Net Mineralization Rates Research Articles

Drained coastal peatlands: A potential nitrogen source to marine ecosystems under prolonged drought and heavy storm events—A microcosm experiment

Over the past several decades there has been a massive increase in coastal eutrophication, which is often caused by increased runoff input of nitrogen from landscape alterations. Peatlands, covering 3% of land area, have stored about 12–21% of global soil organic nitrogen (12–20Pg N) around rivers, lakes and coasts over millennia and are now often drained and farmed. Their huge nitrogen pools may be released by intensified climate driven hydrologic events—prolonged droughts followed by heavy storms—and later transported to marine ecosystems. In this study, we collected peat monoliths from drained, natural, and restored coastal peatlands in the Southeastern U.S., and conducted a microcosm experiment simulating coupled prolonged-drought and storm events to (1) test whether storms could trigger a pulse of nitrogen export from drought-stressed peatlands and (2) assess how differentially hydrologic managements through shifting plant communities affect nitrogen export by combining an experiment of nitrogen release from litter.During the drought phase, we observed a significant temporal variation in net nitrogen mineralization rate (NMR). NMR spiked in the third month and then decreased rapidly. This pattern indicates that drought duration significantly affects nitrogen mineralization in peat. NMR in the drained site reached up to 490±110kgha−1year−1, about 5 times higher than in the restored site. After the 14-month drought phase, we simulated a heavy storm by bringing peat monoliths to saturation. In the discharge waters, concentrations of total dissolved nitrogen in the monoliths from the drained site (72.7±16.3mgL−1) was about ten times as high as from the restored site. Our results indicate that previously drained peatlands under prolonged drought are a potent source of nitrogen export. Moreover, drought-induced plant community shifts to herbaceous plants substantially raise nitrogen release with lasting effects by altering litter quality in peatlands.

Mechanisms behind the stimulation of nitrification by N input in subtropical acid forest soil

Input of N as NH4 + is known to stimulate nitrification and to enhance the risk of N losses through NO3 − leaching in humid subtropical soils. However, the mechanisms responsible for this stimulation effect have not been fully addressed. In this study, an acid subtropical forest soil amended with urea at rates of 0, 20, 50, 100 mg N kg−1 was pre-incubated at 25 °C and 60 % water-holding capacity (WHC) for 60 days. Gross N transformation rates were then measured using a 15N tracing methodology. Gross rates of mineralization and nitrification of NH4 +-N increased (P < 0.05), while gross rate of NO3 − immobilization significantly decreased with increasing N input rates (P < 0.001). A significant relationship was established between the gross nitrification rate of NH4 + and the gross mineralization rate (R 2 = 0.991, P < 0.01), so was between net nitrification rate of NH4 + and the net mineralization rate (R 2 = 0.973, P < 0.05). Stimulation effect of N input on the gross rate of nitrification of NH4 +-N in the acid soil, partially, resulted from stimulation effect of N input on organic N mineralization, which provides pH-favorable microsites for the nitrification of NH4 + in acid soils (De Boer et al., Soil Biol Biochem 20:845–850, 1988; Prosser, Advan Microb Physiol 30:125–181, 1989). The stimulated gross nitrification rate with the decreased gross NO3 − immobilization rate under the elevated N inputs could lead to accumulation of NO3 − and to enhance the risk of NO3 − loss from humid forest soils.

Do warming-induced changes in quantity and stoichiometry of root exudation promote soil N transformations via stimulation of soil nitrifiers, denitrifiers and ammonifiers?

Despite the recognized importance of root exudation to soil biogeochemical processes and soil function in forest ecosystems, few studies have investigated the impact of quantity and stoichiometry of root exudation on soil nitrogen (N) transformations. In addition, the potential mechanism of how climate warming influences soil N cycling via root-soil interactions remains unclear. We performed an experiment in which in situ root exudates were collected from Picea asperata seedling to examine the response of root carbon (C) & N exudation rates and the C: N ratio stoichiometry along with the soil N transformation rates to warming using infrared heaters in the Eastern Tibetan Plateau. The results showed that warming enhanced root C exudation rates (ug C g−1 root biomass h−1), with higher C: N ratio stoichiometry simultaneously. Experimental warming also significantly increased the soil net mineralization rates, net nitrification rates, and denitrification rates. Then, we conducted an in vitro experiment in which three bacterial strains (Bacillus subtilis, Nitrobacter hamburgensis, and Brachymonas denitrificans) representing ammonifiers, nitrifiers, and denitrifiers, respectively, were cultured with exudates as the culture media. The results indicated that root exudates collected in warmed plots significantly stimulated the growth of the three soil N transformation bacteria, compared with the treatments in control plots. Collectively, our results, to some degree, evidence a linkage that climate warming-induced changes in C fluxes and C: N ratio stoichiometry of root exudation stimulated the growth and reproduction of soil N transformation bacteria, consequently accelerating the soil organic matter decomposition and soil N transformations.

Differences in soil type drive the intraspecific variation in the responses of an earthworm species and, consequently, tree growth to warming

The magnitude of the change in tree growth caused by climate warming can be highly dependent on biological context, including factors such as the presence of associated decomposers and their behaviors. We test the hypothesis that 1) the presence of earthworms modifies the response of soil inorganic N and tree growth to warming via the change of cast production; and furthermore 2) the type of soil as food for earthworm differentiate the earthworm response and its effects on the plant-soil system. We conducted a microcosm experiment rearing a geophageous earthworm species (Aporrectodea rosea) and birch (Betula ermanii) seedlings with two types of soil (sedimentary soil and serpentine soil) under two temperature regimes (ambient and ambient +3.3 °C). While the warming increased the cast production by the earthworms in the serpentine soil, it did not influence cast production in the sedimentary soil. The net nitrogen mineralization rate and the net inorganic N production showed similar dynamics to cast production under the warmer conditions, although the net nitrification rate did not. A significant increase in tree height from warming was observed only in the serpentine soil with earthworms, and the tree height was positively correlated with inorganic N production. The context-dependent change of soil N dynamics to warming could be caused by differential feeding activity of earthworm, and it results in the modification of the growth characteristics of birch seedlings.

Phosphorus in the coarse soil fraction is related to soil organic phosphorus mineralization measured by isotopic dilution

A sound prediction of soil organic phosphorus (Po) mineralization would be helpful to improve fertilizer recommendations. However, in most mineral soils expensive and time consuming isotopic dilution experiments are required to assess Po mineralization. A proposed method to estimate Po mineralization is the quantification of P in the coarse fraction (CF, >53 μm) of the soil. The aim of this study was to evaluate if P in the CF can effectively predict Po mineralization. Soil samples from three North American long-term arable field experiments (>10 years) with different textures (sand content 15–82%) and different fertilization treatments (non-P-fertilized, mineral P fertilizer, poultry litter) were analyzed. Soil samples were physically fractioned into CF and fine fraction (FF, <53 μm). Total P (Pt), Po, and total carbon (Ct) were determined in each fraction. Gross and net Po mineralization rates as well as soil respiration were determined in a 13-day-incubation experiment. The cumulative gross Po mineralization over 13 days ranged from 8.3 to 38.6 mg P kg−1, while Pt in CF varied between 9.7 and 90.7 mg P kg−1 and Po in CF between 6.1 and 17.7 mg kg−1. No association between Pt in CF and gross or net mineralized Po was observed. However, a significant linear relationship between Po in CF and net (R2:0.68, p < 0.01) and gross mineralized Po (R2:0.72, p < 0.01) was observed after 6 days and 13 days of incubation. Additionally, gross and net mineralized Po could also be predicted from C respiration, and these models were significantly improved by incorporating the Ct:Po ratio of the CF. Our results suggest that the quantification of Po in the CF as well as the measurement of respiration corrected by the Ct:Po ratio of the CF are promising non-isotopic indicators of Po mineralization rates.

Research on soil net nitrogen mineralization in Stipa grandis grassland with different stages of degradation

Net nitrogen mineralization is one of the nitrogen cycling process, and it is very important to understand nitrogen mineralization characteristics in degradation grassland. In the upper 0–10 cm soil layer of Stipa grandis P. A. Smirn. grassland in the Xilin river basin, Inner Mongolia, we studied the soil net nitrogen mineralization and nitrification among three varying stages of degradation grassland and analyzed the relationship between the mineralization rate of the soil net nitrogen and environmental factors, using the resin-core incubation method. Our results demonstrated that, from May to October 2012, the accumulated net nitrogen mineralization was 21.620 mg kg–1, 12.486 mg kg–1, and 48.053 mg kg–1 in the slightly, medium, and heavily degraded grasslands, respectively and varied greatly among the three stages of degradation shown by the S. grandis grassland plots. During the cultivation period, the variation of net nitrogen mineralization and nitrification was similar to the variation of soil mineral nitrogen. The higher soil mineral nitrogen content indicated higher mineralization of net nitrogen. In July and August, soil moisture was positively correlated with the net nitrogen mineralization rate, and the correlation coefficients between these two factors of 0.73, 0.58, and 0.79 for the slightly, medium, and heavily degraded plots, respectively. The rates of net nitrogen mineralization and nitrification were negatively correlated with soil total nitrogen, but positively correlated with C/N ratio, and weakly correlated with other physicochemical properties.

Biochemical properties of highly mineralised and infertile soil modified by acacia and spinifex plants in northwest Queensland, Australia

Root zone soil properties can significantly influence the establishment of revegetated plant communities and alter their development trajectories in mined landscapes, due to closely coupled biogeochemical linkages between soil and plant systems. The present study aimed to characterise physicochemical and biochemical conditions in soil colonised by slow-growing native plant species: Acacia chisholmii (C3, native leguminous shrub) and Triodia pungens (spinifex C4 grass) in Mt Isa, North-west Queensland, Australia. This is to provide the basis for engineering growth media and root zones suitable for supporting target native plant communities to be revegetated in mined landscapes under subtropical and semiarid climatic conditions. Litter chemistry, soil physicochemical properties, and microbial community structure based on phospholipid fatty acids (PLFAs) biomarker method and activities (basal respiration, net mineralisation, dehydrogenase, invertase, urease and neutral phosphatase activities) were characterised in the surface soils beneath the keystone native plant species. Results showed that soils sampled were generally infertile with low levels of total organic carbon (TOC), available nutrients and slow cycling processes with bacteria dominant microbial communities supporting the native plant species. Surface soils underneath acacia and spinifex were modified by in situ litter return, in terms of TOC, and structure and functions of microbial communities. The levels of soil microbial biomass C and N, basal respiration rate and net mineralisation rate in the acacia soil were twice as much as those in the spinifex. Microbial communities in the acacia soil had a greater fungal:bacterial ratio than in the spinifex. On this basis, growth media and root zones for revegetating native acacia-spinifex communities at local mined landscapes may be engineered by using plant organic matter remediation to supply available nutrients and to rehabilitate suitable microbial communities for in situ litter decomposition and nutrient cycling.

Effects of climate warming and elevated CO2 on autotrophic nitrification and nitrifiers in dryland ecosystems

Global climate change is predicted to enhance atmospheric temperature and CO2, with important consequences on biogeochemical nitrogen cycling in dryland ecosystems, which are highly vulnerable and characterized by extremely low nutrient availability. Belowground nitrification processes, predominantly mediated by ammonia-oxidizing archaea (AOA) and bacteria (AOB), are central to plant nitrogen availability and soil N2O emissions, but their responses to future climatic scenarios in drylands remain largely unknown. Here we investigated the impact of factorial combinations of elevated CO2 (+200 ppm) and elevated temperature (+3 °C) on dynamics of ammonia oxidizers and nitrification in three dryland soils planted with Eucalyptus tereticornis. Soil properties (including total carbon, H2O%, and nitrate) and potential nitrification rates were strongly impacted by elevated temperature after nine months, accompanied by significantly higher AOA abundance in two soils and a gradual decrease in AOB abundance under elevated temperature. DNA-stable isotope probing showed increased assimilation of 13CO2 by AOA, but not AOB, under warming, indicating that AOA were actively growing and utilizing the 13CO2 substrates, which was coupled with significantly higher net nitrogen mineralization and nitrification rates. High-throughput microarray analysis revealed temperature selections of particular AOA assemblages and a significant reduction in diversity and co-occurrence of the metabolically active AOA phylotypes. Although these responses were soil specific, structural equation modelling by compiling all the data together showed that warming had significant direct and indirect impacts on soil nitrification which were driven by changes in AOA community structure, but no obvious effects of elevated CO2 could be identified. Our findings suggest that warming has stronger effects than elevated CO2 on autotrophic nitrification, and AOA are more responsive to elevated temperature than AOB in the tested dryland ecosystems.

Spatial variation in nutrient dynamics among five different peatland types in the Alberta oil sands region

AbstractWetlands are found extensively throughout the Western Boreal Plain, a region under pressure because of disturbance by the oil and gas industries. To understand how wetland systems may respond to disturbance and set targets for reclamation efforts, it is necessary to understand natural variability in nutrient dynamics in the landscape. The purpose of this study was to characterize spatial variability in peatland nutrient (nitrogen, N, and phosphorus, P) dynamics in the Athabasca Oil Sands (AOS) region. N and P availability and net mineralization rates in the upper 10 cm layer of peat were examined during the peak growing season in five peatlands that fell along an apparent moisture gradient. N and P dynamics within and among the sites were related to water table position, peat moisture content and temperature. Phosphorus supply rates and total inorganic N pools and supply rates were generally elevated under wetter conditions, whereas nitrate (NO3−) pools and supply rates and P pools did not vary along a moisture gradient. In general, net immobilization was observed at the wetter sites where nutrient pools were elevated and net mineralization was observed at drier sites where nutrient pools were lower. Nutrient transformation rates were most strongly driven by warmer temperatures. Nutrient availability and immobilization rates were anomalously high at one peatland (a disturbed fen with a semi‐permanent road and decommissioned well pads). We suggest that reclamation and management practices should focus on regulating peatland hydrologic conditions, optimizing these for the most desirable nutrient levels for vegetation growth. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Soil-nitrogen net mineralization increased after nearly six years of continuous nitrogen additions in a subtropical bamboo ecosystem

In order to understand the effects of increasing atmospheric nitrogen (N) deposition on the subtropical bamboo ecosystem, a nearly six-year field experiment was conducted in a Pleioblastus amarus plantation in the rainy region of SW China, near the western edge of Sichuan Basin. Four N treatment levels—control (no N added), low-N (50 kg N ha−1 a−1), medium-N (150 kg N ha−1 a−1), and high-N (300 kg N ha−1 a−1)—were applied monthly in the P. amarus plantation starting in November 2007. In June 2012, we collected intact soil cores in the bamboo plantation and conducted a 30-day laboratory incubation experiment. The results showed that the soil N net mineralization rate was 0.96 ± 0.10 mg N kg−1 day−1, under control treatment. N additions stimulated the soil N net mineralization, and the high-N treatment significantly increased the soil N net mineralization rate compared with the control. Moreover, the soil N net mineralization rate was significantly and positively correlated with the fine root biomass, the soil microbial biomass nitrogen content and the soil initial inorganic N content, respectively, whereas it was negatively correlated with the soil pH value. There were no significant relationships between the soil N net mineralization rate and the soil total nitrogen (TN) content and the soil total organic carbon content and the soil C/N ratio and the soil microbial biomass carbon content, respectively. These results suggest that N additions would improve the mineral N availability in the topsoil of the P. amarus plantation through the effects of N additions on soil chemical and physical characteristics and fine-root biomass.

Urbanization effects on soil nitrogen transformations and microbial biomass in the subtropics

As urbanization can involve multiple alterations to the soil environment, it is uncertain how urbanization effects soil nitrogen cycling. We established 22–0.04 ha plots in six different land cover types—rural slash pine (Pinus elliottii) plantations (n = 3), rural natural pine forests (n = 3), rural natural oak forests (n = 4), urban pine forests (n = 3), urban oak forests (n = 4) and urban lawns (n = 5) to investigate how net soil nitrogen mineralization rates and soil microbial biomass differed between urban forests and rural forests and between urban forests and urban lawns in the Florida panhandle. Urban forest sites have 2.5 times as much net total nitrogen mineralized than rural forest sites based on the mean daily rates averaged over the 2 years study (2010–2012). Urbanization may increase soil microbial biomass and activity (potential carbon mineralization rates) and this may be influencing the soil nitrogen mineralization rates in the forest sites. To include an urban lawn (turfgrass) component in the study, one time measurements of soils from the aforementioned forest sites and from urban lawn sites (no fertilization, no irrigation) were collected in 2012. Urban forest sites and urban lawns sites do not differ in their potential carbon mineralization rates, potential net total nitrogen mineralization rates or microbial biomass carbon and nitrogen contents. However, lawns have a higher potential net nitrification rate compared to urban forests.

Nitrogen addition and mowing affect microbial nitrogen transformations in a C4 grassland in northern China

SummaryMicrobial nitrogen (N) transformations play a key role in regulating N cycling in grassland ecosystems. However, there is still little information on how management of semi‐arid grassland such as mowing and/or N fertilizer application affects microbial activity and N transformations. In a field experiment in northern China, N was added at a rate of 10 g N m−2 year−1 as NH4NO3 to mown and unmown plots (4 × 4 m2) and in situ rates of net ammonification (Ramm), nitrification (Rnit) and mineralization (Rmin) were followed at monthly intervals for the vegetation growth periods in the years 2006–2009. In addition, we also measured soil microbial biomass carbon (MBC) and nitrogen (MBN), microbial respiration (MR) and peak above‐ground biomass in August of each measurement year. Driven by the pronounced inter‐annual variability of rainfall, all the properties investigated varied markedly across years. Nevertheless, we were able to demonstrate that over the 4 years N addition significantly stimulated Rnit, Rmin and MBN, on average, by 288, 149 and 11.6%, respectively. However, N addition decreased MBC significantly as well as the ratio of MBC:MBN by, on average, 10 and 23%, respectively, whereas an effect of N addition on MR could not be demonstrated. Mowing decreased MBN, MR and qCO2 significantly by 9, 28 and 24%, respectively, but no effects were found on microbial net N transformation rates and MBC. N addition and mowing interactively affected Ramm and Rmin, and MBN, MBC:MBN. In summary, our results indicate a positive effect of N addition but a negative effect of mowing on microbial N transformation in this C4 grassland in northern China.

Wood ash and N fertilization in the Canadian boreal forest: Soil properties and response of jack pine and black spruce

Wood ash fertilization has yet to be investigated in Canadian boreal forests. Ash often improves soil acid–base status, but without N addition it seldom increases tree growth on poor mineral soils. We report results of a large scale experiment conducted in a boreal jack pine (Pinus banksiana Lamb) stand growing on sandy acidic soil in Northeastern Canada. The experiment was completely random with four replications (1ha each) of five treatments (0, 1, 2, 4 and 8 dry Mgha−1) of loose fly ash with and without urea (280kgNha−1). Soils were sampled in the 0, 2, and 8Mgha−1 treatment up to eight years after application. Foliar nutrition and stand growth were assessed in all treatments one and two years and five years after treatment, respectively.Even under low ash loading, forest floor exchangeable base cations, pH, and base saturation increased within a year of application. Ash application also resulted in a swift decrease in forest floor organic C and an increase in N potential net mineralization rate. The initial dominant pattern of upper mineral soil properties in relation to ash loading was a curvilinear relationship with the highest values observed in the 2Mgha−1 treatment. Eight years after ash application, significant linear relationships were found between ash loading and base cations and base saturation in the forest floor and mineral soil (0–10cm, 10–20cm). Contrary to N fertilization, ash had no effect on jack pine foliar nutrition and on its five-year growth. However, a decrease of 30% in relative growth rate was observed between the control and the 8Mgha−1 ash treatment for large (⩾10cm DBH) black spruces (Picea mariana (Mill.) BSP). Black spruce is the dominant commercial species of Canadian eastern boreal forests and thus, additional studies are needed to validate the deleterious effect of ash on spruce growth and to elucidate the mechanisms involved.

Gross organic phosphorus mineralization rates can be assessed in a Ferralsol using an isotopic dilution method

Mineralization of organic phosphorus (Po) may be of great importance for plant nutrition in soils containing very little available inorganic phosphorus (Pi). Gross organic P mineralization rates can be quantified by an isotopic dilution method using 33P labeling of soil. However, its application remains a challenge in tropical soils in which the concentration of phosphate ions in the soil solution is below the detection limit of traditional colorimetric methods. This limitation can potentially be overcome by the hexanol concentration method, which uses hexanol to concentrate the blue-colored phosphomolybdate complex from large volumes. We applied the isotopic dilution method in combination with the hexanol concentration method to a Ferralsol from the highlands of Madagascar which had been preincubated in the presence or absence of plant residues for 90days before the start of the experiment. The limits of detection (DL) and quantification (QL) of the gross Po mineralization rate were 0.2 and 0.7mgPkg−1soilday−1, respectively. Basal gross Po mineralization rates after 7days of incubation were 0.8±0.5 and 1.7±0.2mgPkg−1soilday−1 in non-amended and residue-amended soils, respectively. These rates are plausible, suggesting that the isotopic dilution method is applicable in highly weathered tropical soils with Pi concentrations in the soil solution below the detection limit of traditional colorimetric methods. Net Po mineralization which sustains the plant available P pool remains to be quantified. Gross and net Po mineralization rates should now be assessed in highly weathered soils under a range of land uses.

Carbon Sequestration and Nitrogen Transformation in Soil Cooperated with Organic Composts and Biochar during Corn (&amp;lt;i&amp;gt;Zea mays&amp;lt;/i&amp;gt;) Cultivation

Background: Objectives of this study were to investigate changes of soil carbon contents and to evaluate N mineralization and nitrification rates in soils cooperated with organic composts and biochar during the 2nd year corn cultivation. Methods and Results: For the experiment, the soil texture used in this study was clay loam. Application rates of chemical fertilizer were 480-150- 260 kg/ha (N-P2O5-K2O) as recommended amount after soil test. Biochar application was 0.2% to soil weight. The soil samples were periodically taken at every 15-day interval during corn cultivation periods. The treatments were consisted of cow compost (CC), pig compost (PC), swine digestate from aerobic digestion system (AD), and their biochar cooperation. TC contents in treatments cooperated with biochar at harvesting stages were ranged from 0.96% to 1.24%, and its CC applied plot was highest at 1.24%. It was observed that TC contents with biochar treatments were higher than the compost treatment only. Therefore, it was observed to be carbon sequestration into corn field cooperated with biochar. For nitrogen transformation in soil cooperated with organic composts and biochar, net mineralization rates were dramatically decreased at 44 days after sowing, but nitrification rates were abruptly increased at 73 days after sowing. For N mineralization and nitrification rates, it was shown that they were generally low in the soil cooperated with biochar as compared to the only application plots of different organic composts. Also, it was observed to be highest in the application plot of pig compost manure. Conclusion: Overall, application of biochar in the cropland could be an important role for mitigation of greenhouse gas as well as carbon sequestration.

Evidence of a change in water chemistry in Canada's subarctic associated with enhanced winter streamflow

AbstractEnhanced winter streamflow is a characteristic of a nival/pluvial regime that has emerged in parts of the subarctic Canadian Shield because of increasingly common late summer rains. This phenomenon is part of a widespread trend toward higher winter streamflow in watersheds across the circumpolar north. There may be implications for biogeochemical systems as streamflow regimes undergo these types of changes associated with climate warming. Streamflow and geochemical fluxes were observed over 2 years with different winter flow conditions in a subarctic Canadian Shield catchment. Results show that higher wintertime loads of carbon and solutes associated with enhanced winter streamflow were in association with an expansion of contributing areas to run off over what would have existed during typical winter recession. Furthermore, the wet fall conditions that lead to enhanced winter streamflow require water tables close to the topographic surface in highly conductive organic soil layers, which is a similar to the condition during the spring melt. Fall rainfall‐runoff leaves an ample volume of water in the lakes that are ubiquitous in this landscape. This water maintains winter streamflow during a time when it traditionally would have ceased. A slowing of biological activity under lake ice increases net mineralization and nitrification rates. This convergence of nitrogen cycling and winter streamflow produced a disproportionate flux of inorganic nitrogen from the study catchment. A conceptual model of how enhanced winter streamflow changes water chemistry in a lake‐dominated shield landscape is proposed and may be used as a benchmark to guide hypotheses of process interactions, change in other landscapes, or across scales.

Soil bacterial community structure remains stable over a 5-year chronosequence of insect-induced tree mortality.

Extensive tree mortality from insect epidemics has raised concern over possible effects on soil biogeochemical processes. Yet despite the importance of microbes in nutrient cycling, how soil bacterial communities respond to insect-induced tree mortality is largely unknown. We examined soil bacterial community structure (via 16S rRNA gene pyrosequencing) and community assembly processes (via null deviation analysis) along a 5-year chronosequence (substituting space for time) of bark beetle-induced tree mortality in the southern Rocky Mountains, USA. We also measured microbial biomass and soil chemistry, and used in situ experiments to assess inorganic nitrogen mineralization rates. We found that bacterial community structure and assembly—which was strongly influenced by stochastic processes—were largely unaffected by tree mortality despite increased soil ammonium () pools and reductions in soil nitrate () pools and net nitrogen mineralization rates after tree mortality. Linear models suggested that microbial biomass and bacterial phylogenetic diversity are significantly correlated with nitrogen mineralization rates of this forested ecosystem. However, given the overall resistance of the bacterial community to disturbance from tree mortality, soil nitrogen processes likely remained relatively stable following tree mortality when considered at larger spatial and longer temporal scales—a supposition supported by the majority of available studies regarding biogeochemical effects of bark beetle infestations in this region. Our results suggest that soil bacterial community resistance to disturbance helps to explain the relatively weak effects of insect-induced tree mortality on soil N and C pools reported across the Rocky Mountains, USA.

Do Earthworms Have a Greater Influence on Nitrogen Dynamics Than Atmospheric Nitrogen Deposition?

Elevated levels of inorganic nitrogen (N) deposition and earthworm invasion have the potential to alter N dynamics in eastern North American temperate forests. A regional comparison was conducted across 21 sugar maple (Acer saccharum Marsh) stands in southern Ontario, where forest floor C:N ratios ranged from 17 to 38 showed that, similar to many other studies, rates of potential net mineralization and nitrification increased below a forest floor C:N ratio threshold of approximately 25 and that nitrification rates are positively correlated with foliar N concentration. However, detailed measurements at four representative stands, receiving between 9.8 and 19 kg N ha−1 y−1 in throughfall, showed that foliar N levels were highest at the site with the lowest N deposition. The primary difference amongst these sites was the presence of invasive earthworms. Specifically, sites without earthworms had significantly higher forest floor N with a lower C:N ratio than the sites with earthworms. There was no significant difference in the rate of sugar maple litter decomposition or chemistry amongst the sites assessed after 540 days using fine (2-mm mesh) litter bags, suggesting that differences in forest floor N levels were most likely due to consumption of litter by large earthworm species and that the lower C:N ratio of the forest floor in sites without earthworms is brought about primarily by a much longer residence time. This work supports the conclusions that forest floor N concentration (or C:N ratio) has a very strong control on N dynamics in forests, but shows that the presence of earthworms can have an impact on forest floor C:N ratio and hence N dynamics that is greater than current levels of atmospheric inorganic N deposition in temperate forests of Ontario.

Water addition regulates the metabolic activity of ammonia oxidizers responding to environmental perturbations in dry subhumid ecosystems.

Terrestrial arid and semi-arid ecosystems (drylands) constitute about 41% of the Earth's land surface and are predicted to experience increasing fluctuations in water and nitrogen availability. Mounting evidence has confirmed the significant importance of ammonia-oxidizing archaea (AOA) and bacteria (AOB) in nitrification, plant nitrogen availability and atmospheric N2 O emissions, but their responses to environmental perturbations in drylands remain largely unknown. Here we evaluate how the factorial combinations of irrigation and fertilization in forests and land-use change from grassland to forest affects the dynamics of AOA and AOB following a 6-year dryland field study. Potential nitrification rates and AOA and AOB abundances were significantly higher in the irrigated plots, accompanied by considerable changes in community compositions, but their responses to fertilization alone were not significant. DNA-stable isotope probing results showed increased (13) CO2 incorporation into the amoA gene of AOA, but not of AOB, in plots receiving water addition, coupled with significantly higher net mineralization and nitrification rates. High-throughput microarray analysis revealed that active AOA assemblages belonging to Nitrosopumilus and Nitrosotalea were increasingly labelled by (13) CO2 following irrigation. However, no obvious effects of land-use changes on nitrification rates or metabolic activity of AOA and AOB could be observed under dry conditions. We provide evidence that water addition had more important roles than nitrogen fertilization in influencing the autotrophic nitrification in dryland ecosystems, and AOA are increasingly involved in ammonia oxidation when dry soils become wetted.

Indicators and trade-offs of ecosystem services in agricultural soils along a landscape heterogeneity gradient

Soil functions can be classified as supporting (nutrient cycling) and provisioning (crop production) ecosystem services (ES). These services consist of multiple and dynamic functions and are typically assessed using indicators, e.g. microbial biomass as an indicator of supporting services. Agricultural intensification negatively affects indicators of soil functions and is therefore considered to deplete soil ES. It has been suggested that incorporating leys into crop rotations can enhance soil ES. We examined this by comparing indicators of supporting soil services – organic carbon, nitrogen, water holding capacity and available phosphorous (carbon storage and nutrient retention); net nitrogen mineralisation rate and microbial biomass (nutrient cycling and retention) – in barley fields, leys and permanent pastures along a landscape heterogeneity gradient (100, 500 and 1000m radii). In addition, barley yields (provisioning service) were analysed against these indicators to identify trade-offs among soil services. Levels of most indicators did not differ between barley and ley fields and were consistently lower than in permanent pastures. Leys supported greater microbial biomass than barley fields. Landscape heterogeneity had no effect on the indicators or microbial community composition. However, landscape heterogeneity correlated negatively with yield and soil pH, suggesting that soils in heterogeneous landscapes are less fertile and therefore have lower yields. No trade-offs were found between increasing barley yield and the soil indicators. The results suggest that soil ES are determined at the field level, with little influence from the surrounding landscape, and that greater crop yields do not necessarily come at the expense of supporting soil services.