Rapid soil organic matter loss from forest dieback in a subalpine coniferous ecosystem

Forest dieback caused by climate-change associated stresses and insect outbreaks has emerged as a global concern, and the biogeochemical consequences of this phenomenon need to be elucidated. We measured biological and chemical traits of soil beneath live trees or trees recently killed by a mountain-pine-beetle outbreak in a subalpine coniferous forest in the Front Range of Colorado. We focused on the top 5 cm of mineral soil just beneath the O horizon and measured microbial biomass, soil invertebrate abundance and composition, and soil chemical characteristics. With the termination of inputs from rhizodeposition, mycorrhizal fungal turnover and fine root turnover, soil total carbon (C) and total nitrogen (N) in the mineral soil at three sites decreased by 38–49% and 26–45%, respectively. Tree mortality was associated with reduced soil microbial biomass but soil nematode and microarthropod densities were unchanged. Nematode trophic structure was altered with an increased proportion of bacterial feeders. Soil inorganic N concentrations were inversely correlated to microbial C:N ratios. Tree death was associated with increased soil pH, a possible loss of calcium (Ca²⁺), but an accumulation of soil inorganic N, largely as NH₄⁺. Our results suggest that forest dieback results in rapid C and N loss from surface mineral soils and that the accumulation of soil inorganic N, the reduction in microbial biomass, and the more bacterial-based soil food web increase the potential of enhanced N loss from affected ecosystems.     

Nutrient limitations to soil microbial biomass and activity in loblolly pine forests

We performed an assay of nutrient limitations to soil microbial biomass in forest floor material and intact cores of mineral soil collected from three North Carolina loblolly pine (Pinus taeda) forests. We added solutions containing C, N or P alone and in all possible combinations, and we measured the effects of these treatments on microbial biomass and on microbial respiration, which served as a proxy for microbial activity, during a 7-day laboratory incubation at 22 °C. The C solution used was intended to simulate the initial products of fine root decay. Additions of C dramatically increased respiration in both mineral soil and forest floor material, and C addition increased microbial biomass C in the mineral soil. Additions of N increased respiration in forest floor material and increased microbial biomass N in the mineral soil. Addition of P caused a small increase in forest floor respiration, but had no effect on microbial biomass.     

Influence of inorganic fertilizers and organic amendments on soil organic matter and soil microbial properties under tropical conditions

 Soil organic matter level, mineralizable C and N, microbial biomass C and dehydrogenase, urease and alkaline phosphatase activities were studied in soils from a field experiment under a pearl millet-wheat cropping sequence receiving inorganic fertilizers and a combination of inorganic fertilizers and organic amendments for the last 11 years. The amounts of soil organic matter and mineralizable C and N increased with the application of inorganic fertilizers. However, there were greater increases of these parameters when farmyard manure, wheat straw or Sesbania bispinosa green manure was applied along with inorganic fertilizers. Microbial biomass C increased from 147 mg kg^–1 soil in unfertilized soil to 423 mg kg^–1 soil in soil amended with wheat straw and inorganic fertilizers. The urease and alkaline phosphatase activities of soils increased significantly with a combination of inorganic fertilizers and organic amendments. The results indicate that soil organic matter level and soil microbial activities, vital for the nutrient turnover and long-term productivity of the soil, are enhanced by use of organic amendments along with inorganic fertilizers. Received: 6 May 1998     

Twenty years of intensive fertilization and competing vegetation suppression in loblolly pine plantations: Impacts on soil C, N, and microbial biomass

Silvicultural treatments of fertilization (F) and competing vegetation suppression (H) have continued to increase as demands for forest products have grown. The effects of intensive annual F and H treatments on soil C, N, microbial biomass, and CO₂ efflux were examined in a two-way factorial experiment (control, F, H, FxH) in late-rotation (20+ years) loblolly pine stands. This study is unique in testing the cumulative effects of continual H and repeated F treatments for the first 20 years of stand growth, an uncommon operational practice, and in having treatments replicated upon four different soil types in the state of Georgia, USA. Annual fertilization included applications of N, P, K and periodic additions of micronutrients while competing vegetation suppression was maintained for all non-pine vegetation with herbicides throughout the rotation. Measurements included total O-horizon (forest floor) organic matter, C, and N, and 0-10 cm mineral soil pH, C, N, microbial biomass C and N, and surface CO₂ efflux. Sample collections and analyses were conducted seasonally for 1.5 yrs. Competing vegetation suppression was associated with a decrease of total soil C, soil microbial biomass C and N, and soil surface CO₂ efflux, while increasing O-horizon C:N. The fertilization treatment greatly reduced soil microbial biomass C and N, soil pH, and O-horizon C:N, while increasing O-horizon mass, N content, and soil carbon. No significant interactions between F and H were found. The combination of F and H treatments acted additively to achieve the greatest loss of soil microbial biomass, which may possibly have negative implications for long-term soil fertility.     

Tree girdling provides insight on the role of labile carbon in nitrogen partitioning between soil microorganisms and adult European beech

Nitrogen (N) cycling in terrestrial ecosystems is complex since it involves the closely interwoven processes of both N uptake by plants and microbial turnover of a variety of N metabolites. Major interactions between plants and microorganisms involve competition for the same N species, provision of plant nutrients by microorganisms and labile carbon (C) supply to microorganisms by plants via root exudation. Despite these close links between microbial N metabolism and plant N uptake, only a few studies have tried to overcome isolated views of plant N acquisition or microbial N fluxes. In this study we studied competitive patterns of N fluxes in a mountainous beech forest ecosystem between both plants and microorganisms by reducing rhizodeposition by tree girdling. Besides labile C and N pools in soil, we investigated total microbial biomass in soil, microbial N turnover (N mineralization, nitrification, denitrification, microbial immobilization) as well as microbial community structure using denitrifiers and mycorrhizal fungi as model organisms for important functional groups. Furthermore, plant uptake of organic and inorganic N and N metabolite profiles in roots were determined.Surprisingly plants preferred organic N over inorganic N and nitrate (NO₃⁻) over ammonium (NH₄⁺) in all treatments. Microbial N turnover and microbial biomass were in general negatively correlated to plant N acquisition and plant N pools, thus indicating strong competition for N between plants and free living microorganisms. The abundance of the dominant mycorrhizal fungi Cenococcum geophilum was negatively correlated to total soil microbial biomass but positively correlated to glutamine uptake by beech and amino acid concentration in fine roots indicating a significant role of this mycorrhizal fungus in the acquisition of organic N by beech. Tree girdling in general resulted in a decrease of dissolved organic carbon and total microbial biomass in soil while the abundance of C. geophilum remained unaffected, and N uptake by plants was increased. Overall, the girdling-induced decline of rhizodeposition altered the competitive balance of N partitioning in favour of beech and its most abundant mycorrhizal symbiont and at the expense of heterotrophic N turnover by free living microorganisms in soil. Similar to tree girdling, drought periods followed by intensive drying/rewetting events seemed to have favoured N acquisition by plants at the expense of free living microorganisms.     

Microbial performance under increasing nitrogen availability in a Mediterranean forest soil

In forest ecosystems, the external nitrogen (N) inputs mainly involve wet and dry depositions that potentially alter inorganic N availability in the soil and carbon (C) turnover. This study assesses the effect of a slow increase of inorganic N availability on microbial community activity and functionality in a Mediterranean forest soil. A four-month incubation experiment was performed with soil collected from the organic layer of a forest site and fertilized with a solution of ammonium nitrate. The fertilizer was supplied at an equivalent of 0, 10, 25, 50 and 75 kg N ha⁻¹ (0, 0.3, 0.7, 1.3 and 2 mg N g⁻¹ for control N0 and treatments N1, N2, N3 and N4, respectively). The incubation was carried out under optimal conditions, with the addition of the nutritive solution in small aliquots once a week to mimic the phenomenon of N deposition. In order to isolate the effect of N, the pH of the NH₄NO₃ solutions was adjusted to soil pH, and phosphorus was added in order to prevent any nutrient limitation effect. Inorganic N, C-mineralization, the activity of one oxidative enzyme (o-diphenol oxidase) and 8 hydrolitic enzymes (α-glucosidase, β-glucosidase, N-acetyl-β-d-glucosaminidase, cellulase, leucine amino-peptidase, acid phosphatase, butyric esterase and β-xylosidase) and the community level physiological profile (CLPP) were measured and analyzed during the whole incubation and at the end of the experiment as a proxy for microbial decomposition activity. In the first month, the highest N availability (N4) repressed the microbial respiration activity but stimulated microbial enzymatic activity, suggesting a change of C-pathways from spilling to enzymes and biomass investment. The treatments N1, N2 and N3 had no effect in the same period. Throughout the incubation, a general stress condition affected all the treated soils. As a consequence, treated soils exhibited higher respiration rates than the control. This was accompanied by a loss of functional diversity and an end-detected decline in biomass C. Although at the end of incubation most of the soil features showed a clear correlation with the inorganic N pool, the organic C content was strongly affected by different patterns of microbial activity during the experiment: the highest N treatment (N4) showed a lower C loss than the N3 treatment. Overall, the experiment showed how inorganic N availability can potentially alter the C cycle in a Mediterranean forest soil. The effect is non linear, depending on microbial community dynamics, on the community’s ability to adapt given the time scale of the process, and on N supply amount. Our study also revealed a common pattern in the short-term response to N addition in other, similar ecosystems with different climatic conditions.     

Microbial biomass and mineralization-immobilization of nitrogen in some agricultural soils

Summary The chloroform fumigation-incubation method (CFIM) was used to measure the microbial biomass of 17 agricultural soils from Punjab Pakistan which represented different agricultural soil series. The biomass C was used to calculate biomass N and the changes occurring in NH₄ ⁺-N and NO₃ ^–-N content of soils were studied during the turnover of microbial biomass or added C source. Mineral N released in fumigated-incubated soils and biomass N calculated from biomass C was correlated with some N availability indexes.The soils contained 427–1240 kg C as biomass which represented 1.2%–6.9% of the total organic C in the soils studied. Calculations based on biomass C showed that the soils contained 64–186 kg N ha^–1 as microbial biomass. Immobilization of NCO₃ ^–-N was observed in different soils during the turnover of microbial biomass and any net increase in mineral N content of fumigated incubated soils was attributed entirely to NH₄ ⁺-N.Biomass N calculated from biomass C showed non-significant correlation with different N availability indexes whereas mineral N accumulated in fumigated-incubated soils showed highly significant correlations with other indexes including N uptake by plants.     

Tree species effects on soil enzyme activities through effects on soil physicochemical and microbial properties in a tropical montane forest on Mt. Kinabalu, Borneo

Tree species influence on the soil mineralization process can regulate overall nutrient cycling in a forest ecosystem, which may occur through their effects on substrate quality, soil physicochemical properties and soil microbial community. We investigated tree species effects on soil enzyme activities in a tropical montane forest on Mt. Kinabalu, Borneo. Specifically, we analyzed C- and P-degrading enzyme activities, as well as the relationships among the enzyme activities, soil physicochemical properties, substrate quality (C, N, and P concentrations), and microbial composition in the top 5 cm soils beneath conifers (Dacrycarpus imbricatus and Dacrydium gracilis) and broadleaves (Lithocarpus clementianus, Palaquium rioence, and Tristaniopsis clementis). Activities of acid phosphatase and β-d-glucosidase were significantly different among the tree species. Soil moisture, total C and N content and microbial lipid abundance (a proxy for microbial composition) could influence the enzyme activities although the relative contributions of microbial composition to the enzyme activities might be smaller. A higher acid phosphatase activity beneath Dacrydium than those beneath the other tree species can compensate for a lower concentration of P in available fractions beneath Dacrydium. This localized mineralization activity could subsequently influence soil nutrient availability in this forest.     

Long‐term changes of aggregate‐associated and labile soil organic carbon and nitrogen after conversion from forest to grassland and cropland in northern Turkey

Soil management systems can have great effect on soil chemical, physical and biological properties. Conversion of forest to grassland and cropland can alter C and N dynamics. The objective of this study was to evaluate the changes in aggregate‐associated and labile soil organic C and N fractions after conversion of a natural forest to grassland and cropland in northern Turkey. This experiment was conducted on plots subject to three different adjacent land uses (forest, grassland and cropland). Soil samples were taken from 0–5, 5–15 and 15–30 cm depths from each land use. Some soil physical (soil texture, bulk density), chemical (soil pH, soil organic matter, lime content, total organic C and N, inorganic N, free and protected organic C) and biological (microbial biomass C and N, mineralizable C and N) properties were measured. The highest and lowest bulk densities were observed in grassland (1.41 g cm⁻³) and cropland (1.14 g cm⁻³), respectively. Microbial biomass C and total organic C in forest were almost twice greater than grassland and four‐times greater than cropland. Cultivation of forest reduced total organic N, mineralizable N and microbial biomass N by half. The great portion of organic C was stored in macroaggregates (>250 µm) in all the three land uses. Free organic C comprised smaller portion of soil organic C in all the three land uses. Thus, this study indicated that long‐term conversion of forest to grassland and cropland significantly decreased microbial biomass C, mineralizable C and physically protected organic C and the decreases were the greatest in cropland. Copyright © 2011 John Wiley & Sons, Ltd.     

The influence of American Chestnut (Castanea dentata) on nitrogen availability, organic matter and chemistry of silty and sandy loam soils

American chestnut trees once dominated vast areas of deciduous forest in eastern North America, but the exotic chestnut blight almost eliminated the species from the region. Introduction of blight-resistant American chestnut hybrids will probably start in the next decade after many years of tree breeding. What were the historic effects of chestnut on forest soils, and what changes may follow reintroduction of hybrid chestnuts? A site in southern Wisconsin provided an opportunity to examine the effect of chestnut trees on soil properties. At this site, 600 km northwest of chestnut's historic distribution, naturalized chestnuts have spread throughout an intact mixed-species forest from nine planted trees. The site contains soil developed on a silty loess-mantled ridge that abuts sandier hillslopes, allowing the effects of individual chestnuts to be examined on two soil types. I sampled and analyzed forest floor and mineral soils beneath canopies of individual American chestnuts and the surrounding mixed-species deciduous forest on fine-silt and sandy-loam soil types. On sandy loam soils, total soil carbon (C) and nitrogen (N), inorganic N and net mineralization and nitrification rates were 10–17% higher beneath chestnut canopies compared to soils beneath mixed-species deciduous forest. The pool of total soil N beneath chestnut canopies was positively related to the silt content of the sandy loam soils. In contrast, there were no differences between properties of chestnut canopy and mixed-species deciduous forest soils on the fine silt texture class. On sandy loam soil conditions common throughout the pre-blight distribution of American chestnut, soil biogeochemical processes differ beneath individual chestnut trees relative to a diverse mixture of deciduous species. These findings suggest that widespread chestnut reintroduction has the potential to alter both stand- and watershed-scale processes.     

Earthworms increase soil microbial biomass carrying capacity and nitrogen retention in northern hardwood forests

Earthworms have been shown to produce contrasting effects on soil carbon (C) and nitrogen (N) pools and dynamics. We measured soil C and N pools and processes and traced the flow of ¹³C and ¹⁵N from sugar maple (Acer saccharum Marsh.) litter into soil microbial biomass and respirable C and mineralizable and inorganic N pools in mature northern hardwood forest plots with variable earthworm communities. Previous studies have shown that plots dominated by either Lumbricus rubellus or Lumbricus terrestris have markedly lower total soil C than uncolonized plots. Here we show that total soil N pools in earthworm colonized plots were reduced much less than C, but significantly so in plots dominated by contain L. rubellus. Pools of microbial biomass C and N were higher in earthworm-colonized (especially those dominated by L. rubellus) plots and more ¹³C and ¹⁵N were recovered in microbial biomass and less was recovered in mineralizable and inorganic N pools in these plots. These plots also had lower rates of potential net N mineralization and nitrification than uncolonized reference plots. These results suggest that earthworm stimulation of microbial biomass and activity underlie depletion of soil C and retention and maintenance of soil N pools, at least in northern hardwood forests. Earthworms increase the carrying capacity of soil for microbial biomass and facilitate the flow of N from litter into stable soil organic matter. However, declines in soil C and C:N ratio may increase the potential for hydrologic and gaseous losses in earthworm-colonized sites under changing environmental conditions.     

Influence of single trees on spatial and temporal patterns of belowground properties in native pine forest

Linkages between forest dynamics and ecosystem processes are poorly understood and this limits our ability to adequately estimate future changes in forest ecosystems due to human-induced global change. In particular at the single tree level, our understanding of temporal and spatial changes of belowground properties during forest succession is limited. Thus, our aim was to test whether we find a spatial and temporal gradient in nutrient availability and an associated shift in microbial community structure with increasing distance and age of single trees. We found that inorganic nitrogen was less available below the crown of single trees, while soluble organic carbon (DOC) was much more abundant, in particular in the inner zone of influence, i.e. close to the stem. The fungal:bacterial PLFA ratio was greater while microbial biomass carbon (MicC) was lower below the tree crown, indicating a strong influence of trees on spatial patterns of microbial biomass and community structure. Moreover, the positive correlation between MicC and total extractable N, and the negative correlation between fungal:bacterial biomass and δ¹⁵N, suggested that the microbial biomass was N limited below the tree crown and as a consequence nutrient cycling was presumably decelerated compared to open conditions. We also found a temporal pattern of increasing surface soil C and N content with increasing tree age (up to 250 years), underlining the significant role of single trees in creating spatial and temporal heterogeneity in forests.     

Effect of cultivation on microbial carbon and nitrogen in dry tropical forest soil

Summary Fifteen- and forty-year-old cropfields developed from a dry tropical forest were examined for soil organic C and total N and soil microbial C and N. The 15-year-old field had never been manured while the 40-year-old field had been fertilized with farmyard manure every year. The native forest soil was also examined. The results indicated that the native forest soil lost about 57% and 62% organic C and total N, respectively, in the 0–10 cm layer after 15 years of cultivation. The microbial C and N contents of the forest soil were greater than those of the cultivated soils. Application of farmyard manure increased the biomass-C and -N levels in the cultivated soil but the values were still markedly lower than in the forest soil. There was an appreciable seasonal variation in biomass C and N, the values being highest in summer and lowest in the rainy season. During an annual cycle, biomass-C contents varied from 180 to 727 g g^–1 and N from 20 to 80 g g^–1 dry soil, and both were linearly related. Microbial biomass C represented 1.6%–3.6% of total soil organic C and microbial biomass N represented 1.7% 1–4.4% of soil organic N.     

Catabolic and genetic microbial indices,and levels of nitrate,ammonium and organic carbon in soil from the black locust (<Emphasis Type="Italic"> Robinia pseudo-acacia</Emphasis>) and tulip poplar (<Emphasis Type="Italic"> Liriodendron tulipifera</Emphasis>) trees in a Pennsylvania forest

Soil samples were collected from the upper soil horizon within 4 m of black locust ( Robinia pseudo-acacia) and tulip poplar trees ( Liriodendron tulipifera) from the same mixed forest in south-central Pennsylvania. The soil samples were analyzed for organic C levels, pH, NO₃ ^-, NH₄ ⁺, catabolic diversity (Shannon diversity index; catabolic H), catabolic evenness (Simpson-Yule index; catabolic E), genetic H, and genetic E. The catabolic H and genetic H of microbes in these soils were found to correlate well with the levels of mineralized N, organic C, and pH. Significant variations in these parameters were found between the soils from near black locust and tulip poplar trees. Conditions in the soil near the black locust trees were more favorable to nitrification as indicated by the elevated pH, organic C, NO₃ ^-, and total mineral N levels, along with lower NH₄ ⁺ levels. The microbial genetic H and E were greater and the catabolic H and E were lower in the black locust soils than in the tulip poplar soils. This suggests that a more specialized environment exists in the soil near the black locust trees which selects for enhanced nitrification and the use of fewer, but preferred catabolic pathways by a more genetically diverse group of microbes that grow to a greater biomass. Conversely, the soils from near the tulip poplar trees are such that they do not select for some dominant catabolic pathways, rather they allow for the use of a greater variety of catabolic pathways by a less diverse microbial population, which appear to grow to a lower biomass. We believe that the combined application of the microbial genetic and catabolic diversity analyses, microbial biomass estimates, and traditional physico-chemical characteristics in soil studies provides information not easily available that can be useful during assessment of soil processes in different terrestrial habitats.     

Dry-rewetting cycles regulate wheat carbon rhizodeposition,stabilization and nitrogen cycling

Drying and rewetting of soil can have large effects on carbon (C) and nitrogen (N) dynamics. Drying-rewetting effects have mostly been studied in the absence of plants, although it is well known that plant–microbe interactions can substantially alter soil C and N dynamics. We investigated for the first time how drying and rewetting affected rhizodeposition, its utilization by microbes, and its stabilization into soil (C associated with soil mineral phase). We also investigated how drying and rewetting influenced N mineralization and loss. We grew wheat (Triticum aestivum) in a controlled environment under constant moisture and under dry-rewetting cycles, and used a continuous ¹³C-labeling method to partition plant and soil organic matter (SOM) contribution to different soil pools. We applied a ¹⁵N label to the soil to determine N loss. We found that dry-rewetting decreased total input of plant C in microbial biomass (MB) and in the soil mineral phase, mainly due to a reduction of plant biomass. Plant derived C in MB and in the soil mineral phase were positively correlated (R² = 0.54; P = 0.0012). N loss was reduced with dry rewetting cycles, and mineralization increased after each rewetting event. Overall drying and rewetting reduced rhizodeposition and stabilization of new C, primary through biomass reduction. However, frequency of rewetting and intensity of drought may determine the fate of C in MB and consequently into the soil mineral phase. Frequency and intensity may also be crucial in stimulating N mineralization and reducing N loss in agricultural soils.     

The role of tree leaf mulch and nitrogen fertilizer on turfgrass soil quality

 The influence of tree leaf amendment and N fertilization on soil quality in turfgrass environments was evaluated. Our objective was to assess changes in soil quality after additions of leaf materials and N fertilization by monitoring soil chemical and physical parameters, microbial biomass and soil enzymes. Established perennial ryegrass (Lolium perenne) plots were amended annually with maple (Acer spp.) leaves at three different rates (0, 2240, and 4480 kg ha^–1 year^–1) and treated with three nitrogen rates (0, 63, and 126 kg N ha^–1 year^–1). Tree leaf mulching did not significantly affect water infiltration or bulk density. However, trends in the data suggest increased infiltration with increasing leaf application rate. Tree leaf mulching increased total soil C and N at 0–1.3 cm depth but not at 1.3–9.0 cm. Extracted microbial phospholipid, an indicator of microbial biomass size, ranged from 28 to 68 nmol phospholipid g^–1 soil at the 1.3–9.0 cm depth. The activity of β-glucosidase estimated on samples from 0–1.3 cm and 1.3–9.0 cm depths, and dehydrogenase activity estimated on samples from 1.3–9.0 cm were significantly increased by leaf mulching and N fertilizer application. Changes in microbial community composition, as indicated by phospholipid fatty acid methyl ester analysis, appear to be due to seasonal variations and did not reflect changes due to N or leaf amendment treatments. There were no negative effects of tree leaf mulching into turfgrass and early data suggest this practice will improve soil chemical, physical, and biological structure. Received: 10 December 1997     

Soil carbon and nitrogen dynamics as affected by long-term tillage and nitrogen fertilization

Quantifying seasonal dynamics of active soil C and N pools is important for understanding how production systems can be better managed to sustain long-term soil productivity especially in warm subhumid climates. Our objectives were to determine seasonal dynamics of inorganic soil N, potential C and N mineralization, soil microbial biomass C (SMBC), and the metabolic quotient of microbial biomass in continuous corn (Zea mays L.) under conventional (CT), moldboard (MB), chisel (CH), minimum tillage (MT), and no-tillage (NT) with low (45kgNha^–1) and high (90kgNha^–1) N fertilization. An Orelia sandy clay loam (fine-loamy, mixed, hyperthermic Typic Ochraqualf) in south Texas, United States, was sampled before corn planting in February, during pollination in May, and following harvest in July. Soil inorganic N, SMBC, and potential C and N mineralization were usually highest in soils under NT, whereas these characteristics were consistently lower throughout the growing season in soils receiving MB tillage. Nitrogen fertilization had little effect on soil inorganic N, SMBC, and potential C and N mineralization. The metabolic quotient of microbial biomass exhibited seasonal patterns inverse to that of SMBC. Seasonal changes in SMBC, inorganic N, and mineralizable C and N indicated the dependence of seasonal C and N dynamics on long-term substrate availability from crop residues. Long-term reduced tillage increased soil organic matter (SOM), SMBC, inorganic N, and labile C and N pools as compared with plowed systems and may be more sustainable over the long term. Seasonal changes in active soil C and N pools were affected more by tillage than by N fertilization in this subhumid climate. Received: 20 September 1996     

Coarse particulate organic matter is the primary source of mineral N in the topsoil of three beech forests

Forest soils contain a variable amount of organic N roughly repartitioned among particles of different size, microbial biomass and associated with mineral compounds. All pools are alimented by annual litter fall as main input of organic N to the forest floor. Litter N is further subject to mineralization/stabilization recognized as the crucial process for the turnover of litter N. Although it is well documented that different soil types have different soil N stocks, it is presently unknown how different soil types affect the turnover of recent litter N. Here, we compared the potential mineralization of the total soil organic N with that of recent litter-released N in three beech forests varying in their soil properties. Highly ¹⁵N-labelled beech litter was applied to stands located at Aubure, Ebrach, Collelongo, which differ in humus type, soil type and soil chemistry. After 4-5 years of litter decomposition, the upper 3 cm of the organo-mineral A horizon was sampled and the net N mineralization was measured over 112 days under controlled conditions. The origin of mineralized N (litter N versus soil organic N) was calculated using ¹⁵N labeling. In addition, soils were fractionated according to their particle size (>2000 μm, 200-2000 μm, 50-200 μm, <50 μm) and particulate organic matter (POM) was separated from the mineral fraction in size classes, except the <50 μm fraction. Between 41 and 69% of soil organic N was recovered as POM. Litter-released ¹⁵N was mainly to be found in the coarse POM fractions >200 μm. On a soil mass basis, N mineralization was two-fold higher at Aubure and Collelongo than at Ebrach, but, on a soil N basis, N mineralization was the lowest at Collelongo and the highest at Ebrach. On a soil N (or ¹⁵N) basis, mineralization of litter ¹⁵N was two to four-fold higher than mineralization of the average soil N. Furthermore, the δ¹⁵N of the mineral N produced was closer to that of POM than to that of the mineral-bound fraction (<50 μm). Highest rates of ¹⁵N mineralization happened in the soil with the lowest N content, and we found a negative relationship between accumulations of N in the upper A horizon and the mineralization of ¹⁵N from the litter. Our results show that mineral N is preferentially mineralized from POM in the upper organo-mineral soil irrespective of the soil chemistry and that the turnover rate of litter N is faster in soils with a low N content.     

Influence of soil phosphorus status and nitrogen addition on carbon mineralization from 14C-labelled glucose in pasture soils

 This study examines the effect of soil P status and N addition on the decomposition of ¹⁴C-labelled glucose to assess the consequences of reduced fertilizer inputs on the functioning of pastoral systems. A contrast in soil P fertility was obtained by selecting two hill pasture soils with different fertilizer history. At the two selected sites, representing low (LF) and high (HF) fertility status, total P concentrations were 640 and 820 mg kg^–1 and annual pasture production was 4,868 and 14,120 kg DM ha^–1 respectively. Soils were amended with ¹⁴C-labelled glucose (2,076 mg C kg^–1 soil), with and without the addition of N (207 mg kg^–1 soil), and incubated for 168 days. During incubation, the amounts of ¹⁴CO₂ respired, microbial biomass C and ¹⁴C, microbial biomass P, extractable inorganic P (P_i) and net N mineralization were determined periodically. Carbon turnover was greatly influenced by nutrient P availability. The amount of glucose-derived ¹⁴CO₂ production was high (72%) in the HF and low (67%) in the LF soil, as were microbial biomass C and P concentrations. The ¹⁴C that remained in the microbial biomass at the end of the 6-month incubation was higher in the LF soil (15%) than in the HF soil (11%). Fluctuations in P_i in the LF soil during incubation were small compared with those in HF soil, suggesting that P was cycling through microbial biomass. The concentrations of P_i were significantly greater in the HF samples throughout the incubation than in the LF samples. Net N mineralization and nitrification rates were also low in the LF soils, indicating a slow turnover of microorganisms under limited nutrient supply. Addition of N had little effect on biomass ¹⁴C and glucose utilization. This suggests that, at limiting P fertility, C turnover is retarded because microbial biomass becomes less efficient in the utilization of substrates. Received: 18 October 1999     

The response of organic matter mineralisation to nutrient and substrate additions in sub-arctic soils

Global warming in the Arctic may alter decomposition rates in Arctic soils and therefore nutrient availability. In addition, changes in the length of the growing season may increase plant productivity and the rate of labile C input below ground. We carried out an experiment in which inorganic nutrients (NH₄NO₃ and NaPO₄) and organic substrates (glucose and glycine) were added to soils sampled from across the mountain birch forest-tundra heath ecotone in northern Sweden (organic and mineral soils from the forest, and organic soil only from the heath). Carbon dioxide production was then monitored continuously over the following 19 days. Neither inorganic N nor P additions substantially affected soil respiration rates when added separately. However, combined N and P additions stimulated microbial activity, with the response being greatest in the birch forest mineral soil (57% increase in CO₂ production compared with 26% in the heath soil and 8% in the birch forest organic soil). Therefore, mineralisation rates in these soils may be stimulated if the overall nutrient availability to microbes increases in response to global change, but N deposition alone is unlikely to enhance decomposition. Adding either, or both, glucose and glycine increased microbial respiration. Isotopic separation indicated that the mineralisation of native soil organic matter (SOM) was stimulated by glucose addition in the heath soil and the forest mineral soil, but not in the forest organic soil. These positive ‘priming’ effects were lost following N addition in forest mineral soil, and following both N and P additions in the heath soil. In order to meet enhanced microbial nutrient demand, increased inputs of labile C from plants could stimulate the mineralisation of SOM, with the soil C stocks in the tundra-heath potentially most vulnerable.