Amino acid,peptide and protein mineralization dynamics in a taiga forest soil

The availability of inorganic N has been shown to be one of the major factors limiting primary productivity in high latitude ecosystems. The factors regulating the rate of transformation of organic N to nitrate and ammonium, however, remain poorly understood. The aim of this study was to investigate the nature of the soluble N pool in forest soils and to determine the relative rate of inorganic N production from high and low molecular weight (MW) dissolved organic nitrogen (DON) compounds in black spruce forest soils. DON was found to be the dominant N form in soil solution, however, most of this DON was of high MW of which >75% remained unidentified. Free amino acids constituted less than 5% of the total DON pool. The concentration of NO₃⁻ and NH₄⁺ was low in all soils but significantly greater than the concentration of free amino acids. Incubations of low MW DON with soil indicated a rapid processing of amino acids, di- and tri-peptides to NH₄⁺ followed by a slower transformation of the NH₄⁺ pool to NO₃⁻. The rate of protein transformation to NH₄⁺ was slower than for amino acids and peptides suggesting that the block in N mineralization in taiga forest soils is the transformation of high MW DON to low MW DON and not low MW DON to NH₄⁺ or NH₄⁺ to NO₃⁻. Calculated turnover rates of amino acid-derived C and N immobilized in the soil microbial biomass were similar with a half-life of approximately 30 d indicating congruent C and N mineralization.     

Microbial activity differentially regulates the vertical mobility of nitrogen compounds in soil

Alongside nitrate, dissolved organic nitrogen (DON) represents a significant N loss pathway in many agroecosystems. To better understand the factors controlling DON leaching in soil we followed the vertical movement of ¹⁵N-labeled NO₃⁻, NH₄⁺, alanine and trialanine in packed soil columns in response to a simulated rainfall event. We show that in autoclaved (sterile) soil where sorption is assumed to be the dominant regulating factor, leaching followed the series NO₃⁻ > trialanine > alanine > NH₄⁺. In the non-sterile packed soil columns, the rapid rate of NO₃⁻ leaching was unaffected whilst the movement of the amino acid, peptide and NH₄⁺ was almost completely prevented due to microbial immobilization. Our results support the view that (1) DON loss from agricultural soils occurs mainly in the form of recalcitrant compounds (e.g. humic DON) rather than in the form of labile low MW DON (e.g. oligopeptides and amino acids), and (2) that although nitrate was bioavailable, it was not a preferred N form for the C-limited microbial biomass.     

Effects of NH4+ and NO3− on litter and soil organic carbon decomposition in a Chinese fir plantation forest in South China

Soil organic carbon (SOC) dynamics and nutrient availability determine the soil quality and fertility in a Chinese fir plantation forest in subtropical China. Uniformly ¹³C-labeled Chinese fir (Cunninghamia lanceolata) and alder (Alnus cremastogyne) leaf litter with or without 100 mg NH₄⁺ or NO₃⁻ were added to the soil. The purpose was to investigate the influence of N availability on the decomposition of the litter and native SOC. The production of CO₂, the natural abundance of ¹³C–CO₂, and the inorganic N dynamics were monitored. The results showed that Chinese fir (with a high C:N ratio) and alder (with a low C:N ratio) leaf litter caused significant positive priming effects (PEs) of 24% and 42%, respectively, at the end of the experiment (235 d). The PE dynamics showed that positive PE can last for at least 87 d. However, the possible occurrence of a significant negative PE with a sufficient incubation period is difficult to confirm. The application of both NH₄⁺ and NO₃⁻ was found to have a stimulating effect on the decomposition of Chinese fir and alder leaf litter in the early stage (0–15 d) of incubation, but an adverse effect in the late stage. Compared with NO₃⁻, NH₄⁺ caused a greater decrease in the PE induced by both Chinese fir and alder leaf litter. The effects of NH₄⁺ and NO₃⁻ on the PE dynamics had different patterns for different incubation stages. This result may indicate that the stability or recalcitrance of SOC, especially in such plantation forest soils, strongly depends on available leaf litter and application of N to the soil.     

Heterotrophic nitrification is the predominant NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> production mechanism in coniferous but not broad-leaf acid forest soil in subtropical China

A study was carried out to investigate the potential gross nitrogen (N) transformations in natural secondary coniferous and evergreen broad-leaf forest soils in subtropical China. The simultaneously occurring gross N transformations in soil were quantified by a ¹⁵N tracing study. The results showed that N dynamics were dominated by NH₄⁺ turnover in both soils. The total mineralization (from labile and recalcitrant organic N) in the broad-leaf forest was more than twice the rate in the coniferous forest soil. The total rate of mineral N production (NH₄⁺ + NO₃⁻) from the large recalcitrant organic N pool was similar in the two forest soils. However, appreciable NO₃⁻ production was only observed in the coniferous forest soil due to heterotrophic nitrification (i.e. direct oxidation of organic N to NO₃⁻), whereas nitrification in broad-leaf forest was little (or negligible). Thus, a distinct shift occurred from predominantly NH₄⁺ production in the broad-leaf forest soil to a balanced production of NH₄⁺ and NO₃⁻ in the coniferous forest soil. This may be a mechanism to ensure an adequate supply of available mineral N in the coniferous forest soil and most likely reflects differences in microbial community patterns (possibly saprophytic, fungal, activities in coniferous soils). We show for the first time that the high nitrification rate in these soils may be of heterotrophic rather than autotrophic nature. Furthermore, high NO₃⁻ production was only apparent in the coniferous but not in broad-leaf forest soil. This highlights the association of vegetation type with the size and the activity of the SOM pools that ultimately determines whether only NH₄⁺ or also a high NO₃⁻ turnover is present.     

Wetting and drying events determine soil N pools in two Mediterranean ecosystems

To improve our knowledge of how nutrient cycling in Mediterranean environments responds to climate change, we evaluated the effects of the continuous changes in soil nitrogen (N) pools during natural wetting and drying events. We measured soil N pools (microbial biomass [MB-N], dissolved organic nitrogen [DON], NH₄⁺ and NO₃⁻) and N ion exchange resins at weekly intervals for one year in two contrasting Mediterranean ecosystems. All soil N fractions in both ecosystems showed high intraseasonal and interseasonal variability that was greater in inorganic soil fractions than in organic N soil fractions. MB-N, DON and resin-NH₄⁺ showed increased concentrations during wetting events. Only the soil NO₃⁻ and resin-NO₃⁻ showed the opposite trend, suggesting a different response to water pulses compared to the other soil variables. Our results show that N pools are continuously changing, and that this high variability is not associated with the total amount of organic matter and labile soil carbon (C) and N soil fractions found in each ecosystem. The highest variability was found for inorganic N forms, which suggests that organic N forms are more buffered in soils exposed to wetting-drying cycles. Our results suggest that the changes in wetting-drying cycles expected with global climate change may have a significant impact on the availability and turnover of organic and inorganic N.     

Microorganisms in sewage sludge added to an extreme alkaline saline soil affect carbon and nitrogen dynamics

There are plans to vegetate soil of the former lake Texcoco and use wastewater sludge to provide nutrients. However, the Texcoco soil is N depleted, so the amount of N available to the vegetation might be limited and the dynamics of C and N affected. We investigated how emissions of CO₂, N₂O and N₂, and dynamics of mineral N were affected when different types of N fertilizer, i.e. NH₄⁺, NO₃⁻, or unsterilized or sterilized wastewater sludge, were added to the Texcoco soil. An agricultural soil served as control. Sewage sludge added to an alkaline saline soil (Texcoco soil) induced a decrease in concentrations of NH₄⁺ and NO₃⁻. Addition of sewage sludge increased the CO₂ emission rate > two times compared to soil amended with sterilized sludge. The NH₄⁺ concentration was lower when sludge was added to an agricultural soil for the first 28 days and in the Texcoco soil for 56 days compared to soil amended with sterilized sludge. Production of N₂O in the agricultural soil was mainly due to nitrification, even when sludge was added, but denitrification was the main source of N₂O in the Texcoco soil. Microorganisms in the sludge reduced N₂O to N₂, but not the soil microorganisms. It was found that microorganisms added with the sludge accelerated organic material decomposition, increased NH₄⁺ immobilization, and induced immobilization of NO₃⁻ (in Texcoco soil). These results suggest that wastewater sludge improves soil fertility at Otumba (an agricultural soil) and would favour the vegetation of the Texcoco soil (alkaline saline).     

Simulating the dynamics of glucose and NH4+ in alkaline saline soils of the former Lake Texcoco with the Detran model

The turnover of organic matter determines the availability of plant nutrients in unfertilized soils, and this applies particularly to the alkaline saline soil of the former Lake Texcoco in Mexico. We investigated the effects of alkalinity and salinity on dynamics of organic material and inorganic N added to the soil. Glucose labelled with ¹⁴C was added to soil of the former Lake Texcoco drained for different lengths of time, and dynamics of ¹⁴C, C and N were investigated with the Detran model. Soil was sampled from an undrained plot and from three drained for 1, 5 and 8 years, amended with 1000 mg ¹⁴C‐labelled glucose kg^?1 and 200 mg NH₄⁺‐N kg^?1, and incubated aerobically. Production of ¹⁴CO₂ and CO₂, dynamics of NH₄⁺, NO₂^– and NO₃^–, and microbial biomass ¹⁴C, C and N were monitored and simulated with the Detran model. A third stable microbial biomass fraction had to be introduced in the model to simulate the dynamics of glucose, because > 90 mg ¹⁴C kg^?1 soil persisted in the soil microbial biomass after 97 days. The observed priming effect was mostly due to an increased decay of soil organic matter, but an increased turnover of the microbial biomass C contributed somewhat to the phenomenon. The dynamics of NH₄⁺ and NO₃^– in the NH₄⁺‐amended soil could not be simulated unless an immobilization of NH₄⁺ into the microbial biomass occurred in the first day of the incubation without an immediate incorporation of it into microbial organic material. The dynamics of C and a priming effect could be simulated satisfactorily, but the model had to be adjusted to simulate the dynamics of N when NH₄⁺ was added to alkaline saline soils.     

Influence of temperature and vegetation cover on soluble inorganic and organic nitrogen in a spodosol

In this study, the influence of temperature and vegetation cover on soluble inorganic and organic nitrogen in a spodosol from north east Scotland was investigated. Firstly, soil cores were incubated at 5, 10 and 15°C for up to 8 weeks. Net mineralisation was observed at all temperatures with larger rates observed at higher temperatures. In contrast, water extractable dissolved organic nitrogen (DON) displayed no clear trend with time and showed little response to temperature. Secondly, intact cores of the same soil, with and without vegetation, were leached with artificial rain for 6 weeks at 6.5 and 15°C. Temperature and the presence of vegetation interacted to have a significant (P<0.01) effect on the concentration of NO₃⁻ in leachates; highest concentrations were observed in leachates from cores without vegetation at 15°C, whereas lowest concentrations were observed in leachates from cores with vegetation at 6.5°C. In contrast, concentrations of DON and dissolved organic carbon (DOC) were significantly (P<0.001) higher in leachates from cores with vegetation than without vegetation and were not affected by temperature. The cumulative amounts of DON and DOC leached from the cores with vegetation were 4 and 2.5 times greater, respectively, than those leached from the cores without vegetation. Comparison of soil solution (extracted by centrifugation at 0–5 and 5–10 cm depth) after leaching for 6 weeks, showed that the upper layer contained more than twice the amount of DON than the 5–10 cm layer and that the difference in concentration between the two depths was enhanced in the presence of vegetation. The results indicate that vegetation is an important source of DON and DOC. However, the removal of vegetation did not lead to an increase in the quantity of total dissolved nitrogen (TDN) in soil water, but resulted in a change in the dominant N fraction from DON to NO₃⁻. In addition, the results show that DON, in both the incubated and leached cores, did not change as inorganic N was mineralised. This suggests that if water extractable DON was acting as a source of NH₄⁺ or NO₃⁻, then it was being replenished by, and in equilibrium with, a large reserve of organic N. Evidence of such a pool was indirect in the form of additional DON (equivalent to 2 g N m⁻²) being extracted by 0.5 M K₂SO₄.     

Soil nitrogen conservation mechanisms in a pristine south Chilean Nothofagus forest ecosystem

A ¹⁵N tracing study was carried out to identify microbial and abiotic nitrogen (N) transformations in a south Chilean Nothofagus betuloides forest soil which is characterized by low N inputs and absence of human disturbance. Gross N transformation rates were quantified with a ¹⁵N tracing model in combination with a Markov chain Monte Carlo sampling algorithm for parameter estimation. The ¹⁵N tracing model included five different N pools (ammonium (NH₄⁺), nitrate (NO₃⁻), labile (N_lab) and recalcitrant (N_rec) soil organic matter and adsorbed NH₄⁺), and ten gross N transformation rates. The N dynamics in the N. betuloides ecosystem are characterized by low net but high gross mineralization rates. Mineralization in this soil was dominated by turnover of N_lab, while immobilization of NH₄⁺ predominantly entered the N_rec pool. A fast exchange between the NH₄⁺ and the adsorbed NH₄⁺ pool was observed, possibly via physical adsorption on and release from clay lattices, providing an effective buffer for NH₄⁺. Moreover, high NH₄⁺ immobilization rates into the N_rec pool ensure a sustained ecosystem productivity. Nitrate, the most mobile form of N in the system, is characterized by a slow turnover and was produced in roughly equal amounts from NH₄⁺ oxidation and organic N oxidation. More than 86% of the NO₃⁻ produced was immediately consumed again. This study showed for the first time that dissimilatory nitrate reduction to ammonium (DNRA) was almost exclusively (>99%) responsible for NO₃⁻ consumption. DNRA rather than NO₃⁻ immobilization ensures that NO₃⁻ is transformed into another available N form. DNRA may therefore be a widespread N retention mechanism in ecosystems that are N-limited and receive high rainfalls.     

Differing N status and N retention processes of soils under old-growth lowland forest in Eastern Amazonia,Caxiuanã, Brazil

Indirect evidence of the nitrogen (N) status of tropical forests strongly suggests that in heavily weathered soils under old-growth lowland tropical forests nitrogen is in relative excess. However, within the lowland forests of the Amazon basin, there is substantial evidence that soil texture influences soil NH₄⁺ and NO₃^? concentrations and hence possibly N availability and retention in the soil. Here, we evaluate the soil N status of two heavily weathered soils which contrast in texture (sandy versus clay Oxisol). Using ¹⁵N pool dilution, we quantified gross rates of soil N cycling and retention. We also measured the δ¹⁵N signatures from the litter layer down to 50-cm depth mineral soil and calculated the overall ¹⁵N enrichment factor (ε) for each soil type. The clay soil showed high gross N mineralization and nitrification rates and a high overall ¹⁵N enrichment factor, signifying high N losses. The sandy soil had low gross rates of N cycling and ¹⁵N enrichment factor, manifesting a conservative soil N cycling. Faster turnover rates of NH₄⁺ compared to NO₃^? indicated that NH₄⁺ cycles faster through microorganisms than NO₃^?, possibly contributing to better retention of NH₄⁺ than NO₃^?. However this was opposite to abiotic retention processes, which showed higher conversion of NO₃^? to the organic N pool than NH₄⁺. Our combined results suggest that clay Oxisol in Amazonian forest have higher N availability than sandy Oxisol, which will have important consequences for changes in soil N cycling and losses when projected increase in anthropogenic N deposition will occur.     

Severe soil frost reduces losses of carbon and nitrogen from the forest floor during simulated snowmelt: A laboratory experiment

Considerable progress has been made in understanding the impacts of soil frost on carbon (C) and nitrogen (N) cycling, but the effects of soil frost on C and N fluxes during snowmelt remain poorly understood. We conducted a laboratory experiment to determine the effects of soil frost on C and N fluxes from forest floor soils during snowmelt. Soil cores were collected from a sugar maple (Acer saccharum)–American beech (Fagus grandifolia) and a red spruce (Picea rubens)–balsam fir (Abies balsamea) forest at the Hubbard Brook Experimental Forest in New Hampshire, U.S.A. Soils were exposed to one of three temperature treatments, including severe (?15 °C), mild (?0.5 °C), and no soil frost (+5 °C) conditions. After one week the soils were incubated at +5 °C and snow was placed on top of the soils to simulate spring snowmelt. NO₃^? losses were up to 5.5 mg N kg^?1 soil greater in the mild soil frost treatment than the severe soil frost treatment. Net losses of NH₄⁺ and DON in leachate were up to 19 and 18 mg N kg^?1 soil greater in the no soil frost and mild soil frost treatments, respectively, than the severe soil frost treatment. In contrast, soil frost did not have a significant impact on dissolved organic C or cumulative gaseous fluxes of C and N throughout the snowmelt period. However, the total cumulative flux of C (i.e. dissolved organic C + CO₂ + CH₄) and N (i.e. dissolved organic N + NH₄ + NO₃ + N₂O) in the severe soil frost treatment were between one quarter and one half that observed in the no soil frost treatment for both forest types. Together, the results of this study show that total fluxes of N in leachate, as well as total cumulative C and N fluxes (gases + leachate), were significantly reduced following severe soil frost. We conclude that the extent to which C and N cycling during snowmelt is altered in response to changes in winter climate depend on both the presence and severity of soil frost.     

Evidence that fungi can oxidize NH4+ to NO3− in a grassland soil

The contribution of bacteria and fungi to NH₄⁺ and organic N (N_org) oxidation was determined in a grassland soil (pH 6.3) by using the general bacterial inhibitor streptomycin or the fungal inhibitor cycloheximide in a laboratory incubation study at 20°C. Each inhibitor was applied at a rate of 3 mg g^?1 oven‐dry soil. The size and enrichment of the mineral N pools from differentially (NH₄¹⁵NO₃ and ¹⁵NH₄NO₃) and doubly labelled (¹⁵NH₄¹⁵NO₃) NH₄NO₃ were measured at 3, 6, 12, 24, 48, 72, 96 and 120 hours after N addition. Labelled N was applied to each treatment, to supply NH₄⁺‐N and NO₃^?‐N at 3.15 μmol N g^?1 oven‐dry soil. The N treatments were enriched to 60 atom % excess in ¹⁵N and acetate was added at 100 μmol C g^?1 oven‐dry soil, to provide a readily available carbon source. The oxidation rates of NH₄⁺ and N_org were analysed separately for each inhibitor treatment with a ¹⁵N tracing model. In the absence of inhibitors, the rates of NH₄⁺ oxidation and organic N oxidation were 0.0045 μmol N g^?1 hour^?1 and 0.0023 μmol N g^?1 hour^?1, respectively. Streptomycin had no effect on nitrification but cycloheximide inhibited the oxidation of NH₄⁺ by 89% and the oxidation of organic N by more than 30%. The current study provides evidence to suggest that nitrification in grassland soil is carried out by fungi and that they can simultaneously oxidize NH₄⁺ and organic N.     

Effect of elevated CO2 on soil N dynamics in a temperate grassland soil

The response of terrestrial ecosystems to elevated atmospheric CO₂ is related to the availability of other nutrients and in particular to nitrogen (N). Here we present results on soil N transformation dynamics from a N-limited temperate grassland that had been under Free Air CO₂ Enrichment (FACE) for six years. A ¹⁵N labelling laboratory study (i.e. in absence of plant N uptake) was carried out to identify the effect of elevated CO₂ on gross soil N transformations. The simultaneous gross N transformation rates in the soil were analyzed with a ¹⁵N tracing model which considered mineralization of two soil organic matter (SOM) pools, included nitrification from NH₄⁺ and from organic-N to NO₃⁻ and analysed the rate of dissimilatory NO₃⁻ reduction to NH₄⁺ (DNRA). Results indicate that the mineralization of labile organic-N became more important under elevated CO₂. At the same time the gross rate of NH₄⁺ immobilization increased by 20%, while NH₄⁺ oxidation to NO₃⁻ was reduced by 25% under elevated CO₂. The NO₃⁻ dynamics under elevated CO₂ were characterized by a 52% increase in NO₃⁻ immobilization and a 141% increase in the DNRA rate, while NO₃⁻ production via heterotrophic nitrification was reduced to almost zero. The increased turnover of the NH₄⁺ pool, combined with the increased DNRA rate provided an indication that the available N in the grassland soil may gradually shift towards NH₄⁺ under elevated CO₂. The advantage of such a shift is that NH₄⁺ is less prone to N losses, which may increase the N retention and N use efficiency in the grassland ecosystem under elevated CO₂.     

An inverse abundance approach to separate soil nitrogen pools and gaseous nitrogen fluxes into fractions related to ammonium,nitrate and soil organic nitrogen

The soil nitrogen (N) cycle exhibits a variety of complex biochemical reactions in which N species such as NO₂⁻, NO and N₂O are produced and consumed by co‐existing processes that respond differently to the local environmental conditions. Key to understanding the soil N cycle in its full complexity is the development and application of methods that allow a quantification of individual pathways and processes that are responsible for the build‐up and/or emission of N compounds. Triplet ¹⁵N tracer experiments (TTE) have been developed and applied to allow a source‐related quantification of N species such as NO₂⁻, and N₂O by different biochemical pathways (e.g. ammonia oxidation and nitrate reduction) that are related to multiple N sources (NH₄⁺, NO₃⁻ and N_org). An analysis of a TTE requires the application of either a numerical or analytical model. Because of the ease of application it is desirable to use analytical models. However, available analytical solutions suffer from serious drawbacks concerning the quantification of N fluxes related to soil organic N. In this paper we describe the development and application of a new inverse abundance approach (IAA) to analyse a TTE. Theoretical and experimental data sets of soil N₂O release were analysed by the new method. The IAA was also applied to an already existing data set to identify fractions of the soil nitrite pool related to NH₄⁺, NO₃⁻ and N_org. We show that the IAA provides a reliable and comprehensive data evaluation of a TTE.     

Role of dissolved organic nitrogen (DON) in soil N cycling in grassland soils

Dissolved organic nitrogen (DON) represents a significant pool of soluble N in many soils and freshwaters. Further, the low molecular weight (LMW) component of DON represents an important source of N for microorganisms and can also be utilized directly by some plants. Our purpose was to determine which of the pathways in the decomposition and subsequent ammonification and nitrification of organic N represented a significant block in soil N supply in three agricultural grassland soils. The results indicate that the conversion of insoluble organic N to LMW-DON and not LMW-DON to NH₄⁺ or NH₄⁺ to NO₃⁻ represents a major constraint to N supply. We hypothesize that there are two distinct DON pools in soil. The first pool comprises mainly free amino acids and proteins and is turned over very rapidly by the microbial community, so it does not accumulate in soil. The second pool is a high molecular weight pool rich in humic substances, which turns over slowly and represents the major DON loss to freshwaters. The results also suggest that in NO₃⁻ rich soils the uptake of LMW-DON by soil microorganisms may primarily provide them with C to fuel respiration, rather than to satisfy their internal N demand.     

Analysis and behavior of soluble organic nitrogen in forest soils

Background, aim, and scope  

A large proportion of soil nitrogen (N; >80%) is present in organic form. Current research on plant N uptake in terrestrial ecosystems has focused mainly on inorganic N such as ammonium (NH₄ ⁺) and nitrate (NO₃ ⁻), while soluble organic N (SON) has received little attention. In recent years, the increasing evidence showing the direct uptake of various amino acids by plants and the predominance of the organic form in N loss by leaching in many forest ecosystems has drawn attention to critically re-examine the nature and the ecological role of soil SON in terrestrial N cycling. However, little is known about the sources and dynamics, chemical nature, and ecological functions of soil SON in forest ecosystems. This paper reviews recent advances in the areas of research on current techniques for characterizing soil SON and the size, nature, and dynamics of soil SON pools in forest ecosystems.     

Emission of carbon dioxide and dynamics of inorganic N in a gradient of alkaline saline soils of the former lake Texcoco

A plan was developed to apply biosolid to soil of the former lake Texcoco to fertilize the pioneer vegetation. Because, no information exists about how differences in electrolytic conductivity (EC) might affect mineralization of biosolid and dynamics of C and N in soil, 20 soil samples forming a gradient in EC ranging from 22 to 150 dS m⁻¹ were characterized, amended with 500 mg biosolid C kg⁻¹ dry soil and incubated aerobically at 22 ± 2 °C while production of CO₂, concentrations of ammonium (NH₄⁺), nitrite (NO₂⁻), and nitrate (NO₃⁻), and NH₃ volatilization were monitored at 22 ± 2 °C for 70 days. Soil characteristics showed large variations with maximum values often >10-times larger than minimum values. The production of CO₂ in the unamended soil ranged from 25 to 159 mg CO₂-C kg⁻¹ day⁻¹ and NH₃ volatilization from 0 to 189 μg NH₃-N kg⁻¹ day⁻¹_. Application of biosolid increased production of CO₂ significantly 1.4-fold and volatilization of NH₃ 11.5-fold. The EC explained most of the variation in production of CO₂, while particle size distribution explained most of the variation in volatilization of NH₃. The concentration of NH₄⁺ in the biosolid-amended soil decreased sharply in the first 14 days, with the EC explaining most of the variation found, and remained constant thereafter with a small increase at day 70. Significant increases in the concentration of NO₃⁻ were generally found in soil with EC < 64 dS m⁻¹. The EC explained most of the variation in production of CO₂, and dynamics of NH₄⁺ and NO₃⁻ while clay positively and sand content negatively affected NH₃ volatilization. It was found that increases in EC inhibited C and N mineralization in soil of the former lake Texcoco.     

Experimental nitrogen deposition alters the quantity and quality of soil dissolved organic carbon in an alpine meadow on the Qinghai-Tibetan Plateau

Dissolved organic matter (DOM) plays a central role in driving biogeochemical processes in soils, but little information is available on the relation of soil DOM dynamics to microbial activity. The effects of NO₃⁻ and NH₄⁺ deposition in grasslands on the amount and composition of soil DOM also remain largely unclear. In this study, a multi-form, low-dose N addition experiment was conducted in an alpine meadow on the Qinghai–Tibetan Plateau in 2007. Three N fertilizers, NH₄Cl, (NH₄)₂SO₄ and KNO₃, were applied at four rates: 0, 10, 20 and 40 kg N ha⁻¹ yr⁻¹. Soil samples from surface (0–10 cm) and subsurface layers (10–20 cm) were collected in 2011. Excitation/emission matrix fluorescence spectroscopy (EEM) was used to assess the composition and stability of soil DOM. Community-level physiological profile (CLPP, basing on the BIOLOG Ecoplate technique) was measured to evaluate the relationship between soil DOC dynamics and microbial utilization of C resources. Nitrogen (N) dose rather than N form significantly increased soil DOC contents in surface layer by 23.5%–35.1%, whereas it significantly decreased soil DOC contents in subsurface layer by 10.4%–23.8%. Continuous five-year N addition significantly increased the labile components and decreased recalcitrant components of soil DOM in surface layer, while an opposite pattern was observed in subsurface layer; however, the humification indices (HIX) of soil DOM was unaltered by various N treatments. Furthermore, N addition changed the amount and biodegradability of soil DOM through stimulating microbial metabolic activity and preferentially utilizing organic acids. These results suggest that microbial metabolic processes dominate the dynamics of soil DOC, and increasing atmospheric N deposition could be adverse to the accumulation of soil organic carbon pool in the alpine meadow on the Qinghai-Tibetan Plateau.     

Charcoal production at kiln sites affects C and N dynamics and associated soil microorganisms in Quercus spp. temperate forests of central Mexico

Temperate forests dominated by Quercus spp. cover large parts of Central Mexico and rural communities depend on these forests for wood and charcoal. The impacts of charcoal production on selected chemical properties including C and N dynamics, and populations of ammonifiers, nitrifiers and denitrifiers were investigated on surface soils (0–15 cm) collected during the dry and rainy season of these forests. Organic C was halved in soil at the kiln sites compared to undisturbed forest soil. Concentrations of exchangeable Ca²⁺, K⁺ and Mg²⁺ increased >1.6 times at kiln sites and pH increased from 4.5 in undisturbed soil to 7.0 at kiln sites. The kiln sites had 1.3 times and 2.4 times lower microbial biomass C and N, respectively, than undisturbed forest sites during the rainy season. Although the effect of charcoal production on NH₄⁺, NO₂^? and NO₃^? concentrations was small, the ammonifying, nitrifying and denitrifiers were 16 times lower at the kiln sites than in the undisturbed forest soil. This research found that the charcoal production had a negative effect on the cultivable microorganisms involved in N cycling and the soil microbial biomass C and N compared to undisturbed forest soil. Differences in inorganic N dynamics were more affected by seasonality, i.e. precipitation, than by charcoal production.     

Impact of nitrogen and phosphorus additions on soil gross nitrogen transformations in a temperate desert steppe

Nutrient addition has a significant impact on plant growth and nutrient cycling. Yet, the understanding of how the addition of nitrogen (N) or phosphorus (P) significantly affects soil gross N transformations and N availability in temperate desert steppes is still limited. Therefore, a ¹⁵N tracing experiment was conducted to study these processes and their underlying mechanism in a desert steppe soil that had been supplemented with N and P for 4 years in northwestern China. Soil N mineralization was increased significantly by P addition, and N and P additions significantly promoted soil autotrophic nitrification, rather than NH₄⁺-N immobilization. The addition of N promoted dissimilatory NO₃⁻ reduction to NH₄⁺, while that of P inhibited it. Soil NO₃⁻-N production was greatly increased by N added alone and by that of N and P combined, while net NH₄⁺-N production was decreased by these treatments. Soil N mineralization was primarily mediated by pH, P content or organic carbon, while soil NH₄⁺-N content regulated autotrophic nitrification mainly, and this process was mainly controlled by ammonia-oxidizing bacteria rather than archaea and comammox. NH₄⁺-N immobilization was mainly affected by functional microorganisms, the abundance of narG gene and comammox Ntsp-amoA. In conclusion, gross N transformations in the temperate desert steppe largely depended on soil inorganic N, P contents and related functional microorganisms. Soil acidification plays a more key role in N mineralization than other environmental factors or functional microorganisms.