Plant 15N Research Articles

Are vascular epiphytes nitrogen or phosphorus limited? A study of plant 15N fractionation and foliar N : P stoichiometry with the tank bromeliad Vriesea sanguinolenta

Although there is unambiguous evidence for vascular epiphytic plants to be limited by insufficient water and nutrient supply under natural conditions, it is an open debate whether they are primarily phosphorus (P) or nitrogen (N) limited. Plant (15) N fractionation and foliar N : P stoichiometry of a tank epiphyte (Vriesea sanguinolenta), and its response to combined N-P fertilization, were studied under semi-natural conditions over 334 d to clarify the type of nutrient limitation. Plants collected in the field and experimental plants with limited nutrient supply showed significant plant (15) N fractionation (mean 5‰) and plant N : P ratios of c. 13.5. Higher relative growth rates and declines in plant (15) N fractionation (0.5‰) and in foliar N : P ratios to 8.5 in the high N-P treatment indicated that these epiphytes were P limited in situ. The critical foliar N : P ratio was 10.4, as derived from the breakpoint in the relationship between plant (15) N fractionation and foliar N : P. We interpret the widespread (15) N depletion of vascular epiphytes relative to their host trees as deriving from (15) N fractionation of epiphytes as a result of P limitation. High foliar N : P ratios (> 12) corroborate widespread P limitation (or co-limitation by N and P) of epiphytic bromeliads and, possibly, other epiphyte species.

Stem labeling results in different patterns of 14C rhizorespiration and 15N distribution in plants compared to natural assimilation pathways

AbstractTo investigate C and N rhizodeposition, plants can be 13C‐15N double‐labeled with glucose and urea using a stem‐feeding method (wick method). However, it is unclear how the 13C applied as glucose is released into the soil as rhizorespiration in comparison with the 13C applied as CO2 using a natural uptake pathway. In the present study, we therefore compared the short‐term fate of 14C and 15N in white lupine and pea plants applied either by the wick method or the natural pathways of C and N assimilation. Plants were pulse‐labeled in 14CO2‐enriched atmosphere and 15N urea was applied to the roots (atmosphere–soil) following the natural assimilation pathways, or plants were simultaneously labeled with 14C and 15N by applying a 14C glucose–15N urea solution into the stem using the wick method. Plant development, soil microbial biomass, total rhizorespiration, and distribution of N in plants were not affected by the labeling method used but by plant species. However, the 15N : N ratio in plant parts was significantly (p < 0.05) affected by the labeling method, indicating more homogeneous 15N enrichment of plants labeled via root uptake. After 14CO2 atmosphere labeling of plants, the cumulated 14CO2 release from roots and soil showed the common saturation dynamics. In contrast, after 14C‐glucose labeling by the wick method, the cumulated 14CO2 release increased linearly. These results show that 14C applied as glucose using the wick method is not rapidly transferred to the roots as compared to a short‐term 14CO2 pulse. This is partly due to a slower 14C uptake and partly due to slow distribution within the plant. Consequently, 14C‐glucose application by the wick method is no pulse‐labeling approach. However, the advantages of the wick method for 13C‐15N double labeling for estimating rhizodeposition especially under field conditions requires further methodological research.

The arbuscular mycorrhizal fungus Glomus hoi can capture and transfer nitrogen from organic patches to its associated host plant at low temperature

Arbuscular mycorrhizal fungi have been suggested to be of potential benefit in achieving sustainable agriculture systems. However, there is conflicting information on the degree to which arbuscular mycorrhizal fungi (AMF) can grow and function at soil temperatures typical of temperate regions. To resolve this conflict we grew Plantago lanceolata L. inoculated with Glomus hoi (UY 110) in microcosm units maintained at 12/10 °C (day/night). The microcosms had two compartments, one planted and one not. The root-free compartment contained either an organic ( 15N: 13C labelled milled shoot material) or a sand patch. When permitted access, G. hoi proliferated hyphae extensively in the organic patch material. Plant 15N content was a simple function of length density of extra-radical mycelium (ERM) in the patch and c. 6% of host plant N was derived from the patch. These results indicate that G. hoi not only grew at these realistic soil temperatures, but also conferred a nutritional benefit to its host.

Replacing bare fallow with cover crops in a maize cropping system: Yield, N uptake and fertiliser fate

Cover crops in dry regions have been often limited by low nutrient and water-use efficiency. This study was conducted during 3.5 years to determine the effect of replacing bare fallow by a cover crop on yield, N uptake, and fate of labeled fertiliser in an intensive maize production system. Three treatments were studied: barley ( Hordeum vulgare L.), vetch ( Vicia villosa L.) and bare fallow during the intercropping period of maize ( Zea mays L.). All treatments were irrigated and fertilised following the same procedure, and a microplot in each plot was established with 210 kg N ha −1 of double labeled ammonium nitrate. Crop yield and N uptake, soil mineral N (N min), and recovery of 15N in plant and soil were determined after maize harvest and killing the cover crop. Replacing bare fallow with cover crops did not affect subsequent maize yield but affected N uptake. Vetch increased N supply by legume residues after the second year, and the N content in grain by the third. Nitrogen recover from fertiliser was not affected by treatment and averaged 46%. Barley recovered more 15N during the autumn–winter period than vetch or fallow. Under representative conditions, average barley N content was 47, vetch 51, and spontaneous vegetation content 0.8 kg N ha −1. Recovery of 15N in barley comprised 19% of total N content in aerial biomass, while only 4% in vetch. Vetch enhaced soil 15N recovery more than other treatments, suggesting its presence in a fairly stable organic fraction unavailable for maize uptake or lost. Replacing bare fallow by a cover crop only reduced fertiliser losses in a year with abundant precipitation. Nevertheless, reduction in soil N min in vetch and bare fallow treatments was similar, showing that N losses can be reduced in this cropping system, either by replace bare fallow with barley or smaller N fertiliser applicationto maize.

Foliar and soil 15N natural abundances provide field evidence on nitrogen dynamics in temperate and boreal forest ecosystems

The natural abundance of 15N (δ15N) in plants and soils is an ideal tool for assessing ecosystem N dynamics. However, many of the mechanisms driving the variability of foliar and soil δ15N values within and across ecosystems are still unclear. In this study, we analyzed the patterns of N concentrations and δ15N values in leaves, bulk soils and soil mineral N as well as soil N turnover rates across four temperate and boreal forest ecosystems along a mountain transect. The results showed that plant species and soil properties directly controlled soil δ15N patterns and climate factors (air temperature and precipitation) indirectly affected foliar δ15N patterns. Foliar N concentrations varied consistently with the concentrations of soil available N and soil NO3−-N, whereas foliar δ15N was most closely associated with the δ15N of soil NH4+, the most abundant form of N in soil solution. 15N enrichment in surface mineral soil in high elevation forests was mainly attributed to 15N-enriched organic N accumulation. Furthermore, the foliar enrichment factor (ep/s = δ15Nfoliage−δ15Nsoil) was significantly correlated with N transformation and loss rates, and was negatively correlated with the ratio of NH4+ to total inorganic N. These results suggest that foliar δ15N value and foliar N concentration together accurately reflect the N availability of forest ecosystems. Foliar ep/s can act as an integrated proxy to reflect the status of N cycling within or across forest ecosystems. Soil nitrification and species’ NH4+ to NO3− uptake ratios are key processes controlling foliar δ15N patterns in N-limited forest ecosystems. Our findings improve the mechanistic understanding of the commonly observed variability in foliar and soil δ15N within and across forest ecosystems.

Short-term effects of fire on soil and plant nutrients in palmetto flatwoods

Fire may have different effects on the relative availability of nitrogen (N) and phosphorus (P) because N volatilization occurs at lower temperatures than P volatilization, and fire-mediated changes in soil nutrient availability may affect foliar nutrient concentrations. We assessed the short-term effects of fire on soil and plant nutrients and 15N isotopic signatures in a palmetto flatwoods ecosystem in central Florida. Fire caused a short-term increase in extractable ammonium (NH4+) and phosphate (PO43-). The increase in PO43- was greater than the increase in NH4+, resulting in a decrease in the soil extractable N:P ratio shortly after fire. Similarly, foliar %P of the palmetto Serenoa repens (W. Bartram) Small increased more than foliar %N, resulting in a decrease in foliar N:P ratios shortly after fire. Soil δ15N and the difference between foliar and soil δ15N did not vary with time since fire; however, foliar δ15N of S. repens decreased after fire. Foliar %N of Quercus geminata Small and ericaceous shrubs was positively correlated with soil extractable inorganic N, while foliar %P of S. repens was positively correlated with soil extractable PO43-. Variation in foliar δ15N after fire and the positive relationship between soil and foliar nutrients suggest that both increased soil nutrient availability and reallocation of nutrients from below- to aboveground can be important for plant nutrient status after fire in palmetto flatwoods.

Dynamics and recovery of fertilizer 15N in soil and winter wheat crop under minimum versus conventional tillage

The development of reduced tillage in cropping systems raises the question of adaptation of N fertilization of crops in response to changes in the chemical, physical and biological soil caused by the cessation of ploughing. The purpose of the work was to investigate the fate of fertilizer 15N applied in spring to a winter wheat crop ( Triticum aestivum L.) under minimum tillage (MT) and conventional tillage (CT) and to examine the hypothesis that in MT systems plants face stronger microbial competition for N. The experiment was conducted in Northern France on a silt loam in two different rotations [sugar beet ( Beta vulgaris L.)–winter wheat–maize ( Zea mays L.)–winter wheat and seed flax ( Linum usitatissimum L.)–winter wheat–pea ( Pisum sativum L.)–winter wheat] from which we selected two crop sequences, pea–wheat and maize–wheat. At the beginning of stem elongation, 15N labelled fertilizer was applied at the rates of 100 kg N ha −1 for wheat after pea and 120 kg N ha −1 for wheat after maize. The dynamics of fertilizer 15N in soil and plant was assessed at six dates between N application and wheat harvest. In addition, soil respiration was monitored continuously on the maize–wheat CT and MT plots as a way of assessing heterotrophic microbial activity. The type of tillage (MT vs CT) had little effect on fertilizer N dynamics in the soil. The amount of labelled N immobilized was low with values ranging from 3.5 to 11.9% of applied 15N. Plant 15N recovery (above ground biomass + roots) amounted to 59–63% at flowering and harvest. The 15N balance at harvest time (i.e. the sum of 15N recovered from all plant and soil compartments) indicates that approximately 29% of the fertilizer N applied was lost whichever the tillage considered. The type of rotation (i.e. the nature of the preceding crop) had more influence on wheat N uptake than the type of tillage. The lack of observed effect of tillage on the dynamics of nitrogen fertilizer and plant recovery is in agreement with most published results obtained in other soil and climate conditions.

Natural variation of nitrate uptake and nitrogen use efficiency in Arabidopsis thaliana cultivated with limiting and ample nitrogen supply

Eighteen accessions of Arabidopsis thaliana were grown with low (N-) and high (N+) nitrogen supply. N uptake was monitored by feeding plants with 15N-enriched nutritive solution over 24 h. Biomass [fresh matter (FM) and dry matter (DM)], N concentration (N%), and 15N content were monitored and computed to determine the nitrogen use efficiency (NUE) and nitrogen uptake efficiency (NupE). NUE has been estimated as the ratio between biomass and N concentration (DM/N%) and NupE as the concentration of 15N in plants [microg (g(-1) DM)]. Accession traits were analysed to detect common and individual genotype features. The genetic variation in NUE at high N input was mainly explained by variation in N uptake. Even though plants managed N uptake and N metabolism differently under N+ and N-, NUE was similar in these two conditions, showing that NUE was exclusively genetically determined. Hierarchical classification revealed that the physiological classes arising were similar under N- and N+. Both wasteful and efficient genotypes were detected. Three extreme genotypes, Col-0, Bur-0, and Tsu-0, were noted. Bur-0 and Tsu-0 exhibited high NUE and large biomass. Col-0 showed the reverse: low NUE and low biomass. Bur-0 appeared poorly tolerant of a high N supply. The present data will facilitate the choice of Arabidopsis accessions as parents of recombinant inbred line populations suitable for the mapping of quantitiative trait loci related to NUE, NupE, and N storage capacity.

Contrasts Among Mycorrhizal Plant Guilds in Foliar Nitrogen Concentration and δ15N Along Productivity Gradients of a Boreal Forest

The distribution of plant species in boreal forest understories is hypothesized to reflect mycorrhizal guilds and associated adaptations for organic nitrogen (N) acquisition. In this study of a natural edaphic gradient, where supply rates of inorganic N increase with site productivity, we noted a decline in understory ectomycorrhizal, ericoid, and arbutoid plant communities on productive sites, in contrast to a positive response by most arbuscular species. We then assessed the rate of change in foliar N concentration (Nconc) and abundance of 15N (δ15N) of select plants from these mycorrhizal guilds. Two arbuscular plant species (Rubus parviflorus and Viburnum edule) had the sharpest increases in foliar Nconc with enhanced supplies of NH4 + and NO3 −, but with no differences in foliar δ15N. An ectomycorrhizal species, Abies lasiocarpa, and ericoid species, Vaccinium membranaceum, had parallel increases in both Nconc and δ15N with soil N supply. The foliar δ15N of two arbutoid plants (Orthilia secunda and Pyrola asarifolia) were as enriched as ectomycorrhizal sporocarps, likely indicating N transfer from mycorrhizal networks. The depletion of foliar δ15N by ectomycorrhizal and ericoid plants on poorer sites likely reflected a high degree of N retention and photosynthate demand by fungi, whereas arbuscular plants may have had a less significant δ15N response because of a more passive role by fungi in scavenging organic N. The results suggest differences in how mycorrhiza exploit diverse soil N supplies (recalcitrant and labile organic N, NH4 +, NO3 −, and parasitized N) could be an important factor in boreal plant community composition.

Interactive effects of warming and increased nitrogen deposition on 15N tracer retention in a temperate old field: seasonal trends

AbstractThe extent to which increased atmospheric nitrogen (N) deposition will drive changes in plant productivity and species composition over the next century will depend on how other influential global change factors, such as climate warming, affect the N retention of ecosystems. We examined the interactive effects of simulated climate warming and N deposition on the recoveries of 15N‐labeled ammonium and 15N‐labeled nitrate tracers added as a pulse to grass‐dominated, temperate old‐field plots at spring thaw. In addition to the year‐round warming treatment, a winter‐only warming treatment was applied to a set of plots to explore the contribution of this component of climate warming to the overall warming effect. By the end of the plant growing season, there was approximately twice as much 15N enrichment in the plant roots and bulk soil from 15NH4+‐addition plots than from 15NO3−‐addition plots, but there were no effects of warming or N fertilization on 15N recovery. Over winter, approximately half of the excess 15N present in plant shoots was lost, which corresponded with large 15N losses from bulk soil in N fertilized plots and large 15N increases in bulk soil in nonfertilized plots. By the next spring, there was decreased 15N recovery in plants in response to N fertilization, which was largely offset by increases in plant 15N recovery in response to year‐round warming. However, 15N retention in bulk soil, where the major part of the 15N label was recovered, was approximately 40% higher in nonfertilized plots than in N fertilized plots. Overall, our results indicate that climate warming increases plant N sequestration in this system but this effect is overwhelmed by the overall effect of nitrogen deposition on ecosystem N losses.

Nitrogen isotope composition characteristics of modern plants and their variations along an altitudinal gradient in Dongling Mountain in Beijing

Through the systematic investigation of nitrogen isotope composition (δ15N) in modern plants in Dongling Mountain in Beijing, the characteristics of δ15N in plants, the differences of δ15N among plants with different functional types, and the altitudinal trends of plant δ15N values revealed the environmental effects on δ15N. Our results are: (1) the values of δ15N of plants in Dongling Mountain range between −8.0‰ and 14.0‰, with an average value of −1.03‰; (2) δ15N values are variant among different plant functional types. Generally, the δ15N of shrubs are smaller than that of trees, and larger than that of herbaceous plants. The main reason may be the selective absorption of variant nitrogen resource with different δ15N values in soil; (3) there is a second-order polynomials relationship between the altitudes and the average δ15N values of plants, which shows a negative and positive correlation below and above 1350 m a.s.l., respectively. This indicates that the precipitation is a main controlling factor of plant 15N fractionation for the former, and the temperature for the latter; (4) three different, i.e., positive, negative, and no significant, correlations are performed between δ15N of individual species and altitudes. This may be related to different inherited characteristics and the process of nitrogen metabolize; and (5) variant altitudinal trends of plant δ15N are associated with different 15N composition of soil at different altitude, for which the key influencing factors are the changing temperature and precipitation with altitude rather than nitrogen content in leaf.

Evaluation of the wick method for in situ 13C and 15N labelling of annual plants using sugar-urea mixtures

To investigate the amount and fate of root-derived C and N, often tracer techniques are used, where plants are labelled with isotopes. In the present study, we evaluated the suitability of the cotton wick method for in situ labelling of peas (Pisum sativum L.) and oats (Avena sativa L.) with 13C and 15N simultaneously. With two greenhouse experiments we investigated how the wick method and aqueous urea and sugar solutions at a variety of concentrations affected plant development. In addition, we investigated the distribution of 13C and 15N in plants from column experiments under outdoor conditions. Solution was taken up by the plant from a small vial connected to the stem by a cotton wick which was passed through a hole in the stem of the plants. Generally, solution uptake varied between individual plants and decreased with increasing sugar concentrations. Below-ground, above-ground and total plant dry matter, were not significantly affected by the wick method and the applied solutions. Mixtures of aqueous glucose solutions at 2 to 4% and aqueous urea solutions at 1% are useful carriers of 13C and 15N. However, in the investigated plants isotopes were not homogeneously distributed among plant parts. Above-ground plant biomass was preferentially enriched with 13C and 15N, whereas below-ground plant biomass was generally lower enriched. Moreover, isotope distribution ratio of individual plants varied considerably, independent of plant part or timing of labelling. This must be taken into account when estimating root-derived C and N. Future studies comparing labelling methods need to present the isotope distribution ratios among plant parts to allow a true comparison of the methods and the evaluation of their suitability for estimating rhizodeposition.

Modeling the flow of 15N after a 15N pulse to study long‐term N dynamics in a semiarid grassland

Many aspects of nitrogen (N) cycling in terrestrial ecosystems remain poorly understood. Progress in studying N cycling has been hindered by a lack of effective measurements that integrate processes such as denitrification, competition for N between plants and microbes, and soil organic matter (SOM) decomposition over large time scales (years rather than hours or days). Here I show how long-term measurements of 15N in plants, microbes, and soil after a one-time addition of 15N ("labeled" N) can provide powerful information about long-term N dynamics in a semiarid grassland. I develop a simple dynamic model and show that labeled-N fractions in plant and microbial-N pools (expressed as a fraction of total N in each pool) can change long after 15N application (> or = 5 years). These 15N dynamics are closely tied to the turnover times of the different N pools. The model accurately simulated the labeled-N fractions in aboveground biomass measured annually during five years after addition of 15N to a semiarid grassland. I also tested the sensitivity of five different processes on labeled-N fractions in aboveground plant biomass. Changing plant/microbial competition for N had very little effect on the labeled-N fraction in aboveground biomass in the short and long-term. Changing microbial activity (N mineralization and immobilization), N loss, or N resorption/re-translocation by plants affected the labeled-N fraction in the short-term, but not in the long-term. Large long-term effects on the labeled-N fraction in aboveground biomass could only be established by changing the size of the active soil-N pool. Therefore, the significantly greater long-term decline in the labeled-N fraction in aboveground biomass observed under elevated CO2 in this grassland system could have resulted from an increased active soil-N pool under elevated CO2 (i.e., destabilization of soil organic matter that was relatively recalcitrant under ambient CO2 conditions). I conclude that short- and long-term labeled-N fractions in plant biomass after a 15N pulse are sensitive to processes such as N mineralization and immobilization, N loss, and soil organic matter (de-)stabilization. Modeling these fractions provides a useful tool to better understand N cycling in terrestrial ecosystems.

Grazers and soil moisture determine the fate of added 15NH4 + in Yellowstone grasslands

The influence of ungulate grazers on nutrient cycling and ecosystem productivity in grasslands has been shown to differ with moisture, nutrient availability, and feedbacks between above- and belowground activities. We examined the movement of nitrogen (N), applied as (15NH4)2SO4, through both dry and mesic sites in the northern range of Yellowstone National Park to test the hypothesis that plants were more able to acquire added N in grazed relative to ungrazed sites. Previous studies showed enhanced N mineralization in grazed areas, and detritus removal by grazers was predicted to enhance early-season plant growth. Thirteen months after tracer addition, there were no differences in plant 15N as a function of grazing, but historically ungrazed sites retained more 15N in accumulated litter than at grazed sites. This result demonstrated the importance of detritus in regulating redistribution of incoming N and the role of grazers in this process. Site moisture status influenced 15N recovery in all pools—soils, microbial biomass, and plants—and greater plant 15N acquisition occurred in roots at dry relative to mesic sites. Understanding how grazers influence nutrient cycling at the landscape scale requires further investigation of interactions among soil moisture, plant production, litter accumulation, grazing intensity, and belowground processes.

Bone stable isotopic signatures (15N, 18O) as tracers of temperature variation during the Late‐glacial and early Holocene: case study on red deer Cervus elaphus from Rochedane (Jura, France)

AbstractStable isotopes in mammal bones are mainly used to reconstruct dietary preferences and their use as palaeoclimatic indicators is less developed. However, important variations in 15N abundances observed in the bone collagen of large mammals during the Late‐glacial and early Holocene have been tentatively linked to a general increase in temperature. In order to test this hypothesis, we analysed nitrogen and oxygen isotopic abundances from bones of red deer (Cervus elaphus) from the Rochedane site (Jura, France). We observe a clear linear relationship between 15N and 18O that demonstrates the effect of temperature on the abundance of 15N in red deer bone collagen. These results suggest that an increase in soil maturation during global warming of the Late‐glacial and early Holocene led to an increase of 15N in soils and plants that was passed on to their consumers. Red deer seem to be particularly suited for palaeoclimatic reconstruction based on the isotopic signatures of their bones. Copyright © 2009 John Wiley & Sons, Ltd.

Effects of salinity and fertigation practice on cotton yield and 15N recovery

The purpose of optimal water and nutrient management is to maximize water and fertilizer use efficiency and crop production, and to minimize groundwater pollution. In this study, field experiments were conducted to investigate the effect of soil salinity and N fertigation strategy on plant growth, N uptake, as well as plant and soil 15N recovery. The experimental design was a 3 × 3 factorial with three soil salinity levels (2.5, 6.3, and 10.8 dS m −1) and three N fertigation strategies (N applied at the beginning, end, and in the middle of an irrigation cycle). Seed cotton yield, dry matter, N uptake, and plant 15N recovery significantly increased as soil salinity level increased from 2.5 to 6.3 dS m −1, but they decreased markedly at higher soil salinity of 10.8 dS m −1. Soil 15N recovery was higher under soil salinity of 10.8 dS m −1 than those under soil salinity of 6.3 dS m −1, but was not significantly different from that under soil salinity of 2.5 dS m −1. The fertigation strategy that nitrogen applied at the beginning of an irrigation cycle had the highest seed cotton yield and plant 15N recovery, but showed higher potential loss of fertilizer N from the root zone. While the fertigation strategy of applying N at the end of an irrigation cycle tended to avoid potential N loss from the root zone, it had the lowest cotton yield and nitrogen use efficiency. Total 15N recovery was not significantly affected by soil salinity, fertigation strategy, and their interaction. These results suggest that applying nitrogen at the beginning of an irrigation cycle has an advantage on promoting yield and fertilizer use efficiency, therefore, is an agronomically efficient way to provide cotton with fertilizer N under the given production conditions.

Natural 15N 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 δ 15N 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 ( r 2 = 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 ( r 2 = 0.998, P < 0.01) with a slope of 122 g N kg −1 of compost-N. Plant- δ 15N increased curvilinearly with increasing compost application, but this increase was insignificant between SC2 and SC3 treatments. The δ 15N of soil inorganic-N (particularly NO 3 −–N) increased with compost application. We found that plant- δ 15N reflected the N isotope signal of soil NO 3 −–N at each measurement during plant growth, and that δ 15N of inner leaves and soil NO 3 −–N was similar when initial NO 3 − in the compost was abundant. Therefore, we concluded that δ 15N of whole plant (more obviously in newer plant parts) and soil NO 3 −–N could reveal whether compost application was excessive, suggesting a possible use of δ 15N in plants and soil as evidence of excess compost application.

Rhizodeposition-induced decomposition increases N availability to wild and cultivated wheat genotypes under elevated CO 2

Elevated CO 2 may increase nutrient availability in the rhizosphere by stimulating N release from recalcitrant soil organic matter (SOM) pools through enhanced rhizodeposition. We aimed to elucidate how CO 2-induced increases in rhizodeposition affect N release from recalcitrant SOM, and how wild versus cultivated genotypes of wheat mediated differential responses in soil N cycling under elevated CO 2. To quantify root-derived soil carbon (C) input and release of N from stable SOM pools, plants were grown for 1 month in microcosms, exposed to 13C labeling at ambient (392 μmol mol −1) and elevated (792 μmol mol −1) CO 2 concentrations, in soil containing 15N predominantly incorporated into recalcitrant SOM pools. Decomposition of stable soil C increased by 43%, root-derived soil C increased by 59%, and microbial- 13C was enhanced by 50% under elevated compared to ambient CO 2. Concurrently, plant 15N uptake increased (+7%) under elevated CO 2 while 15N contents in the microbial biomass and mineral N pool decreased. Wild genotypes allocated more C to their roots, while cultivated genotypes allocated more C to their shoots under ambient and elevated CO 2. This led to increased stable C decomposition, but not to increased N acquisition for the wild genotypes. Data suggest that increased rhizodeposition under elevated CO 2 can stimulate mineralization of N from recalcitrant SOM pools and that contrasting C allocation patterns cannot fully explain plant mediated differential responses in soil N cycling to elevated CO 2.

Spatial variation of the stable nitrogen isotope ratio of woody plants along a topoedaphic gradient in a subtropical savanna

Variation in the stable N isotope ratio (delta15N) of plants and soils often reflects the influence of environment on the N cycle. We measured leaf delta15N and N concentration ([N]) on all individuals of Prosopis glandulosa (deciduous tree legume), Condalia hookeri (evergreen shrub), and Zanthoxylum fagara (evergreen shrub) present within a belt transect 308 m long x 12 m wide in a subtropical savanna ecosystem in southern Texas, USA in April and August 2005. Soil texture, gravimetric water content (GWC), total N and delta15N were also measured along the transect. At the landscape scale, leaf delta15N was negatively related to elevation for all the three species along this topoedaphic sequence. Changes in soil delta15N, total N, and GWC appeared to contribute to this spatial pattern of leaf delta15N. In lower portions of the landscape, greater soil N availability and GWC are associated with relatively high rates of both N mineralization and nitrification. Both soil delta15N and leaf [N] were positively correlated with leaf delta15N of non-N2 fixing plants. Leaf delta15N of P. glandulosa, an N2-fixing legume, did not correlate with leaf [N]; the delta15N of P. glandulosa's leaves were closer to atmospheric N2 and significantly lower than those of C. hookeri and Z. fagara. Additionally, at smaller spatial scales, a proximity index (which reflected the density and distance of surrounding P. glandulosa trees) was negatively correlated with leaf delta15N of C. hookeri and Z. fagara, indicating the N2-fixing P. glandulosa may be important to the N nutrition of nearby non-N2-fixing species. Our results indicate plant 15N natural abundance can reflect the extent of N retention and help us better understand N dynamics and plant-soil interactions at ecosystem and landscape scales.

Cotton Nitrogen Management in a High‐Residue Conservation System: Cover Crop Fertilization

Nitrogen is required for adequate residue production from cereal cover crops used in no‐till cotton (Gossypium hirsutum L.) production, but residues can immobilize N needed by cotton. We conducted a 3‐yr field study on a Decatur silt loam (clayey, kaolinitic, thermic Rhodic Paleudult) in northern Alabama to test N fertilizer practices for cotton grown with a rye (Secale cereale L.) cover crop and conservation tillage. Nitrogen rates applied to the rye cover crop were 0, 34, and 67 kg N ha−1 and cotton N rates were 0, 45, 90, and 135 kg N ha−1 Additionally, 15N microplots were established in cover crop N treatments of 34 kg N ha−1 and in cotton treatments of 90 kg N ha−1 Data collected included cover crop aerial biomass, cover crop C/N ratios, cotton leaf N at first flower, lint yield, lint quality, and 15N in plant and soil samples. Cotton grown in unfertilized rye treatments needed 57 to 60% (38–40 kg N ha−1) more N to maximize yields above median conventional tillage N recommendations (67 kg N ha−1). Cover crop N rates of 67 kg N ha−1 maximized cover crop biomass production for soil protection and soil organic matter aggradation. If the cover crop was fertilized, minimum cotton N applications of 70 and 76 kg N ha−1 were needed for economic optimum and maximum lint yield, respectively. We speculate that cotton N rates may be decreased in the future as new N and C pool equilibria are reached.