Net Nitrogen Mineralization Research Articles

The combined effects of atrazine and lead (Pb): Relative microbial activities and herbicide dissipation

The experiment was conducted to investigate the effects of single and combined pollution from different concentrations of atrazine (field rate, FR, 2.0mgkg−1 and 5 times FR, 10mgkg−1) and lead (Pb) (300mgkg−1 and 600mgkg−1) on enzyme activity, basal soil respiration (BSR), and net nitrogen (N) mineralization (NNM) in soil after exposure for 0, 7, 14, 21, and 28 days. In addition, residual atrazine was measured in the samples of combined contamination. Results showed that the notable effects of either or both contaminants on the microbial activity and biological processes. Enzyme activity data demonstrated that the order of sensitivity to contamination was urease>invertase>catalase. BSR was strongly stimulated by atrazine/Pb at the early exposure (0–7 days for single contaminant and 7–14 days for combined contaminants). The stimulation effects on BSR were higher at low concentrations of the contamination (FR and Pb300). The combined treatments of 5FR+Pb600 inhibit BSR and NNM. Overall, the parameters associated with N cycling (urease and NNM) were more sensitive than others. Both Pb concentrations (300 and 600mg/kg) had little influence on the dissipation of high concentrations of atrazine (5FR) during the 28-day-incubation. This study has provided useful information on potential ecotoxicology effects of combined contamination of atrazine and Pb on relative microbial biological process.

Aerobic microbial activity in four tropical earthworm-soil systems. A mesocosm experiment

Soil organic matter (SOM) quality and carbon (C) availability may be major features influencing the effect of earthworms on the aerobic processes in clayey tropical soils. In this study, we assessed the effect of an anecic (Polypheretima elongata), an endogeic (Pontoscolex corethrurus) and an epigeic (Eudrilus eugeniae) earthworm on the aerobic microbial activity of two tropical soils, a calcic Vertisol and an acid Ferralsol, with clay content >70% and very different organic C content and SOM stability. The soil–earthworm interaction was studied in a 6-month mesocosm experiment in a greenhouse using soils with and without (control soil) earthworm addition. Potential C mineralisation, actual net nitrogen (N) mineralisation and dehydrogenase activity (DHA), as indicators of the aerobic activity of the soils, and phosphorus (P) availability were determined during the trial. DHA was used as an indicator of the global aerobic activity. Earthworms had little effect on potential C mineralisation but significantly increased actual net N mineralisation. The increase in N mineralisation in the Vertisol was twice as great as, and longer (6 v. 3 months) than for the Ferralsol. Differences between soils for N mineralisation were associated with a less recalcitrant SOM in the Vertisol. Available P increased 10% in the earthworm treatments. Earthworm activity improved N and P availability. DHA was 15 times higher for the Vertisol than for the Ferralsol, but the positive effect of earthworms on DHA was greater for the Ferralsol. This effect was greater for E. eugeniae, probably because of surface burrows generated by this epigeic earthworm, which favoured oxygen entry into the soil. Differences between the two soils were greater for DHA than for C and N mineralisation, and this was observed for the control soils as well as for the earthworm treatments. This indicates that earthworm activity modified the rate of the aerobic processes but it did not affect the intrinsic biological properties of these tropical soils, which were controlled mainly by SOM quality and C availability.

A FUNCTION DESCRIBING NITROGEN AVAILABILITY FROM ORGANIC SOURCES

A simple first-order mathematical function is presented to describe net nitrogen (N) immobilization and mineralization from organic materials spread on or incorporated into horticultural soils. To facilitate the representation of immobilization, kinetics are determined by a conveniently measured carbon dioxide (CO2-C) evolution rate. Other parameters include a substrate conversion efficiency, the C:N ratio of microbial decomposers, and the initial C and N contents of the materials. The model was parameterized using a previously reported incubation study of uncomposted lawn trimming materials (Sullivan et al., 2004). To avoid overfitting, parameters are lumped between the different treatments whenever reasonable. All parameters were lumped except for two labile/recalcitrant partition factors, one for C and one for N. Results for each of eight lawn trimming materials represented evolving C very well (r2=0.99). Nitrogen immobilization and release fits were reasonable (r2=0.71), predicting immobilization and mineralization appropriately. Parameterization options based on proximate carbon analysis were also introduced.

雪被斑块对川西亚高山两个森林群落冬季土壤氮转化的影响

为了解气候变暖情景下雪况变化对高寒森林冬季土壤氮转化的影响，测定了川西亚高山冷杉（<em>Abies faxoniana</em>）+红桦（<em>Betula albo-sinensis</em>）混交林（MF）和冷杉次生林（SF）三类雪被斑块（浅雪被、中厚度雪被和厚雪被）内冬季土壤氮矿化特征。结果表明:经过一个冬季（2011-2012），两个森林群落土壤净氮氨化量都为负值，净氮硝化量都为正值，且净氮硝化量显著高于净氮氨化量；冬季土壤氮氨化、硝化、矿化和固持量都是中度雪被厚度最高，但各雪被斑块之间都未达到显著水平。各雪被斑块下，冷杉次生林土壤氮矿化参数都显著高于针阔混交林，但雪被斑块和林型交互作用对冬季土壤氮矿化无显著影响。这表明，该区冬季土壤氮矿化以硝化过程为主，硝化和氨化过程可能受不同微生物群落调控；短时期内，未来气候变化所导致的雪被减少对该区森林冬季土壤氮转化影响可能不明显。;A consensus in global ecology is that climate change is profoundly altering the process of terrestrial ecosystem. In particular, more and more attentions have been focused on the effects of snow pack on soil process in cold biomes recently, since climate change characterized by winter warming and extreme is changing the pattern of seasonal snow cover and freeze-thaw cycles in the high-latitude and high-altitute regions. As yet, the results concerning the effects of seasonal cover and freeze-thaw cycles on soil process remain uncertainty, and little information has been available on the effects of snow pack on soil nitrogen tansformation in the subalpine forest of western Sichuan, China. In order to deeply understand the process of soil nitrogen transformation in the subalpine forest of western Sichuan under climate change scenarios, therefore, wintertime net nitrogen mineralization, net nitrogen ammonification, net nitrogen nitrification and microbial nitrogen immobilization in soils under three depths of snowpacks (deep snowpack, middle snowpack and shallow snowpack) were measured in two subalpine forests of western Sichuan, which locates at the upper reaches of Yangtze River and the eastern Qinghai-Tibet Plateau. The top of soil columns sampled from<em> Abies faxoniana </em>+ <em>Betula albo-sinensis</em> mixed forest and secondary fir forest were closed, and then <em>in-situ</em> incubated in the forest floor under deep snowpack, middle snowpack and shallow snowpack of the corresponding forest from November 2011 to April 2012, respectively. The results indicated that wintertime soil net nitrogen ammonification ranged from -2.37mg/kg to -2.22 mg/kg under different snowpacks in the mixed forest, and from -7.73 mg/kg to -7.10 mg/kg under different snowpacks in the secondary fir forest. Wintertime soil net nitrogen nitrification varied from 71.98 mg/kg to 82.69 mg/kg under different snowpacks in the mixed forest, and from 101.56 mg/kg to 124.89 mg/kg under different snowpacks in the secondary fir forest. Net nitrogen nitrification was significantly (<em>P</em> < 0.05) higher than net nitrogen ammonification. The highest rates of soil nitrogen ammonification, nitrification, mineralization and microbial nitrogen immobilization were observed in the middle snow pack, but no significant (<em>P</em> > 0.05) differences were observed among snow packs. Meanwhile, all measured indexes in each snow pack were significantly (<em>P</em> < 0.05) higher in the secondary fir forest than those in the fir-birch mixed forest. However, the interaction of snow pack and forest type was not significant on wintertime soil nitrogen transformation. The results suggested that soil nitrogen nitrification be a major process of wintertime soil nitrogen mineralization, and the process of soil nitrogen nitrification and ammonification be regulated by different microbial communities in the subalpine forests of western Sichuan, China. The result also implies that the decline of snowpack caused by climate change, in the short-run, has little effects on soil nitrogen mineralization in our study area.

Effects of temperature on soil net nitrogen mineralisation in two contrasting forests on the eastern Tibetan Plateau, China

Intact soil cores from two adjacent forest ecosystems (natural coniferous forest and dragon spruce plantation) were incubated in the laboratory to examine effects of temperature, reforestation and their interactions on rates of nitrogen (N) mineralisation, nitrification and ammonification in the subalpine forest of the eastern Tibetan Plateau. Two contrasting soils were incubated at five temperatures (–5, 0, 5, 15 and 25°C) for 4 weeks. Rates of N mineralisation and nitrification were insensitive to temperature at lower temperatures (0°C and 5°C) but increased over higher temperatures (15°C and 25°C). Large amounts of ammonium were released for each incubation time in both soils when the incubation temperature was –5°C. Therefore, the rates of mineralisation and ammonification at –5°C were significantly higher than at the other temperatures. Both the accumulations of inorganic N and rates of N transformation were significantly higher in the natural forest than in the plantation. Moreover, temperature sensitivity of net nitrification and N mineralisation were greater in the natural forest than the spruce plantation. Effects of temperature on accumulations of inorganic N and rates of N transformation were dependent on incubation time and forest ecosystem. Our results suggested that –5°C might be a key low temperature for N mineralisation in subalpine forest on the eastern Tibetan Plateau; the effect of projected warming on soil N transformation rate may be less significant in plantation forests than natural forests in this specific region.

Nitrogen mineralization kinetics and parameter estimates in soil amended with Neem seed cake and inorganic fertilizer in Samaru, Nigeria

A field study was conducted to assess the effect of neem seed cake (NSC) on the kinetics of net nitrogen mineralization and parameter estimates. The experiment consisted of sorghum plots to which milled NSC (with %N > 3) and urea (inorganic N source) were applied. Ammonium-nitrogen and nitrate-nitrogen were determined after 0, 2, 4, 6, 8, 12, 19 and 34 weeks from soils covered with PVC tubes inserted into the sixteen treatment combination plots. Inorganic N concentration in the amended plots and the control were analysed using five mathematical N mineralization models. N0, Ne, h and k were estimated from the fit of each mathematical model. The treatments were highly variable in their respective N mineralization characteristics, but the peaks for mineralized N content were observed at early incubation periods, i. e., 10 -28 days. Potential respective mineralizable N (N0) content of the first order rate, consecutive (h ‚ k), consecutive (h = k), Gompertz and mixed order rate constant was -927E-22, 430.90, 1.9E-21, 568.40 and 427.20 mg kg-1 soil and the respretively rate constant (k) was -5E+21, 0.2269, 2E+23, 0.0954 and 0.8116.respretively Net N mineralization was best described by a mixed order rate model (R2 = 0.992) and residual mean square error (RMSE)was = 28.41. A Gompertz function also fitted the data closely (R2 = 0.999; RMSE = 52.47). The two models recognize that N mineralization is not just a function of substrate N concentration but also of the capacity of the microbial community to adjust to substrate quality and composition affecting their growth and activity. Key words : mineralization model, neem seed cake, N mineralization, parameter estimates, PVC tubes

No effect of seed source on multiple aspects of ecosystem functioning during ecological restoration: cultivars compared to local ecotypes of dominant grasses

Genetic principles underlie recommendations to use local seed, but a paucity of information exists on the genetic distinction and ecological consequences of using different seed sources in restorations. We established a field experiment to test whether cultivars and local ecotypes of dominant prairie grasses were genetically distinct and differentially influenced ecosystem functioning. Whole plots were assigned to cultivar and local ecotype grass sources. Three subplots within each whole plot were seeded to unique pools of subordinate species. The cultivar of the increasingly dominant grass, Sorghastrum nutans, was genetically different than the local ecotype, but genetic diversity was similar between the two sources. There were no differences in aboveground net primary production, soil carbon accrual, and net nitrogen mineralization rate in soil between the grass sources. Comparable productivity of the grass sources among the species pools for four years shows functional equivalence in terms of biomass production. Subordinate species comprised over half the aboveground productivity, which may have diluted the potential for documented trait differences between the grass sources to influence ecosystem processes. Regionally developed cultivars may be a suitable alternative to local ecotypes for restoration in fragmented landscapes with limited gene flow between natural and restored prairie and negligible recruitment by seed.

Self-reinforcing impacts of plant invasions change over time

Returning native species to habitats degraded by biological invasions is a critical conservation goal. A leading hypothesis poses that exotic plant dominance is self-reinforced by impacts on ecosystem processes, leading to persistent stable states. Invaders have been documented to modify fire regimes, alter soil nutrients or shift microbial communities in ways that feed back to benefit themselves over competitors. However, few studies have followed invasions through time to ask whether ecosystem impacts and feedbacks persist. Here we return to woodland sites in Hawai'i Volcanoes National Park that were invaded by exotic C4 grasses in the 1960s, the ecosystem impacts of which were studied intensively in the 1990s. We show that positive feedbacks between exotic grasses and soil nitrogen cycling have broken down, but rather than facilitating native vegetation, the weakening feedbacks facilitate new exotic species. Data from the 1990s showed that exotic grasses increased nitrogen-mineralization rates by two- to fourfold, but were nitrogen-limited. Thus, the impacts of the invader created a positive feedback early in the invasion. We now show that annual net soil nitrogen mineralization has since dropped to pre-invasion levels. In addition, a seedling outplanting experiment that varied soil nitrogen and grass competition demonstrates that the changing impacts of grasses do not favour native species re-establishment. Instead, decreased nitrogen availability most benefits another aggressive invader, the nitrogen-fixing tree Morella faya. Long-term studies of invasions may reveal that ecosystem impacts and feedbacks shift over time, but that this may not benefit native species recovery.

Potential climate change impacts on temperate forest ecosystem processes

Large changes in atmospheric CO2, temperature, and precipitation are predicted by 2100, yet the long-term consequences for carbon (C), water, and nitrogen (N) cycling in forests are poorly understood. We applied the PnET-CN ecosystem model to compare the long-term effects of changing climate and atmospheric CO2 on productivity, evapotranspiration, runoff, and net nitrogen mineralization in current Great Lakes forest types. We used two statistically downscaled climate projections, PCM B1 (warmer and wetter) and GFDL A1FI (hotter and drier), to represent two potential future climate and atmospheric CO2 scenarios. To separate the effects of climate and CO2, we ran PnET-CN including and excluding the CO2 routine. Our results suggest that, with rising CO2 and without changes in forest type, average regional productivity could increase from 67% to 142%, changes in evapotranspiration could range from –3% to +6%, runoff could increase from 2% to 22%, and net N mineralization could increase 10% to 12%. Ecosystem responses varied geographically and by forest type. Increased productivity was almost entirely driven by CO2 fertilization effects, rather than by temperature or precipitation (model runs holding CO2 constant showed stable or declining productivity). The relative importance of edaphic and climatic spatial drivers of productivity varied over time, suggesting that productivity in Great Lakes forests may switch from being temperature- to water-limited by the end of the century.

Linking aboveground net primary productivity to soil carbon and dissolved organic carbon in complex terrain

Factors influencing soil organic matter (SOM) stabilization and dissolved organic carbon (DOC) content in complex terrain, where vegetation, climate, and topography vary over the scale of a few meters, are not well understood. We examined the spatial correlations of lidar and geographic information system‐derived landscape topography, empirically measured soil characteristics, and current and historical vegetation composition and structure versus SOM fractions and DOC pools and leaching on a small catchment (WS1) in the H.J. Andrews Experimental Forest, located in the western Cascades Range of Oregon, USA. We predicted that aboveground net primary productivity (ANPP), litter fall, and nitrogen mineralization would be positively correlated with SOM, DOC, and carbon (C) content of the soil based on the principle that increased C inputs cause C stores in and losses from in the soil. We expected that in tandem, certain microtopographical and microclimatic characteristics might be associated with elevated C inputs and correspondingly, soil C stores and losses. We confirmed that on this site, positive relationships exist between ANPP, C inputs (litter fall), and losses (exportable DOC), but we did not find that these relationships between ANPP, inputs, and exports were translated to SOM stores (mg C/g soil), C content of the soil (% C/g soil), or DOC pools (determined with salt and water extractions). We suggest that the biogeochemical processes controlling C storage and lability in soil may relate to longer‐term variability in aboveground inputs that result from a heterogeneous and evolving forest stand.

Impact of snowpack decrease on net nitrogen mineralization and nitrification in forest soil of northern Japan

Winter climate change is an important environmental driver that alters the biogeochemical processes of forest soils. The decrease in snowpack amplifies soil freeze–thaw cycles and decreases the snowmelt water supply to soil. This study examined how snow decrease affects nitrogen (N) mineralization and nitrification in forest soil in northern Japan by conducting an in situ experimental snowpack manipulation experiment and a laboratory incubation of soil with different moisture, temperature and freeze–thaw magnitudes. For the incubation studies, surface mineral soil (0–10 cm) was collected from a cool-temperate natural mixed forest and incubated using the resin core method during the winter. In the field, there were two treatments: 50 and 100 % snow removal and control plots. The increase in the soil freeze–thaw cycle increased net N mineralization and marginally decreased the net nitrification in soil. The dissolved organic carbon (DOC) and DOC/DON ratio in soil increased with the decrease in snowpack especially during the snow melt period. These results suggested that the change in substrate quality by the increase in freeze–thaw cycles caused the significant enhancement of microbial ammonium production in soil. The lower soil moisture and higher gross immobilization of inorganic N by soil microbes may be maintaining the slow net nitrification and low nitrate leaching in freeze–thaw cycles with less snowpack. The results indicate that winter climate change would strongly impact N biogeochemistry through the increase in ammonium availability in soil for plants and microbes, whereas it would be unlikely that nitrate loss from surface soil would be enhanced.

Effects of livestock grazing on the spatial heterogeneity of net soil nitrogen mineralization in three types of Mongolian grasslands

Small-scale soil heterogeneity relates to productivity and biodiversity and is crucial to understand. Soil heterogeneity could be affected by vegetation structure, and large mammal grazers could modify it through herbivory and excretion. The objective is to clarify the effects of livestock grazing on the small-scale (∼3 m) soil heterogeneity in three types of Mongolian grasslands. We sampled soils from inside (ungrazed) and outside (grazed) exclosures in three vegetation types: forest-steppe, shrub-steppe, and desert-steppe. We measured laboratory rates of soil net nitrogen (N) mineralization and net nitrification and geostatistically analyzed heterogeneity. Average rates of net N mineralization and net nitrification were lower at shrub-steppe and desert-steppe and were decreased by grazing. Semivariograms showed vegetation-induced heterogeneity in ungrazed plots, except for net nitrification at forest-steppe. We found linear change with distance under dense and uniform vegetation at forest-steppe, 1.3 m patch under patchy vegetation at shrub-steppe, and linear change, but with much smaller semivariance, under sparse and poor vegetation at desert-steppe. At forest-steppe, grazing randomized the spatial patterns of net N mineralization and net nitrification. At shrub-steppe and desert-steppe, grazing greatly decreased the semivariances of net N mineralization and net nitrification as well as their averages, and the soil heterogeneity was virtually disappeared. Grazing in Mongolian grasslands homogenized the spatial patterns of net N mineralization and net nitrification, irrespective of their original spatial patterns determined by the differences in vegetation structure.

Net mineralization nitrogen and soil chemical changes with application of organic wastes with ‘Fermented Bokashi Compost’

The use of organic wastes in agricultural soils is one of the possible ways to employ these materials. The aims of this study were to evaluate the effectiveness of organic wastes and Fermented Bokashi Compost (FBC), to establish the most efficient use of organic wastes for a soil, changing the net nitrogen mineralization and soil chemical properties. The experimental design was completely randomized in a 6 x 2 x 5 factorial, being five organic wastes plus an control (soil without waste), with or without FBC, evaluated at 0, 7, 42, 70 and 91 days of incubation, with three replicates, under laboratory conditions. The organic wastes enhanced the soil chemical properties and increased nitrogen concentration in soil. However, the net nitrogen mineralization was affected by C/N ratio of wastes and incubation time. The FBC mixed with the wastes accelerated and enhanced organic matter degradation, resulting in quickly available quantity of net nitrogen. The wastes can be considered potentially useful as organic fertilizer but their usefulness appears to depend on knowing the C/N ratio of each one. The FBC can be used when one wants a more accelerated degradation, resulting in a quicker quantity of available nutrients to the plants.

The effect of free-living nematodes on nitrogen mineralisation in undisturbed and disturbed soil cores

Soil fauna, particularly nematodes, are considered to strongly contribute to nitrogen mineralisation through grazing on microflora during decomposition. Demonstration of this effect has mostly relied on calculation-based soil food web analyses or experiments involving simplified and artificially constructed food webs. We carried out an incubation experiment in which defaunated soil cores were reinoculated with entire soil nematode populations extracted from bulk soil and during which nitrogen mineralisation was measured. Both undisturbed and disturbed cores were prepared to investigate whether a representative pore structure influences the effect of entire free-living nematode populations on nitrogen mineralisation. Cores were subjected to a 5 kGy gamma irradiation dose sufficient to eliminate all soil fauna while leaving the microbial biomass largely intact. Half of the irradiated cores were reinoculated with nematodes extracted from a corresponding volume of bulk soil and incubated for 82 days. The microbial biomass was not strongly affected by gamma irradiation or nematode addition but declined strongly in all treatments during incubation. Reinoculation of nematodes was successful in establishing populations of a similar size and composition as in the control samples. Net nitrogen mineralisation from indigenous soil organic matter was observed in all treatments throughout the incubation, but was always more pronounced in irradiated cores. Total mineral nitrogen concentrations did not differ significantly between simply irradiated and irradiated then reinoculated cores. However by the end of the incubation period nematode addition resulted in 87% and 23% more NO3−–N g−1 dry soil in undisturbed and disturbed cores respectively, while NH4+–N g−1 dry soil decreased by 50% in both core types. We found no convincing evidence for a contribution of free-living nematodes on total nitrogen mineralisation, but the activity of nitrifying organisms was clearly stimulated by nematode grazing.

Effect of seasonal freeze–thaw cycle on net nitrogen mineralization of soil organic layer in the subalpine/alpine forests of western Sichuan, China

Nitrogen mineralization, a main way that soil organic nitrogen converts to mineral nitrogen, is one of the key processes in soil nitrogen cycle. The mineral nitrogen has an important role in plant growth in the growing season. It has been widely accepted that soil freezing in winter can kill a number of microorganisms, weakening soil nitrogen mineralization. However, more and more recent studies have documented that soil microorganisms still have high activity during the deep freezing period, and obvious nitrogen mineralization in winter. Seasonal freeze–thaw cycle is a common phenomenon in the subalpine/alpine forest region, which may have a strong effect on soil ecological processes. Furthermore, the changing pattern of seasonal freeze–thaw cycles might have a significant influence on soil nitrogen mineralization in this region in the scenarios of global warming. As yet, little attention has been given to nitrogen mineralization of soil organic layer as affected by changed seasonal freeze–thaw pattern, although the increasing studies have demonstrated that winter warming might give strong effects on the litter decomposition and microbial activity in the subalpine/alpine forest regions. Therefore, a method of intact soil core incubation in combination with natural environmental gradient was employed by transferring forest soils from 3582m (A1) of altitude to 3298m (A2) of altitude and 3023m (A3) of altitude in the subalpine/alpine forests of western Sichuan, respectively. The amounts and rates of net nitrogen mineralization in soil organic layer were measured. The incubation period included the growing season and the freeze–thaw season from May 24, 2010 to April 19, 2011. The results suggested that significant net nitrogen mineralization was only observed in soil organic layer at low altitude (A3) during the whole incubation period. Forest soils at higher altitudes (A1 and A2) showed obvious soil nitrogen immobilization. In comparison with the growing season which showed remarkable nitrogen immobilization characteristic, the freeze–thaw season showed obvious nitrogen mineralization at lower altitudes (A2 and A3). In contrast, the nitrogen immobilization amounts at high altitude (A1) in freeze–thaw period were less than those in the growing season. Besides, the maximum of net nitrogen mineralization amounts and rates at high altitude (A1) in soil organic layer mainly occurred in the late stage of growing season and the onset of freezing, soil nitrogen mineralization at the middle altitude (A2) mainly occurred in the onset of freezing and the deep freezing period, while the highest amount and rate of net nitrogen mineralization at low altitude (A3) occurred in the early stage of thawing and the late stage of growing season. Furthermore, the amount and rate of soil net nitrogen mineralization during the freeze–thaw season were increasing with the decrease of altitude, which correlated with soil freeze–thaw cycle and freezing process at different altitudes. These results indicated that increasing soil temperature in the future could not only significantly enhance soil nitrogen mineralization in the freeze–thaw season, but also improve soil nitrogen mineralization by increasing freeze–thaw cycle times and shortening freeze–thaw period. However, the processes were significantly influenced by soil micro-environment of subalpine/alpine forest regions.

Net Nitrogen Mineralization in Natural Ecosystems across the Conterminous US

Nitrogen is the primary nutrient limiting ecosystem productivity over most of the US. Although soil nitrogen content is important, knowledge about its spatial extent at the continental scale is limited. The objective of this study was to estimate net nitrogen mineralization for the conterminous US (CONUS) using an empirical modeling approach by scaling up site level measurements. Net nitrogen mineralization and total soil nitrogen data across the CONUS were obtained from three different ecosystems: low elevation forests, high elevation forests, and grasslands. Equations to predict net nitrogen mineralization were developed through stepwise linear regression using total Kjeldahl nitrogen, air temperature, precipitation, and nitrogen deposition as predictor variables for four categories: low elevation high temperature forests (coefficient of determination, R 2 = 0.83), low elevation low temperature forests (R 2 =

Temporal Variability of Soil Biological Properties in Temperate Grasslands and Croplands Amended with Organic and Inorganic Fertilizers

Soil biological properties have been widely used as indicators of soil quality, but the wide use of these properties as soil quality indicators is hindered by the uncertainty of their seasonal patterns. In our work we have assessed the seasonal variation of several biological properties (biomass carbon and nitrogen, net nitrogen mineralization, soil basal respiration, dissolved organic nitrogen, and dissolved organic carbon) over an almost 3-year period, starting in February 2008 and finishing in December 2010, in two different trials: (A) grazed grasslands and (B) corn cultivated cropland. In trial A we studied the influence of stock density on soil biological properties, whereas in trial B we studied how biological properties are affected by fertilizer type. Trials, located in a temperate area (43° 14′ N latitude, 8° 15′ W longitude, and 97 m above sea level) of northwestern Spain, were carried out as a completely randomized block design with three replicates. Our data show a high seasonality for the studied properties and a consistent influence of management.

Net and Potential Nitrogen Mineralization in Soil with Sugarcane Vinasse

The increasing ethanol production brings many advantages to Brazil. However, it generates high quantities of vinasse from 10 to 15 l per liter of alcohol produced. This residue is applied as a fertilizer in sugarcane crops but, it presents a potential risk to soil quality when high doses of vinasse are applied in the same area over years. So, one solution would be to transport it to distant locations, but this would mean an increase in costs. Thus, concentrated vinasse, wherein the volume is reduced by evaporation process, becomes a viable alternative and is being increasingly used by sugarcane mills in Brazil. The aim of this study was to evaluate the net nitrogen mineralization and potential nitrogen mineralization of soil that received three doses of concentrated and not concentrated vinasse. The not concentrated vinasse is a good alternative to be applied, but higher doses promote N losses by denitrification due to the high water content. The highest values of potential nitrogen mineralization and, lowest values of constant of mineralization in the treatments with concentrated vinasse indicate that this residue releases N at a slower speed than in treatments with not concentrated vinasse, suggesting that there will be N availability in the subsequent crops.

Post-Fire Spatial Patterns of Soil Nitrogen Mineralization and Microbial Abundance

Stand-replacing fires influence soil nitrogen availability and microbial community composition, which may in turn mediate post-fire successional dynamics and nutrient cycling. However, fires create patchiness at both local and landscape scales and do not result in consistent patterns of ecological dynamics. The objectives of this study were to (1) quantify the spatial structure of microbial communities in forest stands recently affected by stand-replacing fire and (2) determine whether microbial variables aid predictions of in situ net nitrogen mineralization rates in recently burned stands. The study was conducted in lodgepole pine (Pinus contorta var. latifolia) and Engelmann spruce/subalpine fir (Picea engelmannii/Abies lasiocarpa) forest stands that burned during summer 2000 in Greater Yellowstone (Wyoming, USA). Using a fully probabilistic spatial process model and Bayesian kriging, the spatial structure of microbial lipid abundance and fungi-to-bacteria ratios were found to be spatially structured within plots two years following fire (for most plots, autocorrelation range varied from 1.5 to 10.5 m). Congruence of spatial patterns among microbial variables, in situ net N mineralization, and cover variables was evident. Stepwise regression resulted in significant models of in situ net N mineralization and included variables describing fungal and bacterial abundance, although explained variance was low (R2<0.29). Unraveling complex spatial patterns of nutrient cycling and the biotic factors that regulate it remains challenging but is critical for explaining post-fire ecosystem function, especially in Greater Yellowstone, which is projected to experience increased fire frequencies by mid 21st Century.

Soil pH has contrasting effects on gross and net nitrogen mineralizations in adjacent forest and grassland soils in central Alberta, Canada

Soil pH can be affected by land use change and acid deposition and is one of the primary regulators of nutrient cycling in the soil. In this study, two soils from adjacent forest and grassland sites in central Alberta were subjected to different pH treatments to evaluate the short-term effects of pH on soil gross N transformations using the 15N tracing technique and calculated by the numerical model FLUAZ. For the forest soil, gross NH4+ immobilization increased faster than gross N mineralization rates with increasing soil pH, leading to a declining pattern in net N mineralization rates; however, none of those rates changed with pH in the grassland soil. In contrast, the increase in pH significantly stimulated gross and net nitrification rates while soil acidification decreased gross and net nitrification rates for both the forest and grassland soils. The ratio of gross nitrification to gross NH4+ immobilization rates (N/IA) was significantly increased by KOH addition but declined to nearly zero by HCl addition for each soil. The low and high KCl addition treatments partially or completely inhibited gross nitrification rates, respectively, but gross mineralization was less sensitive to salt additions than the nitrification process. We conclude that based on the short-term laboratory incubation experiments both pH and salt (osmotic effect) affected gross N transformations and pH had contrasting effects on gross and net nitrogen mineralization but not on nitrification in the adjacent forest and grassland soils.