Manipulating the N release from N labelled celery residues by using straw and vinasses

The aim of this laboratory study was to investigate the effect of straw and vinasses on the nitrogen (N) mineralization-immobilization turnover of celery residues during two periods (each simulating a time period from autumn till spring) under laboratory conditions. During the first period (1-198 d), ¹⁵N-labelled celery residues (1.1 g dry matter (DM) kg⁻¹ soil) were incubated together with straw (8.1 g DM kg⁻¹ soil), aiming to immobilize the N released from celery residues, followed by an incorporation of vinasses (1.9 g DM kg⁻¹ soil) after 84 d, with a view to remineralizing the immobilized celery-N. During the second period (198-380 d), the experimental set-up was repeated, except that non-labelled celery residues were used. Total N, mineral N and their ¹⁵N enrichments as well as microbial biomass N were determined at regular time intervals. During both periods, mixing celery residues with straw significantly increased microbial biomass N (90.5 and 40.5 mg N kg⁻¹ extra compared to celery only treatment) and decreased the amount of mineral N (reduction of 56.1 and 45.9 mg N kg⁻¹ soil compared to celery only treatment) and the celery-derived mineral ¹⁵N (0% of mineral celery-derived ¹⁵N in straw treatment compared to 35% of mineral celery-derived ¹⁵N in celery only treatment). After maximum immobilization, a natural remineralization (without addition of vinasses) of 32.2 (at day 198) and 11.1 mg N kg⁻¹ soil (at day 380) occurred in the straw treatment, but the mineral N content remained significantly lower than in the celery only treatment during the complete experiment, and the amount of remineralized celery-¹⁵N was very low (5.4% of celery-derived ¹⁵N after 380 d). Vinasses caused no real priming effect, although it did slightly increase the amount of remineralized celery-¹⁵N (+6.4% of celery-derived ¹⁵N at day 380 compared to the straw treatment), probably due an apparent added N interaction caused by displacement reactions with the soil microbial biomass.     

Priming effect of biuret addition on native soil N mineralisation under laboratory conditions

This study was carried out to quantify the priming effect of biuret on native soil nitrogen (N) mineralisation during a 112-day incubation. Addition of biuret (100 mg ¹⁵N-labelled biuret kg⁻¹ soil) increased the turnover rate constant of soil organic matter and had a positive priming effect on native soil N mineralisation in two soils. The additional mineralisation was 0.65% of the total soil N (equivalent to 47.1 kg N ha⁻¹) in a sandy loam soil and 0.62% of the soil N (equivalent to 46.5 kg N ha⁻¹) in a silt loam soil.     

Manipulating N mineralization from high N crop residues using on- and off-farm organic materials

We have studied the possibilities of manipulating N mineralization from high N vegetable crop residues by the addition of organic materials, with the aim of initially immobilizing the mineralized residue N with a view to stimulating remineralization at a later stage. Residues of leek (Allium porrum) were incubated with soil, alone and in combination with straw, two types of green waste compost (with contrasting C:N ratios) and tannic acid. Evolution of mineral N was monitored by destructive sampling. After 15 weeks, molasses was added to part of the samples in each treatment, and incubation continued for another 12 weeks. All materials added during the first incubation stage, except the low C:N compost, resulted in significant immobilization of the residue N. The immobilization with the high C:N compost (41.4 mg N kg⁻¹ soil) was significantly larger than with tannic acid and straw (both immobilized about 26 mg N kg⁻¹ soil). In the straw treatment, remineralization started in the first stage of incubation from day 50 onwards. The addition of molasses caused a strong and significant remineralization in the second stage (equivalent to 73% of the N initially immobilized) in the treatment with the high C:N ratio compost. In the case of tannic acid, there was no consistent effect on mineralization from addition of molasses. This was attributed to the fact that the immobilization observed was due to chemical rather than biological fixation of the residue N. A number of non-toxic organic wastes could be considered for use in mediating release of immobilized N from high N crop residue materials in an attempt to synchronize residue N availability with crop N demand.     

Cyanobacterial inoculation of heated soils: effect on microorganisms of C and N cycles and on chemical composition in soil surface

Physiological groups of soil microorganisms, total C and N and available nutrients were investigated in four heated (350 °C, 1 h) soils (one Ortic Podsol over sandstone and three Humic Cambisol over granite, schist or limestone) inoculated (1.5 μg chlorophyll a g⁻¹ soil or 3.0 μg chlorophyll a g⁻¹ soil) with four cyanobacterial strains of the genus Oscillatoria, Nostoc or Scytonema and a mixture of them.Cyanobacterial inoculation promoted the formation of microbiotic crusts which contained a relatively high number of NH₄⁺-producers (7.4×10⁹ g⁻¹ crust), starch-mineralizing microbes (1.7×10⁸ g⁻¹ crust), cellulose-mineralizing microbes (1.4×10⁶ g⁻¹ crust) and NO₂⁻ and NO₃⁻ producers (6.9×10⁴ and 7.3×10³ g⁻¹ crust, respectively). These crusts showed a wide range of C and N contents with an average of 293 g C kg⁻¹ crust and 50 g N kg⁻¹ crust, respectively. In general, Ca was the most abundant available nutrient (804 mg kg⁻¹ crust), followed by Mg (269 mg kg⁻¹ crust), K (173 mg kg⁻¹ crust), Na (164 mg kg⁻¹ crust) and P (129 mg kg⁻¹ crust). There were close positive correlations among all the biotic and abiotic components of the crusts.Biofertilization with cyanobacteria induced great microbial proliferation as well as high increases in organic matter and nutrients in the surface of the heated soils. In general, cellulolytics were increased by four logarithmic units, amylolytics and ammonifiers by three logarithmic units and nitrifiers by more than two logarithmic units. C and N contents rose an average of 275 g C kg⁻¹ soil and 50 g N kg⁻¹ soil while the C:N ratio decreased up to 7 units. Among the available nutrients the highest increase was for Ca (315 mg kg⁻¹ soil) followed by Mg (189 mg kg⁻¹ soil), K (111 mg kg⁻¹ soil), Na (109 mg kg⁻¹ soil) and P (89 mg kg⁻¹ soil). Fluctuations of the microbial groups as well as those of organic matter and nutrients were positively correlated.The efficacy of inoculation depended on both the type of soil and the class of inoculum. The best treatment was the mixture of the four strains and, whatever the inoculum used, the soil over lime showed the most developed crust followed by the soils over schist, granite and sandstone. In the medium term there were not significant differences between the two inocula amounts tested.These results showed that inoculation of burned soils with alien N₂-fixing cyanobacteria may be a biotechnological means of promoting microbiotic crust formation, enhancing C and N cycling microorganisms and increasing organic matter and nutrient contents in heated soils.     

Effects of decreasing water potential on gross ammonification and nitrification in an acid coniferous forest soil

Changes in the soil water regime, predicted as a consequence of global climate change, might influence the N cycle in temperate forest soils. We investigated the effect of decreasing soil water potentials on gross ammonification and nitrification in different soil horizons of a Norway spruce forest and tested the hypotheses that i) gross rates are more sensitive to desiccation in the Oa and EA horizon as compared to the uppermost Oi/Oe horizon and ii) that gross nitrification is more sensitive than gross ammonification. Soil samples were adjusted by air drying to water potentials from about field capacity to around −1.0 MPa, a range that is often observed under field conditions at our site. Gross rates were measured using the ¹⁵N pool dilution technique. To ensure that the addition of solute label to dry soils and the local rewetting does not affect the results by re-mineralization or preferential consumption of ¹⁵N, we compared different extraction and incubation times.T₀ times ranging from 10 to 300 min and incubation times of 48 h and 72 h did not influence the rates of gross ammonification and nitrification. Even small changes of water potential decreased gross ammonification and nitrification in the O horizon. In the EA horizon, gross nitrification was below detection limit and the response of the generally low rates of gross ammonification to decreasing water potentials was minor. In the Oi/Oe horizon gross ammonification and nitrification decreased from 37.5 to 18.3 mg N kg⁻¹ soil d⁻¹ and from 15.4 to 5.6 mg N kg⁻¹ soil d⁻¹ when the water potential decreased from field capacity to −0.8 MPa. In the Oa horizon gross ammonification decreased from 7.4 to 4.0 mg N kg⁻¹ soil d⁻¹ when the water potential reached −0.6 MPa. At such water potential nitrification almost ceased, while in the Oi/Oe horizon nitrification continued at a rather high level. Hence, only in the Oa horizon nitrification was more sensitive to desiccation than ammonification. Extended drought periods that might result from climate change will cause a reduction in gross N turnover rates in forest soils even at moderate levels of soil desiccation.     

Differential and successive effects of residue quality and soil mineral N on water-stable aggregation during crop residue decomposition

Residue quality has been shown to influence soil water-stable aggregation (WSA) during crop residue decomposition, but there is still little information about its interactive effect with soil mineral N availability. The aim of this study was to determine the effect of soil mineral N on WSA during the decomposition of two high-C/N crop residues (wheat straw with C/N = 125.6 and miscanthus straw with C/N = 311.3). The two crop residues were combined with three mineral N addition rates (0, 60, and 120 mg N kg⁻¹ dry soil). Respiration, soil mineral N content, and WSA (expressed as mean-weight diameter, MWD) were measured on several dates during a 56-d incubation. The effect of decomposing crop residues on WSA followed two phases. (i) Between 0 and 7 d, the increase in WSA was related to intrinsic residue quality with higher decomposability of the wheat straw resulting in higher WSA. (ii) Thereafter, and until the end of the experiment, mineral N addition rates had a predominant but negative influence on WSA. In this second phase, the average MWD of residue-treated soils was 0.92, 0.55, and 0.44 mm for the 0, 60 and 120 mg N kg⁻¹ dry soil addition rates, respectively. Mineral N addition which did result in higher crop residue decomposition did not lead to higher WSA. WSA during crop residue decomposition is therefore not simply positively related to the induced microbial activity, and changes in microbial community composition with differential effects on WSA must be involved. The impact of high-C/N crop residues inputs on WSA, initially assumed to be low, could actually be strong and long-lasting in situations with low soil mineral N content.     

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.     

Land use effects on gross nitrogen mineralization, nitrification, and N2O emissions in ephemeral wetlands

Stable ¹⁵N isotope dilution and tracer techniques were used in cultivated (C) and uncultivated (U) ephemeral wetlands in central Saskatchewan, Canada to: (1) quantify gross mineralization and nitrification rates and (2) estimate the relative proportion of N₂O emissions from these wetlands that could be attributed to denitrification versus nitrification-related processes. In-field incubation experiments were repeated in early May, mid-June and late July. Mean gross mineralization and nitrification rates (10.3 and 3.1 mg kg⁻¹ d⁻¹, respectively) did not differ between C and U wetlands on any given date. Despite these similarities, the mean NH₄⁺ pool size in the U wetlands (17.2 mg kg⁻¹) was two to three times that of the C wetlands (6.7 mg kg⁻¹) whereas the mean NO₃⁻ pool size in U wetlands (2.2 mg kg⁻¹) was less than half that of C wetlands (5.8 mg kg⁻¹). Mean N₂O emissions from the C wetlands decreased from 112.8 to 17.0 ng N₂O m² s⁻¹ from May to July, whereas mean U-wetland N₂O emissions ranged only from 31.8 to 51.1 ng N₂O m² s⁻¹ over the same period. This trend is correlated to water-filled pore space in C wetlands, demonstrating a soil moisture influence on emissions. Denitrification is generally considered the dominant emitter of N₂O under anaerobic conditions, but in the C wetlands, only 49% of the May emissions could be directly attributed to denitrification, decreasing to 29% in July. In contrast, more than 75% of the N₂O emissions from the U wetlands arose from denitrification of the soil NO₃⁻ pool throughout the season. These land use differences in emission sources and rates should be taken into consideration when planning management strategies for greenhouse gas mitigation.     

Chemical composition, or quality, of agroforestry residues influences N2O emissions after their addition to soil

Emissions of N₂O were measured following addition of ¹⁵N-labelled (2.6-4.7 atom% excess ¹⁵N) agroforestry residues (Sesbania sesban, mixed Sesbania/Macroptilium atropurpureum, Crotalaria grahamiana and Calliandra calothyrsus) to a Kenyan oxisol at a rate of 100 mg N kg soil⁻¹ under controlled environment conditions. Emissions were increased following addition of residues, with 22.6 mg N m⁻² (124.4 mg N m⁻² kg biomass⁻¹; 1.1 mg ¹⁵N m⁻²; 1.03% of ¹⁵N applied) emitted as N₂O over 29 d after addition of both Sesbania and Macroptilium residues in the mixed treatment. Fluxes of N₂O were positively correlated with CO₂ fluxes, and N₂O emissions and available soil N were negatively correlated with residue lignin content (r=−0.49;P<0.05), polyphenol content (r=−0.94;P<0.05), protein binding capacity (r=−0.92;P<0.05) and with (lignin+polyphenol)-to-N ratio (r=−0.55;P<0.05). Lower emission (13.6 mg N m⁻² over 29 d; 94.5 mg N m⁻² kg biomass⁻¹; 0.6 mg ¹⁵N m⁻²; 0.29% of ¹⁵N applied) after addition of Calliandra residue was attributed to the high polyphenol content (7.4%) and high polyphenol protein binding capacity (383 μg BSA mg plant⁻¹) of this residue binding to plant protein and reducing its availability for microbial attack, despite the residue having a N content of 2.9%. Our results indicate that residue chemical composition, or quality, needs to be considered when proposing mitigation strategies to reduce N₂O emissions from systems relying on incorporation of plant biomass, e.g. improved-fallow agroforestry systems, and that this consideration should extend beyond the C-to-N ratio of the residue to include polyphenol content and their protein binding capacity.     

Denitrification enzyme activity and potential of subsoils under grazed grasslands assayed by membrane inlet mass spectrometer

The importance of subsoil denitrification on the fate of agriculturally derived nitrate (NO₃) leached to groundwater is crucial for budgeting N in an ecosystem and for identifying areas where the risk of excess NO₃ is reduced. However, the high atmospheric background of di-nitrogen (N₂) causes difficulties in assessing denitrification enzyme activity (DEA) and denitrification potential (DP) in soils directly. Here, we apply Membrane Inlet Mass Spectrometry (MIMS) technique to investigate indirectly DEA and DP in soils by measuring N₂/Ar ratio changes in headspace water over soil. Soils were collected from 0-10, 15-25 and 60-70 cm depths of a grazed ryegrass and grass-clover. The samples were amended with helium-flushed deionized water containing ranges of NO₃ and carbon (glucose-C) and were incubated for six hours in the dark at 21 °C. The peaks for N₂/Ar ratio, declined with increasing soil depth, indicating a reduced substrate requirements to initiate DEA en-masse (15-30 mg NO₃-N alone or with 60-120 mg glucose-C, kg⁻¹ soil). The dissolved N₂O concentrations were very small (0.004-0.269 μg N kg⁻¹ soil) but responded well to the added N and C, showing a reduction in DEA with soil depth. In three separate studies, only subsoils were incubated for 3 days at 12 °C with 20-30 mg NO₃-N ± 40-60 mg glucose-C, kg⁻¹ soil. Denitrification capacity (DC, NO₃ only treatment) was not statistically different to the control (no amendment) within a land use (0.03-0.05 vs. 0.07-0.22 mg N kg⁻¹ soil d⁻¹), the highest being in ryegrass subsoils receiving groundwater. The DP was significantly (P < 0.0001) higher in subsoils under ryegrass than under grass-clover (0.50-0.71 vs. 1.15 mg N kg⁻¹ soil d⁻¹). The rates of DP (NO₃ + glucose-C) increased significantly (P < 0.0001) in unsaturated and saturated subsoils (0.92 and 2.19 mg N kg⁻¹ soil d⁻¹, respectively) of grass-clover, due to the higher reductive state resulting from the 10 day pre-incubation. Available C accelerated denitrification in soils and superseded the temporary elevation in oxidative state due to NO₃ addition. The substrates load differences between the land uses regulated the degree of denitrification rates. Results suggest that both dissolved N₂O measured by gas chromatography and N₂/Ar ratio measured by MIMS to indirectly determine DEA, and the latter to quantify total DC/DP in soils can be used. However, interference of oxygen in the MIMS system should be considered if available C is added or is naturally elevated in soil or groundwater.     

Stabilization and plant uptake of N from N-labelled pea residue 16.5 years after incorporation in soil

The decline of N from ¹⁵N-labelled mature pea residues was followed in unplanted soil over 16.5 yr. Eight years after residue incorporation, 24% of the residue ¹⁵N input was still present in the soil and, after 16.5 yr, 16% of the residue ¹⁵N input remained. A double exponential model successfully described the decay of N from ¹⁵N-labelled pea residues. The total residual ¹⁵N declined with average decay constants of 1.45 yr⁻¹ for the 30 d to 1 yr period and of 0.07 yr⁻¹ for the 1-16 yr period. Sixteen years following incorporation of the residues, indicator plants growing in residues-amended soils were obtaining 1.7% of their N from residue N. This is, to our knowledge, the longest study on decay of N in soils from ¹⁵N-labelled crop residues. The current study thus provides a unique data set for our empirical understanding of N-dynamics in agricultural systems, which is a prerequisite to parameterize and validate N-simulation models.     

Rates of decomposition of plant residues and available nitrogen in soil, related to residue composition through simulation of carbon and nitrogen turnover

The dynamics of inorganic N in soil following the application of plant residues depends on their composition. We assumed that all plant materials are composed of similar components, each decomposing at a specific rate, but differ in the proportions of the various components. The NCSOIL model that simulates C and N turnover in soil was used to link the rates of residue decomposition to their composition, defined as soluble, cellulose-like and lignin-like C and N, and thereby integrate short and long-term effects of residues on available N dynamics in soil. Five plant residues in a wide range of C:N ratios were incubated in soil for 24 weeks at 30 °C, during which C and N mineralization were measured. The materials with large C:N ratios (corn, rice hulls and wheat straw) were also incubated with NH₄⁺-N to avoid N deficiency. The residues were analyzed for total and soluble C and N. The partitioning of insoluble C and N between cellulose- and lignin-like pools was optimized by best fit of simulated C and N mineralization to measured results. The decomposition rate constants of the soluble and lignin-like pools were assumed to be 1.0 and 10⁻⁵ d⁻¹, respectively, and that of the cellulose-like pool, obtained by model optimization against mineralization of cellulose with NH₄⁺-N in soil, was 0.051 d⁻¹. The optimized, kinetically defined lignin-like pool of all residues was considerably larger than lignin contents normally found in plant residues by the Van Soest procedure. Gross N mineralization of tobacco and rape residues was similar, but N recovery from tobacco was larger, because a larger fraction of its C was in the lignin-like pool. N in rice hulls, corn and wheat residues was mostly recalcitrant, yet rice hulls did not cause N deficiency, because most of its C was recalcitrant too. The soluble components of the residues had strong short-term effects on available N in soil, but the cellulose-like pool was equally important for short and medium-term effects. Soluble and cellulose-like C were 29 and 42% of total C, respectively, in corn and 7 and 50% in wheat. Maximal net inorganic N losses, measured in both residue treatments after 2 weeks, were 42 mg g⁻¹ C applied as corn and 31 mg g⁻¹ C applied as wheat, or 84 and 110 mg g⁻¹ decomposed C of corn and wheat, respectively. Rice hulls immobilized N slowly, but by the end of 24 weeks all three residues immobilized 26-27 mg N kg⁻¹ C applied. The different dynamics of N immobilization demonstrated the need to determine the decomposability of C and N rather than their total contents in plant residues.     

Influence of selenium on oxidoreductive enzymes activity in soil and in plants

The aim of this greenhouse experiment was the assessment of the influence of H₂SeO₃ at soil concentrations of 0.05, 0.15 and 0.45 mmol kg⁻¹, on the activity of selected oxidoreductive enzymes in wheat (Triticum aestivum). The wheat plants were grown in 2 dm³ pots filled with dust-silt black soil of pH 7.7. Applied H₂SeO₃ caused activation of plant nitrate reductase at all concentrations, but activation of plant polyphenol oxidase at only two lower concentrations. The highest concentration caused inhibition of polyphenol oxidase and peroxidase. Plant catalase activity decreased under the influence of 0.15 and 0.45 mmol kg⁻¹ concentration. After the final analysis Se was quantified in plants and soil. The amounts in plants were: control (unamended soil) 1.95 mg kg⁻¹; I dose (0.05 mmol kg⁻¹) 18.27 mg kg⁻¹; II dose (0.15 mmol kg⁻¹) 33.20 mg kg⁻¹ and III dose (0.45 mmol kg⁻¹) 38.37 mg kg⁻¹, in soil: 0.265 mg kg⁻¹; 3.61 mg kg⁻¹; 10.53 mg kg⁻¹; 30.53 mg kg⁻¹; respectively. Simultaneously, a laboratory experiment was performed, where the activity of soil catalase and peroxidase were tested after 1, 3, 7, 14, 28, 56, and 112 days after Se treatment. Peroxidase activity in soil decreased with increasing Se content, over the whole experiment. The lowest dose of Se caused activation a significant 10% increase in catalase activity, but the influence of others doses was unclear.     

Natural N abundance of plant and soil inorganic-N as evidence for over-fertilization with compost

To test the hypothesis that N isotope composition can be used as evidence of excessive compost application, we measured variation in patterns of N concentrations and corresponding δ¹⁵N values of plants and soil after compost application. To do so, a pot experiment with Chinese cabbage (Brassica campestris L. cv. Maeryok) was conducted for 42 days. Compost was applied at rates of 0 (SC0), 500 (SC1), 1000 (SC2), and 1500 mg N kg⁻¹ soil (SC3). Plant-N uptake linearly increased with compost application (r² = 0.956, P < 0.05) with an uptake efficiency of 76 g N kg⁻¹ of compost-N at 42 days after application, while dry-mass accumulation did not show such linear increases. Net N mineralized from compost-N increased linearly (r² = 0.998, P < 0.01) with a slope of 122 g N kg⁻¹ of compost-N. Plant-δ¹⁵N increased curvilinearly with increasing compost application, but this increase was insignificant between SC2 and SC3 treatments. The δ¹⁵N of soil inorganic-N (particularly NO₃⁻-N) increased with compost application. We found that plant-δ¹⁵N reflected the N isotope signal of soil NO₃⁻-N at each measurement during plant growth, and that δ¹⁵N of inner leaves and soil NO₃⁻-N was similar when initial NO₃⁻ in the compost was abundant. Therefore, we concluded that δ¹⁵N of whole plant (more obviously in newer plant parts) and soil NO₃⁻-N could reveal whether compost application was excessive, suggesting a possible use of δ¹⁵N in plants and soil as evidence of excess compost application.     

Gross nitrogen transformations in adjacent native and plantation forests of subtropical Australia

The impact of land-use change on soil nitrogen (N) transformations was investigated in adjacent native forest (NF), 53 y-old first rotation (1R) and 5 y-old second rotation (2R) hoop pine (Araucaia cunninghamii) plantations. The ¹⁵N isotope dilution method was used to quantify gross rates of N transformations in aerobic and anaerobic laboratory incubations. Results showed that the land-use change had a significant impact on the soil N transformations. Gross ammonification rates in the aerobic incubation ranged between 0.62 and 1.78 mg N kg⁻¹ d⁻¹, while gross nitrification rates ranged between 2.1 and 6.6 mg N kg⁻¹ d⁻¹. Gross ammonification rates were significantly lower in the NF and the 1R soils than in the 2R soils, however gross nitrification rates were significantly higher in the NF soils than in the plantation soils. The greater rates of gross nitrification found in the NF soil compared to the plantation soils, were related to lower soil C:N ratios (i.e. more labile soil N under NF). Nitrification was found to be the dominant soil N transformation process in the contrasting forest ecosystems. This might be attributed to certain site conditions which may favour the nitrifying community, such as the dry climate and tree species. There was some evidence to suggest that heterotrophic nitrifiers may undertake a significant portion of nitrification.     

Labile, recalcitrant, and microbial carbon and nitrogen pools of a tallgrass prairie soil in the US Great Plains subjected to experimental warming and clipping

Carbon (C) and nitrogen (N) fluxes are largely controlled by the small but highly bio-reactive, labile pools of these elements in terrestrial soils, while long-term C and N storage is determined by the long-lived recalcitrant fractions. Changes in the size of these pools and redistribution among them in response to global warming may considerably affect the long-term terrestrial C and N storage. However, such changes have not been carefully examined in field warming experiments. This study used sulfuric acid hydrolysis to quantify changes in labile and recalcitrant C and N fractions of soil in a tallgrass prairie ecosystem that had been continuously warmed with or without clipping for about 2.5 years. Warming significantly increased labile C and N fractions in the unclipped plots, resulting in increments of 373 mg C kg⁻¹ dry soil and 15 mg N kg⁻¹ dry soil, over this period whilst clipping significantly decreased such concentrations in the warmed plots. Warming also significantly increased soil microbial biomass C and N in the unclipped plots, and increased ratios of soil microbial/labile C and N, indicating an increase in microbial C- and N-use efficiency. Recalcitrant and total C and N contents were not significantly affected by warming. For all measured pools, only labile and microbial biomass C fractions showed significant interactions between warming and clipping, indicating the dependence of the warming effects on clipping. Our results suggest that increased soil labile and microbial C and N fractions likely resulted indirectly from warming increases in plant biomass input, which may be larger than warming-enhanced decomposition of labile organic compounds.     

A survey of symbiotic nitrogen fixation by white clover grown on metal contaminated soils

There is conflicting evidence about toxic effects of heavy metals in soil on symbiotic nitrogen fixation. This study was set-up to assess the general occurrence of such effects. Soils with metal concentration gradients were sampled from six established field trials, where sewage sludge or metal salts have been applied, or from a transect in a sludge treated soil. Additional contaminated soils were sampled near metal smelters, in floodplains, in sludge amended arable land and in a metalliferous area. Symbiotic nitrogen fixation was measured with ¹⁵N isotope dilution in white clover (Trifolium repens L.) grown in potted soil that was not re-inoculated, and using ryegrass (Lolium perenne L.) as reference crop. The fraction nitrogen in clover derived from fixation (N_dff) varied from 0 to 88% depending on soil. Pronounced metal toxicity on N_dff was only confirmed in a sludge treated soil where nitrogen fixation was halved from the control value at soil total metal concentration of 737 mg Zn kg⁻¹, 428 mg Cu kg⁻¹ and 10 mg Cd kg⁻¹. The N_dff was significantly reduced by increasing metal concentration in soils from two other sites where N_dff was low throughout and where these effects might be attributed to confounding factors. No significant effects of metals on N_dff were identified in all other gradients even up to elevated total metal concentration (e.g. 55 mg Cd kg⁻¹). The variation of N_dff among all soils (n=48), is mainly explained by the number of rhizobia in the soil (log MPN, log (cells g⁻¹ soil)), whereas correlations with total or soil solution metal concentrations were weak (R²<0.25). The is significantly affected by the presence or absence of the host plant at the sampling site. No effects of metals were identified at even at total Zn concentrations of about 2000 mg Zn kg⁻¹, whereas metal toxicity could be identified at lower most probable number (MPN) values. This survey shows that the metal toxicity on symbiotic nitrogen fixation cannot be generalized and that survival of a healthy population of the microsymbiont is probably the critical factor.     

Bulk soil C to N ratio as a simple measure of net N mineralization from stabilized soil organic matter in sandy arable soils

Investigations of 23 northwestern German sandy Ap horizons (mean clay content 35 g kg⁻¹), that had higher organic matter (OM) levels than expected for sands, showed that the bulk soil C to N ratio reliably indicated the release of N from stabilized OM. Soils were incubated at 35 °C for 200 days under aerobic conditions. Cumulative N release curves were split into N released from fresh materials (N_fast) and N released from the larger pool of stabilized, older OM (N_slow rates, 0.06-0.77 μg N g⁻¹ soil d⁻¹, or 0.7-49 μg N g⁻¹ OM). Correlating the N_slow rates with total N contents of soils yielded no satisfactory relationships while their relationship with C to N ratios was very close (negative exponential, R²=0.88). Low rates of N release (N_slow) per unit of OM occurred if C to N exceeded 15. This was associated with historical factors like podzolization, calluna heathland, plaggen fertilization or a combination of these.     

Possibilities to reduce rice straw-induced global warming potential of a sandy paddy soil by combining hydrological manipulations and urea-N fertilizations

Global warming potential (GWP) of sandy paddy soils may be reduced by trade-offs between N₂O, CH₄ and CO₂ emissions. Laboratory experiments using either rice straw (1% or 0.5%) or together with urea-N (25 or 50 mg N kg⁻¹ soil) at various levels of soil water were carried out for 30 days each, to test this assumption. Waterlogging combined with urea-N increased total N₂O emissions, with greater release upon rewaterlogging (7.4 mg N kg⁻¹ soil) than experienced by removing waterlogging only. Rice straw±urea-N either emitted small amounts of N₂O or resulted in negative values at all water levels, including saturated and aerobic. Total CH₄ fluxes declined with the decreased water levels and amount of rice straw (<193 mg C kg⁻¹ soil), and also for CO₂ with the latter (<1340 mg C kg⁻¹ soil), and rewaterlogging had little influence on both. N₂O under rewaterlogged and waterlogged±urea-N, CH₄ under waterlogged with rice straw, and CO₂ for the remainder were the major contributors to GWP. Results show that waterlogging following aerobic decomposition of rice straw (1%) with urea-N, applied either at the beginning or at the end of the aerobic conditions, could decrease GWP by 56-64% and 32-42% over the sole addition of rice straw (1% and 0.5%) under waterlogged and saturated conditions, respectively.