impact of heating, that the high severity wildfire would reduce the size of the microbial biomass and possibly favor bacteria over fungi because of the apparent greater sensitivity of fungi to heat. We also expected that wildfire would not only alter the quantity of available C, but it would also change the quality of C remaining in the soil, and these two factors would alter the physiological capacity of the microbial community. Community level physiological profiles (Biolog), and laboratory estimates of C and N transformations suggest that there was a large change in the function of the soil microflora following a high-severity burn. Phospholipid fatty acid (PLFA) analysis of the microbial population shows significant decrease in the fungal and bacterial populations in the 0-5 cm layer of soil immediately following the fire. Wildfire resulted in a reduction in the size of the microbial biomass and a change in its functional and structural composition.

The direct extraction method for estimating microbial biomass-N, which involves chloroform fumigation and extraction without an incubation step, affords the opportunity to analyze soils for which the incubation step is problematic. We tested the direct extraction method on freshly sampled grassland and forest soils of California, which experience seasonal wetting and drying cycles and labile substrate inputs. Time-course profiles of extractable-N during fumigation varied among soils, with the forest samples achieving peak N-flush after only 1-day fumigation, while the grassland soils required 5–7 days. When dry grassland soil was wetted to different moisture contents before fumigation, direct extraction efficiency was positively related to soil moisture. Direct extraction efficiency appeared to vary seasonally in the forest soils. Comparisons of N-flushes from direct extraction and fumigation-incubation methods are equivocal because of differential microbial immobilization during incubation. Agreement between the two methods was best when a variable k N was used to convert the fumigation-incubation data to microbial biomass-N. Although the direct extraction procedure offers some promise for evaluating a labile fraction of the microbial biomass-N, which is difficult to evaluate in these soils by any current method, uncertainties remain regarding duration of fumigation, consistency of water content and conversion to biomass-N.