Net Nitrogen Mineralization Rates Research Articles

The Impact of Sheep Grazing on Net Nitrogen Mineralization Rate in Two Temperate Salt Marshes

Abstract: Nitrogen mineralization rate was studied in grazing trials with three different stocking rates (0, 3, 10 sheep ha‐1) in two man‐made salt marshes, viz. a Puccinellia maritima‐dominated low salt marsh and a high salt marsh dominated by Festuca rubra. Mineralization rates were derived from the amounts of mineral N which accumulated in situ during six‐week incubation periods in tubes containing undisturbed soil cores from the upper 10 cm soil layer. The annual rates of net N mineralization were significantly higher in the better drained, high salt marsh (71 ‐ 81 kg ha‐1 yr‐1) than in the low salt marsh (39 ‐ 49 kg ha‐1 yr‐1). High amounts of belowground litter accumulated in the low salt marsh due to frequent water logging. Both N mineralization and nitrification rate were negatively correlated with soil water content. In the Puccinellia maritima salt marsh, grazing had neither an effect on N mineralization rates during any of the incubation periods nor on annual mineralization rates. In the Festuca rubra salt marsh, N mineralization rates increased earlier during spring at the intensively grazed site than at the moderately grazed and the ungrazed site. N mineralization and nitrification rates were significantly higher at the ungrazed site than at the intensively grazed site during the period of peak net N mineralization from the end of April until mid‐June. Although sheep grazing affected the seasonal pattern of N mineralization in the high marsh, grazing did not affect the annual rate of net N mineralization.

Muskrat (Ondatra zibethicus) Disturbance to Vegetation and Potential Net Nitrogen Mineralization and Nitrification Rates in a Freshwater Tidal Marsh

The muskrat (Ondatra zibethicus) is a wetland mammal whose disturbance activities include grazing, burrowing and lodge construction. We evaluated the effects of these disturbances on plant biomass, species richness and diversity, stem density and potential net nitrogen mineralization and nitrification rates in a freshwater tidal marsh on the Hudson River in New York. We hypothesized that muskrats increase floristic richness and diversity by decreasing the biomass of narrowleaf cattail (Typha angustifolia) and that muskrats increase potential net nitrogen mineralization and nitrification rates through aeration and reduced plant uptake because of herbivory. Because muskrats commonly build lodges on or close to creek banks, we separated the disturbance effects of muskrats from the disturbance effects of the creek bank by sampling quadrats along transects placed perpendicular to creek banks at lodge sites. Muskrats decreased biomass, particularly of cattail, but had no measurable effect on stem count, species richness or species diversity. Muskrats increased potential net nitrogen mineralization and nitrification rates; however, this effect was limited to active sites. Creek bank disturbance increased stem count but had no effect on the other variables. Although muskrats did not significantly affect floristic diversity in this study, their disturbance activities did influence soil nitrogen dynamics, which is an important component of wetland function.

Soil nitrogen turnover in proximal and distal stem areas of European beech trees

In European beech (Fagus sylvatica L.) forests, a large proportion of the water and ion input to the soil results from stemflow which creates a soil microsite of high element fluxes proximal to the tree trunk. The soil proximal to the stem is considered to have different rates of nitrogen turnover which might influence the estimation of N-turnover rates at the stand scale. In a previous study we reported high nitrate fluxes with seepage proximal to the stems in a forest dominated by European beech in Steigerwald, Germany. Here, we investigated the soil nitrogen turnover in the top 15 cm soil in proximal (defined as 1 m2 around beech stems) and distal stem areas. Laboratory incubations and in situ sequential coring incubations were used to determine the net rates of ammonification, nitrification, and root uptake of mineral nitrogen. In the laboratory incubations higher rates of net nitrogen mineralization and nitrification were found in the forest floor proximal to the stem as compared to distal stem areas. No stem related differences were observed in case of mineral soil samples. In contrast, the in situ incubations revealed higher rates of nitrification in the mineral soil in proximal stem areas, while net nitrogen mineralization was equal in proximal and distal areas. In the in situ incubations the average ratio of nitrification/ammonification was 0.85 in proximal and 0.34 in distal stem areas. The net nitrogen mineralization was 4.4 g N m-2 90 day-1 in both areas. Mineralized nitrogen was almost completely taken up by tree roots with ammonium as the dominant nitrogen species. The average ratio of nitrate/ammonium uptake was 0.69 in proximal and 0.20 in distal areas. The higher water content of the soil in proximal stem areas is considered to be the major reason for the increased rates of nitrification. Different nitrogen turnover rates in proximal stem areas had no influence on the nitrogen turnover rates in soil at the stand scale. Consequently, the observed high nitrate fluxes with seepage proximal to stems are attributed to the high nitrogen input by stemflow rather than to soil nitrogen turnover.

Changes in Soil Chemical and Physical Characteristics in Japanese Cypress (Chamaecyparis obtusa Endl.) Stands by Mixture of Deciduous Broad-leaved Trees in the Northern Kanto Region of Japan

In order to clarify the effects of a mixture of deciduous broad-leaved trees on soil fertility, we investigated litter biomass accumulation, mineral soil chemical and physical characteristics, characteristics of nitrogen mineralization, and the mutual relationships between them in Japanese cypress (Chamaecyparis obtusa) stands mixed with deciduous broad-leaved trees at different ratios (mixture ratio; MR = 0, 16, 33, 43, 100% by basal area) in the northern Kanto region of Japan. Litter biomass in the forest floor and mineral soil was 19.1 Mg ha−1 in MR 0% and decreased approximately 60 % in MR 33%, MR 43% and MR100%. The permeability at 0–5 cm soil depth in MR100% was twice as much as that in MR 0%. Increases in soil permeability were likely due to larger soil pores in the higher MR with much accumulated deciduous broad-leaves. At 0–5 cm soil depth, the differences in carbon concentration among the plots were not clear. On the other hand, carbon concentrations at 5–10 cm depth increased from 90 g kg−1 to 147 g kg−1 with increases in MR from 0% to 100%. Concentrations of exchangeable bases increased two to four times with increases in MR from 0 to 100% at 0–10 cm depth. Soil pH (H2O) generally increased with increases in MR at each depth. The rates of net nitrogen mineralization at 0–5 cm depth in vitro increased from 25 to 87 mg kg−1 2 weeks−1 with increases in MR from 0 to 100%. However, increases in nitrification with increases in MR were not clear compared with nitrogen mineralization. These results indicated that a mixture of deciduous broad-leaved trees in a Japanese cypress stand was effective in preventing soil fertility decline.

Seasonal variation of inorganic nitrogen and net mineralization in a salt marsh ecosystem

Inorganic nitrogen pools and net mineralization were estimated in three sites of a Tagus estuary salt marsh in Portugal throughout 1 year. Ammonium (NH4 +) was the major form of inorganic nitrogen found in the salt marsh soil. Extractable NH4 + concentrations showed a marked seasonal pattern with a concentration peak during the hotter months of July/August. The great majority (>99%) of the total nitrogen in the soil was found in sedimented organic matter, not readily available for plant uptake. Net nitrogen mineralization, determined using a field incubation method, showed a peak during the months of June/July which resulted in an increase on nitrogen availability. With the exception of the lower salt marsh, estimated rates of in situ net nitrogen mineralization in the soil during summer were well related to the increase in plant above‐ground biomass and plant nitrogen pools, indicating that the process is an important source of available nitrogen for plant uptake and growth. Annual net nitrogen mineralization ranged between 2.4 and 4.5 g N m−2 yr− 1 being significantly higher for the lower salt marsh site. Rates of net nitrogen mineralization were relatively low during most of the year with a particularly active period from June to August, possibly due to an effect of temperature on soil microbial activity.

Urea hydrolysis and nitrification in arctic salt-marsh soils: Possible constraints on the growth of forage plants

Lesser snow geese graze intensively on graminoid vegetation of intertidal and non-tidal salt marshes at La Pérouse Bay, Manitoba. The replacement of defoliated plant tissue is limited by the supply of nitrogen. Nitrogen in goose faeces may become available for plant uptake through hydrolysis of urea and through nitrification. In recent years, heavy grazing and grubbing by the growing goose population have damaged the vegetation at some sites. Soils beneath damaged swards become hypersaline and grubbed soils exhibit low rates of net nitrogen mineralization and low ammonium concentrations. We measured effects of temperature and salinity on the rate of urea hydrolysis and on the nitrification potential of soils. Rates of urea hydrolysis increased with rising temperatures (1°C - 30°C) but declined with increasing salinity. Rates of urea hydrolysis were low in soils where the vegetation had been removed by grubbing but high in the more organic soils of the non-tidal inland marsh. Rates of nitrification were consistently low, particularly in soils from the inland marsh. The nitrification rate increased with temperature, and was higher at the salinity of sea water than at lower or higher salinities. Watering turves with river water for six weeks before measuring rates of nitrification did not significantly change the results in soils from either the intertidal marsh or the inland marsh. It is probable that most mineral nitrogen available to plants growing in these soils is in the form of ammonium. Low soil temperatures and hypersaline conditions limit rates of urea hydrolysis, and likely restrict available nitrogen for the growth of forage plants.

Nitrogen mineralization associated with birch and fir under different soil moisture regimes

We examined net nitrogen mineralization rates in forest floor beneath birch-dominated or fir-dominated canopy plots in a high-elevation mixed-species forest in New Hampshire during the 1995 and 1996 growing seasons. Soil moisture was significantly greater in 1996 than in 1995 (season averages were 2.1 times and 1.4 times dry soil mass, respectively). Net mineralization was significantly greater in both plot types in 1996 than in 1995. The magnitude of difference, however, was much greater in fir plots. Nitrification increased in birch plots and significantly decreased in fir plots from 1995 to 1996. Results of a three-way ANOVA showed significant year and species main effects for net mineralization and a significant species × year interaction for nitrification. There were no significant correlations between net N mineralization and measured soil chemistry variables in 1995. In 1996 there were significant positive correlations between total N, and net mineralization and nitrification in birch plots and between soil moisture and net mineralization in fir plots. These results support a growing body of research suggesting that species feedbacks influence rates of net N mineralization in mixed species forests. Further, this study provides novel evidence that rates of net N mineralization may respond differently to changing abiotic conditions depending on the local canopy species in a mixed-species forest.

Indicators of nitrate export from forested watersheds of the mid‐Appalachians, United States of America

Soil net nitrogen mineralization and nitrification rates were studied on nine undisturbed, forested watersheds in an effort to explain large variations in nitrate export in streamflow within the mid‐Appalachian region. Rates of soil net nitrogen mineralization and net nitrification were measured in the upper 10 cm of mineral soil over a 5‐week summer incubation period (June–July) using nine buried bags in each of the three major soil types on each watershed. Watersheds with high, medium, and low nitrate export rates exhibited high, medium, and low mean net nitrogen mineralization and net nitrification rates, respectively. Exchangeable calcium (an index to site fertility), C/N ratios, and soil moisture content together explained 63% of the variation in soil nitrogen mineralization rates, and exchangeable calcium and soil moisture content explained 61% of the variation in soil nitrification rates using multiple regression analysis. The variation in watershed nitrate export was best explained by total nitrogen in the upper 10 cm of mineral soil (explained 46%) and the percentage of mineralization due to nitrification (explained 42%). Estimated rates of wet and dry atmospheric deposition of nitrogen were not significantly correlated with watershed nitrate export. Results from this study demonstrate that soil nitrogen pools and dynamics are the most critical factors controlling nitrate export from forested watersheds in the mid‐Appalachians. Long‐term changes in site fertility, C/N ratios, and soil moisture, which largely control microbial nitrogen cycling, should have a significant effect on long‐term trends in nitrate leaching.

Contingent effects of plant species on soils along a regional moisture gradient in the Great Plains.

The central grassland region of the United States encompasses major gradients in temperature and precipitation that determine the distribution of plant life forms, which in turn may influence key ecosystem processes such as nutrient cycling and soil organic matter dynamics. One such gradient is the threefold increase in precipitation from the eastern Colorado shortgrass-steppe, in the rain shadow of the Rocky Mountains, to the tallgrass prairie in eastern Kansas. We investigated the relative roles of plant species and plant cover in influencing soil C and N cycling in three sites along this gradient. Plant cover (i.e., the presence or absence of an individual plant) was relatively more important than plant species in explaining variability in soil properties at the dry site, the Central Plains Experimental Range in␣northeastern Colorado. However, plant species explained relatively more of the variability in soil properties than did plant cover at the two wetter sites, Hays and Konza, in central and eastern Kansas. The wetter sites had more continuous plant cover, resulting in less plant-cover-induced variation in soil C and N, than did the dry site, which had distinct patches of bare ground. Plant species at the wetter sites had higher and more variable levels of tissue C:N than plant species at the dry site, due to both within species changes and changes in species composition. Aboveground tissue C:N was better correlated with net nitrogen mineralization rates at the wet sites than the dry site. Thus, tissue chemistry appears to exert more control on nitrogen dynamics at the wet than the dry sites. The results suggest that plant species traits that are relevant to nutrient cycling (e.g., tissue C:N ratios, spatial patterns, productivity) reflect environmental limitations as well as species' physiological potentials. Furthermore, a dominant environmental driver such as precipitation may ameliorate or exaggerate the importance of individual species traits for nutrient cycling.

Net nitrogen mineralization and net nitrification rates in soils following deforestation for pasture across the southwestern Brazilian Amazon Basin landscape.

Previous studies of the effect of tropical forest conversion to cattle pasture on soil N dynamics showed that rates of net N mineralization and net nitrification were lower in pastures compared with the original forest. In this study, we sought to determine the generality of these patterns by examining soil inorganic N concentrations, net mineralization and nitrification rates in 6 forests and 11 pastures 3 years old or older on ultisols and oxisols that encompassed a wide variety of soil textures and spanned a 700-km geographical range in the southwestern Brazilian Amazon Basin state of Rondônia. We sampled each site during October-November and April-May. Forest soils had higher extractable NO3--N and total inorganic N concentrations than pasture soils, but substantial NO3--N occurred in both forest and pasture soils. Rates of net N mineralization and net nitrification were higher in forest soils. Greater concentrations of soil organic matter in finer textured soils were associated with greater rates of net N mineralization and net nitrification, but this relationship was true only under native forest vegetation; rates were uniformly low in pastures, regardless of soil type or texture. Net N mineralization and net nitrification rates per unit of total soil organic matter showed no pattern across the different forest sites, suggesting that controls of net N mineralization may be broadly similar across a wide range of soil types. Similar reductions in rates of net N transformations in pastures 3 years old or older across a range of textures on these soils suggest that changes to soil N cycling caused by deforestation for pasture may be Basin-wide in extent. Lower net N mineralization and net nitrification rates in established pastures suggest that annual N losses from largely deforested landscapes may be lower than losses from the original forest. Total ecosystem N losses since deforestation are likely to depend on the balance between lower N loss rates from established pastures and the magnitude and duration of N losses that occur in the years immediately following forest clearing.

Effects of soil extracts from repeated plantation woodland of Chinese-fir on microbial activities and soil nitrogen mineralization dynamics

Effects of soil extracts from repeated plantation woodlands of Chinese-fir on soil fungi growth, the activities of microbial communities, and rates of net soil nitrogen mineralization were investigated. Soil extracts from replanted woodlands significantly inhibited soil non-pathogenic fungi growth, reduced soil respiration activities, and net soil nitrogen mineralization rates. However, soil extracts from replanted woodland increased the growth of pathogenic fungi. The combination of soil extracts and pathogenic fungi did not significantly reduce the growth of Chinese-fir seedlings when compared to the soil extracts alone. The combination of soil extracts with pathogenic and non-pathogenic fungi significantly increased the growth of Chinese-fir seedlings when compared to the combination of soil extracts and pathogenic fungi. The results suggest that the allelochemicals from soil extracts, rather than pathogenic fungi, are the key factor regulating the productivity and nitrogen cycling in repeated plantation woodlands.

Resource heterogeneity generated by shrubs and topography on coastal sand dunes

In early stages of primary succession, colonizing plants can create resource patches that influence the abundance and distribution of other species. To test whether different colonizing shrubs generate contrasting patches on coastal sand dunes, we compared soil characteristics and light availability under the nitrogen-fixing shrub Lupinus arboreus, under the non-nitrogen-fixing shrub Artemisia pycnocephala, and between shrubs on dunes at a site in northern California. Concentrations of inorganic nitrogen and net nitrogen mineralization rates were generally 1–10 times greater in soil under Lupinus than under Artemisia or between shrubs. Soil water content was mostly lower under shrubs. Mean photon flux density near ground level was reduced by at least 80% at ≥ 35 cm inside shrub canopies. Topography appeared to have more effect on soil moisture but less direct effect on nitrogen availability than did Lupinus. However, Lupinus probably increases nitrogen levels more on higher, drier dunes. Microhabitats under and between nitrogen-fixing shrubs constitute a mosaic of individually poor but complementary patches in which high levels of light and moderate levels of soil nitrogen are present but tend not to occur together.

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

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.

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.

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.

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.

Relationships between litter quality and nitrogen availability in Rocky Mountain forests

We assessed the relationships between net nitrogen mineralization, litter quality, and litter decomposition rates in 30 Colorado forests to test three hypotheses of the LINKAGES forest ecosystem model: (i) nitrogen release from litter decay in the forest floor correlates with total (forest floor plus mineral soil) net nitrogen mineralization; (ii) net nitrogen mineralization can be predicted from litter chemistry; and (iii) lignin:nitrogen ratio is the litter quality parameter most closely correlated with decomposition rates. Net nitrogen mineralization was measured for 1 year in 10 stands each of lodgepole pine (Pinuscontorta var. latifolia Engelm.), aspen (Populustremuloides Michx.), and Engelmann spruce–subalpine fir (Piceaengelmannii Parry and Abieslasiocarpa (Hook.) Nutt.), using a sequence of on-site incubations in closed-top tubes. We analyzed foliage litter and fine-root samples for initial concentrations of nitrogen, lignin, carbon, and cellulose, and determined litter mass remaining after 2-, 4-, and 6 months of laboratory decomposition. Net nitrogen mineralization in the forest floor correlated highly with total net nitrogen mineralization both within and across species. Net nitrogen mineralization also correlated significantly with litter quality across species, as did litter mass remaining after each decay period. Initial lignin:nitrogen ratio showed as strong a relationship with net nitrogen mineralization and litter decay rates as any of the litter quality variables assessed. The three tested hypotheses were thus broadly supported in these forest ecosystems.

Nitrogen cycling in acid forest soils subject to increased atmospheric nitrogen input

Nitrogen cycling was studied in four forest ecosystems in the Netherlands, characterized by increased atmospheric nitrogen input. The study involved in situ incubation experiments to measure nitrogen transformation rates in the ectorganic (L + F + H) layer and the top 5 cm of the mineral soil. Throughfall and lysimeter data were collected from two of these sites to determine nitrogen solute fluxes and the proton production related to nitrogen cycling. The annual nitrogen input by throughfall ranged from 22 to 55 kg N ha −1 year −1. On the basis of the nitrogen budget, three of the forests could be characterized as nitrogen saturated. In two of these three forests high nitrification rates (16 kg N ha −1 year −1) were measured in the ectorganic layer and in the top of the mineral soil. At the other sites relatively low nitrate production rates (5–6 kg N ha −1 year −1) were measured. Net mineralization rates ranged from 26 to 71 kg N ha −1 year −1 in the investigated layers. A strong positive relationship between net nitrogen mineralization and nitrification rate was found at the two sites with high nitrification rates. Nitrification rate relative to net mineralization rate ranged from 22 to 29%. A comparison between incubation and lysimeter data revealed a quantitative discrepancy. It was concluded that the in situ incubation technique underestimates nitrogen transformation rates. Nitrogen solute fluxes indicated that in the nitrifying site, high nitrate production rates in the ectorganic layer coincided with large nitrate fluxes underneath this layer, while the nitrate produced in the top of the mineral soil had no net effect on the nitrate flux underneath that layer. As a result, high proton production rates (4.7 kmol H + ha −1 year −1) as a result of nitrogen transformations, were calculated for the ectorganic layer of this forest.

Methane fluxes in a northern hardwood forest ecosystem in relation to acid precipitation

Methane fluxes were measured periodically in 1990 and 1991 in upland forest and peatland soils of a northern hardwood forest in the Adirondack region of New York State. A portion of the study area received lime (10 Mg CaCO 3 ha -1) in October 1989 to mitigate several decades of chronic acid precipitation (H + deposition in 1981 = 653 eq ha −1 yr −1). Consumption of atmospheric methane by forest soils averaged −0.60 mg CH 4 m −2 d −1 and showed no difference between treatments (unlimed versus limed soils). Eliminating root growth and removing leaf litter from the soil seemed to decrease rates of methane consumption. Related responses of soil nitrogen transformations to the lime indicated lower nitrification rates and higher net nitrogen mineralization rates compared to unlimed soils, yet rates of methane consumption were unaffected by these changes in soil nitrogen transformations. Methane efflux from the peatland soils into the atmosphere averaged 12.1 mg CH 4 m −2 d −1, with no significant difference in rates between treatments. Laboratory measurements revealed that liming resulted in less consumption of atmospheric methane (measured with atmospheres amended with methane) and potentially more methane production. Overall, methane efflux from these peatland soils is much lower than that for more northern counterparts that currently experience cooler climate but less acid precipitation. This suggests that several decades of acid precipitation may play a role in limiting methane emissions from northern peatlands.