Net Nitrogen Mineralization Research Articles

Rates of net nitrogen mineralization in disturbed and undisturbed soils

Quantification of net nitrogen mineralization (NNM) in soils is indispensable in order to optimize N fertilization of crops. Two long-term laboratory incubation methods were applied to determine rates of net nitrogen mineralization (rNNM) of soils from two sites of arable land (sandy loam soil, silty loam soil) at four temperature levels (2°C, 8°C, 14°C, 21°C). Since variability within replicates was small, the modified 12-week incubation method of Stanford and Smith (1972) using disturbed soils allowed to establish reliable Arrhenius functions with reasonable expenditure. The fit of the functions derived from the 5-month incubation of 23 undisturbed soil columns (4420 cm3) was worse. This was caused by greater variability and less differentiation between temperature levels. Results of both experiments could be described best by zero-order kinetics. Mean mineralization rates of disturbed samples were approximately twice as high than those of undisturbed samples. The suitability of both methods for the prediction of NNM at site conditions is discussed. Actual respiration (AR) at incubation temperatures and substrate induced respiration (SIR) were measured at the end of the incubation of undisturbed soil columns. The results presented reveal that soil microbial communities develop in a different manner during long-term incubation at different temperatures. This behavior offends the underlying assumption that soil microbes remain in steady-state during incubation and that rising rates are physiological reactions to temperature enhancement. Therefore soil microbial biomass (SMB) dynamics during the experiment has to be accounted for when rates of NNM and Arrhenius functions are established. R Merck Section editor

Nitrogen additions (NaNO 3 and NH 4NO 3) at Aber forest, Wales: II. Response of trees and soil nitrogen transformations

A manipulation experiment was carried out to investigate the impacts of increased nitrogen deposition on a mature Sitka stand which has symptoms of nitrogen saturation. Sodium nitrate at 35 or 75 kg N ha −1 year −1 and ammonium nitrate at 35 kg N ha −1 year −1 were applied weekly to the forest floor as a spray. A water-only control (or zero-N) and no-spray control were included in the design to enable water and nitrogen effects to be differentiated. Data from in situ polythene bag incubations indicate that there was no significant impact on annual soil net N-mineralisation or nitrate accumulation rates in the forest floor material following 2.5 years of treatment. Net nitrogen mineralisation in the forest floor was estimated to be 68–90 kg N ha −1 year −1 and nitrate accumulation 6–22 kg N ha −1 year −1, with large within-treatment variability. No changes in litter decomposition rates in response to treatments was observed. The main response of the trees appeared to be increased water availability in the sprayed plots, indicating that the trees at this site are normally drought stressed. There was a trend towards increased mean needle weight and %N in the current year foliage in all of the sprayed plots, with no relationship to nitrogen loading. Increased water availability also increased the standing crop of roots (diameter less than 5 mm) in the forest floor in the water-only control. Increases in nitrate loading, however, reversed this effect in the sodium nitrate treatment plots, as a decrease in root mass in the sodium nitrate treatment plots relative to the water-only control treatment was observed. Increased above-ground litterfall in the third winter appeared to be positively associated with nitrate loading with no change in litter chemistry observed. More long-term monitoring is needed to determine the future impacts of these changes on tree vitality and the long term impact of ammonium on soil nitrogen dynamics.

Temperature Effects on Kinetics of Microbial Respiration and Net Nitrogen and Sulfur Mineralization

AbstractGlobal climate change may impact the cycling of C, N, and S in forest ecosystems because increased soil temperatures could alter rates of microbially mediated processes. We studied the effects of temperature on microbial respiration and net N and S mineralization in surface soils from four northern hardwood forests in the Great Lakes region. Soil samples were incubated in the laboratory at five temperatures (5, 10, 15, 20, and 25°C) for 32 wk. Headspace gas was analyzed for CO2‐C at 2‐wk intervals, and soils were extracted to determine inorganic N and S. Cumulative respired C and mineralized N and S increased with temperature at all sites and were strongly related (r2 = 0.67 to 0.90, significant at P = 0.001) to an interaction between temperature and soil organic C. Production of respired C and mineralized N was closely fit by first‐order kinetic models (r2 ≥ 0.94, P = 0.001), whereas mineralized S was best described by zero‐order kinetics. Contrary to common assumptions, rate constants estimated from the first‐order models were not consistently related to temperature, but apparent pool sizes of C and N were highly temperature dependent. Temperature effects on microbial respiration could not be accurately predicted using temperature‐adjusted rate constants combined with a constant pool size of labile C. Results suggest that rates of microbial respiration and the mineralization of N and S may be related to a temperature‐dependent constraint on microbial access to substrate pools. Simulation models should rely on a thorough understanding of the biological basis underlying microbially mediated C, N, and S transformations in soil.

Modelling the influence of moisture and temperature on net nitrogen mineralization in a forested sandy soil

Soil from beneath an 11-year-old P. radiata plantation was incubated under a factorial combination of temperature (5,10,15,20 or 25°C) and moisture (10,20,40,60,80 or 100% of field capacity; 0.26 g g −1) for 15, 30 or 60 days. Mineral N production was a linear function of time for the majority of treatments and could be represented by zero-order kinetics. The influence of soil temperature or moisture on net N mineralization could be explained using simple exponential and logistic functions which respectively explained 98 and 91% of the variation in mineralization. These simple functions were combined in a simple linear model which accounted for 95% of the variation in in vitro mineralization. The model was used, in conjunction with field measurements of soil moisture and temperature, to predict in situ net N mineralization on four consecutive periods between October 1992 and May 1993. Good agreement was obtained between modelled and measured in situ mineralization for three of the four periods. The model was used to examine the effect of constant cf. varying soil moisture content on estimated in situ mineralization. The results suggest that in situ procedures which maintain soil at a constant moisture content may fail to approximate true rates where moisture contents are subject to temporal variation.

Light-fraction soil organic matter: origin and contribution to net nitrogen mineralization

Two investigations were conducted to examine the nature of light-fraction (< 1.75 Mg m −3) soil organic matter (OM): (1) determination of the relative contributions of above- and belowground litter to the light fraction (LF) pool in two oak stands (Noe Woods and Wingra Woods) that are part of a long-term (>30yr) litter manipulation study in southern Wisconsin, U.S.A.; and (2) evaluation of the monthly variation in LF mass and LF N mineralization potential, plus the contribution of LF to total net N mineralization, for a cornfield, pine stand, and maple stand in western Massachusetts, U.S.A. The long-term treatments at the Wisconsin plots include no-litter, 2 × litter, and one-time removal of the A horizon. In the first study, LF mass under the no-litter treatment was 64 and 32% lower for the Noe and Wingra sites, respectively, relative to the controls (unaltered litter inputs); LF mass under the 2 × litter treatment was 85% higher at Noe, but 15% lower at the Wingra site. The LF represented only 6.7 and 4.3% of the total mineral-soil C losses under no-litter and 7.0% (Noe alone) of the total mineral-soil C gain under 2 × litter. The results suggest that aboveground foliar litter contributed one-third (Wingra) to two-thirds (Noe) of the LF pool. Additionally, the heavy fraction (HF) declined quickly in response to relatively short-term changes in litter input and functioned as the primary long-term C sink. In the second study, LF represented 11% (corn), 13% (pine), and 2% (maple) of the N mineralization potential (anaerobic incubation) for the whole mineral soil; contribution of LF to net N mineralization could not be determined for the cornfield, but was calculated to be 3 kg N ha −1 yr −1 for both the pine and maple stands. N mineralization potential for the pine and maple sites was consistently lower for LF OM than for HF OM. Both LF mass (corn and pine) and LF N mineralization potential (all sites) showed differences by sampling month. LF constituted 5% (maple), 13% (corn), and 14% (pine) of mineral soil OM. Findings suggest that HF is the primary N source in coarse-textured mineral soils, and also that LF is a relatively minor N source in forest stands with mor-type soil.

Microbial-feeding nematodes and protozoa in soil: Their effectson microbial activity and nitrogen mineralization in decomposition hotspots and the rhizosphere

Food web studies from a range of ecosystems have demonstrated that the fauna contributes about 30% of total net nitrogen mineralization. This results mainly from the activities of microbial-feeding microfauna (nematodes and protozoa). Microbial and microfaunal activity is concentrated at spatially discrete and heterogeneously distributed organic substrates, including the rhizosphere. The dynamics of microfauna and their effect on nutrient cycling and microbial processes at these sites is reviewed. The potential manipulation of microfauna, either as an experimental tool to further understand soil microbial ecology or as a practical means of managing nutrient flows in agroecosystems, is discussed.

Landscape Patterns in Soil Microbial Processes in the Serengeti National Park, Tanzania

The Serengeti ecosystem contains some of Africa's most geobotanically diverse landscapes and supports some of the highest primary and secondary production on Earth. In an attempt to characterize landscape patterns in soil microbial processes across the Serengeti, 17 study sites from nine landscape regions were sampled for soil physical/chemical characteristics and laboratory determination of soil microbial biomass, 20—d net turnover rates of carbon and nitrogen, and respiratory and nitrogen mineralization responses to carbon and nitrogen amendments. A large variation in soil physical/chemical characteristics across landscapes and a high degree of intercorrelation among these soil properties were found. Soil microbial biomass carbon ranged from 587 to 8971 μg/g soil dry mass, constituting between 3.4 and 9.4% of the total soil carbon at the Central Hills and Southern Plains landscape sites, respectively. Soil respiration rates (as carbon loss per unit soil dry mass) ranged from 9 μ—1.d—1 in the Northwest to 57 mg.g—1.d—1 on the Southern Plains and were positively correlated with soil microbial biomass. Regression models incorporating percent water—holding capacity and total organic carbon were highly predictive of levels of microbial biomass and soil respiration across all landscapes. Net nitrogen mineralization rates per unit soil dry mass, averaging between ‐0.48 and 1.09 mg°g—1°d—1, were positively correlated with soil respiration rates, but unrelated to soil mineral nitrogen pools or soil microbial biomass. (NH4)2SO4 additions significantly reduced both soil respiration rates and net nitrogen mineralization rates, but significantly increased net nitrate production, suggesting that nitrification is limited, in part, by ammonium availability. Low phosphorus availability may not only restrict nitrate production, but also limit ammonium production, thus having fundamental impacts on the nitrogen economy in this ecosystem. The interaction between N and P cycling is likely most significant in the tallgrass Northwest and Northeast landscapes, where granitic, P—deficient parent materials predominate, and nitrogen competition between plants and soil microbes limits aboveground nitrogen flow. On the Southern, Southeast, and Northwest Plains, where grazing intensity is greatest, soils have the highest levels of soil microbial biomass and lower C:P ratios, and microbial growth and nitrogen turnover rates appeared to be more C than N limited. The Eastern and Western Corridor, which support lower levels of sustained, intensive grazing, showed intermediate levels of soil microbial biomass and carbon and nitrogen turnover rates. The observation that mean annual rainfall is positively correlated with net primary production but negatively correlated with soil fertility across African savannas is supported by our data, which generally show that regional patterns of soil fertility and soil microbial processes are negatively correlated with mean annual rainfall across the Serengeti. We found soil microbial processes to be closely correlated with landscape patterns of herbivore use and intensity of herbivory. Herbivores track plant growth, which is highly variable both temporally and spatially, and in doing so, profoundly impact nutrient cycling processes where they feed. Thus, the bursts in soil microbial processes and plant growth that follow rainfall events are accompanied by the direct impact of herbivores on these processes.

Net nitrogen mineralization in soils under six indigenous tree species, an abandoned pasture and a secondary forest in the atlantic lowlands of costa rica

Nitrogen mineralization, nitrification potentials, pH, total N, C, extractable P and cations were measured in soils under 4-year-old, mono-specific stands of six fast-growing, native tree species, an abandoned pasture, and a 20-year-old secondary forest, as part of a study on the use of indigenous tree species for rehabilitation of soil fertility on degraded pastures at the La Selva Biological Station in the Atlantic humid lowlands of Costa Rica. Soil net nitrification potential rates were higher under two N-fixing, leguminous species,Stryphnodendron microstachyum Poepp. et Endl. (1.1–1.9 mg kg−1 day−1) andDalbergia tucurensis Donn. Smith (0.7–1.5 mg kg−1 day−1), than under the non-N-fixing trees in the plantation,Vochysia guatemalesis Don. Sm.,Vochysia ferruginea Mart,Dipteryx panamensis (Pittier) Record and Mell andHyeronima alchorneoides Fr. Allemao (0.2–0.8 mg kg−1 day−1). Values under the N-fixing trees were comparable to those found in secondary forest. There were no statistically significant differences in soil total N or in other nurtients between the species. Results of pH measurements done before and after incubation did not show any clear evidence of a pH drop attributable to nitrification.

Net nitrogen mineralization and net nitrification along a tropical forest-to-pasture chronosequence

Soil inorganic nitrogen pools, net mineralization and net nitrification rates were compared during the dry season along a chronosequence of upland (terra firme) forest, 3-, 9- and 20-year-old pastures in the western Brazilian Amazon Basin state of Rondonia to investigate the influence of forest conversion to pasture on soil nitrogen cycles. Surface soil (0 to 10 cm) from forest had larger extractable inorganic nitrogen pools than pasture soils. In the forest, NO 3 − pools equaled or exceeded NH 4 + pools, while pasture inorganic N pools consisted almost exclusively of NH 4 + . Rates of net N mineralization and net nitrification in seven -day laboratory incubations were higher in the seven - day forest than in the pastures. Net N mineralization rates did not differ significantly among different-aged pastures, but net nitrification rates were significantly lower in the 20-year-old pasture. Higher net N mineralization and net nitrification rates were measured in laboratory and in situ incubations of sieved soil, compared with in situ incubations of intact soil cores. Rates calculated in seven-day incubations were higher than determined by longer incubations. Sieving may increase N mineralization and/or decrease N immobilization compared with intact cores. We concluded that 7-day laboratory incubation of sieved soil was the most useful index for comparing N availability across the chronosequence of forest and pasture sites. High net nitrification rates in forest soils suggest a potential for NO 3 − losses either through leaching or gaseous emissions.

A preliminary study to predict net nitrogen mineralisation in a flooded rice soil using anaerobic incubation

Complementary field and laboratory studies were conducted to determine whether laboratory measurements of net nitrogen (N) mineralisation under anaerobic conditions could be used to predict field rates in a flooded soil and N uptake by a rice crop. The laboratory experiment consisted of measurements of ammonium accumulation at 10, 20, 30, and 40�C for 7, 14, and 28 days of anaerobic incubation. There was no accumulation of ammonium at 10�C, but increasing ammonification rate at temperatures of 20�C was observed, except for a slower rate at 40�C after 14 days. Two models were tested on the data: a zero-order reaction in which rate of mineralisation was a linear function of temperature; a first-order reaction in which net N mineralisation rate was a proportion of a depleting pool of labile organic N. In the second model, the rate was also linearly related to temperature. Both models fitted the laboratory data well (R2 = 0.94 and 0.97, respectively), but the second model accounted better for mineralisation at 40�C for the 28-day incubation. These models were then run, using daily mean temperatures over a rice-growing season, to predict net mineralisation in the field. The predictions were compared with measured net N mineralisation in a flooded soil and N uptake by the crop measured throughout the season in the field from which the incubated soil was sampled. Net N mineralisation and crop uptake increased throughout the season, reaching maximum values of 115 and 111 kg N/ha at maturity. The zero-order and first-order models both predicted net N mineralisation accurately until the middle of the season, after which the zero-order model overestimated net N mineralisation but the first-order model predicted the reduction in the rate of net N mineralisation with reasonable accuracy. The close agreement between the laboratory incubations and field measurements of net mineralisation and crop N uptake suggest that incubation tests may provide useful information for including in a model to assist rice growers' decisions about N fertiliser.

Nitrogen and Carbon Mineralization in Cultivated Acrisols and Vertisols in a Sub‐Humid Tropical Climate

AbstractIn the southern part of the Republic of Benin (West Africa), investigations were carried out on two contrasting sites (Ferrali‐Haplic Acrisols and Stagni‐Eutric Vertisols), with the aim of assessing the effect of soil type and micro‐climate on carbon and nitrogen turnover under subhumid tropical conditions. The Acrisols in Southern Benin are characterized by a sandy topsoil with high water permeability. In contrast, the Vertisols have a clayey topsoil and show stagnic properties. Maize was planted during two years i.e. four consecutive cropping seasons (no‐till, no organic or mineral fertilizer, weeding by hoe) on a total of fourteen Acrisol and nine Vertisol plots. Fresh organic material like fallow vegetation or crop residues were systematically removed. In spite of their higher clay content the Vertisols showed significantly higher carbon mineralization rates (12.0%) under field conditions than Acrisols (5.2% of Corg per year). The net nitrogen mineralization rates were, however, similar on both sites (13.8% and 13.0% of N1 per year respectively). Differences between Vertisols and Acrisols were due to the drier microclimate in the Acrisols. Incubation studies with Acrisol topsoil samples under controlled conditions demonstrated that decreasing soil matrix potential and temperature fluctuations, as they occur in the field, reduce CO2 evolution more than the net nitrogen mineralization, which was not the case in Vertisol topsoil samples. Carbon mineralization is, therefore, under tropical sub‐humid conditions highly related to soil microclimate, hence site specific. Carbon and nitrogen mineralization may differ considerably from each other depending on site conditions.

Soil nitrogen availability and nitrification in Mediterranean shrublands of varying fire history and successional stage

The short-term effect of a single fire, and the long-term effect of recent fire history and successional stage on total and mineral N concentration, net nitrogen mineralization, and nitrification were evaluated in soils from a steep semi-arid shrubland chronosequence in southeast Spain. A single fire significantly increased soil mineral N availability and net nitrification. Increasing fire frequency in the last few decades was. associated with a sharp decrease in surface soil organic matter and total N concentrations and pools, and with changes in the long-term N dynamic patterns. The surface-soil extractable NH4+:NO3− ratio increased throughout the chronosequence. All net mineralized N in laboratory incubations from all sites was converted to nitrate, suggesting that allelochemic inhibition of net nitrification is probably not important in this system. Net nitrification in samples during incubation increased through the sere. The maximum rate of net nitrification (kmax) increased through the first three stages of the sere. A linear relationship was found between total soil N and N mineralization, and both kmax and net nitrification for the first three stages of the sere, suggesting that total N and ammonification are likely to be the control mechanisms of nitrification within the sere. The oldest site exhibited the lowest specific kmax and the highest, potential soil respiration rate suggesting that a lower N quality and increasing competition for ammonium might also limit nitrification at least in the long-unburned garrigue site.

Soil Microbial Biomass Dynamics and Net Nitrogen Mineralization in Northern Hardwood Ecosystems

AbstractSoil temperature, water potential, and substrate (C) availability are the primary constraints on microbial activity within soil and display substantial seasonal variation within northern deciduous forests. Following autumn litterfall, soil C availability is relatively high, and N should be assimilated by soil microorganisms to maintain or form new biomass. Conversely, N should be mineralized from microbial biomass during midsummer when C availability is relatively low and soil temperatures are high. Because N availability is directly controlled by microbial activity, we hypothesized that microbial biomass and net N mineralization are inversely related on a seasonal basis. To test this hypothesis, we studied the temporal relationship between microbial biomass (C and N) and rates of net N mineralization in two different northern hardwood ecosystems, one dominated by sugar maple (Acer saccharum Marsh.) and basswood (Tilia americana L.) and the other by sugar maple and red oak (Quercus rubra L.). In situ buried bags were used to estimate net N mineralization and nitrification at monthly intervals for 1 yr. Microbial C and N contents of the incubated soil were determined using the chloroform fumigation‐incubation method. Net N mineralization displayed marked seasonal variability, ranging from 35 to 115 mg N m−2 d−1 during the growing season. In contrast, microbial biomass C and N were relatively constant throughout the year, averaging 112 g C m−2 and 17 g N m−2. Neither microbial biomass (C or N) nor the change in microbial biomass between sampling dates were significantly inversely correlated with mean daily rates of net N mineralization. As such, our data do not support the idea that N availability is controlled by large seasonal fluctuations in soil microbial biomass. Rather, our results suggest that N availability is primarily controlled by changes in the turnover rate of microbial biomass such that a relatively constant pool is maintained through time. In addition, mean annual rates of net N mineralization and nitrification did not differ significantly from those previously measured by others in the same stands, suggesting that annual rates may be relatively consistent in climatically similar years.

Biogeochemistry of nitrogen in a coastal dune succession on Spiekeroog (Germany) and the impact of climate

The nitrogen cycle of terrestrial plant-dominated ecosystems is explained by means of a compartment model. Its main parts are described: the plant-internal cycle, the ecosystem-internal cycle and the global N-cycle, including input-output relations and biological N-fixation. The distribution of total nitrogen (N t ) as well as the net mineralization of nitrogen in the soils of a dune succession on Spiekeroog Island (Germany) are described. Values for 13 different sites are given, ranging from foredunes to climax dune forests in a chronosequence from 0 to > 250 years. In the course of development, the N-pools in the soils increase from approximately 150 kg to more than 3000 kg N t . ha −1 . 30 cm −1

The spatial variability of soil resources following long-term disturbance.

The spatial distributions of selected soil properties in two adjacent sites in southwest Michigan were examined to evaluate the potential effects of chronic disturbance on resource heterogeneity. One site was a cultivated field that had been cleared, plowed, and cropped annually for decades prior to sampling while the other, uncultivated field was cleared of original forest in 1960 after which it was mown annually but never plowed or cropped. We took replicate samples from a 330-point unaligned grid across the sites for soil pH, gravimetric moisture, inorganic phosphorus, total carbon, and net nitrification and nitrogen mineralization potentials. Soils in the cultivated site contained less than half as much carbon as in the uncultivated site, but had higher levels of inorganic phosphorus and moisture, and higher soil pH. Potential net nitrogen mineralization and nitrification rates did not differ between sites. Geostatistical analysis showed that almost all properties examined were strongly autocorelated within each site; structural variance as a proportion of sample variance ranged from 30-95% for all properties, and for any given property differed little between sites. The distance over which this dependence was expressed, however, was for all properties but pH substantially less in the uncultivated site (7-26 m) as compared to the tilled site (48-108m), especially for total C and net nitrification and N mineralization. These results suggest that the spatial pattern and scale of soil variability can differ markedly among edaphically identical sites and that these differences can be related to disturbance history.

Effectiveness of antibiotics to distinguish the contributions of fungi and bacteria to net nitrogen mineralization, nitrification and respiration

Streptomycin and cycloheximide were applied singly or in combination to an arable or a forest soil to determine their effects on N mineralization, nitrification, CO 2 evolution, and microbial biomass C and N. Only slight differences were observed in concentrations of both inorganic N forms, CO 2 evolution and amounts of microbial biomass C and N between the streptomycin-treated arable soil and the respective control. Cycloherimide singly or in combination with streptomycin stimulated N mineralization, depressed NO − 3 production as well as the increase in microbial biomass C and N observed in the untreated arable soil. The C : N ratio of soil microbial biomass fluctuated around 4 with eycloheximide singly or in combination with streptomycin during the 0–10 day incubation; the ratio was generally higher than 4.5 but never exceeded 5 in the streptomycin-treated arable soil and in the respective control. The inhibition of nitrification in the arable soil by the fungicide prompted the question concerning the relative importance of autotrophic and heterotrophic nitrification rates; the inhibition of the NO − 3 formation by nitrapyrin and the absence of stimulation by peptone indicated that the potential for autotrophic nitrification in this soil is greater than that for heterotrophic nitrification. The addition of antibiotics to the forest soil increased respiration and NH + 4-N concentration with the highest effect with both anibiotics. Both biocides singly or in combination depressed the decrease of NO − 3-N concentration observed in the control; they markedly depressed microbial biomass C and N during the early incubation period; then both microbial biomass values increased probably because surviving microorganisms utilized organic compounds released from antibiotic-killed cells.

Variation in Foliar 15N Abundance and the Availability of Soil Nitrogen on Walker Branch Watershed

Spatial patterns in natural 15N abundance (°15) in soil, soil solutions, and non—N2—fixing plants were studied in the deciduous forest on Walker Branch Watershed near Oak Ridge, Tennessee. This study was undertaken to test the hypothesis that foliar °15N values are related to the availability of inorganic nitrogen in mineral soil. Soils collected in or near valley bottoms on the watershed had higher levels of net nitrogen mineralization and net nitrification potential than those sampled from ridges and slopes. More positive foliar °15N values occurred in valley bottoms, which, relative to other positions on the watershed, were characterized by greater availability of soil nitrogen and lower C—to—N ratios in the Oi—horizon, in the surface mineral soil, and in autumn leaf fall. Although leaf nitrogen concentrations changed significantly over the course of the growing season, there was little seasonal variation in foliar °15N values. A hypothesis about the relative importance of different sources of nitrogen to the forest and how nitrogen cycling varies with topography in this nitrogen—deficient ecosystem was derived, in part, from spatial patterns in natural 15N abundance. There appear to be two processes affecting the topographic patterns in foliar 15N abundance on this watershed: (1) greater uptake from isotopically heavy pools of inorganic soil nitrogen by plants in valley bottoms, and (2) uptake of isotopically light ammonium—N in atmospheric deposition by plants on ridges and slopes (where the availability of inorganic soil nitrogen to plant roots is more limited). Results from this study indicate that foliar °15N values are positively correlated with net nitrification potential in surface soil.

Nutrient cycling in forests.

Studies of nutrient cycling in forests span more than 100 yr. In earlier years, most attention was given to the measurement of the pools of nutrients in plants and soil and of the return of nutrients from plant to soil in litterfall. The past 20 yr or so have seen a major concentration on the processes of nutrient cycling, with particular emphasis on those processes by which the supply of nutrients to the growing forest is sustained. In the more highly productive forests, up to 10 tonnes of litter of low nutritional quality is deposited annually on the forest floor. The decomposition of this litter, the mineralization of the nutrients it holds, and the uptake of nutrients by tree roots in the carbon-rich environment which results are the themes of this review. Studies of decomposition of litter in forests have been dominated by the role of nitrogen as a limiting factor, a domination which reflects the preponderance of studies of temperate forests in the Northern Hemisphere. For many forests of the world growing on soils of considerable age, it seems more probable that growth and nutrient cycling are limited by phosphorus (or some other element). There is increasing evidence for a number of forests that phosphorus is immobilized in the first stages of decomposition to a significantly greater extent than is nitrogen. Advances in research will depend, as with studies of soil organic matter, in denning and developing analytical techniques for studying biologically active forms of potentially limiting nutrients, rather than total elemental concentrations. The availability of phosphorus in forests is sustained by phosphorus cycling. More than 50% of the total phosphorus in the surface soils is in organic forms and much of the more labile phosphorus is in the form of diesters. Phosphorus availability is determined by competition between biological and geochemical sinks, and it is clear that the sinks in the rhizosphere (plant roots, microorganisms, soil mineral and organic components) are extensively modified by active processes (e.g. production of exudates, nutrient storage in a variety of organic or polymeric forms and nutrient transport away from sites of uptake). There is abundant evidence that roots of many species exude compounds which have the ability to solubilize sources of phosphorus of otherwise low availability. The significance of root exudates (for example, phosphatases, organic acids) in the functioning of perennial ecosystems has yet to be quantified and there are conflicting reports as to the effects of simple organic acids on phosphorus availability. The distribution of phosphorus sinks and their relative competitiveness and their modification are topics of fundamental importance for future research. In contrast to the mineralization of phosphorus, our knowledge of transformations and availability of nitrogen in forest soils is well-developed. Net nitrogen mineralization rates approximate rates of nitrogen return in litterfall but the contribution of nitrification is variable. Nitrification is not inhibited by the low pH of many forest soils and there is increasing evidence of nitrate immobilization by microorganisms and of increased diversity and better competitiveness for NH4 + of nitrifying microorganisms than has previously been accepted. Variability in rates of nitrification is often interpreted as being due to allelopathy. Hypotheses invoking allelopathy are more or less untestable, and it seems likely that new techniques using 15 N in situ will lead to a more fundamental understanding of nitrogen transformations in forest soils. Recent studies in coniferous forest soils have highlighted the short (< 1 d) turnover time of NH4 + . Finally, it seems that forest soils are resistant to major changes in patterns of nitrogen mineralization (and certainly, because of the large number of sinks, in patterns of phosphorus mineralization) following disturbance by natural events such as wind-throw and fire, and by man-made events such as logging and fertilizing. The long-term disturbance by acid rain is a more complex matter since forest ecosystems are not adequate buffers for nitrate. Contents Summary 561 I. Introduction 562 II. Linking nutrient cycling to nutrient availability - Setting the themes 563 III. The nature of soil organic matter 566 IV. Tree roots and the availability of nutrients 566 V. The decomposition of forest litter 569 VI. Mineralization of organically-bound nutrients 571 Acknowledgements 576 References 576.

Above-ground net primary productivity and nitrogen mineralization in a mixed mesophytic forest of eastern Kentucky

Above-ground net primary productivity (ANPP) and potential nitrogen (N) mineralization were measured in four forest types along a soil fertility gradient in eastern Kentucky to test the hypothesis that ANPP in the mixed mesophytic forest region is related to both species composition and mineralizable N. N mineralization differed significantly among the four forest types, with the lowest values occurring on sites containing oak forest and the highest rates occurring on sites containing a mixed mesophytic forest. However, ANPP did not differ among forest types, and was unrelated to mineralizable N. ANPP averaged 6306 kg ha −1 year −1, of which 56% was woody production and 44% was above-ground litter production. The poor relationship between ANPP and site fertility may reflect differential patterns of nutrient use efficiency among the deciduous species which make up the vegetation gradients of eastern Kentucky.

Changes in soil organic matter and net nitrogen mineralization in heathland soils, after removal, addition or replacement of litter from Erica tetralix or Molinia caerulea

The effects of different litter input rates and of different types of litter on soil organic matter accumulation and net N mineralization were investigated in plant communities dominated by Erica tetralix L. or Molinia caerulea (L.) Moench. Plots in which the litter on the soil had repeatedly been removed were compared with plots in the same plant community in which litter had been added to the soil. In another treatment, litter was removed and replaced by litter from the other plant community. Net N mineralization was measured in situ after 5 years. Less soil organic matter and soil N was found in plots in which litter had been removed, compared with control plots, or plots to which litter had been added, but these differences were significant for the Erica sp. soils only. Plots in which litter had been replaced and control plots did not differ significantly in the amount of soil organic matter. However, in both plant communities, the differences agreed with the faster decomposition rate of Molinia sp. litter compared with Erica sp. litter. The gravimetric soil moisture content was correlated positively with the amount of soil organic matter, both in the Erica sp. soils and the Molinia sp. soils. Net N mineralization rates (g N m-2) differed significantly between treatments for Erica sp. soils but no for Molinia sp. soils. For Erica sp. soils, net N mineralization rates increased with increasing amounts of soil organic matter and soil N. Replacing the litter with Molinia sp. litter (which differs in chemical composition) had no clear additional effect on the net N mineralization rate.