Transformation Rates In Soils Research Articles

Effects of 18 years repeated N fertilizer applications on gross N transformation rates in a subtropical rain-fed purple soil

In this study, a 15N tracing experiment was conducted to determine gross N transformation rates in soils with 18 years of repeated N fertilizer applications and to quantify the underlying mechanisms to explain the variability of crop yield in a subtropical rain-fed purple soil. Six N fertilizer treatments were evaluated: unfertilized control (CK), mineral N fertilizer (NPK), pig manure (OM), mineral N fertilizer plus pig manure (OM-NPK), crop residue (RSD) and mineral N fertilizer plus crop residue (RSD-NPK). The results showed that the soil organic carbon (SOC) concentration in OM (average 13.79 g kg−1) and RSD-NPK (average 13.15 g kg−1) were significantly higher than the other treatments (varied from 5.91 to 11.23 g kg−1) after 18 years. Higher maize yields were observed in NPK, OM, OM-NPK, and RSD-NPK, and especially in RSD-NPK and OM-NPK after 18 years, compared with the data measured in the 11th years. Soil gross N mineralization rates (M) were significantly higher in all N fertilizer treatments (varied from 13.00 to 31.17 mg N kg−1 soil d−1) compared to CK (average 5.53 mg N kg−1 soil d−1). After 18 years of repeated N fertilizer applications, M were significantly higher than those in the 11th years for the same N fertilizer treatment, indicating that long-term repeated N fertilizer application, whether mineral N or organic fertilizer, continuously enhances M. M was positively correlated with SOC (p < 0.05) and total N (TN) (p < 0.05), suggesting that soil organic substrates are key factors regulating M. Maize yield was positively correlated with M (p < 0.05), suggesting that available N produced through soil N mineralization played the important role in yield formation. All N fertilizer treatments continuously stimulated gross autotrophic nitrification rates (ONH4). ONH4 increased with increasing M (p < 0.01) and ammonia-oxidizing bacteria (AOB) abundance (p < 0.05). However, compared to the 11th year, rates of dissimilatory nitrate reduction to ammonium (DNRA) and immobilization of NH4+ (except RSD) and NO3− (except OM-NPK) decreased significantly in the N fertilizer treatments in the 18th year, indicating that N retention capacity is reduced over time and the risk of N losses increased. Considering the best balance among food security, soil C sequestration, and reduction of N leaching, mineral N in combination with straw return (RSD-NPK) can be recommended for regions with similar soils and climates.

Assessing the long-term impact of urease and nitrification inhibitor use on microbial community composition, diversity and function in grassland soil

Reductions in ammonia (NH3) and nitrous oxide (N2O) emissions from agricultural systems are critical for achievement of sustainability targets that underpin international efforts on climate and biodiversity. Urease inhibitors (UI) such as N-(n-butyl) thiophosphoric triamide (NBPT) and nitrification inhibitors (NI) such as dicyandiamide (DCD) slow down microbial and chemical N transformation rates in soil, resulting in decreased environmental N losses. To date there has been minimal assessment of the long-term non-target impacts of UI and NI on soil microbial communities and biological function in grasslands. Utilising a temperate grassland field experiment where fertilisers (with or without inhibitors) were repeatedly applied over a five year period, we assessed the impact of individual or combined inhibitor use on microbial community composition, abundance and function via a combination of functional assays, quantitative polymerase chain reaction (qPCR) assays and amplicon sequence analysis. We also investigated the effect of N inhibitor use on the N functional community, and whether the form of applied N fertiliser (i.e. calcium ammonium nitrate (CAN) or Urea) affected microbial community composition and function. Treatments included a Control (no N); CAN; Urea; Urea + NBPT (UI); Urea + DCD (NI); and Urea + DCD + NBPT (NI & UI). There was no impact of either UI or NI use on non-target microbial community composition or abundance. Function and the abundance of N cycling communities were mainly unaffected by fertilisation or the use of inhibitors. The observed effect of NI was primarily on the nitrification process. There was a significant reduction in nitrification potential associated with the use of NI, and in the case of the Urea + DCD treatment a reduction in COMAMMOX nitrifier abundance, and an increase in potential N mineralisation and N2O emissions. Finally, there was a significant impact of fertilisation and fertiliser type (i.e. CAN or Urea) on the fungal community structure but no impact on bacterial community structure. These results provide a knowledge base that will inform policy regarding the utilisation of N inhibitors as a mitigation measure for reducing gaseous N losses in grasslands.

Grazing rest versus no grazing stimulates soil inorganic N turnover in the alpine grasslands of the Qinghai-Tibet plateau

Little information is available on soil N cycling in the grasslands of the Qinghai-Tibet plateau under different grazing regimes, which is required to evaluate management practices with respect to soil N availability and dynamics. In this study, a 15N-labeled (15NH4NO3 and NH415NO3) incubation experiment was conducted to investigate gross N transformation rates in soils of the southeast region of the Qinghai-Tibet plateau (China) under no grazing, continuous grazing and grazing rest conditions. Compared with no grazing, continuous grazing significantly reduced the rates of mineralization, microbial NH4+ immobilization, autotrophic nitrification and NO3− consumption, but increased the ratio of autotrophic nitrification to microbial NH4+ immobilization and the ratio of the rates of total NO3− production to total NO3− consumption. This indicated that continuous grazing greatly decreased both the inorganic N supply capacity and N turnover but increased the NO3− production potential of the soil. By contrast, mineralization and microbial NH4+ immobilization rates did not significantly differ in the soils of the grazing rest and no grazing treatments, suggesting that a higher soil inorganic N supply capacity is maintained by grazing rest than by continuous grazing. In addition, grazing rest stimulated NO3− turnover, by increasing the rates of autotrophic nitrification, microbial NO3− immobilization and dissimilatory NO3− reduction to NH4+. Taken together, our study suggests that the most effective management practice for the sustainable utilization of local grasslands is a regime based on grazing rest, as it improved physicochemical and biological properties, which in turn increases N supply and N turnover.

The effects of silver nanoparticles on the microbial nitrogen cycle: a review of the known risks.

The nitrogen cycle is an integral biogeochemical function for maintaining healthy environments. Nitrogen is a key nutrient that must be continuously replenished through recycling mechanisms to sustain ecosystems, disruption to which can result in compromised ecosystem functioning. Certain stages in the microbial conversion of nitrogen compounds are performed by a limited range of micro-organisms making these key functional species in ecosystems. The growing industrial use of silver nanoparticles (AgNPs) potentially poses significant risks for microbial nitrogen cycling species. AgNPs possess potent antimicrobial properties and are expected to reach a range of natural environments through several routes of exposure. Certain functional nitrogen cycling microbes have been shown to be highly susceptible to AgNP toxicity. The current literature indicates that AgNPs can negatively affect certain nitrogen fixing, nitrifying and denitrifying microbes in vitro. In vivo studies investigating the effect of AgNPs on nitrogen cycling microbial communities and nitrogen transformation rates in soil, sediment and sludge environments have also indicated disruption of these functional processes. This review provides a comprehensive description of the current state of knowledge regarding the toxicity of AgNPs to nitrogen cycling communities. The aim of the review is to highlight the most susceptible stages in the nitrogen cycle and the implications for the affected ecosystems.

Are nitrate production and retention processes in subtropical acidic forest soils responsive to ammonium deposition?

Changes in soil N-cycling and retention processes in subtropical/tropical acidic forest ecosystems under anthropogenic N inputs are not well understood. We conducted a laboratory 15N tracing study on an acid soil (pH values: 4.6 to 5.0) from a subtropical forest fertilized for more than 2.5 years at a rate of 0, 40, and 120 kg NH4ClN ha−1 yr−1, respectively. To get a better resolution of mechanistic changes in soil N cycling and retention processes under NH4+ additions, we used a conceptual 15N tracing model to quantify process-specific and pool-specific N transformation rates in soils. Gross N mineralization rates decreased at high NH4+ additions, which were paralleled by a reduction in fungal biomass and mineralization of recalcitrant organic N. Gross NH4+ immobilization rates did not show a change with increasing NH4+ additions. Interestingly, soil NO3− production (heterotrophic, autotrophic, and gross nitrification) and retention (NO3− immobilization and dissimilatory nitrate reduction to ammonium) showed insensitivity to increasing additions of NH4+. The mechanisms behind the lack of response of heterotrophic nitrification were unclear, but possibly related to the absence of significant changes in soil C: N ratio and soil acidity under increased NH4+ additions. Because of the low autotrophic nitrification potential and the lack of NH4+ limitation to autotrophic nitrifiers, autotrophic nitrification was unresponsive to NH4+ additions. NO3− immobilization rates appeared to be controlled by the NO3− produced from heterotrophic nitrification, as indicated by the positive relationship between NO3− immobilization and heterotrophic nitrification (R2 = 0.59, p = 0.015), thus showing a lack of a change under increased NH4+ additions. DNRA seemed to be inherently less responsive to environmental changes such as NH4+ deposition. Our work demonstrates that enhanced NH4+ deposition has a low potential to stimulate soil NO3− production and weaken soil retention of NO3− in this, and perhaps other subtropical/tropical acidic forest ecosystems.

A 33P tracing model for quantifying gross P transformation rates in soil

Due to rapid sorption of phosphate released by mineralization processes, in most soils net organic P mineralization rates cannot be derived from changes in extractable inorganic P over time. Besides, a mechanistic understanding is only obtained if the individual gross P transformation rates are known. Available techniques for the quantification of gross P transformation rates rely on isotopic dilution principles. To unambiguously analyse the full dynamics and quantify all relevant simultaneously occurring P transformation rates in soil, we developed a numerical 33P tracing model. The tracing model combines a process-based numerical model with a parameter optimization routine to estimate gross P rates. Each rate can follow either zero-order, first-order or Michaelis–Menten kinetics. The tracing model was used to analyse a previously published dataset by Bünemann et al. (2012). The model was able to simulate the observed dynamics. However, gross organic P mineralization rates were lower than previously published. Net organic P mineralization rates tended towards zero, confirming the overriding dominance of microbial immobilization and re-mineralization processes in the studied grassland soil. The P tracing model presented in this study removes the need for the baseline of isotopic dilution due to physicochemical processes. The dynamic 33P tracing model, which takes into account simultaneous dilution and enrichment, presents an important advancement over the conventional estimation of gross P transformations in isotopic dilution experiments.

Transformation of Chloride to Organic Chlorine in Terrestrial Environments: Variability, Extent, and Implications

Chloride (Clin) is a key component in a number of biogeochemical risk assessment models and, as a rule, these models assume that Clin behaves conservatively in soil. However, this assumption is challenged by the growing number of studies on natural formation–mineralization of organically bound chlorine (Clorg) in terrestrial environments. The authors synthesized and scrutinized previously reported transformation rates of Clin to Clorg in terrestrial environments to assess whether the process is of sufficient size to impact risk assessment models significantly. Their conclusion is that there is increasing evidence that considerable amounts of Clin are transformed to Clorg in or on plants and that the process consistently runs at relatively high rates. The findings imply that risk assessment models that include uptake-transport of Clin by vegetation are likely to be influenced by the transformation of Clin to Clorg in or on vegetation. The review confirms previous observations that transformation rates in soils depend on a combination of a number of factors such as concentration or deposition of Clin, organic matter content, nutrient availability, redox, and season. The review reveals that there seems to be at least two pools of Clorg in the terrestrial environment: one with slow transformation rates, low mobility, and with a comparably low chlorine-to-carbon ratio (leaves, litter, and bulk-soil), and a smaller pool with considerably higher transformation rates, higher mobility and higher chlorine-to-carbon ratios (organic matter in through fall and soil leachate). Formation of chlorinated organic matter has a strong biotic component, implying that models built on physicochemical parameters (e.g., sorption and Kd values) are insufficient to predict movement and fate of 36Cl. The study clearly demonstrates that it is necessary to develop reliable rate estimates and methods that enable a deeper understanding of how and why the Clin is transformed to Clorg and how this influences transport, distribution, and fate of 36Cl in different environments.

Soil drainage and vegetation controls of nitrogen transformation rates in forest soils, southern Quebec

We investigated the influence of soil drainage class and tree species on nitrogen (N) mineralization and nitrification rates in two forest catenas in southern Quebec. Monthly net N mineralization and nitrification rates were determined along transects running from well‐drained to poorly drained soils for 2 years through in situ incubation of homogenized soils. Potential N transformation rates in soils under American beech, sugar maple, and eastern hemlock trees were determined through incubation of homogenized soils in the laboratory under two different moisture regimes (50 and 100% water by volume) mimicking well‐drained and poorly drained soil conditions in the two watersheds. Field‐based N mineralization rates averaged 38 ± 6 mg m−2 d−1 in well‐drained soils, while those in the poorly drained soils averaged 17 ± 5 mg N m−2 d−1. Similarly, net nitrification rates in well‐drained soils (18 ± 4 mg N m−2 d−1) were 3 times greater than those in poorly drained soils (6 ± 3 mg N m−2 d−1). Laboratory‐based potential N mineralization rates in soils ranked sugar maple &gt; American beech &gt; eastern hemlock under both well‐drained (incubated at 50% water by volume) and poorly drained soil conditions (incubated at 100% water by volume). Potential nitrification rates ranked sugar maple &gt; American beech &gt; eastern hemlock under well‐drained soil conditions, while under poorly drained conditions, American beech &gt; sugar maple ≥ eastern hemlock. Nitrification enzyme activity determined through a soil slurry method correlated significantly with field‐based nitrification rates. Differences in soil volumetric water contents, leaf litter N input, and soil C:N ratios, as surrogates of soil drainage and floristic heterogeneity, respectively, correlated significantly with field‐based N mineralization and nitrification rates. Field‐based N mineralization and nitrification rates were higher in summer than in early spring and autumn. Soil drainage class and tree species exert marked controls over N transformation rates in forested landscapes and need to be incorporated when characterizing and/or modeling internal N cycling at watershed scales.

Retention of available P in acid soils of tropical and subtropical evergreen broad-leaved forests

Precipitation of mineral phosphate is often recognized as a factor of limiting the availability of P in acidic soils of tropical and subtropical forests. For this paper, we studied the extractable P pools and their transformation rates in soils of a tropical evergreen forest at Xishuangbanna and a subtropical montane wet forest at the Ailao Mountains in order to understand the biogeochemical processes regulating P availability in acidic soils. The two forests differ in forest humus layer; it is deep in the Ailao forest while little is present in the Xishuangbanna forest. The extractable P pools by resin and sodium-bicarbonate decreased when soil organic carbon content was reduced. The lowest levels of extractable P pools occurred in the surface (0–10 cm) mineral soils of the Xishuangbanna forest. However, microbial P in the mineral soil of the Xishuangbanna forest was twice that in the Ailao forest. Potential rates of microbial P immobilization were greater than those of organic P mineralization in mineral soils for both forests. We suggest that microbial P immobilization plays an essential role in avoiding mineral P precipitation and retaining available P of plant in tropical acidic soils, whereas both floor mass accumulation and microbial P immobilization function benefit retaining plant available P in subtropical montane wet forests.

15N tracing models with a Monte Carlo optimization procedure provide new insights on gross N transformations in soils

15N tracing studies in combination with analyses via process-based models are the current “state-of-the-art” technique to quantify gross nitrogen (N) transformation rates in soils. A crucial component of this technique is the optimization algorithm which primarily decides how many model parameters can simultaneously be estimated. Recently, we published a Markov chain Monte Carlo (MCMC) method which has the potential to simultaneously estimate large number of parameters in 15N tracing models [Müller et al., 2007. Estimation of parameters in complex 15N tracing models by Monte Carlo sampling. Soil Biology & Biochemistry 39, 715–726]. Here, we present the results of a reanalysis of datasets by Kirkham and Bartholomew [1954. Equations for following nutrient transformations in soil, utilizing tracer data. Soil Science Society of America Proceedings 18, 33–34], Myrold and Tiedje [1986. Simultaneous estimation of several nitrogen cycle rates using 15N: theory and application. Soil Biology & Biochemistry 18, 559–568] and Watson et al. [2000. Overestimation of gross N transformation rates in grassland soils due to non-uniform exploitation of applied and native pools. Soil Biology & Biochemistry 32, 2019–2030] using the MCMC technique. Analytical solutions such as the ones derived by Kirkham and Bartholomew [1954. Equations for following nutrient transformations in soil, utilizing tracer data. Soil Science Society of America Proceedings 18, 33–34] result in gross rates without uncertainties. We show that the analysis of the same data sets with the MCMC method provides standard deviations for gross N transformations. The standard deviations are further reduced if realistic data uncertainties are considered. Reanalyzing data by Myrold and Tiedje [1986. Simultaneous estimation of several nitrogen cycle rates using 15N: theory and application. Soil Biology & Biochemistry 18, 559–568] (Capac soil) resulted in a model fit similar to the one of the original analysis but with more precise estimates of gross N transformations. In addition, our analysis showed that small N transformations such as heterotrophic nitrification, which was neglected in the original analysis, could be quantified for this soil. Watson et al. [2000. Overestimation of gross N transformation rates in grassland soils due to non-uniform exploitation of applied and native pools. Soil Biology & Biochemistry 32, 2019–2030] provided evidence of a non-uniform exploitation of applied and native N that led to an overestimation of gross N transformations. Reanalyzing the data (CENIT soil, low N application) with the Müller et al. [2007. Estimation of parameters in complex 15N tracing models by Monte Carlo sampling. Soil Biology & Biochemistry 39, 715–726] model where NH 4 + oxidation was set to Michaelis–Menten kinetics resulted in a satisfactory fit between modeled and observed data, indicating that the observed artifact by Watson et al. [2000. Overestimation of gross N transformation rates in grassland soils due to non-uniform exploitation of applied and native pools. Soil Biology & Biochemistry 32, 2019–2030] was mainly due to inappropriate kinetic settings. Our study shows that the combination of a MCMC method with 15N tracing models is able to consider more complex and possibly more realistic models and kinetic settings to estimate gross N transformation rates and thus overcomes restriction of previous 15N tracing techniques.

Estimation of parameters in complex 15N tracing models by Monte Carlo sampling

The most widely used method to quantify gross N transformation rates in soils is based on 15N dilution and enrichment principles. To identify rate parameters, 15N-tracing experiments are analysed by models that are linked to algorithms that try to minimize the misfit between modelled and observed data. In currently available 15N-tracing models optimization algorithms are based on the Levenberg–Marquardt method that is suitable for the determination of small number of parameters. Therefore, these models are restricted to a few processes. Methods based on Monte Carlo sampling have the potential to overcome restrictions on parameter numbers but have not been tested for application in 15N-tracing models. Here, for the first time, we use a Markov chain Monte Carlo (MCMC) method with a tracing model to simultaneously determine the probability density functions (PDFs) of the whole set of parameters for a previously published data set [Müller, C., Stevens, R.J., Laughlin, R.J., 2004. A 15N tracing model to analyse N transformations in old grassland soil. Soil Biology & Biochemistry 36, 619–632]. We show that the MCMC method can simultaneously determine PDFs of more than 8 parameters and demonstrate for the first time that it is possible to optimize models where transformations are described by Michaelis–Menten kinetics. Setting the NH 4 + oxidation rate to Michaelis–Menten kinetics reduced the misfit by 19%. Together with monitoring diagnostics for parameter convergence, the MCMC method is a very efficient and robust technique to determine PDFs for parameters in 15N-tracing models that contain large number of N transformations and complex process descriptions.

Short-term nitrogen transformation rates in riparian wetland soil determined with nitrogen-15

N transformation rates in soil from a riparian wetland that receives runoff from adjacent pastoral land were investigated in a short-term (250 min), anaerobic laboratory incubation (20°C). A joint 15N tracing-isotope dilution technique was employed that used paired incubations of labelled (99 atom % 15N) NO3–-unlabelled NH4+ and unlabelled NO3–-labelled (99 atom % 15N) NH4+ at three N input levels (0.4, 4 and 24 μg N g–1 soil). At each N input level NO3– and NH4+ were added in equal proportions (0.2, 2 and 12 μg N g–1 soil). Soil and gas samples were analysed after 10, 70 and 250 min, and the fate of 15N and N transformation rates were determined for each time period; 0–10 min (phase 1), 10–70 min (phase 2) and 70–250 min (phase 3). N transformation rates for all processes except gross NH4+ mineralisation were very high during phase 1. Processes favoured by aerobic conditions, NO3– immobilisation (0–17% NO3– removal, 0–8.2 μg N g–1 soil h–1), autotrophic nitrification (~2% NH4+ removal, 0.58–0.88 μg N g–1 soil h–1) and heterotrophic nitrification (11–35 μg N g–1 soil h–1) increased with increased N input while the anaerobic dissimilatory NO3– reduction to NH4+ process (1–6% NO3– removal, 0.48–0.62 μg N g–1 soil h–1) decreased, presumably due to the oxidising effect of higher NO3– inputs. Denitrification (8–78% NO3– removal, 3.8–9.6 μg N g–1 soil h–1) exhibited no clear trend related to N input levels. NH4+ immobilisation (39–72% NH4+ removal, 15–19 μg N g–1 soil h–1) was higher than NO3– immobilisation. Gross NH4+ mineralisation (0.27–0.80 μg N g–1 soil h–1) was the only process not detected in phase 1 and one of few processes measurable in phases 2 or 3.

Nitrogen and phosphorus release from humus and mineral soil under black spruce forests in central Quebec

In the black spruce ( Picea mariana (Mill.) BSP) forests which span North America, low annual temperatures, high nitrogen and phosphorus absorption by feathermosses and small litter inputs contribute to reduced annual N and P transformation rates in soils of these ecosystems. In past studies of nutrient dynamics in these systems, concentrations of dissolved organic N (DON) and P (DOP) in soil extracts have equaled or exceeded those of mineral N and P; therefore, organic forms of N and P may be an important source of nutrition to plants growing in this region. Our objective was to determine if DON and DOP were important constituents in repeated extractions of laboratory incubated organic material and mineral soils taken from recently burned, recently harvested and fully stocked black spruce stands in central Quebec. Cumulative concentrations of DON ranged from 7 to 17% and 31 to 45% of total N extracted from the organic material and mineral soils, respectively. Cumulative concentrations of DOP ranged from 35 to 44% and 37 to 48% of total P extracted from the organic material and mineral soils, respectively. We detected a pulse of CO 2-C release from the organic material after thawing, and weekly CO 2-C release was related to NH 4 +-N release ( R 2=0.36, P=0.0001). These results suggest that increases in temperature after a winter freeze result in a pulse of microbial activity and NH 4 +-N mineralization in organic layers and that DON and DOP are an important part of N and P cycling in these boreal systems.

Gross nitrogen transformation rates in soil at a surface coal mine site reclaimed for prime farmland use

Organic wastes were used to increase N fertility at a surface mine reclamation site, with the ultimate goal to stimulate microbial activity and improve the reclaimed soil's chemical and physical properties. Gross N transformation rates are indicators of microbial activity but are undocumented in such reconstructed ecosystems. We measured gross nitrification, N mineralization and N immobilization in waste-amended and unamended soil using 15N pool dilution techniques. Measurements were made in June, July and November 1993 at the reclamation site in western Kentucky, and compared to net N transformation rates. The premise that organic waste amendment stimulates microbial activity in reclaimed soils was supported by the data. Gross N mineralization, nitrification and immobilization rates were as much as 4.5 times greater in waste-amended soil than unamended soil. Gross N mineralization and nitrification rates and gross NH 4 + and NO 3 − immobilization rates were significantly greater than net rates in waste-amended and unamended soil. There was net immobilization of NH 4 + and NO 3 − in waste-amended soil, whereas there was net N mineralization in unamended soil. This was consistent with using substrates containing high C-to-N ratios. Reclamation practices created soil environments in which gross N transformation rates were of the same magnitude as those measured for less disturbed soil ecosystems.