Estimating N2 fixation by Sesbania rostrata and S. cannabina (syn. S. aculeata) in lowland rice soil by the 15N dilution method

Summary A field experiment in concrete-based plots was conducted to estimate the contribution of N derived from air (Ndfa) or biological N₂ fixation in Sesbania rostrata and S. cannabina (syn. S. aculeata), using various references, by the ¹⁵N dilution method. The two Sesbania species as N₂-fixing reference plants and four aquatic weed species as non-N₂-fixing references were grown for 65 days after sowing in two consecutive crops, in the dry and the wet seasons, under flooded conditions. Soil previously labeled with ¹⁵N at 0.26 atom % ¹⁵N excess in mineralizable N was further labeled by ammonium sulfate with 3 and 6 atom % ¹⁵N excess. The results showed that ¹⁵N enrichment of soil NH ₄ ⁺ -N dropped exponentially in the first crop to half the original level in 50 days while in the second crop, it declined gradually to half the level in 130 days. The decline in ¹⁵N enrichment, in both N₂-fixing and non-fixing species, was also steeper in the first crop than in the second crop. Variations in ¹⁵N enrichment among non-fixing species were smaller in the second crop. The ratio of the uptake of soil N to that of fertilizer N in N₂-fixing and non-fixing species was estimated by the technique of varying the ¹⁵N level. In the second crop, this ratio in non-fixing species was higher than that in N₂-fixing species. Comparable estimates of % Ndfa were obtained by using ¹⁵N enrichment of various non-fixing species. There was also good agreement between the estimates obtained by using ¹⁵N enrichment of non-fixing species and those by using soil NH ₄ ⁺ -N, particularly in the second crop. By 25 days after sowing, the first crop of both Sesbania spp. had obtained 50% of total N from the atmosphere and the second crop had obtained 75%. The contribution from air increased with the age of the plant and ranged from 70% to 95% in 45–55 days. S. rostrata fixed substantially higher amounts of N₂ due to its higher biomass production compared with S. cannabina. Mathematical considerations in applying the ¹⁵N dilution method are discussed with reference to these results.     

In situ mineral <Superscript>15</Superscript>N dynamics and fate of added <Superscript>15</Superscript>NH<Subscript>4</Subscript><Superscript>+</Superscript> in hoop pine plantation and adjacent native forest in subtropical Australia

Background, aim, and scope  Hoop pine (Araucaria cunninghamii) is a nitrogen (N) demanding indigenous Australia softwood species with plantations in Southeast Queensland, Australia. Soil fertility has declined with increasing rotations and comparison study of N cycling between hoop pine plantations, and adjacent native forest (NF) is required to develop effective forest management for enhancing sustainable forest production and promoting environmental benefits. Field in situ mineral ¹⁵N transformations in these two forest ecosystems have not been studied. Hence, the present study was to compare the differences in soil nutrients, N transformations, ¹⁵N fluxes, and fate between the hoop pine plantation and the adjacent native forest. Materials and methods  The study sites were in Yarraman State Forest (26°52′ S, 151°51′ E), Southeastern Queensland, Australia. The in situ core incubation method was used in the field experiments. Mineral N was determined using a LACHAT Quickchem Automated Ion Analyzer. ¹⁵N were performed using an isotope ratio mass spectrometer with a Eurovector elemental analyzer. All statistical tests were carried out by the SPSS 11.0 for Windows statistical software package. Results  Soil total C and N were significantly higher in the NF than in the 53-year-old hoop pine plantation. Concentrations of NO₃ ^– were significantly higher in the NF soil than in the plantation soil. The plantation soil had significantly higher ¹⁵N and ¹³C natural abundances than the NF soil. The NF soil had significantly lower C/N ratios than the plantation soil. NO₃ ^––N was dominated in mineral N pools in both NF and plantation soils, accounting for 91.6% and 70.3% of the total mineral N pools, respectively. Rates of net nitrification and net N mineralization were, respectively, four and three times higher in the NF soil than in the plantation soil. The ¹⁵NO₃ ^––N and mineral ¹⁵N were significantly higher in the NF soil than in the plantation soil. Significant difference in ¹⁵NH₄ ⁺–N was found in the NF soil before and after the incubation. Discussion  The NF soil had significantly higher NO₃ ^––N, mineral N, total N and C but lower δ¹⁵N, δ¹³C, and C/N ratios than the plantation soil. Moreover, the rates of soil net N mineralization and nitrification were significantly higher, but ammonification rate was lower in the NF than in the plantation. The NF soil had many more dynamic N transformations than the plantation soil due to the combination of multiple species and layers and, thus, stimulation of microbial activity and alteration of C and N pool sizes in favor of the N transformations by soil microbes. The net rate of N and ¹⁵N transformation demonstrated differences in N dynamic related to the variation in tree species between the two ecosystems. Conclusions  The change of land use and trees species had significant impacts on soil nutrients and N cycling processes. The plantation had larger losses of N than the NF. The NO₃ ^––N and ¹⁵NO₃ ^––N dominated in the mineral N and ¹⁵N pools in both forest ecosystems. Recommendations and perspectives  Native forest soil had strong N dynamic compared with the plantation soil. Composition of multiple tree species with different ecological niches in the plantation could promote the soil ecosystem sustainability. The ¹⁵N isotope dilution technique in the field can be quite useful for studying in situ mineral ¹⁵N transformations and fate to further understand actual N dynamics in natural forest soils.     

Effect of CO<Subscript>2</Subscript> on nitrification and immobilization of NH<Subscript>4</Subscript><Superscript>+</Superscript>-N

A laboratory incubation experiment was conducted to demonstrate that reduced availability of CO₂ may be an important factor limiting nitrification. Soil samples amended with wheat straw (0%, 0.1% and 0.2%) and (¹⁵NH₄)₂SO₄ (200 mg N kg^–1 soil, 2.213 atom% ¹⁵N excess) were incubated at 30±2°C for 20 days with or without the arrangement for trapping CO₂ resulting from the decomposition of organic matter. Nitrification (as determined by the disappearance of NH₄⁺ and accumulation of NO₃^–) was found to be highly sensitive to available CO₂ decreasing significantly when CO₂ was trapped in alkali solution and increasing substantially when the amount of CO₂ in the soil atmosphere increased due to the decomposition of added wheat straw. The co-efficient of correlation between NH₄⁺-N and NO₃^–-N content of soil was highly significant (r =0.99). During incubation, 0.1–78% of the applied NH₄⁺ was recovered as NO₃^– at different incubation intervals. Amendment of soil with wheat straw significantly increased NH₄⁺ immobilization. From 1.6% to 4.5% of the applied N was unaccounted for and was due to N losses. The results of the study suggest that decreased availability of CO₂ will limit the process of nitrification during soil incubations involving trapping of CO₂ (in closed vessels) or its removal from the stream of air passing over the incubated soil (in open-ended systems).     

Soil N<Subscript>2</Subscript>O emissions and N<Subscript>2</Subscript>O/(N<Subscript>2</Subscript>O+N<Subscript>2</Subscript>) ratio as affected by different fertilization practices and soil moisture

The objective of this work was to evaluate the effect of the chemical nature and application frequency of N fertilizers at different moisture contents on soil N₂O emissions and N₂O/(N₂O+N₂) ratio. The research was based on five fertilization treatments: unfertilized control, a single application of 80 kg ha⁻¹ N-urea, five split applications of 16 kg ha⁻¹ N-urea, a single application of 80 kg ha⁻¹ N–KNO₃, five split applications of 16 kg ha⁻¹ N–KNO₃. Cumulative N₂O emissions for 22 days were unaffected by fertilization treatments at 32% water-filled pore space (WFPS). At 100% and 120% WFPS, cumulative N₂O emissions were highest from soil fertilized with KNO₃. The split application of N fertilizers decreased N₂O emissions compared to a single initial application only when KNO₃ was applied to a saturated soil, at 100% WFPS. Emissions of N₂O were very low after the application of urea, similar to those found at unfertilized soil. Average N₂O/(N₂O+N₂) ratio values were significantly affected by moisture levels (p = 0.015), being the lowest at 120% WFPS. The N₂O/(N₂O+N₂) ratio averaged 0.2 in unfertilized soil and 0.5 in fertilized soil, although these differences were not statistically significant.     

Changes in CH<Subscript>4</Subscript> production during different stages of litter decomposition under inundation and N addition

Purpose

Our aim was to examine linkages between mass loss, chemical transformation and CH₄ production during decomposition of leaf litters submerged under water. We hypothesised that (i) labile leaf litters would fuel a rapid, high rate of methane (CH₄) production and that recalcitrant litters would fuel long-lasting but lower emissions, (ii) leaf litters experiencing a greater alteration to chemical properties would stimulate increased CH₄ production and (iii) nitrogen (N) addition would increase CH₄ emissions.

Materials and methods

Litters from six plant species were collected from a riparian ecosystem adjacent to Wyaralong Dam, located in Queensland, Australia, i.e., Lophostemon confertus, Cynodon dactylon, Heteropogon contortus, Chamaecrista rotundifolia, Chrysocephalum apiculatum and Imperata cylindrica. We evaluated the rate of mass loss and CH₄ emissions for 122 days of incubation in inundated microcosms with and without N addition. We quantified the chemical changes in the decomposing litters with ¹³ C-cross polarization and magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectrum.

Results and discussion

The inundation treatment of plant litters significantly affected decomposition rates. All litters decomposed in either inundated or aerobic microcosms were quite distinct with regard to the NMR spectra of their initial litters. N addition altered the NMR spectra under both inundation and aerobic conditions. The N treatment only marginally influenced the decomposition rates of I. cylindrica and C. apiculatum litters. The diurnal patterns of CH₄ production in the H. contortus, C. rotundifolia and C. apiculatum litters under inundation incubation could be expressed as one-humped curves, with the peak value dependent on litter species and N treatment. N addition stimulated CH₄ emission by C. rotundifolia and C. apiculatum litters and inhibited CH₄ emission from microcosms containing the litters of the three gramineous species, i.e., I. cylindrica, C. dactylon and H. contortus.

Conclusions

Our results provide evidence that labile leaf litters could fuel a rapid, high rate of CH₄ production and that recalcitrant litters fuelled a lower CH₄ emission. We did not find that leaf litters with altered chemical properties stimulated increased CH₄ production. We also found that N addition was able to increase CH₄ emissions, but this effect was dependent on the litter species.

     

Alder-poplar associations: Determination of plant nitrogen sources by isotope techniques

Summary Three¹⁵N isotopic dilution methods (¹⁵N natural abundance, labelled mineral fertilizer, and organic matter) were used to determine the proportion of N derived from different available sources in seedlines ofAlnus glutinosa andPopulus nigra planted together or in monoculture under natural climatic conditions. The proportion of N derived from N₂ fixation in associated alders was appreciably higher than that determined in monoculture. The reduction of soil N uptake by associated alders contributed to an increase in total plant N and biomass production in associated poplars. When slightly N-labelled organic matter (alder leaf litter) was incorporated into the soil, 10–15% of its initial N content was recovered in poplar tissues, showing that this N source makes an important contribution to the N yield of associated non-fixing plants. There were no significant differences between the results obtained by¹⁵N natural abundance and those obtained by labelled fertilizer methods, suggesting that the ¹⁵N method could be used to evaluate annual N budgets in natural ecosystems.     

N<Subscript>2</Subscript> fixation by faba bean (<Emphasis Type="Italic">Vicia faba</Emphasis> L.) in a gypsum-amended sodic soil

The response of faba bean to the application of four rates of gypsum (0, 2.5, 5.0, 10.0 t ha⁻¹) to a non-saline, alkaline sodic soil was measured in terms of grain yield, dry matter (DM) production, N accumulation and the proportional dependence of the legume on symbiotic N₂ fixation (P _atm). A yield-independent, time-integrated ¹⁵N-dilution model was used to estimate symbiotic dependence. A significant decrease in the exchangeable sodium percentage and significant increases in exchangeable Ca⁺⁺ and the Ca⁺⁺:Mg⁺⁺ ratio in the 0–10-cm soil layer were measured 30 months after application of 10 t ha⁻¹ gypsum. Despite low and erratic rainfall during crop growth, faba bean DM and N uptake responded positively to gypsum application. The symbiotic dependence of the legume at physiological maturity was little affected by sodicity (P _atm = 0.74 at zero gypsum and 0.81–0.82 at 2.5–10 t ha⁻¹ gypsum). The increase in fixed N due to gypsum application was mainly due to increases in legume DM and total N uptake. At 10 t ha⁻¹ of gypsum, faba bean fixed more than 200 kg N ha⁻¹ in above-ground biomass.     

Net and gross mineral N production rates at three levels of forest canopy retention: evidence that NH<Subscript>4</Subscript><Superscript>+</Superscript> and NO<Subscript>3</Subscript><Superscript>−</Superscript> dynamics are uncoupled

Alternative silvicultural systems were introduced in Coastal Western Hemlock forests of British Columbia, Canada, to reduce disturbance incurred by conventional clear-cutting and to maintain the forest influence on soil nutrient cycling. As we hypothesized, in situ pools and net mineralization of NH₄ ⁺ were lower under no and low disturbance (old-growth forest and shelterwood) compared to clear-cuts (high disturbance); in situ pools and net production of NO₃ ⁻ were very low across all treatments. Gross transformation rates of NH₄ ⁺ increased while those of NO₃ ⁻ decreased with increasing disturbance, suggesting that these processes were uncoupled. We conclude that shelterwood harvesting reduces the impact on forest floor NH₄ ⁺ cycling compared to clear-cutting, and that neither low nor high disturbance intensity results in substantial NO₃ ⁻ accumulation, as what occasionally occurs in other ecosystems. We hypothesize that the uncoupling of NH₄ ⁺ and NO₃ ⁻ dynamics may be due to the predominance of heterotrophic nitrification by lignin-degrading fungi that oxidize organic N rather than NH₄ ⁺–N, and whose activities are suppressed at high NH₄ ⁺ concentrations.     

Improved estimation of biological nitrogen fixation of soybean cultivars (<Emphasis Type="Italic">Glycine max</Emphasis> L. Merril) using <Superscript>15</Superscript>N natural abundance technique

Biological nitrogen fixation (BNF) of 17 soybean cultivars was comparatively estimated by the δ¹⁵N natural abundance technique using two non-nodulation soybeans (Clay and Chippewa) as reference plants. A field study was established on the experimental farm of the University of Abomey-Calavi, Benin on a typical “terre de barre” soil classified by Food and Agriculture Organization-United Nations Educational, Scientific and Cultural Organisation as Rhodic Ferralsol. A nitrogen-free pot trial was also carried out using soil substrate sampled from the Atlantic Ocean beach. In the N-free medium, N content of the whole soybean cultivars ranged from 2.6 to 8.1 mg N per plant compared with an average of 1.8 mg N per plant observed with the non-fixing soybeans. Plant δ¹⁵N of the nodulating soybeans ranged from −2.7756‰ (Jupiter) to 0.1951‰ (Conquista), while the non-nodulating cultivars Chippewa and Clay had 2.67‰ and 9.30‰, respectively. Percentage and amount of N derived from air (Ndfa) were significantly different (P < 0.01) among soybean cultivars, and values depended highly on the selected reference plants. When Clay was used as the reference plant, the average percentage Ndfa was 1.4 times higher than when Chippewa was the reference plant. Both reference plants consistently ranked promiscuous soybean cvs. TG× 1894 3F and TG× 1908 8F as the best cultivars and cv. TG× 1888 29F as the least in percentage Ndfa, suggesting that any of the reference plants could be used in δ¹⁵N method for assessing N₂-fixation. The two identified promiscuous soybean cultivars with greatest capacity to fix N could be included in a soybean extension program for West African farming systems.     

Influence of reference trees on N2-fixation estimates inLeucaena leucocephala andAcacia albida using15N-labelling techniques

Summary We examined the suitability of four reference crops, i.e., two non-fixing trees,Cassia siamea andEucalyptus grandis, and two uninoculated fixing trees,Leucaena leucocephala andAcacia albida, for measuring fixed N₂ fixed in inoculatedL. leucocephala andA. albida grown for 36 weeks in pots. The¹⁵N isotope-dilution (involving the addition of equal amounts of labelled N fertilizer to the non-fixing and the fixing plants) and theA-value (with different amounts of labelled N fertilizer added to the fixing and the non-fixing crops) methods were used. The isotope dilution approach gave several large negative estimates of fixed N₂ inA. albida. Positive and similar values of fixed N₂ were measured in all four reference crops using theA-value approach. ForL. leucocephala the isotope-dilution approach gave different estimates of fixed N₂, with the different reference crops; the uninoculated N₂-fixing crops indicated significantly less fixed N₂ than the non-fixing reference crops. Similar values for N₂ fixed inL. leucocephala were obtained using the two non-fixing trees, either by the isotope-dilution or theA-value method. On average,A. albida derived about twice as much N from fertilizer asL. leucocephala. In both species, the atom %¹⁵N excess declined by about 50% in successive harvests.     

Isotope 15N enrichment of soil-ammonium N as a reference to estimate N2 fixation by stem and root-inoculated S. rostrata

Summary A pot experiment in the greenhouse was conducted to compare the contribution of N derived from the atmosphere or from biological N₂ fixation by Sesbania rostrata inoculated with Azorhizobium caulinodans, applied either to roots or to roots and stems (single or multiple stem inoculation). Two subsequent crops were grown for 50 days under flooded conditions. N derived from air was estimated by ¹⁵N dilution using ¹⁵N enrichment of soil NH _inf4 ^sup+ -N and of Echinochloa crusgalli as the non-N₂-fixing reference datum and compared with estimates obtained by the N-difference method. The first crop was grown to stabilize the ¹⁵N into the soil organic N fraction. The ¹⁵N enrichment of soil NH _inf4 ^sup+ -N in the second crop declined slowly. The extractability ratio (¹⁵N enrichment of extractable soil N to ¹⁵N enrichment of total soil N) decreased from 4.8 to 4.1 50 days after planting. The enrichment of soil NH _inf4 ^sup+ -N was comparable to that of E. crus-galli, resulting in similar estimates of N derived from air when either soil NH _inf4 ^sup+ -N or enrichment of E. crus-galli was used as a non-fixing reference. The N-difference method did not always provide reliable estimates of N derived from air; percentages ranged from 75 to more than 80 by 50 days after planting in both crops and did not differ among treatments. The study demonstrates the potential of using ¹⁵N enrichment of soil NH _inf4 ^sup+ -N as a non-N₂-fixing reference for reliable BNF estimates of crops in lowland puddled soil.     

Denitrification in the rooting zone of cropped soils with regard to methodology and climate: A review

Summary Denitrification N losses can be determined by three methods. The first is by estimating the non-recovery of ¹⁵ N-labelled compounds (¹⁵N-balance method). Using this method, denitrification losses are deduced from the balance of an N budged (¹⁵N-labeled fertilizer), having accounted for transformations in soil, plant uptake, and leaching losses. The evolution of gaseous N from native soil N is not taken into account by this procedure. Studies on arable land with annual crops in the temperate zone have shown that of the fertilizer N applied, about 20–500% (10–70 kg N^* ha^–1) is not recovered at the end of the growth period. The second method of determining denitrification N losses is by in situ field measurement of ¹⁵ N ₂ and ¹⁵ N ₂ O production. Under this procedure, ¹⁵N-enriched N is applied to a plot and the denitrification N losses are determined by covering the soil. The method allows a quantitative estimate of the relative contributions to the emitted gas by both the original enriched source and the native soil N. N-evolution rates measured on arable land under a temperate climate are approximately the same order of magnitude as the N losses estimated by the non-recovery of ¹⁵ N method. The third measuring procedure is based on the acetylene inhibition phenomenon. This principle uses the inhibition of bacterial N₂O reduction to N₂ in the presence of acetylene (C₂H₂). The methoddetermines the denitrification of all NO₃ ^–-N irrespective of its source. Measurements on classical crop production systems show maximum N losses in the temperate climate of about 20–30 kg N^* ha^–1 during the growth period of annual crops. A similar level of denitrification is estimated for grassland sites under the same climate. In the subtropics (mediterranean climate with hot summers and mild winters), from both intensively cultivated arable land and grassland sites, N losses may exceed 200 kg^* ha^–1 year^–1. Without the use of irrigation the denitrification flux is negligible in spite of the high temperatures in this climate.     

Effects of litter quality and microarthropods on N dynamics and retention of exogenous 15N in decomposing litter

Summary Surface additions of (¹⁵NH₄)₂SO₄ were used to measure the immobilization and subsequent movement of exogenous N added to two litter types of contrasting quality (Cornus florida and Quercus prinus). Litterbaskets were used to measure the litter mass loss and N dynamics and to follow the movement of the ¹⁵N label through litter, F layer, and soil pools. Half of the litterbaskets of each species were treated with naphthalene to reduce microarthropod densities. The faster decomposing C. florida litter maintained a higher excess atom % ¹⁵N, and a greater relative concentration of the labeled input (g ¹⁵N g^–1) than did Q. prinus litter. In both litter types the excess atom % ¹⁵N, relative concentration (g ¹⁵N g^–1), and absolute amount of label recovered in the litter declined over time. This occurred during a period of net accumulation of total litter N, implying simultaneous release of the initial input and immobilization of N from other sources. The concentration of ¹⁵N in the soil increased over time, while the F layer apparently acted as an intermediary in the transfer of ¹⁵N from litter to soil. Naphthalene effectively reduced microarthropod numbers in all horizons of the litterbaskets and significantly reduced the decay rates of Q. prinus, but not C. florida litter. Naphthalene did not appear to affect total N dynamics in the litter. However, with all horizons taken together, the naphthalene-treated litterbaskets retained more total ¹⁵N than the control litterbaskets. Naphthalene also changed the vertical distribution of ¹⁵N within litterbaskets, so that the litter retained less of the ¹⁵N-labeled input and the F layer and soil horizons retained more of the labeled input than in control litterbaskets. Our major conclusions are: (1) the N pool of decomposing litter is dynamic, with simultaneous N release and immobilization activating N turnover even during the net accumulation phase; (2) litter quality is an important determinant of immobilization and retention of exogenous N inputs and, therefore, turnover of the litter N pool; and (3) microarthropod activity can significantly affect the incorporation and retention of exogenous N inputs in decomposing litter, although these changes are apparently not reflected in net N accumulation or release during the 1st year of decomposition. However, the naphthalene may have affected microbially mediated N dynamics and this possibility needs to be considered in interpreting the results.     

Elevated CO<Subscript>2</Subscript> concentration affected pine and oak litter chemistry and the respiration and microbial biomass of soils amended with these litters

Elevated atmospheric CO₂ concentration ([CO₂]) may change litter chemistry which affects litter decomposability. This study investigated respiration and microbial biomass of soils amended with litter of Pinus densiflora (a coniferous species; pine) and Quercus variabilis (a deciduous species; oak) that were grown under different atmospheric [CO₂] and thus had different chemistry. Elevated [CO₂] increased lignin/N through increased lignin concentration and decreased N concentration. The CO₂ emission from the soils amended with litter produced under the same [CO₂] regime was greater for oak than pine litter, confirming that broadleaf litter with lower lignin decomposes faster than needle leaf litter. Within each species, however, soils amended with high lignin/N litter grown under elevated [CO₂] emitted more CO₂ than those with low lignin/N litter grown under ambient [CO₂]. Such contrasting effects of lignin/N on inter- and intra-species variations in litter decomposition should be ascribed to the effects of other litter chemistry variables including nonstructural carbohydrate, calcium and manganese as well as inhibitory effect of N on lignin decomposition. The microbial biomass was also higher in the soils amended with high lignin/N litter than those with low lignin/N litter probably due to low substrate use efficiency of lignin by microbes. Our study suggests that elevated [CO₂] increases lignin/N for both species, but increased lignin/N does not always reduce soil respiration and microbial biomass. Further study investigating a variety of tree species is required for more comprehensive understanding of inter- and intra-species variations of litter decomposition under elevated [CO₂].     

BIOMASS ALLOCATION AND NITROGEN DISTRIBUTION IN RYEGRASS UNDER WATER AND NITROGEN SUPPLIES

Ryegrass (Lolium perenne L.) in grassland is known to sustain with water and nitrogen (N). This study investigates biomass and N partitioning in plant organs (roots, main and the youngest tillers) under water-nitrogen interactions. Nitrogen was applied at the rates of 50 and 100 mg N kg^?1 as N₁ (low N) and N₂ (high N) treatments, respectively, with uniform irrigation until 440 growing degree-days (GDD). Thereafter, the water supply was restricted to 50 mL on a weekly basis (W₁) against 50 mL on a daily basis (W₂) and concurrently, N enriched with 1 atom% ¹⁵N isotopes. Cumulative tillers’ biomass increased linearly from 1st to 8th order, but thereafter reached a plateau with further increases in number of negligible weights. Initially tiller mass and number per plan did not differ (P < 0.05) with water and/or N applications but changed at 788 GDD with clear differences at 911 GDD with the highest under N₂W₂ and lowest under N₁W₁. Nitrogen concentration sharply decreased from 530 to 700 GDD and then levelled off with age. The decline was more pronounced in tillers than roots. The high N treatment showed elevated N-concentration under both water treatments. Watering on a daily basis promoted vegetative growth. High water and N levels significantly (P < 0.05) influenced concentration of N absorbed during ¹⁵N labeling (N_L) in all organs with relatively pronounced N_L under N₂. The additive positive effect of W₂ and N₂ was obvious on N_L as compared to N_T, which showed that plants discriminate N-uptake on mass basis. Nitrogen (mobile) was higher in young and ¹⁵N (heavier) was low in young tillers and vice versa. Accumulation of N absorbed during ¹⁵N labeling (¹⁵N_A) was significant knowing that water is a strong determining factor of N concentration in ryegrass organs.     

Effect of Stover Management and Nitrogen Fertilization on N<Subscript>2</Subscript>O and CO<Subscript>2</Subscript> Emissions from Irrigated Maize in a High Nitrate Mediterranean Soil

A high soil nitrogen (N) content in irrigated areas quite often results in environmental problems. Improving the management practices of intensive agriculture can mitigate greenhouse gas (GHG) emissions. This study compared the effect of maize stover incorporation or removal together with different mineral N fertilizer rates (0, 200 and 300 kg N ha^?1) on the emission of nitrous oxide (N₂O) and carbon dioxide (CO₂) on a sprinkler-irrigated maize (Zea mays L.). The trail was conducted in the Ebro Valley (NE Spain) in a high nitrate-N soil (i.e. 200 g NO₃–N kg^?1). Nitrous oxide and CO₂ emissions were sampled weekly using a semi-static closed chamber and quantified using the photoacoustic technique in 2011 and 2012. Applying sidedress N fertilizer tended to increase N₂O emissions whereas stover incorporation did not have any clear effect. Nitrification was probably the main process leading to N₂O. Denitrification was limited by the low soil moisture content (WFPS <?54%), due to an adequate irrigation management. Emissions ranged from ??0.11 to 0.36% of the N applied, below the IPCC (2007) values. Nitrogen fertilization tended to reduce CO₂ emission, but only in 2011. Stover incorporation increased CO₂ emission. Nitrogen use efficiency decreased with increasing mineral fertilizer supply. The application of N in high N soils of the Ebro Valley is not necessary until the soil restores a normal mineral N content, regardless of stover management. This will combine productivity with keeping N₂O and CO₂ emissions under control provided irrigation is adequately managed. Testing soil NO₃ ^?–N contents before fertilizing would improve N fertilizer recommendations.     

Leaf litter decomposition in the pure and mixed plantations of <Emphasis Type="Italic">Cunninghamia lanceolata</Emphasis> and <Emphasis Type="Italic">Michelia macclurei</Emphasis> in subtropical China

Leaf litter decomposition of Cunninghamia lanceolata, Michelia macclurei, and their mixture in the corresponding stands in subtropical China was studied using the litterbag method. The objective was to assess the influence of native evergreen broadleaved species on leaf litter decomposition. The hypotheses were: (1) M. macclurei leaf litter with lower C/N ratio and higher initial N concentration decomposed faster than C. lanceolata litter, (2) decomposition rates in litter mixtures could be predicted from single-species decay rates, and (3) litters decomposed more rapidly at the site that contained the same species as in the litterbag. The mass loss of leaf litter was positively correlated with initial N concentration and negatively correlated with C/N ratio. The decomposition rate of M. macclurei leaf litter was significantly higher than that of C. lanceolata needle litter in the pure C. lanceolata stand. Contrary to what would be predicted, the litter mixture decomposed more slowly than expected based on the results from component species decomposing alone. There was no significant difference in litter decomposition rate between different habitats.     

Evaluation of nitrogen fixation by different strains of theAzolla-Anabaena symbiosis in the presence of a high level of ammonium

Summary The proportion of N derived from N₂ fixation for 99 strains ofAzolla spp. (comprising all known species) in the presence of ammonium (40 mg/1) was assessed using a¹⁵N-dilution technique. The percentage of N derived from air varied from 29.5% to 79.9%. Although the N concentration ofAzolla spp. was not correlated with fertilizer N, it correlated fairly well with N₂ fixation. Regression analysis suggests that the N yield ofAzolla spp. is more dependent on N₂ fixation than on ammonium assimilation. The high correlation between N yield and isotopically determined, fixed N₂ indicates that the N yield could be used as a parameter in the selection ofAzolla spp. strains that are capable of maintaining high N₂ fixation in the presence of a high level of ammonium.     

A yield-independent, 15N-isotope dilution method to estimate legume symbiotic dependence without a non-N2-fixing reference plant

The proportional contribution of atmospheric N₂ to the N nutrition of lupin (P _atm) was estimated in a field experiment following addition of NH₄Cl of KNO₃ to unconfined microplots (1.5 m²) at 2.5 g N m^-2 (10 atom% ¹⁵N). The integrated ¹⁵N enrichment, or mean pool abundance, of nitrate extracted from 0- to 15-cm samples taken under the lupin crop on eight occasion between 28 and 190 days after sowing was used as the reference criterion to estimate P _atm by the ¹⁵N-isotope dilution technique. Estimates of P _atm were similar to those obtained using canola as a non-fixing reference plant, but were higher than estimates obtained using a yield-dependent model. Use of mean pool abundance obviates the need for a non-fixing reference plant, and the frequent sampling and isotope-ratio analysis of the legume biomass required with the yield-dependent model is unnecessary. However, further work is needed to validate a sampling strategy commensurate with the growth of the legume roots.     

<Superscript>15</Superscript>N uptake by mycorrhizal native and invasive plants from a N-eutrophied shrubland: a greenhouse experiment

In the coastal sage scrub (CSS) community of southern California, Artemisia californica and other native shrubs are gradually being replaced by invasive annual grasses, especially Bromus madritensis ssp. rubens. This decline may be attributed, in part, to local atmospheric N deposition, which causes elevated soil NO₃ ^–. Unaffected soils have low N with a relatively higher concentration of NH₄ ⁺ than NO₃ ^–. The objectives in this study were: (1) to compare the short-term uptake of ¹⁵NO₃ ^– and ¹⁵NH₄ ⁺ by B. madritensis and A. californica and (2) to discern whether ¹⁵NO₃ ^– or ¹⁵NH₄ ⁺ uptake is influenced by the mycorrhizal status of either plant species. Analysis of ¹⁵N concentrations indicated that both A. californica and B. madritensis took up more ¹⁵NO₃ ^– than ¹⁵NH₄ ⁺, but overall, B. madritensis took up 6–15 times more of both forms of ¹⁵N than A. californica. Mycorrhizal A. californica had an increased ¹⁵NH₄ ⁺ concentration in roots but not shoots. In B. madritensis, the only mycorrhizal response was a reallocation of ¹⁵NO₃ ^–, with mycorrhizal plants retaining a higher proportion of ¹⁵NO₃ ^– in roots. Overall, arbuscular mycorrhizae had a small effect on ¹⁵N uptake in this short-term study. However, the ability of B. madritensis to take up so much ¹⁵N may explain in part why it has been so successful in replacing A. californica in CSS.