Initial recovery of soil carbon and nitrogen pools and dynamics following disturbance in jack pine forests: A comparison of wildfire and clearcut harvesting

Forests naturally maintained by stand-replacing wildfires are often managed with clearcut harvesting, yet we know little about how replacing wildfire with clearcutting affects soil processes and properties. We compared the initial recovery of carbon (C) and nitrogen (N) pools and dynamics following disturbance in jack pine (Pinus banksiana) stands in northern Lower Michigan, USA, by sampling soils (Oa+A horizons) from three “treatments”: 3-6-year-old harvest-regenerated stands, 3-6-year-old wildfire-regenerated stands and 40-55-year-old intact, mature stands (n=4 stands per treatment). We measured total C and N; microbial biomass and potentially mineralizable C and N; net nitrification; and gross rates of N mineralization and nitrification. Burned stands exhibited reduced soil N but not C, whereas clearcut and mature stands had similar quantities of soil organic matter. Both disturbance types reduced microbial biomass C compared to mature stands; however, microbial biomass N was reduced in burned stands but not in clearcut stands. The experimental C and N mineralization values were fit to a first-order rate equation to estimate potentially mineralizable pool size (C₀ and N₀) and rate parameters. Values for C₀ in burned and clearcut stands were approximately half that of the mature treatment, with no difference between disturbance types. In contrast, N₀ was lowest in the wildfire stands (170.2 μg N g⁻¹), intermediate in the clearcuts (215.4 μg N g⁻¹) and highest in the mature stands (244.6 μg N g⁻¹). The most pronounced difference between disturbance types was for net nitrification. These data were fit to a sigmoidal growth equation to estimate potential NO₃⁻ accumulation (Nit_max) and kinetic parameters. Values of Nit_max in clearcut soils exceeded that of wildfire and mature soils (149.2 vs. 83.5 vs. 96.5 μg NO₃⁻-N g⁻¹, respectively). Moreover, the clearcut treatment exhibited no lag period for net NO₃⁻ production, whereas the burned and mature treatments exhibited an approximate 8-week lag period before producing appreciable quantities of NO₃⁻. There were no differences between disturbances in gross rates of mineralization or nitrification; rather, lower NO₃⁻ immobilization rates in the clearcut soils, 0.20 μg NO₃⁻ g⁻¹ d⁻¹ compared to 0.65 in the burned soils, explained the difference in net nitrification. Because the mobility of NO₃⁻ and NH₄⁺ differs markedly in soil, our results suggest that differences in nitrification between wildfire and clearcutting could have important consequences for plant nutrition and leaching losses following disturbance.     

Effects of repeated fertilizer and cattle slurry applications over 38 years on N dynamics in a temperate grassland soil

The effects of repeated synthetic fertilizer or cattle slurry applications at annual rates of 50, 100 or 200 m³ ha⁻¹ yr⁻¹ over a 38 year period were investigated with respect to herbage yield, N uptake and gross soil N dynamics at a permanent grassland site. While synthetic fertilizer had a sustained and constant effect on herbage yield and N uptake, increasing cattle slurry application rates increased the herbage yield and N uptake linearly over the entire observation period. Cattle slurry applications, two and four times the recommended rate (50 m³ ha⁻¹ yr⁻¹, 170 kg N ha⁻¹), increased N uptake by 46 and 78%, respectively after 38 years. To explain the long-term effect, a ¹⁵N tracing study was carried out to identify the potential change in N dynamics under the various treatments. The analysis model evaluated process-specific rates, such as mineralization, from two organic-N pools, as well as nitrification from NH₄⁺ and organic-N oxidation. Total mineralization was similar in all treatments. However, while in an unfertilized control treatment more than 90% of NH₄⁺ production was related to mineralization of recalcitrant organic-N, a shift occurred toward a predominance of mineralization from labile organic-N in the cattle slurry treatments and this proportion increased with the increase in slurry application rate. Furthermore, the oxidation of recalcitrant organic-N shifted from a predominant NH₄⁺ production in the control treatment, toward a predominant NO₃⁻ production (heterotrophic nitrification) in the cattle slurry treatments. The concomitant increase in heterotrophic nitrification and NH₄⁺ oxidation with increasing cattle slurry application rate was mainly responsible for the increase in net NO₃⁻ production rate. Thus the increase in N uptake and herbage yield on the cattle slurry treatments could be related to NO₃⁻ rather than NH₄⁺ production. The ¹⁵N tracing study was successful in revealing process-specific changes in the N cycle in relationship to long-term repeated amendments.     

Nitrogen transformations in cold and frozen agricultural soils following organic amendments

Though microbial activity is known to occur in frozen soils, little is known about the fate of animal manure N applied in the fall to agricultural soils located in areas with prolonged winter periods. Our objective was to examine transformations of soil and pig slurry N at low temperatures. Loamy and clay soils were either unamended (Control), amended with ¹⁵NH₄-labeled pig slurry, or amended with the pig slurry and wheat straw. Soils were incubated at −6, −2, 2, 6, and 10 °C. The amounts of NH₄, NO₃ and microbial biomass N (MBN), and the presence of ¹⁵N in these pools were monitored. Total mineral N, NO₃ and ¹⁵NO₃ increased at temperature down to −2 °C in the loam soil and −6 °C in the clay soil, indicating that nitrification and mineralization proceeded in frozen soils. Nitrification and mineralization rates were 1.8-4.9 times higher in the clay than in the loamy soil, especially below freezing point (3.2-4.9), possibly because more unfrozen water remained in the clay than in the loamy soil. Slurry addition increased nitrification rates by 3-14 times at all temperatures, indicating that this process was N-limited even in frozen soils. Straw incorporation caused significant net N immobilization only at temperatures ≥2 °C in both soils; the rates were 1.4-3.4 higher in the loam than in the clay soil. Nevertheless, up to 30% of the applied ¹⁵N was present in MBN at all temperatures. These findings indicate that microbial N immobilization occurred in frozen soils, but was not strong enough to induce net immobilization below the freezing point, even in the presence of straw. The Q₁₀ values for estimated mineralization and nitrification rates were one to two orders-of-magnitude larger below 2 °C than above this temperature (13-208 versus 1.5-6.9, respectively), indicating that these processes are highly sensitive to a small increase in soil temperature around the freezing point of water. This study confirms that net mineralization and nitrification can occur at potentially significant rates in frozen agricultural soils, especially in the presence of organic amendments. In contrast, net N immobilization could be detected essentially above the freezing point. Our results imply that fall-applied N could be at risk of overwinter losses, particularly in fine-textured soils.     

Differences in isotopic fractionation of nitrogen in water-saturated and unsaturated soils

Temporal variations in δ¹⁵N of NH₄⁺ and NO₃⁻ in water-saturated and unsaturated soils were examined in a laboratory incubation study. Ammonium sulfate (δ¹⁵N=−2.6‰) was added to 25 g samples of soil at concentrations of 160 mg N kg⁻¹. Soils were then incubated under unsaturated (50% of water holding capacity at saturation, WHC) or saturated (100% of WHC) water conditions for 7 and 36 d, respectively. During 7 d incubation of unsaturated soil, the NH₄⁺-N concentration decreased from 164.8 to 34.4 mg kg⁻¹, and the δ¹⁵N of NH₄⁺ increased from −0.4 to +57.2‰ through nitrification, as evidenced by corresponding increase in NO₃⁻-N concentration and lower δ¹⁵N of NO₃⁻ (product) than that of NH₄⁺ (substrate) at each sampling time. In saturated soil, the concentration of NH₄⁺-N decreased gradually from 162.4 to 24.2 mg kg⁻¹, and the δ¹⁵N values increased from +0.8 to +21.0‰ during 36 d incubation. However, increase in NO₃⁻ concentration was not observed due to loss of NO₃⁻ through concurrent denitrification in anaerobic sites. The apparent isotopic fractionation factors (α_s/p) associated with decrease in NH₄⁺ concentration were 1.04 and 1.01 in unsaturated and saturated soils, respectively. Since nitrification is likely to introduce greater isotope fractionation than microbial immobilization, the higher value for unsaturated soil probably reflected faster nitrification under aerobic conditions. The lower value for saturated soil suggests that immobilization and subsequent remineralization of NH₄⁺ were relatively more dominant than nitrification under the anaerobic conditions.     

Gross N transformation rates and net N mineralisation rates related to the C and N contents of soil organic matter fractions in grassland soils of different age

We investigated the relationship between soil organic matter (SOM) content and N dynamics in three grassland soils (0-10 and 10-20 cm depth) of different age (6, 14 and 50 y-old) with sandy loam textures. To study the distribution of the total C and N content the SOM was fractionated into light, intermediate and heavy density fractions of particulate macro-organic matter (150-2000 μm) and the 50-150 μm and <50 μm size fractions. The potential gross N transformation rates (mineralisation, nitrification, NH₄⁺ and NO₃⁻ immobilization) were determined by means of short-term, fully mirrored ¹⁵N isotope dilution experiments (7-d incubations). The long-term potential net N mineralisation and gross N immobilization rates were measured in 70-d incubations. The total C and N contents mainly tended to increase in the 0-10 cm layer with increasing age of the grassland soils. Significant differences in total SOM storage were detected for the long-term (50 y-old) conversion from arable land to permanent grassland. The largest relative increase in C and N contents had occurred in the heavy density fraction of the macro-organic matter, followed by the 50-150 and <50 μm fractions. Our results suggest that the heavy density fraction of the macro-organic matter could serve as a good indicator of early SOM accumulation, induced by converting arable land to permanent grassland. Gross N mineralisation, nitrification, and (long-term) gross N immobilization rates tended to increase with increasing age of the grasslands, and showed strong, positive correlations with the total C and N contents. The calculated gross N mineralisation rates (7-d incubations) and net N mineralisation rates (70-d incubations) corresponded with a gross N mineralisation of 643, 982 and 1876 kg N ha⁻¹ y⁻¹, and a net N mineralisation of 195, 208 and 274 kg N ha⁻¹ y⁻¹ in the upper 20 cm of the 6, 14 and 50 y-old grassland soils, respectively. Linear regression analysis showed that 93% of the variability of the gross N mineralisation rates could be explained by variation in the total N contents, whereas total N contents together with the C-to-N ratios of the <50 μm fraction explained 84% of the variability of the net N mineralisation rates. The relationship between long-term net N mineralisation rates and gross N mineralisation rates could be fitted by means of a logarithmic equation (net m=0.24Ln(gross m)+0.23, R²=0.69, P<0.05), which reflects that the ratio of gross N immobilization-to-gross N mineralisation tended to increase with increasing SOM contents. Microbial demand for N tended to increase with increasing SOM content in the grassland soils, indicating that potential N retention in soils through microbial N immobilization tends to be limited by C availability.     

Effect of tebuthiuron on soil N mineralization and nitrification

Abstract

Herbicides have potential for economical and efficient site preparation following timber harvest. The effects of tebuthiu‐ron, one of the herbicides approved for this use, on soil nitrogen (N) mineralization and nitrification were determined in laboratory incubations. Tebuthiuron was added at rates from 0 to 1000 μg g^‐1 to three soils. There was no effect of tebuthiuron additions of less than 1 μg g^‐1 on soil N mineralization and nitrification. Tebuthiuron reduced nitrification in all soils at 1000 μg g^‐1 and in two of the soils at 100 μg g^‐1 . All soils had increased net mineralization with tebuthiuron added at 100 and 1000 μg g^‐1. The addition of 50 μg NH⁺ ₄‐N and 1000 μg tebuthiuron g^‐1 resulted in increased net mineralization in the three soils. Nitrification was affected differently in each of the three soils by the addition of both NH⁺ ₄‐N and tebuthiuron. The added NH⁺ ₄‐N either removed the inhibition of nitrification by the herbicide or had no effect on the inhibition in two of the soils. In the third soil, nitrification was reduced by the addition of NH⁺ ₄‐N.

The presence of NO^‐ ₃‐N in these acid soils and the effects of added NH⁺ ₄‐N on NO^‐ ₃‐N production suggest that heterotrophic nitrification occurs in at least two of the soils. The findings of this study indicate that any effects of tebuthiuron on N mineralization and nitrification at the currently recommended application rates are likely to be transient and localized.     

Soil microbial biomass and nitrogen transformations among five tree species of the Catskill Mountains, New York, USA

We measured soil microbial biomass nitrogen (MBN), microbial uptake of ¹⁵N, potential net mineralization and net nitrification in the laboratory to determine the influence of tree species on nitrogen (N) transformations in soils of the Catskills Mountains, New York, USA. Organic horizon soils were taken from single species plots of beech (Fagus grandifolia), hemlock (Tsuga canadensis), red oak (Quercus rubra), sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis). ¹⁵NH₄Cl was added to the soils and N pools were sampled at 1, 3, 10 and 28 days to examine microbial uptake of ¹⁵N over time. Soil MBN was about 60% lower in red oak and sugar maple soils than in the other three species. Soil pools of NO₃⁻ and rates of net nitrification were significantly greater in soils associated with sugar maple than hemlock, red oak and yellow birch. With the exception of sugar maple soils, microbial recovery of ¹⁵N was significantly greater after 10 and 28 days compared to 60 min and 1 day following ¹⁵N tracer addition. Microbial ¹⁵N recovery declined significantly within sugar maple stands within the first 3 days of incubation. Soil carbon to nitrogen ratio (C:N) was lowest in sugar maple soils and highest in red oak soils. However, correlations between soil C:N and MBN or rates of net mineralization and nitrification were not significant. Soil moisture could account for 22% of the variation in MBN and 36% of the variation in net mineralization. Soil microbial transformations of N vary among tree species stands and may have consequences for forest N retention and loss.     

Effect of the nitrification inhibitor nitrapyrin on the fate of nitrogen applied to a soil incubated under laboratory conditions

The aim of this study was to examine the effect of the nitrification inhibitor nitrapyrin on the fate and recovery of fertilizer nitrogen (N) and on N mineralization from soil organic sources. Intact soil cores were collected from a grassland field. Diammonium phosphate (DAP) and urea were applied as N sources. Cores were equilibrated at –5 kPa matric potential and incubated at 20 °C for 42 to 56 days. Changes in NH₄⁺‐N, accumulation of NO₃^–‐N, apparent recovery of applied N, and emission of N₂O (acetylene was used to block N₂O reductase) were examined during the study. A significant increase in NH₄⁺‐N released through mineralization was recorded when nitrapyrin was added to the control soil without N fertilizer application. In the soils to which N was added either as urea or DAP, 50–90 % of the applied N disappeared from the NH₄⁺‐N pool. Some of this N (8–16 %) accumulated as NO₃^–‐N, while a small proportion of N (1 %) escaped as N₂O. Addition of nitrapyrin resulted in a decrease and delay of NH₄⁺‐N disappearance, accumulation of much lower soil NO₃^–‐N contents, a substantial reduction in N₂O emissions, and a 30–40 % increase in the apparent recovery of added N. The study indicates that N recovery can be increased by using the nitrification inhibitor nitrapyrin in grassland soils at moisture condition close to field capacity.     

西藏高原自然生态系统转化为农田后提高了土壤的净矿化和净硝化

A comparative study was conducted to determine the NH4^＋ and NO3^- concentrations in soil profiles and to examine the net nitrogen （N） mineralization and nitrification in adjacent forest, grassland, and cropland soils on the Tibetan Plateau. Cropland soil showed significantly higher inorganic N concentrations in soil profiles compared with forest and grassland soils. NO3^- -N accounted for 70%-90^ of inorganic N in cropland soil, while NH4^＋ -N was the main form of inorganic N in forest and grassland soils. The average net N mineralization rate at 0 20 cm depth was approximately twice in cropland soil （1.48 mg kg^-1 d^-1） as high as in forest （0.83 mg kg^-1 d^-1） or grassland soil （0.72 mg kg^-1 d^-1）. Cropland showed strong net nitrification, with the net rate almost equal to the total net N mineralization. Urea addition stimulated soil respiration, particularly in forest soil. Most urea-N, however, remained as NH4^＋ in forest and grassland soils, while NO3^- was the main form of inorganic N to increase in cropland soil. Higher rates of net nitrification in cropland soils suggest that land use change on the Tibetan Plateau may lead to high N losses through nitrate leaching.     

华北平原农田生态系统土壤C、N净矿化及尿素转化研究

以华北平原区4个农田生态系统[京郊蔬菜大棚(GH)和河北栾城(LF)、河北南皮(NF)、山东惠民(HF)3个粮田]为研究对象,采用室内好气、恒温、避光条件下培养30.d,对比研究了不同海拔和不同农业扰动强度下的农田生态系统中耕层(020.cm)土壤的净N矿化、净硝化、净C矿化以及尿素的转化,旨在探索人类农业扰动强度和地理海拔对土壤供N潜力和尿素N转化的影响。结果表明,4个地区的土壤供N潜力分别为:14.4、13.2,17.7和16.5.mg/kg,说明高度熟化的华北区农田土壤供N潜力相对稳定。以施用有机肥为主的蔬菜大棚和以施用化肥为主的粮田对土壤供N没有显著影响。农田土壤净矿化后的供N形式主要是NO3--N。以施用有机肥为主的蔬菜大棚积累了较高的土壤有机质和全N,但是土壤净C矿化以及施用尿素后CO2的排放量均低于以施用化肥为主的粮田。尿素在各区域农田土壤中水解转化后均主要以NO3--N形式存在,NO3--N占尿素水解后无机N增量的98%9～9%;华北平原农田生态系统施入尿素态N.30d后,水解成有效态无机N的转化率为63.4%8～3.2%,即每克尿素态N在京郊蔬菜大棚(GH)、栾城高产农田(LF)、南皮农田(NF)和惠民农田(HF)土壤中转化为NO3--N的量分别为0.69、0.82、0.64和0.63.g/kg,同时可使相应区域农田的CO2排放量分别增加CO21.20、1.360、.67和1.58.g/kg。     

Role of soil drying in nitrogen mineralization and microbial community function in semi-arid grasslands of north-west Australia

We examined effects of wetting and then progressive drying on nitrogen (N) mineralization rates and microbial community composition, biomass and activity of soils from spinifex (Triodia R. Br.) grasslands of the semi-arid Pilbara region of northern Australia. We compared soils under and between spinifex hummocks and also examined impacts of fire history on soils over a 28 d laboratory incubation. Soil water potentials were initially adjusted to −100 kPa and monitored as soils dried. We estimated N mineralization by measuring changes in amounts of nitrate (NO₃⁻-N) and ammonium (NH₄⁺-N) over time and with change in soil water potential. Microbial activity was assessed by amounts of CO₂ respired. Phospholipid fatty acid (PLFA) analyses were used to characterize shifts in microbial community composition during soil drying. Net N mineralized under hummocks was twice that of open spaces between hummocks and mineralization rates followed first-order kinetics. An initial N mineralization flush following re-wetting accounted for more than 90% of the total amount of N mineralized during the incubation. Initial microbial biomass under hummocks was twice that of open areas between hummocks, but after 28 d microbial biomass was<2 μ g⁻¹ ninhydrin N regardless of position. Respiration of CO₂ from soils under hummocks was more than double that of soils from between hummocks. N mineralization, microbial biomass and microbial activity were negligible once soils had dried to −1000 kPa. Microbial community composition was also significantly different between 0 and 28 d of the incubation but was not influenced by burning treatment or position. Regression analysis showed that soil water potential, microbial biomass N, NO₃⁻-N, % C and δ¹⁵N all explained significant proportions of the variance in microbial community composition when modelled individually. However, sequential multiple regression analysis determined only microbial biomass was significant in explaining variance of microbial community compositions. Nitrogen mineralization rates and microbial biomass did not differ between burned and unburned sites suggesting that any effects of fire are mostly short-lived. We conclude that the highly labile nature of much of soil organic N in these semi-arid grasslands provides a ready substrate for N mineralization. However, process rates are likely to be primarily limited by the amount of substrate available as well as water availability and less so by substrate quality or microbial community composition.     

Activated carbon amendments to soil alters nitrification rates in Scots pine forests

The influence of charcoal on biotic processes in soils remains poorly understood. Charcoal is a natural product of wildfires that burned on a historic return interval of ∼100 years in Scots pine (Pinus sylvestris L.) forests of northern Sweden. Fire suppression and changes in forest stand management have resulted in a lack of charcoal production in these ecosystems. It is thought that charcoal may alter N mineralization and nitrification rates, however, previous studies have not been conclusive. Replicated field studies were conducted at three late-succession field sites in northern Sweden and supporting laboratory incubations were conducted using soil humus collected from these sites. We used activated carbon (AC), as a surrogate for natural-occurring fire-produced charcoal. Two rates of AC (0 and 2000 kg ha⁻¹), and glycine (0 and 100 kg N as glycine ha⁻¹) were applied in factorial combination to field microplots in a randomized complete block pattern. Net nitrification, N mineralization, and free phenol concentrations were measured using ionic and non-ionic resin capsules, respectively. These same treatments and also two rates of birch leaf litter (0 and 1000 kg ha⁻¹) were applied in a laboratory incubation and soils from this incubation were extracted with KCl and analyzed for NH₄⁺ and NO₃⁻. Nitrification rates increased with AC amendments in laboratory incubations, but this was not supported by field studies. Ammonification rates, as measured by NH₄⁺ accumulation on ionic resins, were increased considerably by glycine applications, but some NH₄⁺ was apparently lost to surface sorption to the AC. Phenolic accumulation on non-ionic resin capsules was significantly reduced by AC amendments. We conclude that charcoal exhibits important characteristics that affect regulating steps in the transformation and cycling of N.     

Sources of spatial and temporal variability of inorganic nitrogen in pure and mixed deciduous temperate forests

We investigated the influence of tree canopy composition and structure on the spatial and temporal variability of (i) concentrations of inorganic N (NH₄⁺ and NO₃⁻) and (ii) net N-mineralization and net nitrification, within the temperate forest floor. We compared a pure European beech stand (PS) with a mixed beech-hornbeam one (MS). Three sampling areas were chosen in each stand. Within the PS, the tree locations represented a decreasing gradient of light intensity reaching the forest floor. Within the MS they represented a gradient in the amount of hornbeam leaves present in the litter. In the field NH₄⁺ and NO₃⁻ concentrations were measured in the upper mineral soil (UMS) and the overlying organic layers (OL and OF+OH). Field exposures using buried bags were carried out on UMS over 1 year to measure in situ net N-mineralization and net nitrification. Potential net N-mineralization and net nitrification were investigated in summer with UMS, OL and OF+OH incubated at 28 °C for 28 days in the laboratory. We hypothesize that with the presence of a mull-forming species (hornbeam) within a stand dominated by a moder-forming one (European beech), (i) the spatial and (ii) temporal patterns of soil inorganic N concentrations, net N-mineralization and net nitrification would be different in the two stands. Our main results show that tree species composition has an influence on both spatial and temporal patterns of nitrification. The PS exhibited its highest peaks of UMS NO₃⁻ concentration and net nitrification in spring and early summer while they were highest in the MS in winter. Furthermore, PS exhibited a higher rate of net nitrification than MS. We discuss this unexpected result and suggest that dissolved organic C may be the controlling factor for net nitrification in the MS.     

C and N turnover in structurally intact soils of different texture

The turnover of native and applied C and N in undisturbed soil samples of different texture but similar mineralogical composition, origin and cropping history was evaluated at −10 kPa water potential. Cores of structurally intact soil with 108, 224 and 337 g clay kg⁻¹ were horizontially sliced and ¹⁵N-labelled sheep faeces was placed between the two halves of the intact core. The cores together with unamended treatments were incubated in the dark at 20 °C and the evolution of CO₂-C determined continuously for 177 d. Inorganic and microbial biomass N and ¹⁵N were determined periodically. Net nitrification was less in soil amended with faeces compared with unamended soil. When adjusted for the NO₃-N present in soil before faeces was applied, net nitrification became negative indicating that NO₃-N had been immobilized or denitrified. The soil most rich in clay nitrified least N and ¹⁵N. The amounts of N retained in the microbial biomass in unamended soils increased with clay content. A maximum of 13% of the faeces ¹⁵N was recovered in the microbial biomass in the amended soils. CO₂-C evolution increased with clay content in amended and unamended soils. CO₂-C evolution from the most sandy soil was reduced due to a low content of potentially mineralizable native soil C whereas the rate constant of C mineralization rate peaked in this soil. When the pool of potentially mineralizable native soil C was assumed proportional to volumetric water content, the three soils contained similar proportions of potentially mineralizable native soil C but the rate constant of C mineralization remained highest in the soil with least clay. Thus although a similar availability of water in the three soils was ensured by their identical matric potential, the actual volume of water seemed to determine the proportion of total C that was potentially mineralizable. The proportion of mineralizable C in the faeces was similar in the three soils (70% of total C), again with a higher rate constant of C mineralization in the soil with least clay. It is hypothesized that the pool of potentially mineralizable C and C rate constants fluctuate with the soil water content.     

Gross nitrogen mineralization-, immobilization-, and nitrification rates as a function of soil C/N ratio and microbial activity

A laboratory experiment was designed to challenge the idea that the C/N ratio of forest soils may control gross N immobilization, mineralization, and nitrification rates. Soils were collected from three deciduous forests sites varying in C/N ratio between 15 and 27. They were air-dried and rewetted to induce a burst of microbial activity. The N transformation rates were calculated from an isotope dilution and enrichment procedure, in which ¹⁵NH₄Cl or Na¹⁵NO₃ was repeatedly added to the soils during 7 days of incubation. The experiments suggested that differences in gross nitrogen immobilization and mineralization rates between the soils were more related to the respiration rate and ATP content than to the C/N ratio. Peaks of respiration and ATP content were followed by high rates of mineralization and immobilization, with 1-2 days of delay. The gross immobilization of NH₄⁺ was dependent on the gross mineralization and one to two orders of magnitude larger than the gross NO₃⁻ immobilization. The gross nitrification rates were negatively related to the ATP content and the C/N ratio and greatly exceeding the net nitrification rates. Taken together, the observations suggest that leaching of nitrate from forest soils may be largely dependent on the density and activity of the microbial community.     

How important is N2O production in removing atmospherically deposited nitrogen from UK moorland catchments?

Nitrate (NO₃⁻) leaching due to anthropogenic nitrogen (N) deposition is an environmental problem in many parts of the UK uplands, associated with surface water acidification and affecting lake nutrient balances. It is often assumed that gaseous return of deposited N to the atmosphere as N₂O through denitrification may provide an important sink for N. This assumption was tested for four moorland catchments (Allt a’Mharcaidh in the Cairngorms, Afon Gwy in mid-Wales, Scoat Tarn in the English Lake District and River Etherow in the southern Pennines), covering gradients of atmospheric N deposition and surface water NO₃⁻ leaching, through a combination of field and laboratory experiments. Field measurements of N₂O fluxes from static chambers with and without additions of NH₄NO₃ solution were carried out every 4 weeks over 1 yr. Wetted soil cores from the same field plots were used in experimental laboratory incubations at 5 and 15 °C with and without additions of NH₄NO₃ solution, followed by measurement of N₂O fluxes. Field measurements showed that significant N₂O fluxes occurred in only a very small number of plots with most showing zero values for much of the year. The maximum fluxes were 0.24 kg-N/ha/yr from unamended plots at the River Etherow and 0.49 kg-N/ha/yr from plots with NH₄NO₃ additions at the Allt a’Mharcaidh. Laboratory incubation experiments demonstrated that large N₂O fluxes could be induced by warming and NH₄NO₃ additions, with the top 5 cm of soil cores responsible for the largest fluxes, reaching 11.8 kg-N/ha/yr from a podsol at Scoat Tarn. Acetylene block experiments showed that while N₂ was not likely to be a significant denitrification product in these soils, reduced N₂O fluxes indicated that nitrification was an important source of N₂O in many cases. A simple model of denitrification suggesting that 10-80% of net N inputs may be denitrified from non-agricultural soils was found to greatly over-estimate fluxes in the UK uplands. The proportion of deposition denitrified was found to be much closer to the IPCC suggested value of 1% with an upper limit of 10%. Interception of N deposition by vegetation may greatly reduce the net supply of N from this source, while soil acidification or other factors limiting carbon supply to soil microbes may prevent large denitrification fluxes even where NO₃⁻ supply is not limiting.     

Nitrous oxide flux from soil amino acid mineralization

Nitrous oxide (N₂O) is a greenhouse gas produced during microbial transformation of soil N that has been implicated in global climate warming. Nitrous oxide efflux from N fertilized soils has been modeled using NO₃⁻ content with a limited success, but predicting N₂O production in non-fertilized soils has proven to be much more complex. The present study investigates the contribution of soil amino acid (AA) mineralization to N₂O flux from semi-arid soils. In laboratory incubations (−34 kPa moisture potential), soil mineralization of eleven AAs (100 μg AA-N g⁻¹ soil) promoted a wide range in the production of N₂O (156.0±79.3 ng N₂O-N g⁻¹ soil) during 12 d incubations. Comparison of the δ¹³C content (‰) of the individual AAs and the δ¹³C signature of the respired AA-CO₂-C determined that, with the exception of TYR, all of the AAs were completely mineralized during incubations, allowing for the calculation of a N₂O-N conversion rate from each AA. Next, soils from three different semi-arid vegetation ecosystems with a wide range in total N content were incubated and monitored for CO₂ and N₂O efflux. A model utilizing CO₂ respired from the three soils as a measure of organic matter C mineralization, a preincubation soil AA composition of each soil, and the N₂O-N conversion rate from the AA incubations effectively predicted the range of N₂O production by all three soils. Nitrous oxide flux did not correspond to factors shown to influence anaerobic denitrification, including soil NO₃⁻ contents, soil moisture, oxygen consumption, and CO₂ respiration, suggesting that nitrification and aerobic nitrifier denitrification could be contributing to N₂O production in these soils. Results indicate that quantification of AA mineralization may be useful for predicting N₂O production in soils.     

Microbial populations immobilizing NH4-N and NO3-N differ in their sensitivity to sodium chloride salinity in soil

An incubation experiment was conducted to study the response to sodium chloride (NaCl) salinity of microbial population immobilizing NH₄⁺- and NO₃⁻-N using glucose as an easily oxidizable C source. Immobilization of NH₄⁺-N was faster than that of NO₃⁻-N and was complete within 12 h of -incubation. Presence of NaCl retarded the process of N immobilization; that of NO₃⁻-N being more affected. Remineralization of immobilized N started within 48 h in case of both NH₄⁺- and NO₃⁻-N and was faster for the latter. Both remineralization and nitrification were significantly delayed in the presence of NaCl; inhibition being more at 4000 mg NaCl kg⁻¹ soil. The inhibitory effect of NaCl on remineralization of N was relatively more for NH₄⁺-treated soil. The results of the study suggested a higher sensitivity to NaCl of microorganisms assimilating NO₃⁻. However, remineralization of N from NO₃⁻-assimilating microbial population was less affected by NaCl salinity compared to NH₄⁺-assimilating population.     

Temperature and soil moisture interactively affected soil net N mineralization in temperate grassland in Northern China

Intact soil cores from three adjacent sites (Site A: grazed, Site B: fenced for 4 years, and Site C: fenced for 24 years) were incubated in the laboratory to examine effects of temperature, soil moisture, and their interactions on net nitrification and N mineralization rates in the Inner Mongolia grassland of Northern China. Incubation temperature significantly influenced net nitrification and N mineralization rates in all the three grassland sites. There were no differences in net nitrification or N mineralization rates at lower temperatures (−10, 0, and 5 °C) whereas significant differences were found at higher temperatures (15, 25, and 35 °C). Soil moisture profoundly impacted net nitrification and N mineralization rates in all the three sites. Interactions of temperature and moisture significantly affected net nitrification and mineralization rates in Site B and C, but not in Site A. Temperature sensitivity of net nitrification and N mineralization varied with soil moisture and grassland site. Our results showed greater net N mineralization rates and lower concentrations of inorganic N in the grazed site than those in the fenced sites, suggesting negative impacts of grazing on soil N pools and net primary productivity.