Does antimony affect microbial respiration in Mediterranean soils? A microcosm experiment

The aim of this study was to determine the effects of antimony on soil microbial respiration. Two Mediterranean calcareous soils were sampled: a contaminated soil close to an abandoned lead and silver smelter and a soil far from the pollution source and considered not to be contaminated. Two forms of antimony, antimony trioxide (Sb₂O₃) and potassium antimonyl tartrate trihydrate (C₈H₄K₂O₁₂Sb₁₂·3H₂O), were tested at three concentrations (50, 500 and 5000 ppm) in controlled conditions under short- (3 days) and medium- (3 months) term incubation. Modifications in the substrate-induced respiration (SIR) were assessed by gas chromatography respirometric measurements. Results clearly showed that SIR was immediately and significantly more affected by Sb input in a non-contaminated soil than in a long-term contaminated soil, especially since the concentration was high and Sb was added to a more soluble and available form (tartrate instead of mineral oxide).     

Does the walkley‐black method determine soil charcoal?

Abstract

The Walkley‐Black Method is shown to recover charcoal carbon (C) from both charcoal samples made in the laboratory from a range of plant materials as well as from soils containing various amounts of relic charcoal. The rate of recovery of charcoal C depends on the nature of the material from which it is derived and its particle size but not on its surface area. From the data presented, it is clear that the Walkley‐Black Method recovers charcoal C with a high enough efficiency so that, at the concentrations of charcoal found in soil, given its fine particle size and the potentially diverse nature of its origin, it is not possible to differentiate between charcoal C and other organic forms found in soil by this method.     

Biochar amendment changes temperature sensitivity of soil respiration and composition of microbial communities 3 years after incorporation in an organic carbon-poor dry cropland soil

Topsoil samples were collected from plots in a dry cropland in the North China Plain 3 years after a single incorporation of biochar at 20 and 40 t ha^?1 and analyzed for abundances and composition of microbial community and for respiration under controlled laboratory conditions at 15, 20, and 25 °C. The addition of biochar generally reduced soil respirations at the three temperatures and the temperature sensitivity (Q₁₀) at 15–20 °C. Biochar amendment significantly increased bacterial 16S rRNA gene abundances and fungal ITS gene diversity and induced clear changes in their community compositions due to improvements in soil chemical properties such as soil organic C (SOC) and available N contents and pH. Illumina Miseq sequencing showed that the relative abundances of Actinobacteria, Gammaproteobacteria, Firmicutes, and Alternaria within Ascomycota, capable of decomposing SOC, were significantly decreased under biochar at 40 t ha^?1. The Q₁₀ values at 15–20 °C were significantly correlated with fungal diversity and dehydrogenase activity. Our results suggest that after 3 years a single biochar amendment could induce a shift in microbial community composition and functioning towards a slower organic C turnover and stability to warming, which may potentially reduce soil C loss in dryland under climate warming in the future.     

Vertical distribution of soil macrofauna in an arid ecosystem: Are litter and belowground compartmentalized habitats?

The vertical distribution of soil macroarthropods has been poorly studied despite their importance in understanding the interrelationship between the surface litter and deeper soil layers. Analyzing macrofaunal assemblages in litter and mineral soil layers is especially relevant in soils of arid and semiarid areas, where the litter usually forms a discrete layer that remains separated from the mineral soil and supports a markedly different fauna. In order to analyze the degree of compartmentalization among litter and mineral soil communities, we studied the vertical distribution of macroinvertebrates in an arid area of Southeastern Spain. During 2 years, macroinvertebrates were sampled in the litter and mineral soil beneath shrubs, ant nest mounds and bare soil using cores to a depth of 50 cm. Results showed that macroinvertebrate richness, abundance and biomass decreased gradually with soil depth with small differences between microhabitats. Assemblage composition also varied with depth; an overall vertical stratification was observed, although effects of sampling period, especially in the winter, and microhabitats with higher litter accumulations on the similarity among assemblages were observed. Although the faunal assemblages of the litter and mineral soil habitats displayed some important differences in taxonomic and trophic composition, there were taxa inhabiting both habitats, acting as connectors between litter and the mineral soil. In addition, seasonal differences in the vertical distribution of detritivorous tenebrionid larvae indicate that this connection varies in time, emphasizing the importance of temporal variability in the connection between the surface layer and the below-ground soil.     

Do long-lived ants affect soil microbial communities?

This study was designed to test the hypothesis that desert ant species that build nests that remain viable at a particular point in space for more than a decade produce soil conditions that enhance microbial biomass and functional diversity. We studied the effects of a seed-harvester ant, Pogonomyrmex rugosus, and two generalist ant species, Aphaenogaster cockerelli and Myrmecocystus depilis, on soil microbial communities. Microbial biomass was higher in P. rugosus-modified soils than in reference soils when soil water content was higher than 3%. Microbial biomass was either higher in reference soils or exhibited no difference in reference soils and nest-modified soils of A. cockerelli and M. depilis. There were differences in microbial functional diversity and microbial community level physiological profiles (MicroResp method) between ant-nest-modified and reference soils of the three ant species on some sampling dates. Temporal patterns of soil microbial communities associated with the ant species resulted from differences in soil moisture, density, and species composition of the annual plant communities associated with the ant nests and in reference areas. Differences in annual plant communities associated with ant nests and surrounding areas resulted in different chemical inputs into the soil organic-matter pools. This study shows that generalizations about the effects of long-lived ant nests on soil biota in arid regions must consider feeding behaviors of the ant species and temporal patterns of rainfall.     

Variations in the soil microbial community composition of a tropical montane forest ecosystem: Does tree species matter?

We investigated tree species effects on the soil microbial community in the tropical montane forest on Mt. Kinabalu, in Malaysian Borneo. We investigated microbial composition (lipid profile) and soil physicochemical parameters (pH, moisture, total C, N and phenolics concentration) in top 5-cm soils underneath two conifers (Dacrycarpus imbricatus and Dacrydium gracilis) and three broad-leaves (Lithocarpus clementianus, Palaquium rioence and Tristaniopsis clementis). We found that the primary difference in microbial composition was between conifer versus broad-leaves. The abundance of specific microbial biomarker lipids correlated with soil pH, total C and N. We conclude that tree species have significant impacts on the soil microbial community through their effects on soil pH, total C and N.     

Does biochar interfere with standard methods for determining soil microbial biomass and phenotypic community structure?

Due to its high sorption affinity for organic compounds, biochar may interfere with extraction procedures involving such compounds used for microbially-related assays commonly applied to soils. Here we assessed the impact of two biochars (derived from pine bark and produced at 300 and 600 °C) at three concentrations (0, 12.5, and 50 g kg⁻¹) in three distinct arable soils with contrasting textural classes (loamy sand, sandy loam, and clay) on the determination of soil microbial biomass C by fumigation–extraction, fungal biomass by ergosterol analysis, and microbial community structure as defined by phospholipid fatty acid (PLFA) profiling. Biochar did not affect the apparent concentration of soil microbial biomass C and had no significant impact on apparent PLFA profiles. By contrast, the apparent extraction efficiency of ergosterol was affected dependent on soil type, biochar production temperature, and biochar concentration. Nonetheless, ergosterol contents of biochar-amended soils can be accurately estimated by correcting for reduced recovery using an ergosterol spike.     

Does altered rainfall regime change pesticide effects in soil? A terrestrial model ecosystem study from Mediterranean Portugal on the effects of pyrimethanil to soil microbial communities under extremes in rainfall

In this century, agroecosystems are subjected to multiple global change stressors acting in concert such as alterations in rainfall regimes and pesticide use. Alterations in rainfall regimes, characterised by more extreme intra-annual rainfall regimes, have been forecasted for the Mediterranean region. At the same time, the use of pesticides continues to rise. Here, we report the responses of soil microbial community to a model pesticide, i.e., fungicide pyrimethanil (PYR) under altered rainfall regimes (i.e., drought and heavy rainfall) two and eight weeks after PYR application. We measured the functional responses as enzyme activities, potential nitrification and BIOLOG carbon substrate utilisation. We also characterised the soil bacterial communities using polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE) method. After two weeks, enzyme activities were mainly responsive to PYR and kinetic parameters, calculated from BIOLOG carbon substrate utilisation, indicated interaction effects from PYR and rain treatments. Bacterial band richness increased with PYR treatment under normal rain and drought regimes, but bacterial band richness was higher at 1X than 5X PYR under heavy rainfall. Bacterial community structure was also different with the PYR and rainfall treatments. By week eight, PYR treated soils remained functionally different from untreated soils. Bacterial band richness was consistent across PYR treatment regardless of rain regime. However, the bacterial community structure remained significantly different among the PYR treatments under different rain regimes. We conclude that rainfall extremes can alter the effect of PYR on the soil microbial community structure without altering PYR effects on soil functions (measured as enzyme activities, potential nitrification and BIOLOG carbon substrate utilisation).     

Do general spatial relationships for microbial biomass and soil enzyme activities exist in temperate grassland soils?

In heterogeneous environments such as soil it is imperative to understand the spatial relationships between microbial communities, microbial functioning and microbial habitats in order to predict microbial services in managed grasslands. Grassland land-use intensity has been shown to affect the spatial distribution of soil microorganisms, but so far it is unknown whether this is transferable from one geographic region to another. This study evaluated the spatial distribution of soil microbial biomass and enzyme activities involved in C-, N- and P-cycling, together with physico-chemical soil properties in 18 grassland sites differing in their land-use intensity in two geographic regions: the Hainich National Park in the middle of Germany and the Swabian Alb in south-west Germany. Enzyme activities associated with the C- and N-cycles, namely β-glucosidase, xylosidase and chitinase, organic carbon (C_org), total nitrogen (N_t), extractable organic carbon, and mineral nitrogen (N_min) were higher in the Swabian Alb (Leptosols) than in the Hainich National Park (primarily Stagnosols). There was a negative relationship between bulk density and soil properties such as microbial biomass (C_mic, N_mic), urease, C_org, and N_t. The drivers (local abiotic soil properties, spatial separation) of the enzyme profiles (β-glucosidase, chitinase, xylosidase, phosphatase, and urease) were determined through a spatial analysis of the within site variation of enzyme profiles and abiotic properties, using the Procrustes rotation test. The test revealed that physical and chemical properties showed more spatial pattern than the enzyme profiles. β-glucosidase, chitinase, xylosidase, phosphatase, and urease activities were related to local abiotic soil properties, but showed little spatial correlation. Semivariogram modeling revealed that the ranges of spatial autocorrelation of all measured variables were site specific and not related to region or to land-use intensity. Nevertheless, land-use intensity changed the occurrence of spatial patterns measurable at the plot scale: increasing land-use intensity led to an increase in detectable spatial patterns for abiotic soil properties on Leptosols. The conclusion of this study is that microbial biomass and functions in grassland soils do not follow general spatial distribution patterns, but that the spatial distribution is site-specific and mainly related to the abiotic properties of the soils.     

Do nitrogen isotope patterns reflect microbial colonization of soil organic matter fractions?

Different theories have been brought forward to explain the commonly observed δ¹⁵N enrichment with depth in soil profiles, including the discrimination against ¹⁵N during N decomposition and the buildup of ¹⁵N-enriched microbial residues. A combination of soil organic matter (SOM) size and density fractionations, ¹⁵N determinations, and phospholipid fatty acid (PLFA) analyses was conducted on soils from a pristine N-limited Nothofagus forest in southern Chile. The purpose of this study was to investigate which SOM fractions mostly reflect the ¹⁵N-enrichment pattern and to link ¹⁵N SOM enrichment with microbial community composition. Nitrogen-15 enrichments were greater for the microaggregate (<150 μm) than for the macroaggregate (>150 μm) size fraction, with Rayleigh isotope enrichment factors averaging −8.5‰ and −3.7‰, respectively. The macro-organic matter density fractions (>150 μm) showed intermediate enrichment factors of −5.1‰ and −7.3‰ for the light (<1.37 g cm⁻³) and heavy (>1.37 g cm⁻³) fraction, respectively. The abundance of fungal and bacterial PLFAs was significantly higher in the microaggregate compared to the macroaggregate size fraction, but their relative abundance did not change between aggregate size fractions. Our data link differential ¹⁵N enrichment of SOM fractions to “total” microbial abundance and, as such, corroborates existing theories of microbial-induced ¹⁵N enrichment. Isotopic fractionation during microbial N decomposition processes alone could not explain the large ¹⁵N enrichment in the microaggregate size fraction (−8.5‰) and the heavy density fraction (−7.3‰). We therefore suggest that microbial turnover and accretion of ¹⁵N-enriched microbial (especially fungal) compounds was an additional driver for ¹⁵N enrichment of this soil profile.     

Is the decline of soil microbial biomass in late winter coupled to changes in the physical state of cold soils?

During winter when the active layer of Arctic and alpine soils is below 0 °C, soil microbes are alive but metabolizing slowly, presumably in contact with unfrozen water. This unfrozen water is at the same negative chemical potential as the ice. While both the hydrostatic and the osmotic components of the chemical potential will contribute to this negative value, we argue that the osmotic component (osmotic potential) is the significant contributor. Hence, the soil microorganisms need to be at least halotolerant and psychrotolerant to survive in seasonally frozen soils. The low osmotic potential of unfrozen soil water will lead to the withdrawal of cell water, unless balanced by accumulation of compatible solutes. Many microbes appear to survive this dehydration, since microbial biomass in some situations is high, and rising, in winter. In late winter however, before the soil temperature rises above zero, there can be a considerable decline in soil microbial biomass due to the loss of compatible solutes from viable cells or to cell rupture. This decline may be caused by changes in the physical state of the system, specifically by sudden fluxes of melt water down channels in frozen soil, rapidly raising the chemical potential. The dehydrated cells may be unable to accommodate a rapid rise in osmotic potential so that cell membranes rupture and cells lyse. The exhaustion of soluble substrates released from senescing plant and microbial tissues in autumn and winter may also limit microbial growth, while in addition the rising temperatures may terminate a winter bloom of psychrophiles.Climate change is predicted to cause a decline in plant production in these northern soils, due to summer drought and to an increase in freeze-thaw cycles. Both of these may be expected to reduce soil microbial biomass in late winter. After lysis of microbial cells this biomass provides nutrients for plant growth in early spring. These feedbacks, in turn, could affect herbivory and production at higher trophic levels.     

Field scale variability of nitrogen and δ15N in soil and plants

Abstract

Understanding the factors that influence soil and plant nitrogen (N) spatial variability may improve our ability to develop management systems that maximize productivity and minimize environmental hazards. The objective of this study was to determine the field (65 ha) scale spatial variability of N and δ¹⁵N in soil and corn (Zea mays). Soil, grain, and stover samples were collected from grids that ranged in size from 30 by 30 m to 60 by 60 m. Plant samples, collected following physiological maturity in 1995, were analyzed for total N and δ¹⁵N. Soil samples, collected prior to planting in the spring of 1995 and 1996, were analyzed for inorganic‐N, total N, and δ¹⁵N. All parameters showed strong spatial relationships. In an undrained portion of the field containing somewhat poorly and poorly drained soils there was a net loss of 95 kg N ha^‐1, while in an adjacent area that was tile drained there was a net gain of 98 kg N ha^‐1. Denitrification and N mineralization most likely were responsible for losses and gains, respectively. Differences between the N balances of these areas (193 kg N ha^‐1) provide a relative measure of the impact of tile drainage on plant N availability and greenhouse gas production in a wet year.     

Does salinity enhance Cd toxicity to soil alkaline phosphatase?

The aim of this study was to provide data to assess the additive effects of soil salinity on the toxicity of Cd to soil alkaline phosphatase (EC 3.1.3.1). Two soils (Langroud acid soil and Shervedan calcareous soil) were artificially salinized with NaCl. The natural and salinized soils were treated with CdSO₄ solutions to give a Cd concentration in the range 3–5000 mg kg^?1. Soil alkaline phosphatase activity was measured after 3 days of incubation. Salinity enhanced the extractable Cd concentration in both Langroud and Shervedan soils. The percentage of soil alkaline phosphatase activity inhibited by Cd was significantly increased from 27.8 to 45 in the Langroud acid soil as salinity increased from natural levels to 28 dS m^?1. An increase in the inhibition percentage was not observed in the Shervedan soil. Lower values for the ecological dose causing 50% inhibition (ED₅₀) under saline conditions in the Shervedan soil supported the hypothesis that Cd may be more toxic to soil alkaline phosphatase when the soil is more saline. We conclude that Cd toxicity to soil alkaline phosphatase is salinity dependent and that higher Cd concentrations under saline conditions are probably responsible for the enhanced Cd toxicity to soil alkaline phosphatase.     

Do organic inputs alter resistance and resilience of soil microbial community to drying?

Grassland ecosystems in south-eastern Australia are important for dairy and livestock farming. Their productivity relies heavily on water availability, as well as the ecosystem services provided by soil microbial communities including carbon and nutrient cycling. Management practices such as compost application are being encouraged as a means to improve both soil water holding capacity and fertility, thereby buffering against the impacts of increasing climate variability. Such buffering consists of two complementary processes: resistance, which measures the ability of an ecosystem to maintain community structure and function during a period of stress (such as drying); and resilience, which measures the ability of an ecosystem to recover community structure and function post-stress. We investigated the effects of compost on the resistance and resilience of the grassland soil ecosystem under drying and drying with rewetting events, in a terrestrial model ecosystem. Overall, compost addition led to an increase in soil moisture, greater plant available P and higher plant δ¹⁵N. Soil C:nutrient ratios, mineral N content (NH₄⁺ and NO₃⁻) and soil microbial PLFA composition were similar between amended and unamended soils. Rainfall treatment led to differences in soil moisture, plant above-ground and below-ground biomass, plant δ¹⁵N, soil mineral N content (NH₄⁺ and NO₃⁻) and microbial biomass C, N and P composition but had no effects on soil C:nutrient ratios, plant available P and soil microbial PLFA composition. There was little interaction between rainfall and compost. Generally, the soil microbial community was resistant and resilient to fluctuations in rainfall regardless of compost amendment. However, these properties of the soil microbial community were translated to resilience and not resistance in soil functions. Overall, the results below-ground showed much greater response to rainfall than compost amendment. Water was the key factor shaping the soil microbial community, and nutrients were not strong co-limiting factors. Future projections of increasing rainfall variability will have important below-ground functional consequences in the grassland, including altered nutrient cycling.     

Does microbial habitat or community structure drive the functional stability of microbes to stresses following re-vegetation of a severely degraded soil?

Re-vegetation of eroded soil restores organic carbon concentrations and improves the physical stability of the soil, which may then extend the range of microhabitats and influence soil microbial activity and functional stability through its effects on soil bacterial community structure. The objectives of this study were (i) to evaluate the restorative effect of re-vegetation on soil physical stability, microbial activity and bacterial community structure; (ii) to examine the effects of soil physical microhabitats on bacterial community structure and diversity and on soil microbial functional stability. Soil samples were collected from an 18-year-old eroded bare soil restored with either Cinnamomum camphora (“Eroded Cc”) or Lespedeza bicolour (“Eroded Lb”). An uneroded soil planted with Pinus massoniana (“Uneroded Pm”) and an eroded bare soil served as references. The effect of microhabitats was assessed by physical destruction with a wet shaking treatment. Soil bacterial community structure and diversity were measured using a terminal restriction fragment length polymorphism (T-RFLP) approach, while soil microbiological stability (resistance and resilience) was determined by measuring short-term (28 days) decomposition rate of added barley (Hordeum vulgare) powder following copper and heat perturbations. The results demonstrated that re-vegetation treatment affected the recovery of physical and biological stability, microbial decomposition and the bacterial community structure. Although the restored soils overshot the Uneroded Pm sample in physical stability, they had lower microbial decomposition and less resilience to copper and heat perturbations than the Uneroded Pm samples. Soil physical destruction by shaking had the same effect on soil physical stability, but different effects on soil microbial functional stability. There were significant effects of vegetation treatment and perturbation type, and interactive effects among vegetation treatment, shaking and perturbation type on bacterial community structure. The destruction of aggregate structure increased resilience of the Eroded Lb sample and also altered its bacterial community structure. Both copper and heat perturbations resulted in significantly different community structure from the unperturbed controls, with a larger effect of copper than heat perturbation. Bacterial diversity (Shannon index) increased following the perturbations, with a more profound effect in the Uneroded Pm sample than in the restored soils. The interactive effects of vegetation treatment and shaking on microbial community and stability suggest that soil aggregation may contribute to the generation of bacterial community structure and mediation of biological stability via the protection afforded by soil organic carbon. Differential effects of re-vegetation treatment suggest that the long-term effects are mediated through changes in the quality and quantity of C inputs to soil.     

Humus quantity and quality of an entic Haplustoll under different soil‐crop management systems

Abstract

The effects of different management systems on the level and composition of humified organic matter in an entic Haplustoll from the semiarid Pampean region were studied. The systems were: TPc, wheat‐mixed pasture; TV, wheat (Triticum aestivum), oat (Avena sativa), corn (Zea mays) and triticale grasses; TP, wheat‐cattle grazing; and V, virgin, non cultivated. Humic acids were extracted, fractionated, and analyzed for their organic carbon (OC) content, elemental composition, and E4:E6 spectral ratios. The infrared (IR), electron spin resonance (ESR). and ¹³C‐NMR spectra were registered on these humic acids. The TP rotation showed the lowest humic acid‐carbon to fulvic acid‐carbon (HA‐C:FA‐C) ratio. The lower O:C ratio of humic acids from the cropped soils indicates a higher level of oxidation than that of the virgin one. The comparison of the different methodologies allowed us to conclude that crop rotations and conservation tillage were adequate to mantain the level and composition of the soil organic matter and humus which affected the soil fertility and level of productivity     

Does history matter? Temperature effects on soil microbial biomass and community structure based on the phospholipid fatty acid (PLFA) analysis

Background, aim, and scope

Temperature is an important environmental factor regulating soil microbial biomass, activity, and community. Soils from different climatic regions may have very different dominant soil microbes, which are acclimated to the local conditions like temperature. Changing soil temperature especially warming has been shown to increase the mortality rate of soil microbes. However, little is known about the responses of soil microbes coming from different climatic regions to different incubation temperatures. The objective of this study was to examine the temperature effects on microbial biomass and community of soils collected from cold, intermediate, and hot natural sites.

Materials and methods

Soils were collected from northern (Heilongjiang province), central (Jiangsu province), and southern (Guangxi province) China, these soils having very different temperature histories. The Heilongjiang soil was from the coldest region with a mean annual temperature of 1.2°C, the Jiangsu soil was intermediate with a mean annual temperature of 15.7°C, and Guangxi soil was from the hottest area, with a mean annual temperature of 21.2°C. These three soils were incubated at 4°C, 15°C, 25°C, and 35°C for up to 56 days. Phospholipid fatty acid (PLFA) analyses were conducted on days 0, 3, 7, 14, 28, and 56 to track the dynamics of soil microbes.

Results

Soil microbial biomass indexed by total phospholipid fatty acid concentration decreased with increasing incubation temperature, with that of the Heilongjiang soil decreasing most. At the end of incubation, the biomass at 35°C in the Heilongjiang, Jiangsu, and Guangxi soils had declined to 65%, 72%, and 96% of the initial biomass, respectively. The PLFA patterns shifted with increasing temperatures in all the soils, especially at 35°C; the change was biggest in the Heilongjiang soil.

Discussion

History does have effects on soil microbes responding to environmental stress. Soil microbial biomass and PLFA profiles shifted least in the Guangxi soil with the hottest temperature history and most in the Heilongjiang soil with the coldest temperature, indicating that the distribution of free-living microorganisms is influenced by climatic factors. The majority of soil microorganisms coming from the hot regions are more adapted to high temperature (35°C) compared to those from the cold area. There are some regular changes of PLFA profiles when increasing incubation temperature to 35°C. However, the effect of incubation temperature on soil microbial community structure was inconclusive. As PLFA profile community structure is the phenotypic community structure. Genotype experiments are required to be done in future studies for the better understanding of soil microbes in different climate regions with concerning temperature variation.

Conclusions

With the increasing incubation temperature, soil microbial biomass and PLFA profiles shifted most in the soil with the coldest temperature history and least in the soil with the hottest temperature. History does matter in determining soil microbial dynamics when facing thermal stress.     

Does accelerated soil organic matter decomposition in the presence of plants increase plant N availability?

Plant roots can increase microbial activity and soil organic matter (SOM) decomposition via rhizosphere priming effects. It is virtually unknown how differences in the priming effect among plant species and soil type affect N mineralization and plant uptake. In a greenhouse experiment, we tested whether priming effects caused by Fremont cottonwood (Populus fremontii) and Ponderosa pine (Pinus ponderosa) grown in three different soil types increased plant available N. We measured primed C as the difference in soil-derived CO₂-C fluxes between planted and non-planted treatments. We calculated “excess plant available N” as the difference in plant available N (estimated from changes in soil inorganic N and plant N pools at the start and end of the experiment) between planted and non-planted treatments. Gross N mineralization at day 105 was significantly greater in the presence of plants across all treatments, while microbial N measured at the same time was not affected by plant presence. Gross N mineralization was significantly positively correlated to the rate of priming. Species effects on plant available N were not consistent among soil types. Plant available N in one soil type increased in the P. fremontii treatment but not in the P. ponderosa treatment, whereas in the other two soils, the effects of the two plant species were reversed. There was no relationship between the cumulative amount of primed C and excess plant available N during the first 107 days of the experiment when inorganic N was still abundant in all planted soils. However, during the second half of the experiment (days 108-398) when soil inorganic N in the planted treatments was depleted by plant N uptake, the cumulative amount of primed C was significantly positively correlated to excess plant available N. Primed C explained 78% of the variability in plant available N for five of the six plant-soil combinations. Excess plant available N could not be predicted from cumulative amount of primed C in one species-soil type combination. Possibly, greater microbial N immobilization due to large inputs of rhizodeposits with low N concentration may have reduced plant available N or we may have underestimated plant available N in this treatment because of N loss through root exudation and death. We conclude that soil N availability cannot be determined by soil properties alone, but that is strongly influenced by root-soil interactions.