Patterns Of Nitrogen Mineralization Research Articles

Soil biological indicators associated with nitrogen mineralization patterns in rice soils under long‐term integrated nutrient management

AbstractIn long‐term integrated nutrient management (INM) involving organic amendments with inorganic fertilizer application, understanding nitrogen (N) mineralization is key to assessing soil functioning. In this study, soil samples were collected following rice cropping from treatments consisting of various combinations of inorganic with organic fertilizers, including farm yard manure, wheat straw or Green Leaf Manuring, as well as 100% inorganic and a control (no fertilizer). The soil samples were incubated for 120 days at 30°C for determining N mineralization potential, and the mineralization patterns were fitted to double first‐order kinetic models to separate the potentially mineralizable nitrogen (PMN) into readily available N (PMN‐I) and resistant N (PMN‐II) fractions. The half‐lives of PMN‐I and PMN‐II ranged from 1.2 to 3.3 days and 22.4 to 63.0 days, respectively. In the organically amended soils, we observed that microbial biomass carbon (MBC) and nitrogen (MBN) increased by 43% and 41%, respectively, compared to inorganic fertilizer application alone. Moreover, the soil enzyme activity increased, revealing enhanced biological activity (dehydrogenase—31.96 μg TPF, Urease—35.69 μg urea‐N, glucosidase—57.78 μg p‐nitrophenol, fluorescein diacetate hydrolysis—45.84 μg fluorescein, N‐acetyl β‐glucosaminidase—46.47 μg p‐nitrophenol released g−1 dry soil h−1). We determined that PMN‐I was affected by the soil urease activity; whereas, PMN‐II was regulated by soil organic carbon, MBC, MBN, dehydrogenase activity and fluorescein diacetate hydrolysing capacity. These findings elucidate the soil biological indicators associated with nitrogen (N) mineralization, which can help optimize INM practice.

Conservation agriculture affects soil organic matter distribution, microbial metabolic capacity and nitrogen turnover under Danish field conditions

Conservation agriculture (CA) has been reported to affect nutrient cycling. This study aims to investigate how CA induced soil organic matter stratification affects carbon and nitrogen turnover. A case farm study was established on two Danish farms with conventional ploughed tillage (P) and CA practises. Here, we studied how organic matter stratification patterns to 50 cm soil depth differed between the two systems. Further we investigated differences in carbon and nitrogen mineralization patterns in lab incubation experiments. Average stratification ratio, the ratio between soil C and N content in the upper 5 cm and at 20–30 cm, the depth of the plough layer in the ploughed system, was 1.86 and 1.61 under CA and 1.04 and 1.06 under P. Carbon respiration from intact soil core incubation was affected by soil total carbon content, and showed stronger stratification in CA than in P. Nitrogen mineralization rates from intact soil core incubation was largest in CA top-layer compared to CA 13.5–16.5 cm layer and both P soil layers, with initial mineralization followed by immobilization during the second half of the a four-week incubation. Net change in mineral nitrogen after incubation was only apparent in the 13.5–16.5 cm layer in P, with an average N mineralization rate of 0.08 mg N kg−1 soil d−1. Sieving to 2 mm did not affect N mineralization dynamics. Field-based ammonium to nitrate ratio was higher in CA than in P soils, across varying soil depths and time-points over the entire year. Soil acidity was reduced by one pH unit in CA compared to P. Microbial metabolic capacity was significantly larger in the top 5 cm of CA from the deeper depths, and from the P soils. In conclusion, carbon and nitrogen mineralization, as well as microbial metabolic capacity were strongly affected by an increased stratification ratio of organic matter in CA.

Nitrogen mineralization patterns in Populus deltoides and Tectona grandis based agrisilvicultural practices in Central Himalaya, India

The present study is an attempt to access the effect of two different silvicultural tree species on the process of nitrogen mineralization. Seasonal and depth wise variation in nitrogen mineralization of Populus deltoides (PAS) and Tectona grandis (TAS) based agrisilvicultural systems were studied using buried-bag technique. N-mineralization and nitrification rates differ significantly across the sites and seasons. Net N-mineralization rate varied from 5.88 ± 0.41 to 7.46 ± 1.44 µg g−1 month−1. Ammonium and nitrate concentration decreased with increasing soil depth at both the sites. The annual N-mineralization and nitrification rates were high in PAS than TAS system. The results suggested that variations in rate of N-mineralization and nitrification in the studied agrisilvicultural systems are related to differences in soil moisture content, nutrient status and vegetation cover in combination with other environmental factors.

Peanut residue distribution gradients and tillage practices determine patterns of nitrogen mineralization

Peanut (Arachis hypogaea L.) harvest practices create residue distribution gradients in the field that lead to spatial and temporal variability in available nitrogen (N) during subsequent crop growth. The objective of this study was to quantify N mineralization from peanut residue loading rates that are reflective of postharvest residue distribution gradients under simulated conventional and conservation tillage. A field study was conducted in Florida, USA beginning in September 2015. Fresh shoot residues were placed in litterbags at loading rates (1.1, 2.2, 4.5, and 6.7 Mg ha−1 on an air-dry weight basis) that were based on the residue distribution gradient measured in the field following harvest. Three tillage scenarios were simulated by placing litterbags on the soil surface (no-till) or burying them at 0.10 m depth in fall or spring (fall and spring tillage, respectively). Litterbags were retrieved periodically over 365 days. Buried residues mineralized N faster than surface residues, even when buried residues had high levels of recalcitrant fractions, as was the case with spring-incorporated residues. Exponential models predicted that during a wheat (Triticum aestivum L.) crop, residue loading rates of 1.1, 2.2, 4.5, and 6.7 Mg ha−1 would mineralize 3–6, 5–12, 24–34, and 35–41 kg N ha−1, respectively, depending on tillage practice. At the same loading rates, N mineralization estimates dropped to 2–4, 3–6, 9–11, and 5–18 kg N ha−1 during a hypothetical cotton (Gossypium hirsutum L.) crop planted the following spring. These results suggest that peanut harvest and tillage practices cause large spatial and temporal variability in available N following harvest and may partially explain inconsistencies and spatial variability in subsequent crop performance when peanut residues are relied upon as a N source and mineral N fertilization is reduced.

Patterns of Nitrogen Mineralization under Two Water Regimes in an Acidic Entisol as Influenced by Chemical Composition of Plant Residues

ABSTRACTA 12-week laboratory incubation experiment was conducted to determine the effect of chemical composition of residues of nine plant species on their cumulative net nitrogen (N) mineralization in soil under submergence (SUB) and field capacity (FC) water conditions. The influence of residues and of water regimes on residue-N released over time was significant. Net mineralization was higher in FC than SUB especially in later periods, although differences either among soil water levels or among species were not significant in all the periods sampled. Net N release had significant correlations with initial N and ratios of carbon, lignin or polyphenol or the (lignin + polyphenol) to N concentration of residues under two water conditions. (Lignin + polyphenol):N ratio in SUB and carbon/nitrogen (C:N) ratio in FC showed highest correlations with N mineralization in all periods and explained 64.1% variation in N mineralization from residues under SUB and 64.5% under FC after 2 and 8 weeks, respectively.

Effects of Soil Moisture and Temperature on Carbon and Nitrogen Mineralization in Grassland Soils Fertilized with Improved Cattle Slurry Manure with and without Manure Additive

A seven week laboratory incubation experiment was carried out to determine the effect of three levels of soil moisture and temperature on carbon and nitrogen mineralization in grassland soils fertilized with improved cattle slurry manure with and without manure additive. The result indicated that soils with higher total soil organic carbon and lower CN ratios had higher dissolved organic carbon, higher soil microbial respiration and nitrogen mineralization. Soils treated with improved cattle slurry manure with manure additive have similar carbon and nitrogen mineralization patterns. Soil moisture affected soil microbial respiration and dissolved organic carbon through its effect on oxygen availability. However, soil moisture did not influence nitrogen mineralization. Soil temperature also affected carbon and nitrogen mineralization through its effect on degradation of active pool of soil organic matter and production of stable organic pool that are resistant to microbial degradation. From this study it is concluded that carbon and nitrogen mineralization in grassland soils fertilized with cattle slurry manure with manure additive behaved similarly with those grassland soils treated with cattle slurry manure without manure additive. Moreover, soil moisture affected only carbon mineralization whereas soil temperature affected both carbon and nitrogen mineralization in grassland soils.

Carbon and nitrogen mineralization patterns of two contrasting crop residues in a Mollisol: Effects of residue type and placement in soils

Predicting carbon (C) mineralization of crop residues returned to soils is important for forecasting carbon dioxide (CO2) emissions into the atmosphere and soil nitrogen (N) availability. In this study, a laboratory incubation experiment was conducted to investigate C mineralization of residues of soybean (Glycine max), maize (Zea mays), and their mixture placed on the soil surface and incorporated into the soils in a Mollisol in northeast China. Both the residue type and placement significantly affected C and N mineralization, while no significant interactions between them were observed on cumulative C mineralization. The soybean residue had a higher decomposition rate than the maize residue regardless of their placements; decomposition rates for both soybean and maize residues placed on the soil surface were higher than those of the same residues incorporated into the soils. Moreover, non-additive effects on the contribution of each residue type to C mineralization of the residue mixture were not observed. Our results suggest that crop residue with a low N concentration and a high C/N ratio (such as maize) can be incorporated into soils to immobilize N and decrease CO2 emissions in comparison with crop residue placed on the soil surface.

Patterns of Nitrogen Mineralization in Wetlands of the New Jersey Pinelands along a Shallow Water Table Gradient

Abstract Nitrogen dynamics in wetlands are often assumed to vary with water tables and the resulting patterns of soil moisture, and other soil properties. To test this hypothesis, we have examined N mineralization patterns in two replicate sequences of three forested wetland types located on a shallow water table gradient in the New Jersey Pinelands during a 12 mo sampling period. Each sequence of the three forested wetlands consists of pine wetlands and pine-hardwood swamps with sandy mineral soils and cedar swamps with peat soils along the shallow water table gradient. Although water tables differed between the two mineral-soil wetlands, there were no differences in soil properties between them, including patterns of extractable N or net N mineralization rate. However, peat soils from the cedar swamps had net N mineralization rate 5–10 times higher than the mineral soils from the other two types of wetlands over the sampling period. Although soil moisture was correlated with water table position within ...

In Situ Effect of Some Tropical Litters on N Mineralization

ABSTRACT Soil nitrogen mineralization patterns were investigated under field conditions in the presence of five leaf litters of different qualities, Faidherbia albida, Azadirachta indica, Andropogon gayanus, Casuarina equisetifolia, and Eragrostis tremula. The experimentation was conducted in a tropical sandy soil in central Senegal over 12 months. No relationship could be drawn between litter quality (N content, cellulose, hemicellulose, lignin) and N mineralization during this field experimentation. By contrast, the presence of the litter modified the soil N cycle. At all sampling dates, the patterns of N mineralization were most high in litter amendment. The content of mineral N was highest in Casuarina equisetifolia amendment (17.03 μg N g−1 soil in November 2000, and 28.53 μg N g−1 soil in March 2001). Soil incubated in the absence of the different litters exhibited a different pattern of N mineralization. Net mineralization was observed when the soil had been previously submitted to the influence of Casuarina equisetifolia (4.87 μg N g−1 soil), while net immobilization was observed in the case of Azadirachta indica (8.13 μg N g−1 soil).

Nitrogen Release from Crop Residues and Organic Amendments as Affected by Biochemical Composition

Although amounts and patterns of nitrogen mineralization from decomposing crop residues and other organic materials are known to be affected by their initial chemical composition and quality (e.g., C/N ratio, lignin and soluble polyphenol concentrations), published results differed in which properties correlated best with the N release. Six crop residues (leguminous and non‐leguminous) and two organic materials (spent mushroom compost and dairy pond sludge) were incubated to determine the release of mineral N and its relationships to residue chemical composition. A previously proposed equation for calculating plant residue quality index (PRQI) was modified to PRQIM (Plant residue quality index‐modified) to include C/N, lignin/N and polyphenol/N ratios of crop residues and organic materials so as to integrate these three major variables, which control the residue decomposition and nutrient release. The PRQIM was significantly and highly correlated with N release not only for the data obtained in the present study but also for three independent data sets obtained from the literature for a wide variety of organic materials ranging from arable crop residues, tropical savanna woodland litter and leguminous tree prunings.

Spatial patterns of nitrogen mineralization, fertilizer distribution and roots explain nitrate leaching from mature Amazonian oil palm plantation

Abstract. The African oil palm (Elaeis guineensis) is relatively well adapted to the pedoclimatic conditions of central Amazonia. The clayey upland soils of the region are well supplied with nitrogen, although they are deficient in most other nutrients. Under these conditions, oil palm does not respond to N fertilization with yield increases. In this research, the N status of a central Amazonian upland soil was evaluated after having supported a productive oil palm plantation for 15 years without N fertilization. Mineral N in the upper 2 m of soil showed pronounced spatial patterns, with very low concentrations close to the palms, indicative of efficient N uptake by the palms, and evidence for nitrate leaching into the subsoil in the inter‐tree spaces despite the near‐absence of a leguminous cover crop during the previous ten years. The pronounced increases of mineral N with increasing tree distance were explained by increases in N mineralization and a strong decrease in fine root length density of the palms, especially in the subsoil. Failure of the palms to fully occupy the available soil volume with their roots was apparently related to fertilizer placement close to the stem base, which over the years had led to steep fertility gradients between the soil under the trees and the inter‐tree spaces. Broadcast fertilization would have presumably favoured a more extensive lateral root development of the palms, and consequently improved nutrient and water uptake from the inter‐tree spaces. The incomplete soil occupation by the palm roots also suggests that young oil palms can be associated with shade tolerant crops without much risk of root competition. These conclusions may be valid also for other tree crops and may help to reduce nitrate leaching and consequently the need for N fertilization in Amazonian tree crop agriculture.

Interactions between residues of maize and pigeonpea and mineral N fertilizers during decomposition and N mineralization

Nitrogen mineralization patterns of maize and pigeonpea ( Cajanus cajan) residues were examined in leaching tubes, both in isolation and mixtures, in Malawian soils of varying texture. Senesced pigeonpea leaves (C-to-N ratio 24) induced a short period of nitrogen immobilization which was followed by steady net nitrogen mineralization in all of three soils. The immobilization period lasted between 14 and 28 days and was longer in soils with larger clay contents. Maize residues contained 30% of their N in the form of water-soluble nitrate. Both the sole maize residue (C-to-N ratio 75 after adjustment for nitrate which constituted 28% of the N) and the mixture of maize and senesced pigeonpeas leaves revealed a similar prolonged strong net N immobilization up to 130 days before the two treatments started to diverge slightly. Mixing maize with pigeonpea residues with equal amounts of N failed to substantially alleviate the N immobilization capacity of the maize residues. N immobilization in the mixture was much greater than that predicted from the mineralization patterns of the individual components. When increasing amounts (50, 100 and 150 mg N kg −1 soil) of green pigeonpea leaves, senesced pigeonpea leaves and ammonia-N were added to 50 mg N kg −1 soil of maize residues, N released in the mixtures increased with the increasing amounts of N added to the maize residues with greater increases from residues with larger N concentrations. There was evidence that microbial degradation of maize carbon was limited by N availability. The implications of the results for management of crop residues and mineral N fertilizers in the field are discussed.

Landscape patterns of nitrogen mineralization and nitrification in southern Ohio hardwood forests

This study quantified nitrogen mineralization and nitrification potentials in soils of hardwood forests of southern Ohio at three spatial scales: (1) the regional scale, represented by four study areas of 90–120 ha separated by 3–65 km, (2) the local scale, represented by three contiguous watersheds within each study area, and (3) the topographic scale, represented by xeric, intermediate, and mesic sites within each watershed, as defined by a GIS-generated Integrated Moisture Index (IMI). Organic C, NO3 pool size, net N mineralization, proportional nitrification, and net nitrification potentials all varied among study sites (i.e. at the regional scale). Using path analysis, we were able to construct scale-independent causal models explaining 30–35% of the variance in organic C and potential net N mineralization and ≥70% of the variance in potential net NO3 production. Site- and scale-specific differences in geology and/or land use history among study sites were likely responsible for the variation not explained by the path analysis. At the local scale, there were significant variations in organic C and inorganic N pool sizes among watersheds within a study site in two of the four study sites. In addition, most parameters we measured varied significantly along the topographic gradient (i.e. with long-term soil moisture availability/IMI). Based on our results, scaling up models of nitrification from plot scale to the regional scale should be straightforward, whereas scaling up organic C storage and N mineralization will require incorporation of independent scaling paradigms at three (or more) spatial scales.

Decomposition- and nitrogen-mineralization patterns of Leucaena leucocephala and Cassia siamea mulch under tropical semiarid conditions in Kenya

In agroforestry systems, loppings from trees and shrubs are commonly used, often as mulch, as sources of nutrients for interplanted crops. Therefore, it is important to understand the rates of mulch decomposition. This paper reports the results of a study on the decomposition and nitrogen (N)-mineralization patterns of the leaves, small twigs, and mulch (leaves plus twigs) of Leucaena leucocephala and Cassia siamea in a field experiment in an Alfisol in semiarid Kenya. Plant materials contained in 5mm nylon bags were placed below or on the soil surface in an alleycropping system involving the two tree species, with maize (Zea mays L.) as the intercrop. For plant materials of both species (except Leucaena leaves), there were two phases of decompositions: an initial rapid phase and a slower second one; Leucaena leaves had only a single phase. When placed below the soil surface, the decomposition rates of both Leucaena and Cassia mulch were similar (about 12% week-1 in the first phase and 1% week-1 in the second phase). When placed on the soil surface, Leucaena mulch decomposed 1.3 times more rapidly than Cassia mulch in the first phase. The patterns of N release from the mulch of both species were similar to those of mass loss. In general, mulch-decomposition and N-release rates of both species were related more to their C:N ratios than to polyphenol contents; while C:N ratio predicted the rate of first (rapid) phase, the rate of the second (slow) phase appeared to be regulated by lignin and polyphenol contents.

Modelling impacts of land use change and climate change on nitrate-nitrogen in the River Don, North East Scotland

The river quality model QUASAR (Quality Simulation Along Rivers) has been applied to the River Don system in North East Scotland. This mass balance model for nitrogen has been used to estimate the impacts of land use change and climate change on river water quality. Changing agricultural patterns have been simulated and indicate increased nitrogen levels in the river as catchment land use changes. Climate change will alter flow regimes, temperature and nitrogen mineralization patterns. Simulation runs for a range of scenarios illustrate the impacts of climate change with the most significant effect being mineralization of nitrogen in the soil feeding through into the river system. These higher nitrogen levels are reduced slightly by the increased temperatures and decreased summer flows, both of which enhance denitrification processes.

Rates of soil acidification under different patterns of nitrogen mineralization

Abstract To evaluate the relative significance of acid deposition in a forest-soil system, H+-generating and -consuming conditions in the whole system were quantified based on the determination of the H+ budget. The two sites selected for this study displayed different rates of nitrification, which affected significantly the rates of soil acidification. Since at one site (MIKAMI), 60% of the total acid load was attributed to nitrification, the depletion of the soil acid-neutralizing capacity (ANC(s)) was three and a half times faster than that of the other site (YASU). The decreasing rates of ANC(s) were 5.9 kmol ha−1 y−1 at MIKAMI and 1.7 kmol ha−1 y−1 at YASU, which are classified as sites with “intermediate” and “low” rates of soil acidification, respectively. Atmospheric input of H+ accounted for only a small part (< 10%) of the total acid load. Comparison of the drainage at two depths (40 and SO cm) provided information about the mechanisms by which dissolved AI3+ was retained in soil. The YASU soil showed a high sulfate adsorption capacity, which resulted in rise of the solution pH by one unit, and subsequently in the precipitation of dissolved Al3+ as jurbanite. In contrast, at MIKAMI, acid neutralization did not occur in the soil at 40- to 80-cm depths, but the cation exchange reaction decreased the concentration of dissolved Al3+.

Nitrogen mineralization from leaves and litter of tropical plants: Relationship to nitrogen, lignin and soluble polyphenol concentrations

Rates and patterns of nitrogen mineralization from decomposing plant materials are known to be affected by initial concentrations of N, lignin and soluble polyphenols, but published results show differences in which properties correlated best with N release. We incubated soil with fresh leaves and litter from 12 species commonly used in tropical agroforestry systems, including both legumes and non-legumes, to estimate how release of mineral N was affected by chemical composition. After 16 weeks, net accumulation of mineral N in incubations ranged from 81% of initial N in fresh Sesbania sesban to net depletion equivalent to 70% of initial N in Casuarina equisetifolia litter. Fresh legume leaves generally had net N accumulation, whereas legume litter and non-legume fresh leaves and litter had net depletion. At all sampling dates, the percent of leaf N accumulated was most highly correlated with initial N concentration. Phosphorus and the ratios lignin:N and (lignin + polyphenol):N also strongly correlated with N accumulation, although these indices themselves were correlated with N. We conclude that differences among previous studies in best chemical predictors of mineralization rates arose from relatively small ranges in chemical composition of materials used. Over our wide range of materials, initial soluble polyphenols were secondary to initial N, which explained most of the variation in N accumulation or depletion.

Nitrogen mineralization patterns of leaf-twig mixtures from tropical leguminous trees

Green manure applications in alley cropping systems often include twigs despite their potential to absorb (immobilize) nitrogen (N). To assess the impact of twigs on net N mineralization or immobilization from hedge row cuttings, we separated cuttings fromCalliandra calothyrsus andGliricidia sepium into leaf-only, twig-only, and mixed (leaf + twig) fractions and incubated them with moist soil in the laboratory. Soil extractable inorganic N did no differ among treatments after two weeks, but after four and eight weeks was greatest in leaf-only, and least in twig-only treatments. After two weeks, extractable N from the leaf-only treatment rose steadily, while that from the twig-only and mixed treatments was variable due to periods of net mineralization and net immobilization. The pattern of variation in mixed treatments paralleled that of twig-only, indicating that net immobilization in the mixture was largely caused by the presence of twigs. Extractable N from the mixture was somewhat lower than that predicted from the sum of leaf-only and twig-only treatments. We conclude that twigs in green manure reduce short-term N availability to associated crops in agroforestry systems.

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.

Patterns of nitrogen mineralization and nitrification in floodplain successional soils along the Tanana River, interior Alaska

Cold climatic conditions govern the productivity of taiga forests, yet within a successional sequence the microclimatic and biogeochemical variations also have a major effect on soil microbial activities, thus affecting plant productivity through nutrient availability. Nitrogen mineralization and nitrification were measured in primary-successional floodplain forests of interior Alaska. Forest floor and mineral soils from an early (open willow), middle (poplar–alder), and late (white spruce) successional stage were used. The effects of temperature, moisture, and NH4+ were tested in the laboratory for each of the successional stages. Potential nitrification was estimated using the chlorate-inhibition technique. Surface mineral soils and white spruce forest floor had low to undetectable rates of nitrogen mineralization and nitrification (&lt;1–3 μg N•g−1•d−1). The poplar–alder forest floor had the most pronounced seasonal patterns and the highest rates of net NH4+ mineralization (&lt;1–7 μg N•g−1•d−1) and net nitrification (&lt;1–21 μg N•g−1•d−1). Temperature was limiting in early and midsuccessional stages, and both moisture and temperature were limiting in the later white spruce stage. Ammonium additions increased nitrification only in the poplar–alder forest floor, suggesting the NH4+ is not limiting in the other successional stages. The chlorate inhibition assay indicated that a considerable portion of the nitrification in the poplar–alder forest floor may be due to heterotrophic activity.