Rapid uptake of N-ammonium and glycine-C, N by arbuscular and ericoid mycorrhizal plants native to a Northern California coastal pygmy forest

While it is well established that plants are able to acquire nitrogen in inorganic form, there is less information on their ability to ‘short circuit’ the N cycle, compete with microbes, and acquire nitrogen in organic form. Mycorrhizal fungi, known to enhance nutrient uptake by plants, may play a role in organic N uptake, particularly ericoid mycorrhizas. We asked the question—Can mycorrhizal fungi increase the ability of plants to take up organic N, compared to inorganic N? Here, we report on the abilities of three plant species, ericoid mycorrhizal Rhododendron macrophyllum and Vaccinium ovatum and arbuscular mycorrhizal Cupressus goveniana ssp. pigmaea, to acquire C and/or N from an organic and an inorganic N source. All three species are native to a California coastal pygmy forest growing in acidic, low-fertility, highly organic soils. In a pot study, glycine-α¹³C, ¹⁵N and ¹⁵N-ammonium were applied to pygmy forest soil for 17 or 44 h. Ericoid mycorrhizal species did not demonstrate a preference for either inorganic or organic sources of N while Cupressus acquired more NH₄-N than glycine-N. For all species, glycine-N uptake did not increase after 17 h suggesting glycine uptake and glycine immobilization occurred rapidly. Both glycine-N and glycine-C were recovered in shoots and in roots suggesting that all species acquired some N in organic form. Regression analyses of glycine-N and glycine-C recovery in root tissue indicate that much of the glycine was taken up intact and that the minimum proportion of glycine-N recovered in organic form was 85% (Cupressus) and 70% (Rhododendron). Regressions were non-significant for Vaccinium. For all species, glycine-N remained predominantly in roots while glycine-C was transferred to shoots. In contrast, NH₄-N remained in roots of ericoid plants but was transferred to shoots of arbuscular mycorrhizal Cupressus. Since net N mineralization rates in pygmy forest soils are low, our results suggest that organic N may be an important N source for plants in this temperate coniferous ecosystem regardless of mycorrhizal type. Acquisition of amino acid C by these species also may partially offset the carbon cost to plants of hosting mycorrhizal fungi.     

Organic farming promotes selective uptake of glycine over nitrate uptake by pakchoi

ABSTRACT

Understanding how plants use of various nitrogen (N) sources is important for improving plant N use efficiency in organic farming systems. This study investigated the effects of farming management practices (organic and conventional) on pakchoi short-term uptake of glycine (Gly), nitrate (NO₃ ^?) and ammonium (NH₄ ⁺) under two N level conditions. Results showed that plant N uptake rates and N contributions from the three N forms in the low N (0.15 μg N g^?1 dry soil) treatment did not significantly differ between the organic and conventional soils, except the significantly greater Gly contribution in organic soil at 24 h after tracer addition. Under high N (15 μg N g^?1 dry soil) conditions, the N uptake rates, uptake efficiencies, and N contributions of Gly and NH₄ ⁺-N were significantly greater in pakchoi cultivated in the organic soil compared to conventional soil, whereas the N uptake rates and N contributions from NO₃ ^–-N decreased in pakchoi cultivated in the organic soil. The greater Gly-N uptake in plants grown in high-N treated organic soil may be related to the greater gross N transformation, Gly turnover rate and the increased expression of an amino acid transporter gene BcLHT1. Intact Gly contributed at most 6% to Gly-derived N at 24 h after tracer additions, which accounting for about 1.24% of the total N uptake in organic soil. Our study suggested that Gly-N and other organic source N might serve as a more important compensatory N source for plants in organic farming.     

Effects of nitrogen rate and nitrogen form on glycine uptake by pakchoi (Brassica chinensis L.) under sterile culture

It is well known that plants are capable of taking up intact amino acids. However, how the nitrogen (N) rates and N forms affect amino acid uptake and amino acid nutritional contribution for plant are still uncertain. Effects of the different proportions of nitrate (NO₃^?), ammonium (NH₄⁺) and ¹⁵N-labeled glycine on pakchoi seedlings glycine uptake were investigated for 21 days hydroponics under the aseptic media. Our results showed that plant biomass and glycine uptake was positively related to glycine rate. NO₃^? and NH₄⁺, the two antagonistic N forms, both significantly inhibited plant glycine uptake. Their interactions with glycine were also negatively related to glycine uptake and glycine nutritional contribution. Glycine nutritional contribution in the treatments with high glycine rate (13.4%–35.8%) was significantly higher than that with low glycine rate (2.2%–13.2%). The high nutritional contribution indicated amino acids can serve as an important N source for plant growth under the high organic and low inorganic N input ecosystem.     

无菌砂培条件下三种农作物对甘氨酸的吸收

Seedlings of wheat (Triticum aestivum L.), Chinese cabbage (Brassica campestris L.) and mung bean (Phaseolus radiatus L.) were grown for 30 days in sterile sand media with 6 N treatments, I.e. NH4+-N, glycine-N, 3 different ratios of glycine-N:NH4+-N (NH4+-N was labeled with 15N) and a control receiving no N, to assess the importance of amino acids in excessive N nutrition along with inorganic N interactions. The contribution of nitrogen derived from glycine-N to total plant N was investigated. The total plant N of the three species treated with N was significantly greater (P < 0.05) than the control treatment. Also, seedlings from all the three species had significantly more total N as NH4+-N (P < 0.05) than at least two of the four treatments with glycine-N. However, for all species, differences in total N among treatments with a mixture of glycine-N and NH4+-N were mostly not significant. The contribution of N derived from glycine-N to plant total N content for all species increased with increasing glycine-N:NH4+-N ratio in the treatment solution. These results indicated that agricultural plants could effectively use organic nitrogen from organic nitrogen sources (e.g. glycine) and from organic and inorganic N mixtures (e.g. a glycine-N and NH4+-N mix). There were also genotypic differences in glycine-N and NH4+-N uptake by agricultural species.     

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

Purpose

Input of N as NH₄ ⁺ is known to stimulate nitrification and to enhance the risk of N losses through NO₃ ^? leaching in humid subtropical soils. However, the mechanisms responsible for this stimulation effect have not been fully addressed.

Materials and methods

In this study, an acid subtropical forest soil amended with urea at rates of 0, 20, 50, 100 mg N kg^?1 was pre-incubated at 25 °C and 60 % water-holding capacity (WHC) for 60 days. Gross N transformation rates were then measured using a ¹⁵N tracing methodology.

Results and discussion

Gross rates of mineralization and nitrification of NH₄ ⁺-N increased (P?<?0.05), while gross rate of NO₃ ^? immobilization significantly decreased with increasing N input rates (P?<?0.001). A significant relationship was established between the gross nitrification rate of NH₄ ⁺ and the gross mineralization rate (R ²?=?0.991, P?<?0.01), so was between net nitrification rate of NH₄ ⁺ and the net mineralization rate (R ²?=?0.973, P?<?0.05).

Conclusions

Stimulation effect of N input on the gross rate of nitrification of NH₄ ⁺-N in the acid soil, partially, resulted from stimulation effect of N input on organic N mineralization, which provides pH-favorable microsites for the nitrification of NH₄ ⁺ in acid soils (De Boer et al., Soil Biol Biochem 20:845–850, 1988; Prosser, Advan Microb Physiol 30:125–181, 1989). The stimulated gross nitrification rate with the decreased gross NO₃ ^? immobilization rate under the elevated N inputs could lead to accumulation of NO₃ ^? and to enhance the risk of NO₃ ^? loss from humid forest soils.

     

Organic nitrogen stimulates the heterotrophic nitrification rate in an acidic forest soil

Many previous studies have demonstrated that heterotrophic nitrification processes play an important role in the production of NO₃⁻ in acidic soils. However, it is not clear whether a low concentration of nitrogenous organic compounds support heterotrophic nitrification processes in natural soils. In this study, we performed an ¹⁵N tracer experiment with a glycine concentration gradient (20, 40, 80, and 160 mg N kg⁻¹) to investigate the effect of the organic nitrogen concentration on the heterotrophic nitrification rate and its relative contribution to the total nitrification of the studied acidic forest soil. This experiment demonstrated that ¹⁵N–NO₃⁻ accumulated over time with all nitrogen treatments in the presence of acetylene, confirming that heterotrophic nitrification occurred even at a low organic nitrogen concentration (20 mg kg⁻¹) in the studied acidic forest soil. In the presence of acetylene, the ¹⁵N–NO₃⁻ concentration in the 20 and 40 mg kg⁻¹ glycine-N treatments was significantly lower than in the 80 and 160 mg kg⁻¹ glycine-N treatments (p < 0.05), indicating that a high organic nitrogen concentration stimulated the heterotrophic nitrification rate. There was no significant difference in the average contribution of heterotrophic nitrification to total nitrification among the different nitrogen treatments, suggesting that the organic nitrogen concentration did not affect the relative contribution of heterotrophic nitrification to total nitrification in the studied acidic soil. Our results confirmed that a low concentration of organic N (20 mg kg⁻¹) supported heterotrophic nitrification in the studied soil. The organic nitrogen concentration stimulates the heterotrophic nitrification rate, but does not affect the relative contribution of heterotrophic nitrification to total nitrification in the studied acidic soil.     

Assessing chemical soil tests for predicting nitrogen mineralization in paddy soils of the Dongting Lake region in China

Purpose

Developing routine methods that accurately predict soil nitrogen (N) mineralization is essential for fertilization recommendation; thus, chemical soil testing has received worldwide attention. However, the optimal chemical soil test for predicting soil N mineralization is region specific. This study aimed to determine suitable chemical soil tests for predicting N mineralization in paddy soils of the Dongting Lake region, China.

Materials and methods

Composite surface samples (0–20 cm) of soils (n?=?30) with diverse inherent properties were collected from representative paddy fields across the region. The benchmark indices for soil N mineralization were the net mineralization rate of soil N in a 112-day anaerobic incubation under waterlogged conditions (NMRN₁₁₂) and N mineralization potential (N _o ) estimated using a modified double exponential model. Laboratory-based measurements of soil labile organic N (SLON) were conducted using chemical fractionation methods including 0.01 M NaHCO₃ extraction, hot 2 M KCl hydrolysis, phosphate-borate (PB) buffer hydrolysis, acidic KMnO₄ oxidation, and alkaline KMnO₄ oxidation. These were compared with the benchmark indices to assess their suitability for use as indicators for N mineralization.

Results and discussion

Acidic KMnO₄-oxidative organic N (acidic KMnO₄-N) and PB buffer-hydrolysable organic N (PB_HYDR-N) correlated strongly with NMRN₁₁₂ and N _o (r?=?0.825–0.884, P?<?0.001, n?=?30). Grouping of soils based on soil texture generally provided no improvement in the relationships of chemical soil tests with NMRN₁₁₂ and N _o . Multiple stepwise regression analysis indicated that combining acidic KMnO₄-N and PB_HYDR-N yielded the best prediction of soil N mineralization, explaining 86.1 and 85.5 % of the variation in NMRN₁₁₂ and N _o , respectively, of the 30 tested paddy soils.

Conclusions

The results of acidic KMnO₄-N and PB_HYDR-N as indicators for soil N mineralization were promising, and the operations of acidic KMnO₄ oxidation and PB buffer hydrolysis procedures are simple and cost-effective. Therefore, a combination of acidic KMnO₄-N and PB_HYDR-N shows promise in predicting N mineralization in paddy soils of the Dongting Lake region. However, further calibration through field studies is required and the chemical characteristics of acidic KMnO₄-N and PB_HYDR-N needs to be further clarified.

     

Boiling Potassium Chloride–Extractable Nitrogen as an Index of Potentially Mineralizable and Plant‐Available Nitrogen in Soil

Abstract

Mineralization of soil organic nitrogen (N) and its contribution toward crop N uptake is central to developing efficient N‐management practices. Because biological incubation methods are time consuming and do not fit into the batch‐analysis techniques of soil‐testing laboratories, an analytical procedure that can provide an estimate of the mineralizable N would be useful as a soil‐test method for predicting plant‐available N in soil. In the present studies, the ability of boiling potassium chloride (KCl) to extract potentially mineralizable and plant‐available N in arable soils of semi‐arid India was tested against results from biological incubations and uptake of N by wheat in a pot experiment. Mineralization of organic N in soils was studied in the laboratory by conducting aerobic incubations for 112 days at 32°C and 33 KPa of moisture. Cumulative N mineralization in different soils ranged from 8.2 to 75.6 mg N kg^?1 soil that constituted 2.7 to 8.8% of organic N. The amount of mineral N extracted by KCl increased with increase in length of boiling from 0.5 to 2 h. Boiling for 0.5, 1, 1.5, and 2 h resulted in an increase in mineral‐N extraction by 9.3, 12.7, 19.6, and 26.1%, respectively, as compared to mineral N extracted at room temperature. The boiling‐KCl‐hydrolyzable N (ΔN_i) was directly dependent upon soil organic N content, but the presence of clay retarded hydrolysis for boiling lengths of 0.5 and 1 h. However, for boiling lengths of 1.5, and 2 h, the negative effect of clay was not apparent. The ΔN _i was significantly (P=0.05) correlated to cumulative N mineralized and N‐mineralization potential (N₀). The relationship between N₀ and ΔN _i was curvilinear and was best described by a power function. Boiling length of 2 h accounted for 78% of the variability in N₀. Results of the pot experiment showed that at 21‐ and 63‐day growth stages, dry‐matter yield and N uptake by wheat were significantly correlated to boiling‐KCl‐extractable mineral N. Thus, boiling KCl could be used to predict potentially mineralizable and plant‐available N in these soils, and a boiling time of 2 h was most suitable to avoid the negatively affected estimates of boiling‐KCl‐hydrolyzable N in the presence of clay. The results have implications for selecting length of boiling in soils varying widely in clay content, and this may explain why, in earlier studies, longer boiling times (viz. 2 or 4 h) were better predictors of N availability as compared to 0.5 and 1 h.     

Impact of addition of different rates of rice-residue biochar on C and N dynamics in texturally diverse soils

Impacts of crop residue biochar on soil C and N dynamics have been found to be subtly inconsistent in diverse soils. In the present study, three soils differing in texture (loamy sand, sandy clay loam and clay) were amended with different rates (0%, 0.5%, 1%, 2% and 4%) of rice-residue biochar and incubated at 25°C for 60 days. Soil respiration was measured throughout the incubation period whereas, microbial biomass C (MBC), dissolved organic C (DOC), NH₄⁺-N and NO₃^–N were analysed after 2, 7, 14, 28 and 60 days of incubation. Carbon mineralization differed significantly between the soils with loamy sand evolving the greatest CO₂ followed by sandy clay loam and clay. Likewise, irrespective of the sampling period, MBC, DOC, NH₄⁺-N and NO₃^–N increased significantly with increasing rate of biochar addition, with consistently higher values in loamy sand than the other two soils. Furthermore, regardless of the biochar rates, NO₃^–-N concentration increased significantly with increasing period of incubation, but in contrast, NH₄⁺-N temporarily increased and thereafter, decreased until day 60 in all soils. It is concluded that C and N mineralization in the biochar amended soils varied with the texture and native organic C status of the soils.     

Is CO2 evolution in saline soils affected by an osmotic effect and calcium carbonate?

Salt-affected soils are widespread, particularly in arid climates, but information on nutrient dynamics and carbon dioxide (CO₂) efflux from salt-affected soils is scarce. Four laboratory incubation experiments were conducted with three soils. To determine the influence of calcium carbonate (CaCO₃) on respiration in saline and non-saline soils, a loamy sand (6.3% clay) was left unamended or amended with NaCl to obtain an electrical conductivity (EC) of 1.0 dS?m^?1 in a 1:5 soil/water extract. Powdered CaCO₃ at rates of 0%, 0.5%, 1.0%, 2.5%, 5.0% and 10.0% (w/w) and 0.25-2 mm mature wheat residue at 0% and 2% (w/w) were then added. Cumulative CO₂-C emission from the salt amended and unamended soils was not affected by CaCO₃ addition. To investigate the effect of EC on microbial activity, soil respiration was measured after amending a sandy loam (18.8% clay) and a silt loam (22.5% clay) with varying amount of NaCl to obtain an EC_1:5 of 1.0–8.0 dS?m^?1 and 2.5 g glucose C?kg^?1 soil. Soil respiration was reduced by more than 50% at EC_1:5?≥?5.0 dS?m^?1. In a further experiment, salinity up to an EC_1:5 of 5.0 dS?m^?1 was developed in the silt loam with NaCl or CaCl₂. No differences in respiration at a given EC were obtained between the two salts, indicating that Na and Ca did not differ in toxicity to microbial activity. The effect of different addition rates (0.25–2.0%) of mature wheat residue on the response of respiration to salinity was investigated by adding NaCl to the silt loam to obtain an EC_1:5 of 2.0 and 4.0 dS?m^?1. The clearest difference between salinity levels was with 2% residue rate. At a given salinity level, the modelled decomposition constant ‘k’ increased with increasing residue addition rate up to 1% and then remained constant. Particulate organic carbon left after decomposition from the added wheat residues was negatively correlated with cumulative respiration but positively correlated with EC. Inorganic N (NH ₄ ⁺ -N and NO ₃ ^? -N) and resin P significantly decreased with increasing salinity. Resin P was significantly decreased by addition of CaCl₂ and CaCO₃.     

Immobilization and mineralization of nitrogen in soils incubated with pig slurry or ammonium sulphate

Nitrogen mineralization and immobilization were investigated in two soils incubated with ammonium sulphate or pig slurry over a range of temperatures and moisture contents. A reduction in the mineralization of soil organic N was observed in soils incubated with 100 μg NH₄⁺-Ng^?1 soil as ammonium sulphate at 30°C but not at lower temperatures. Addition of 100 μg NH₄⁺-N g^?1 soil as pig slurry resulted in a period of nett immobilization lasting up to 30 days at 5°C. Although the length of the immobilization phase was shorter at higher temperatures the total N immobilized was similar. The subsequent rate of mineralization in slurry-treated soils was not significantly greater (P = 0.05) than in untreated soils. There was no evidence of any subsequent increased mineralization arising from the immobilized N or slurry organic N for up to 175 days. The rate of immobilization was found to increase with increasing moisture content, though the period of nett immobilization was shorter, so that the amount of N immobilized was similar over a range of moisture contents from 10 to 40%. Approximately 40% of the NH₄⁺-N in the slurry was immobilized under the incubation conditions used.     

Relationships between microbial biomass nitrogen,nitrate leaching and nitrogen uptake by corn in a compost and chemical fertilizer-amended regosol

Abstract

To determine the relationships between microbial biomass nitrogen (N), nitrate–nitrogen leaching (NO₃-N leaching) and N uptake by plants, a field experiment and a soil column experiment were conducted. In the field experiment, microbial biomass N, 0.5 mol L^?1 K₂SO₄ extractable N (extractable N), NO₃-N leaching and N uptake by corn were monitored in sawdust compost (SDC: 20 Mg ha^?1 containing 158 kg N ha^?1 of total N [approximately 50% is easily decomposable organic N]), chemical fertilizer (CF) and no fertilizer (NF) treatments from May 2000 to September 2002. In the soil column experiment, microbial biomass N, extractable N and NO₃-N leaching were monitored in soil treated with SDC (20 Mg ha^?1) + rice straw (RS) at five different application rates (0, 2.5, 5, 7.5 and 10 Mg ha^?1 containing 0, 15, 29, 44 and 59 kg N ha^?1) and in soil treated with CF in 2001. Nitrogen was applied as (NH₄)₂SO₄ at rates of 220 kg N ha^?1 for SDC and SDC + RS treatments and at a rate of 300 kg N ha^?1 for the CF treatment in both experiments. In the field experiment, microbial biomass N in the SDC treatment increased to 147 kg N ha^?1 at 7 days after treatment (DAT) and was maintained at 60–70 kg N ha^?1 after 30 days. Conversely, microbial biomass N in the CF treatment did not increase significantly. Extractable N in the surface soil increased immediately after treatment, but was found at lower levels in the SDC treatment compared to the CF treatment until 7 DAT. A small amount of NO₃-N leaching was observed until 21 DAT and increased markedly from 27 to 42 DAT in the SDC and CF treatments. Cumulative NO₃-N leaching in the CF treatment was 146 kg N ha^?1, which was equal to half of the applied N, but only 53 kg N ha^?1 in the SDC treatment. In contrast, there was no significant difference between N uptake by corn in the SDC and CF treatments. In the soil column experiment, microbial biomass N in the SDC + RS treatment at 7 DAT increased with increased RS application. Conversely, extractable N at 7 DAT and cumulative NO₃-N leaching until 42 DAT decreased with increased RS application. In both experiments, microbial biomass N was negatively correlated with extractable N at 7 DAT and cumulative NO₃-N leaching until 42 DAT, and extractable N was positively correlated with cumulative NO₃-N leaching. We concluded that microbial biomass N formation in the surface soil decreased extractable N and, consequently, contributed to decreasing NO₃-N leaching without impacting negatively on N uptake by plants.     

Nitrogen uptake pattern of herbaceous plants: coping strategies in altered neighbor species

The mechanisms for maintaining the species diversity of plant communities under conditions of resource limitation is an important subject in ecology. How interspecific relationships influence the pattern of nutrient absorption by coexisting species in N-limited ecosystems is still disputed. We investigated the effect of neighbor species on the uptake of inorganic and organic N by three common plant species using ¹⁵N tracer techniques in a semi-arid alpine steppe on the northern Tibet. The results showed that the plant species varied in their capacity to absorb NO₃ ^?-N, NH₄ ⁺-N, and glycine-N with or without neighbor species. Carex moorcroftii and Leontopodium nanum showed much more plasticity in resource utilization than Stipa purpurea when neighbor species were present. When C. moorcroftii and S. purpurea coexisted, they all increased their ¹⁵N uptake for the NO₃ ^?-N (C. moorcroftii 2.2-fold increase and S. purpurea 2.2-fold increase) and glycine-N treatments (C. moorcroftii 2.9-fold increase and S. purpurea 3.4-fold increase), which indicated that neighborhood had a positive effect for N absorption between the two species. However, L. nanum was a less effective competitor for N utilization than the neighbor species across almost all treatments. The dominant species appeared to have an inhibitory effect on N absorption by the accompanying species in this alpine steppe environment. Thus, interspecific neighbor pairs may result in both a mutually beneficial cooperative relationship and a competitive relationship among neighbors in resource use patterns in extreme environments. Resource use plasticity in altered neighbor species may be due to phenotypic plasticity based on the conditions of the realized niche, offering a valuable insight into niche complementarity and providing a general and important mechanism for resource partitioning in an alpine area.     

Production and consumption of N2O during denitrification in subtropical soils of China

Purpose

Nitrous oxide (N₂O) production and reduction rates are dependent on the interactions with each other and it is therefore important to evaluate them within the context of simultaneously operating N₂O emission and reduction. The objective of this study was to quantify the simultaneously occurring N₂O emission and reduction across a range of subtropical soils in China, to gain a mechanistic understanding of potential N₂O dynamics under the denitrification condition and their important drivers, and to evaluate the potential role of the subtropical soils as either sources or sinks of N₂O through denitrification.

Materials and methods

Soils (45, from a range of different land uses and soil parent materials) were collected from the subtropical region of Jiangxi Province, China, and tested for their potential capacity for N₂O emission and N₂O reduction to N₂ during denitrification. N₂O emission and reduction were determined in a closed system under N₂ headspace after the soils were treated with 200?mg?kg^?1 NO ₃ ^? -N and incubation at 30?°C for 28?days. The soil physical and chemical properties, the temporal variations in headspace N₂O concentration, and NO ₃ ^? -N and NH ₄ ⁺ -N concentrations in the soil slurry were measured.

Results and discussion

Variations in N₂O concentration (N) over incubation time (t) were consistent with an equation in which average R ²?=?0.84?±?0.11 (p?<?0.05): $ N = A \times \left( {1 - \exp \left( { - {k_1} \times t} \right)} \right) - B \times \exp \left( {{k_2} \times t} \right) $ , where A is the total N₂O emission during the incubation, B is a constant, and k ₁ and k ₂ are the N₂O emission constant and reduction constants, respectively. The results of the simulation showed that k ₁ was greater than k ₂. The reduced amount of NO ₃ ^? -N in the first 7?days of incubation and the N₂O emission rate (the percentage of A value relative to the amount of NO ₃ ^? -N reduced during the 28-day incubation, R _n) were able to explain 82.9?% (p?<?0.01) of the variation in total N₂O emission (A) during the incubation for the soil samples studied, indicating that the total amount of N₂O emitted was determined predominately by denitrification capacity. Soil organic carbon content and soil nitrogen mineralization are the key factors that determine differences in the amounts of reduced NO ₃ ^? -N among the soil samples. The R _n value decreased with increasing k ₂ (p?<?0.01), indicating that soils with higher N₂O reduction capacity under these incubation conditions would emit less N₂O per unit of denitrified NO ₃ ^? -N than the other soils. Results are valuable in the evaluation of net N₂O emissions in the subtropical soils and the global N budget.

Conclusions

In a closed, anaerobic system, variations in N₂O concentration in the headspace over the incubation time were found to be compatible with a nonlinear equation. Soil organic carbon and the amount of NH ₄ ⁺ -N mineralized from the organic N during the first 7?days of incubation are the key factors that determine differences in the N₂O emission constant (k ₁), the N₂O reduction constant (k ₂), the total N₂O emission during the incubation (A) and the N₂O emission rate (R _n).     

Nitrogen mineralization indicators under semi-arid and semi-humid conditions: influence on wheat yield and nitrogen uptake

The objectives were i) to assess indicators for potential nitrogen (N) mineralization and ii) to analyze their relationships for predicting winter wheat (Triticum aestivum L.) growth parameters (yield and N uptake, N_up) in Mollisols of the semi-arid and semi-humid region of the Argentine Pampas. Thirty-six farmer fields were sampled at 0–20 cm. Several N mineralization indicators, wheat grain yield and N_up at physiological maturity stage were assessed. A principal component (PC) analysis was performed using correlated factors to grain yield and N_up. The cluster analysis showed two main groups: high fertility and low fertility soils. In high fertility soils, combining PCs in multiple regression models enhanced the wheat yield and N_up prediction significantly with a high R² (adj R² = 0.71–0.83). The main factors that explained the wheat parameters were associated with water availability and N mineralization indicator, but they differ according to soil fertility.

Abbreviations: N: nitrogen; SOM: soil organic matter; POM: particulate organic matter; SOC: soil organic carbon; SON: soil organic nitrogen; POM-C: particulate organic carbon; POM-N: particulate organic nitrogen; N_an: anaerobic nitrogen; N_hyd: hydrolyzable N; NO₃-N: cold nitrate; N₂₀₅: N determined by spectrometer at 205 nm; N₂₆₀: N determined by spectrometer at 260 nm; Pe: extractable P; N_up: wheat N uptake; NO₃-N: inorganic N in the form of nitrate; FR: fallow rainfalls (March-Seeding rainfall); FLR: flowering rainfalls (October-December rainfall); GFR: grain filling rainfall (November rainfall); CCR: crop growing season rainfall (June-December rainfall); PCA: principal component analysis; PC: principal component; MR: multiple regression     

N<Subscript>2</Subscript>O production pathways relate to land use type in acidic soils in subtropical China

Purpose

Agricultural practises impact soil properties and N transformation rate, and have a greater effect on N₂O production pathways in agricultural soils compared with natural woodland soils. However, whether agricultural land use affects N₂O production pathways in acidic soils in subtropical regions remains unknown.

Materials and methods

In this study, we collected natural woodland soil (WD) and three types of agricultural soils, namely upland agricultural (UA), tea plantation (TP) and bamboo plantation (BP) soils. We performed paired ¹⁵N-tracing experiment to investigate the effects of land use types on N₂O production pathways in acidic soils in subtropical regions in China.

Results and discussion

The results revealed that heterotrophic nitrification is the dominant pathway of N₂O production in WD, accounting for 44.6 % of N₂O emissions, whereas heterotrophic nitrification contributed less than 2.7 % in all three agricultural soils, due to a lower organic C content and soil C/N ratio. In contrast, denitrification dominated N₂O production in agricultural soils, accounting for 54.5, 72.8 and 77.1 % in UA, TP and BP, respectively. Nitrate (NO₃ ^?) predominantly affected the contribution from denitrification in soils under different land use types. Autotrophic nitrification increased after the conversion of woodland to agricultural lands, peaking at 42.8 % in UA compared with only 21.5 % in WD, and was positively correlated with soil pH. Our data suggest that pH plays a great role in controlling N₂O emissions through autotrophic nitrification following conversion of woodland to agricultural lands.

Conclusions

Our results demonstrate the variability in N₂O production pathways in soils of different land use types. Soil pH, the quantity and quality of organic C and NO₃ ^? content primarily determined N₂O emissions. These results will likely assist modelling and mitigation of N₂O emissions from different land use types in subtropical acidic soils in China and elsewhere.

     

Quantification of nitrogen excretion rates for three lumbricid earthworms using 15N

 Nitrogen excretion rates of ¹⁵N-labeled earthworms and contributions of ¹⁵N excretion products to organic (dissolved organic N) and inorganic (NH₄-N, NO₃-N) soil N pools were determined at 10  °C and 18  °C under laboratory conditions. Juvenile and adult Lumbricus terrestris L., pre-clitellate and adult Aporrectodea tuberculata (Eisen), and adult Lumbricus rubellus (Hoffmeister) were labeled with ¹⁵N by providing earthworms with ¹⁵N-labeled organic substrates for 5–6 weeks. The quantity of ¹⁵N excreted in unlabeled soil was measured after 48 h, and daily N excretion rates were calculated. N excretion rates ranged from 274.4 to 744 μg N g^–1 earthworm fresh weight day^–1, with a daily turnover of 0.3–0.9% of earthworm tissue N. The N excretion rates of juvenile L. terrestris were significantly lower than adult L. terrestris, and there was no difference in the N excretion rates of pre-clitellate and adult A. tuberculata. Extractable N pools, particularly NH₄-N, were greater in soils incubated with earthworms for 48 h than soils incubated without earthworms. Between 13 and 40% of excreted ¹⁵N was found in the ¹⁵N-mineral N (NH₄-N+NO₃-N) pool, and 13–23% was in the ¹⁵N-DON pool. Other fates of excreted ¹⁵N may have been incorporation in microbial biomass, chemical or physical protection in non-extractable N forms, or gaseous N losses. Earthworm excretion rates were combined with earthworm biomass measurements to estimate N flux from earthworm populations through excretion. Annual earthworm excretion was estimated at 41.5 kg N ha^–1 in an inorganically-fertilized corn agroecosystem, and was equivalent to 22% of crop N uptake. Our results suggest that the earthworms could contribute significantly to N cycling in corn agroecosystems through excretion processes. Received: 12 April 1999     

Optimization of arsenic and pentachlorophenol removal from soil using an experimental design methodology

Purpose

In this study, a soil-washing process was investigated for arsenic (As) and pentachlorophenol (PCP) removal from polluted soils. This research first evaluates the use of chemical reagents (HCl, HNO₃, H₂SO₄, lactic acid, NaOH, KOH, Ca(OH)₂, and ethanol) for the leaching of As and PCP from polluted soils.

Materials and methods

A Box–Behnken experimental design was used to optimize the main operating parameters for soil washing. A laboratory-scale leaching process was applied to treat four soils polluted with both organic ([PCP] _i ?=?2.5–30 mg kg^?1) and inorganic ([As] _i ?=?50–250 mg kg^?1, [Cr] _i ?=?35–220 mg kg^?1, and [Cu] _i ?=?80–350 mg kg^?1) compounds.

Results and discussion

Removals of 72–89, 43–62, 52–68, and 64–98 % were obtained for As, Cr, Cu, and PCP, respectively, using the optimized operating conditions ([NaOH]?=?1 N, [cocamidopropylbetaine] _i ?=?2 % w w^?1, t?=?2 h, T?=?80 °C, and PD?=?10 %).

Conclusions

The use of NaOH, in combination with the surfactant, is efficient in reducing both organic and inorganic pollutants from soils with different levels of contamination.     

Importance of organic nitrogen fractions in sandy soils,obtained by electro-ultrafiltration or CaCl2 extraction,for nitrogen mineralization and nitrogen uptake of rape

Summary Sandy soils have low reserves of mineral N in spring. Therefore organic-bound N is the most important pool available for crops. The objective of the present investigation was to study the importance of the organic-bound N extracted by electro-ultrafiltration and by a CaCl₂ solution for the supply of N to rape and for N mineralization. Mitscherlich-pot experiments carried out with 12 different sandy soils (Germany) showed a highly significant correlation between the organic N extracted (two fractions) and the N uptake by the rape (electroultrafiltration extract: r=0.76^***; CaCl₂ extract: r=0.76^***). Organic N extracted by both methods before the application of N fertilizer was also significantly correlated with N mineralization (electro-ultrafiltration extract: r=0.75^***; CaCl₂ extract: r=0.79^***). N uptake by the rape and the mineralization of organic N increased with soil pH and decreased with an increasing C:N ratio and an increasing proportion of sand in the soils. Ninety-eight percent of the variation in N uptake by the rape was determined by the differences in net mineralization of organic N. This show that in sandy soils with low mineral N reserves (NO _inf3 ^sup- -N, NH ₄ ⁺ -N) the organic soil N extracted by electro-ultrafiltration or CaCl₂ solutions indicates the variance in plant-available N. Total soil N was not related to the N uptake by plants nor to N mineralization.     

Nitrification,acidification, and nitrogen leaching from subtropical cropland soils as affected by rice straw-based biochar: laboratory incubation and column leaching studies

Purpose

Few studies have examined the effects of biochar on nitrification of ammonium-based fertilizer in acidic arable soils, which contributes to NO₃ ^? leaching and soil acidification.

Materials and methods

We conducted a 42-day aerobic incubation and a 119-day weekly leaching experiment to investigate nitrification, N leaching, and soil acidification in two subtropical soils to which 300 mg N kg^?1 ammonium sulfate or urea and 1 or 5 wt% rice straw biochar were applied.

Results and discussion

During aerobic incubation, NO₃ ^? accumulation was enhanced by applying biochar in increasing amounts from 1 to 5 wt%. As a result, pH decreased in the two soils from the original levels. Under leaching conditions, biochar did not increase NO₃ ^?, but 5 wt% biochar addition did reduce N leaching compared to that in soils treated with only N. Consistently, lower amounts of added N were recovered from the incubation (KCl-extractable N) and leaching (leaching plus KCl-extractable N) experiments following 5 wt% biochar application compared to soils treated with only N.

Conclusions

Incorporating biochar into acidic arable soils accelerates nitrification and thus weakens the liming effects of biochar. The enhanced nitrification does not necessarily increase NO₃ ^? leaching. Rather, biochar reduces overall N leaching due to both improved N adsorption and increased unaccounted-for N (immobilization and possible gaseous losses). Further studies are necessary to assess the effects of biochar (when used as an addition to soil) on N.