Net Nitrogen Mineralization Research Articles

Effects ofin situfreezing on soil net nitrogen mineralization and net nitrification in fertilized grassland of northern China

AbstractEffects of soil freezing on nitrogen (N) mineralization have been the subject of increased attention in the ecological literature, though fewer studies have examined N mineralization responses to successive mild freezing, severe freezing and cyclic freeze–thaw events. Even less is known about relationships of responses to soil N status. This study measured soil N mineralization and nitrification in the field along an experimental N gradient in a grassland of northern China during the dormant season (October 2005–April 2006), a period in which freezing naturally occurs. Net N mineralization exhibited great temporal variability, with nitrification being the predominant N transformation process. Soil microbial biomass C and N and extractable NH4 + pools declined by 40, 52, and 56%, respectively, in April 2006, compared with their initial concentrations in October 2005; soil NO3–pools increased by 84%. Temporal patterns of N mineralization were correlated with soil microbial biomass C and N. N mineralization and nitrification increased linearly with added N. Microbial biomass C in treated soils increased by 10% relative to controls, whereas microbial N declined by 9%. Results further suggest that freezing events greatly alter soil N dynamics in the dormant season at this site, with considerable available N accumulating during this period.

Understanding plant-microbe interactions for phytoremediation of petroleum-polluted soil.

Plant-microbe interactions are considered to be important processes determining the efficiency of phytoremediation of petroleum pollution, however relatively little is known about how these interactions are influenced by petroleum pollution. In this experimental study using a microcosm approach, we examined how plant ecophysiological traits, soil nutrients and microbial activities were influenced by petroleum pollution in Phragmites australis, a phytoremediating species. Generally, petroleum pollution reduced plant performance, especially at early stages of plant growth. Petroleum had negative effects on the net accumulation of inorganic nitrogen from its organic forms (net nitrogen mineralization (NNM)) most likely by decreasing the inorganic nitrogen available to the plants in petroleum-polluted soils. However, abundant dissolved organic nitrogen (DON) was found in petroleum-polluted soil. In order to overcome initial deficiency of inorganic nitrogen, plants by dint of high colonization of arbuscular mycorrhizal fungi might absorb some DON for their growth in petroleum-polluted soils. In addition, through using a real-time polymerase chain reaction method, we quantified hydrocarbon-degrading bacterial traits based on their catabolic genes (i.e. alkB (alkane monooxygenase), nah (naphthalene dioxygenase) and tol (xylene monooxygenase) genes). This enumeration of target genes suggests that different hydrocarbon-degrading bacteria experienced different dynamic changes during phytoremediation and a greater abundance of alkB was detected during vegetative growth stages. Because phytoremediation of different components of petroleum is performed by different hydrocarbon-degrading bacteria, plants’ ability of phytoremediating different components might therefore vary during the plant life cycle. Phytoremediation might be most effective during the vegetative growth stages as greater abundances of hydrocarbon-degrading bacteria containing alkB and tol genes were observed at these stages. The information provided by this study enhances our understanding of the effects of petroleum pollution on plant-microbe interactions and the roles of these interactions in the phytoremediation of petroleum-polluted soil.

Biochemical characteristics of solid fractions from animal slurry separation and their effects on C and N mineralisation in soil

Solid fractions from separated animal slurry can be used as organic fertilisers on agricultural land. Solid fractions contain variable amounts of inorganic and organic N, so it is important to synchronise their application in the field with crop demand to ensure N availability in the growing season. This study quantified C and N mineralisation for a wide range of solid fractions from slurry separation applied to soil and examined potential correlations between chemical and biochemical characteristics of solid fractions and their C and N turnover. The solid fractions were mixed with soil and incubated at 14°C for 120 days, during which CO2 evolution and inorganic N content of the solid fractions were determined. A two-parameter exponential function fitted to the individual solid fraction C mineralisation patterns explained 98% of the data variation, while a three-parameter Monod-type equation fitted to the net N mineralisation patterns explained 89% of the variation. Between 5% and 45% of initially added C was mineralised within the incubation period, with the largest proportion tending to be mineralised from simple mechanically separated solid fractions (MEC). Nitrogen was initially immobilised by the majority of solid fractions. Solid fractions from decanter centrifuged, anaerobically digested slurry (DEC) and chemically pre-treated and separated slurry (KEM) began to re-mineralise N after 20–40 days, whereas N was continuously immobilised from MEC solid fractions. The carbon mineralisation rate constant was correlated with the C content in the neutral detergent soluble (NDS), hemicellulose and cellulose fractions and the N content in NDS. Net nitrogen mineralisation was correlated with the C/Norg ratio of solid fractions, the N content of NDS and the C content of hemicellulose and cellulose.

Measurement of Net Nitrogen and Phosphorus Mineralization in Wetland Soils Using a Modification of the Resin‐Core Technique

A modification of the resin‐core method was developed and tested for measuring in situ soil N and P net mineralization rates in wetland soils where temporal variation in bidirectional vertical water movement and saturation can complicate measurement. The modified design includes three mixed‐bed ion‐exchange resin bags located above and three resin bags located below soil incubating inside a core tube. The two inner resin bags adjacent to the soil capture NH4+, NO3−, and soluble reactive phosphorus (SRP) transported out of the soil during incubation; the two outer resin bags remove inorganic nutrients transported into the modified resin core; and the two middle resin bags serve as quality‐control checks on the function of the inner and outer resin bags. Modified resin cores were incubated monthly for a year along the hydrogeomorphic gradient through a floodplain wetland. Only small amounts of NH4+, NO3−, and SRP were found in the two middle resin bags, indicating that the modified resin‐core design was effective. Soil moisture and pH inside the modified resin cores typically tracked changes in the surrounding soil abiotic environment. In contrast, use of the closed polyethylene bag method provided substantially different net P and N mineralization rates than modified resin cores and did not track changes in soil moisture or pH. Net ammonification, nitrification, N mineralization, and P mineralization rates measured using modified resin cores varied through space and time associated with hydrologic, geomorphic, and climatic gradients in the floodplain wetland. The modified resin‐core technique successfully characterized spatiotemporal variation of net mineralization fluxes in situ and is a viable technique for assessing soil nutrient availability and developing ecosystem budgets.

The Influence of Condensed Tannin Structure on Rate of Microbial Mineralization and Reactivity to Chemical Assays

We examined how tannin structure influences reactivity in tannin assays and carbon and nitrogen mineralization. Condensed tannins from the foliage of ten tree and shrub species and from pecan shells (Carya illinoensis) had different proportions of: (a) epicatechin (cis) and catechin (trans) isomers, (b) procyanidin (PC) and prodelphinidin (PD) monomers, and (c) different chain lengths. The response of each tannin to several widely used tannin assays was determined. Although there was some variation in response to proanthocyanidin (butanol/HCl) and Folin Ciocalteu assays, we did not deduce any predictable relationship between tannin structure and response to either assay. There was little variation in protein precipitation among the different tannins. To assess biological activity, six of the tannins were incubated with forest humus for 22days. We determined that, while PC-based tannins remained at least partly extractable for the duration of the incubation, tannins with a high proportion of PD subunits rapidly became unextractable from soil. There was a positive correlation between net nitrogen mineralization and cis chemical structure. Carbon mineralization was enhanced initially by the addition of tannins to humus, but after 22days, a negative correlation between the proportion of cis subunits and respiration was determined. Overall, we were not able to demonstrate consistent effects of structure on either microbial mineralization or reactivity to chemical assays; such relationships remain elusive.

Imaging-optode measurements of ammonium distribution in soil after different manure amendments

The choice of manure application technique can affect both the spatial distribution of ammonium in soil and net nitrogen (N) mineralization, and thereby N availability to crops. In this study we compared net N mineralization and spatial ammonium distribution after different degrees of incorporation of solid chicken manure and cattle slurry into soil. Ammonium-specific fluorescing optodes were assembled with manure applied to soil in closed chambers and the spatial distribution of ammonium in different treatments was measured for 2 weeks. The results indicated that much ammonium from the manures was quickly adsorbed to clay particles. Consequently, the ammonium concentration in the soil solution was threefold higher in the sandy soil than in the clay soil studied. Ammonium was distributed over a larger soil volume from manure applied below the soil surface than from manure applied above. Because the optodes excluded ammonium adsorbed to soil particles, net N mineralization was instead studied in separate incubations using extraction with potassium chloride solution for determination of ammonium and nitrate. When manure was kept concentrated in lumps rather than being mixed with soil, nitrate levels were about five times smaller after 1 week and 5–10% more of the manure N occurred as mineral N after 2 weeks. There were no differences in net N mineralization between surface application and subsurface incorporation. In this study a new technique to visualize and measure ammonium patterns around manure in soil proved to be useful for evaluating ammonium distribution and adsorption, but net N mineralization required incubations.

Calcium and phosphorus interact to reduce mid-growing season net nitrogen mineralization potential in organic horizons in a northern hardwood forest

Acid deposition can deplete soil calcium (Ca) and be detrimental to the health of some forests. We examined effects of soil Ca and phosphorus (P) availability on microbial activity and nitrogen (N) transformations in a plot-scale nutrient addition experiment at the Hubbard Brook Experimental Forest in New Hampshire, USA. We tested the hypotheses that (1) microbial activity and N transformations respond to large but not small changes in soil Ca, (2) soil Ca availability influences net N mineralization via the immobilization of N, rather than via changes in microbial activity, and (3) the response to Ca is constrained by P availability. Seasonality was a primary influence on the microbial response to treatments; N cycling processes varied from May to October and treatment effects were only detectable in the mid-growing season, in July. Neither microbial activity (C mineralization) nor gross N mineralization responded to Ca or to P, in either horizon. In the Oa horizon in July net N mineralization was reduced by high Ca and by Ca + P, and gross nitrification was increased by P addition. In the Oe horizon in July net N mineralization was reduced by Ca + P. These results partially supported our hypotheses, suggesting that soil Ca depletion has the potential to increase mid-growing season N availability via subtle changes in N immobilization, and that this effect is sensitive to soil P chemistry. The horizon-specific nature of the responses that we detected suggests that the proportions of Oe and Oa horizons comprising the surface organic layer will influence the relative importance of these processes at the ecosystem scale. Our results highlight the need for further attention to seasonal changes in controls of microbial mineralization/immobilization processes, to functional differences between organic horizons, and to interactions between Ca and P in soils, in order to learn the specific mechanisms underlying the influence of Ca status on nutrient recycling in these northern hardwood ecosystems. ► Calcium addition reduced net N mineralization in the Oa horizon, by increasing N immobilization. ► Phosphorus availability interacted with Ca to reduce net N mineralization in Oe and Oa horizons. ► Soil acidification may be increasing net N mineralization in forests of the northeastern USA. ► These effects are likely sensitive to soil P and proportions of Oe and Oa in the forest floor.

Bottom-Up Factors Influencing Riparian Willow Recovery in Yellowstone National Park

After the elimination of wolves (Canis lupis L.) in the 1920s, woody riparian plant communities on the northern range of Yellowstone National Park (YNP) declined an estimated 50%. After the reintroduction of wolves in 1995-1996, riparian willows (Salix spp.) on YNP's northern range showed significant growth for the first time since the 1920s. However, the pace of willow recovery has not been uniform. Some communities have exceeded 400 cm, while others are still at pre-1995 levels of 250 cm max. height) willow sites where willows had escaped elk (Cervus elaphus L.) browsing with willow sites that could still be browsed. Unlike studies that manipulated willow height with fences and artificial dams, we examined sites that had natural growth differences in height since the reintroduction of wolves. Tall willow sites had greater water availability, more-rapid net soil nitrogen mineralization, greater snow depth, lower soil respiration rates, and cooler summer soil temperatures than nearby short willow sites. Most of these differences were measured both in herbaceous areas adjacent to the willow patches and in the willow patches themselves, suggesting that they were not effects of varying willow height recovery but were instead preexisting site differences that may have contributed to increased plant productivity. Our results agree with earlier studies in experimental plots which suggest that the varying pace of willow recovery has been influenced by abiotic limiting fac- tors that interact with top-down reductions in willow browsing by elk.

Carbon dynamics of terrestrial ecosystems on the Tibetan Plateau during the 20th century: an analysis with a process‐based biogeochemical model

ABSTRACTAim The Tibetan Plateau accounts for about a quarter of the total land area of China and has a variety of ecosystems ranging from alpine tundra to evergreen tropics. Its soils are dominated by permafrost and are rich in organic carbon. Its climate is unique due to the influence of the Asian monsoon and its complex topography. To date, the carbon dynamics of the Tibetan Plateau have not been well quantified under changes of climate and permafrost conditions. Here we use a process‐based biogeochemistry model, the Terrestrial Ecosystem Model (TEM), which was incorporated with a soil thermal model, to examine the permafrost dynamics and their effects on carbon dynamics on the plateau during the past century.Location The Tibetan Plateau.Methods We parameterize and verify the TEM using the existing data for soil temperature, permafrost distribution and carbon and nitrogen from the region. We then extrapolate the model and parameters to the whole plateau.Results During the 20th century, the Tibetan Plateau changed from a small carbon source or neutral in the early part of the century to a sink later, with a large inter‐annual and spatial variability due to changes of climate and permafrost conditions. Net primary production and soil respiration increased by 0.52 and 0.22 Tg C year−1, respectively, resulting in a regional carbon sink increase of 0.3 Tg C year−1. By the end of the century, the regional carbon sink reached 36 Tg C year−1 and carbon storage in vegetation and soils is 32 and 16 Pg C, respectively. On the plateau, from west to east, the net primary production, soil respiration and net ecosystem production increased, due primarily to the increase of air temperature and precipitation and lowering elevation. In contrast, the decrease of carbon fluxes from south to north was primarily controlled by precipitation gradient. Dynamics of air temperature and associated soil temperature and active layer depth resulted in a higher plant carbon uptake than soil carbon release, strengthening the regional carbon sink during the century.Main conclusions We found that increasing soil temperature and deepening active layer depth enhanced soil respiration, increasing the net nitrogen mineralization rate. Together with the effects of warming air temperature and rising CO2 concentrations on photosynthesis, the stronger plant nitrogen uptake due to the enhanced available nitrogen stimulates plant carbon uptake, thereby strengthening the regional carbon sink as the rate of increase net primary production was faster than that of soil respiration. Further, the warming and associated soil thermal dynamics shifted the regional carbon sink from the middle of July in the early 20th century to early July by the end of the century. Our study suggests that soil thermal dynamics should be considered for future quantification of carbon dynamics in this climate‐sensitive region.

Decomposição de serrapilheira em bosque de sabiá na Zona da Mata de Pernambuco

Objetivou-se avaliar a decomposição de frações de serrapilheira de sabiá (Mimosa caesalpiniifolia Benth) utilizando-se a técnica de sacos de náilon. Foram incubadas as seguintes frações de serrapilheira: folha senescente, folha no início da mineralização e ramos com até 20 mm de diâmetro. A incubação foi realizada nos períodos de 0, 4, 8, 16, 64, 100 e 256 dias nos anos de 2006 e 2007. As frações foram distribuídas em blocos ao acaso com cinco repetições. Foram avaliados os desaparecimentos de biomassa, nitrogênio (N) e fósforo (P), as concentrações de nitrogênio e fósforo e a relação carbono/ nitrogênio da serrapilheira ao longo dos períodos de incubação. De modo geral, o modelo exponencial negativo explicou o desaparecimento de biomassa, nitrogênio e fósforo, todavia, houve variação entre anos e, em alguns casos, apesar de significativos, os modelos apresentaram baixa correlação entre dados observados e preditos. A taxa de desaparecimento de biomassa foi lenta, uma vez que apenas 30% de biomassa de folhas foi mineralizada após 256 dias de incubação. A mineralização líquida de nitrogênio apresentou ampla variação entre anos e diferiu entre as frações estudadas. O teor de nitrogênio da serrapilheira incubada aumentou, em média, até os 32 dias (folhas) e até os 64 dias (ramos) de incubação, estabilizando-se em seguida. Foi usado o modelo platô linear para explicar esse processo. Com o passar dos períodos de incubação, a relação carbono/ nitrogênio diminuiu. Apesar de elevado teor de nitrogênio, a decomposição da serrapilheira de sabiá é lenta, o que pode reduzir as perdas de nutrientes no bosque, aumentando sua sustentabilidade e reduzindo os possíveis efeitos deletérios ao ambiente.

Effects of grazing exclusion on soil net nitrogen mineralization and nitrogen availability in a temperate steppe in northern China

This study was conducted to examine the potential effects of ecological restoration by grazing exclusion on soil net nitrogen (N) mineralization over the growing season in the temperate steppe in this area. Both laboratory and in situ incubations with intact soil cores were used. Strong temporal variability in in situ net nitrification, net mineralization and available soil N were observed. Long-term (25 years) grazing exclusion stimulated in situ net nitrification whereas short-term (5 years) grazing exclusion had no effect. Neither long- nor short-term grazing exclusion affected net mineralization. Available soil N increased with time since grazing exclusion. Laboratory incubation showed that soil temperature affected net mineralization significantly and soil moisture effect depended on soil moisture range. The relationships of net mineralization with soil temperature and soil moisture observed in the laboratory incubation could not explain the temporal dynamics of in situ net mineralization. This study suggests that the grazing exclusion plays a positive role in regulating soil N availability, and thus stimulates the plant growth in the temperate steppe ecosystem in northern China.

Contrasting effects of wood ash application on microbial community structure, biomass and processes in drained forested peatlands

The effects of wood ash application on soil microbial processes were investigated in three drained forested peatlands, which differed in nutrient status and time since application. Measured variables included the concentrations of soil elements and phospholipid fatty acids (PLFAs), net nitrogen (N) mineralization, nitrification and denitrification enzyme activity, potential methane (CH(4)) oxidation, CH(4) production and microbial respiration kinetics. Wood ash application had a considerable influence on soil element concentrations. This mirrored a decrease in the majority of the microbial biomarkers by more than one-third in the two oligotrophic peatlands, although the microbial community composition was not altered. The decreases in PLFAs coincided with reduced net ammonification and net N mineralization. Other measured variables did not change systematically as a result of wood ash application. No significant changes in microbial biomass or processes were found in the mesotrophic peatland, possibly because too little time (1 year) had elapsed since the wood ash application. This study suggests that oligotrophic peatlands can be substantially affected by wood ash for a period of at least 4 years after application. However, within 25 years of the wood ash application, the microbial biomass seemed to have recovered or adapted to enhanced element concentrations in the soil.

Impact of land-use types on soil nitrogen net mineralization in the sandstorm and water source area of Beijing, China

Abstract Changes of land-use type (LUT) can affect soil nutrient pools and cycling processes that relate long-term sustainability of ecosystem, and can also affect atmospheric CO2 concentrations and global warming through soil respiration. We conducted a comparative study to determine NH4+ and NO3− concentrations in soil profiles (0–200 cm) and examined the net nitrogen (N) mineralization and net nitrification in soil surface (0–20 cm) of adjacent naturally regenerated secondary forests (NSF), man-made forests (MMF), grasslands and cropland soils from the windy arid and semi-arid Hebei plateau, the sandstorm and water source area of Beijing, China. Cropland and grassland soils showed significantly higher inorganic N concentrations than forest soils. NO3−-N accounted for 50–90% of inorganic N in cropland and grassland soils, while NH4+-N was the main form of inorganic N in NSF and MMF soils. Average net N-mineralization rates (mg kg−1 d−1) were much higher in native ecosystems (1.51 for NSF soils and 1.24 for grassland soils) than in human disturbed LUT (0.15 for cropland soils and 0.85 for MMF soils). Net ammonification was low in all the LUT while net nitrification was the major process of net N mineralization. For more insight in urea transformation, the increase in NH4+ and, NO3− concentrations as well as C mineralization after urea addition was analyzed on whole soils. Urea application stimulated the net soil C mineralization and urea transformation pattern was consistent with net soil N mineralization, except that the rate was slightly slower. Land-use conversion from NSF to MMF, or from grassland to cropland decreased soil net N mineralization, but increased net nitrification after 40 years or 70 years, respectively. The observed higher rates of net nitrification suggested that land-use conversions in the Hebei plateau might lead to N losses in the form of nitrate.

Experimentally testing the role of foundation species in forests: the Harvard Forest Hemlock Removal Experiment

Summary 1. Problem statement– Foundation species define and structure ecological systems. In forests around the world, foundation tree species are declining due to overexploitation, pests and pathogens. Eastern hemlock (Tsuga canadensis), a foundation tree species in eastern North America, is threatened by an exotic insect, the hemlock woolly adelgid (Adelges tsugae). The loss of hemlock is hypothesized to result in dramatic changes in assemblages of associated species with cascading impacts on food webs and fluxes of energy and nutrients. We describe the setting, design and analytical framework of the Harvard Forest Hemlock Removal Experiment (HF‐HeRE), a multi‐hectare, long‐term experiment that overcomes many of the major logistical and analytical challenges of studying system‐wide consequences of foundation species loss. 2. Study design– HF‐HeRE is a replicated and blocked Before‐After‐Control‐Impact experiment that includes two hemlock removal treatments: girdling all hemlocks to simulate death by adelgid and logging all hemlocks >20 cm diameter and other merchantable trees to simulate pre‐emptive salvage operations. These treatments are paired with two control treatments: hemlock controls that are beginning to be infested in 2010 by the adelgid and hardwood controls that represent future conditions of most hemlock stands in eastern North America. 3. Ongoing measurements and monitoring– Ongoing long‐term measurements to quantify the magnitude and direction of forest ecosystem change as hemlock declines include: air and soil temperature, light availability, leaf area and canopy closure; changes in species composition and abundance of the soil seed‐bank, understorey vegetation, and soil‐dwelling invertebrates; dynamics of coarse woody debris; soil nitrogen availability and net nitrogen mineralization; and soil carbon flux. Short‐term or one‐time‐only measurements include initial tree ages, hemlock‐decomposing fungi, wood‐boring beetles and throughfall chemistry. Additional within‐plot, replicated experiments include effects of ants and litter‐dwelling microarthoropods on ecosystem functioning, and responses of salamanders to canopy change. 4. Future directions and collaborations– HF‐HeRE is part of an evolving network of retrospective studies, natural experiments, large manipulations and modelling efforts focused on identifying and understanding the role of single foundation species on ecological processes and dynamics. We invite colleagues from around the world who are interested in exploring complementary questions to take advantage of the HF‐HeRE research infrastructure.

Effects of summer catch crop, residue management, soil temperature and water on the succeeding cucumber rhizosphere nitrogen mineralization in intensive production systems

Nitrogen nutrient management is crucially important in shallow-rooted vegetable production systems characterized by high input and high environmental risk. To investigate the effects of summer catch crop (sweet corn, common bean, garland chrysanthemum and edible amaranth), residue management, and soil temperature and water on the succeeding cucumber rhizosphere nitrogen mineralization in intensive production systems, we determined the rates of net nitrogen mineralization and nitrification in a 4-year field experiment on greenhouse cucumber double-cropping systems. Summer catch crop and its residue significantly increased the succeeding cucumber rhizosphere mineral nitrogen contents, when compared to conventional practices. In general, summer catch crop and its residue significantly increased the rates of both net nitrogen mineralization and net nitrogen nitrification at 4 or 40°C, and increased the rates of net nitrogen immobilization (negative mineralization) and net nitrogen nitrification at 15 or 28°C, in succeeding cucumber rhizosphere after four-year treatment. Soil temperature and water had more influence than catch crops and residue management on N mineralization. The effect of carbon on nitrogen mineralization was more pronounced than that of nitrogen, and the effect of microbial carbon on the different forms of inorganic N was more pronounced than that of organic carbon. When the effects of soil temperature and water content were eliminated, cumulative net nitrogen mineralization and nitrification in catch crop and residue management plots were 296–784 and 57–84% higher, respectively, than conventional practices plots. Catch crops and residue management influenced change of ammonium-N more significantly than that of nitrate-N. Additionally, there were complex relationships between fruit yield and soil N mineralization in catch crop- and residue management-induced systems.

Net nitrogen mineralization and nitrification of different landscape positions in a lowland subtropical rainforest in Taiwan

Measurements of soil nitrogen (N) transformation rate are needed to be able to assess the plant availability and forest ecosystem losses of N, but little is known about lowland subtropical rainforests. The objectives of the present study were to determine the rates of N mineralization and nitrification in different seasons (January and August 2006) and in different landscape positions (footslope and summit) in the Nanjenshan forest of southern Taiwan, where vegetation types and soil properties vary among different landscape positions. Net N mineralization and nitrification were measured using 28-day in-situ open core and capped core incubations. The results from January 2006 showed that the concentrations of ammonium-N (NH4 +-N), mineral N (NH4 +-N plus NO3 −-N) and nitrate-N (NO3 −-N) were not significantly different between open and capped cores, or between summit and footslope. However, the results of August 2006 showed that the NH4 +-N concentrations of the summit soils were significantly higher than those of the footslope soils (P < 0.05), and that NO3 −-N concentrations in the open cores were significantly lower than those in the capped cores because of higher rainfall in the summer. In general, concentrations of mineral N and NO3 −-N in August were higher than those in January (both P = 0.0003), and NH4 +-N concentrations were significantly different between the different landscape positions (P < 0.05). There were larger soil C and N and microbial N pools at the summit position; however, footslope soils showed higher net N mineralization and nitrification rate as expressed on the basis of per unit C or N. Our results suggested that the substrate properties of the footslope position contributed to the higher net N mineralization and nitrification rate, and that the differences in N transformation rates between the landscape positions appeared to be related to vegetation type.

Soils as agents of selection: feedbacks between plants and soils alter seedling survival and performance

Soils are one of the first selective environments a seed experiences and yet little is known about the evolutionary consequences of plant-soil feedbacks. We have previously found that plant phytochemical traits in a model system, Populus spp., influence rates of leaf litter decay, soil microbial communities and rates of soil net nitrogen mineralization. Utilizing this natural variation in plant-soil linkages we examined two related hypotheses: (1) Populus angustifolia seedlings are locally adapted to their native soils; and (2) Soils act as agents of selection, differentially affecting seedling survival and the heritability of plant traits. We conducted a greenhouse experiment by planting seedlings from 20 randomly collected P. angustifolia genetic families in soils conditioned by various Populus species and measured subsequent survival and performance. Even though P. angustifolia soils are less fertile overall, P. angustifolia seedlings grown in these soils were twice as likely to survive, grew 24% taller, had 27% more leaves, and 29% greater above-ground biomass than P. angustifolia seedlings grown in non-native P. fremontii or hybrid soils. Increased survival resulted in higher trait variation among seedlings in native soils compared to seedlings grown in non-native soils. Soil microbial biomass varied significantly across soil environments which could explain more of the variation in seedling performance than soil texture, pH, or nutrient availability, suggesting strong microbial interactions and feedbacks between plants, soils, and associated microorganisms. Overall, these data suggest that a “home-field advantage” or a positive plant soil feedback helps maintain genetic variance in P. angustifolia seedlings.

Net nitrogen mineralization and nitrification rates in forest soil in northeastern Taiwan

We used a laboratory incubation approach to measure rates of net N mineralization and nitrification in forest soils from Fu-shan Experimental Forest WS1 in northern Taiwan. Net mineralization rates in the O horizon ranged from 4.0 to 13.8 mg N kg−1 day−1, and net nitrification rates ranged from 2.2 to 11.6 mg N kg−1 day−1. For mineral (10–20 cm depth) soil, net mineralization ranged from 0.06 to 2.8 mg N kg−1 day−1 and net nitrification rates ranged from 0.02 to 2.8 mg N kg−1 day−1. We did not find any consistent differences in N mineralization or nitrification rates in soils from the upper and lower part of the watershed. We compared the rates of these processes in three soil horizons (to a soil depth of 30 cm) on a single sampling date and found a large decrease in both net N mineralization and nitrification with depth. We estimated that the soil total N pool was 6,909 kg N ha−1. The present study demonstrates the importance of the stock of mineral soil N in WS1, mostly organic N, which can be transformed to inorganic N and potentially exported to surface and ground water from this watershed. Additional studies quantifying the rates of soil N cycling, particularly multi-site comparisons within Taiwan and the East Asia–Pacific region, will greatly improve our understanding of regional patterns in nitrogen cycling.

10.1007/s11184-008-1004-x

The effect of afforestation on the activity of microbiological mineralization of soil organic matter has been studied in Siberia. The results show that this effect concerns mainly net nitrogen mineralization and net nitrification, while carbon mineralization (CO2 formation) does not depend on the type of ecosystem. It is proposed to use the rates of net nitrogen mineralization and nitrification as the most sensitive indicators of changes in an ecosystem.

Responses of soil nitrogen mineralization to temperature and moisture in alpine ecosystems on the Tibetan Plateau

The responses of soil net nitrogen (N) mineralization to temperature and moisture were investigated in four alpine ecosystems of forest, shrub, meadow and steppe by laboratory incubation method with undisturbed soil cores on the Tibetan Plateau. The results indicated the soil net N mineralization varies greatly between alpine ecosystems. The soil net N mineralization rate in three incubating moisture of forest ecosystem rose markedly, and that of meadow ecosystem rose gently from temperature of 5°C to 35°C, while that of shrub and steppe ecosystems increased from temperature of 5°C to 25°C and reduced from temperature of 25°C to 35°C. At the same incubating temperature, the soil net N mineralization of four alpine ecosystems increased in the middle moisture and deceased in the low or high moisture.