15N Retention Research Articles

The Growth and N Retention of Two Annual Desert Plants Varied Under Different Nitrogen Deposition Rates.

Nitrogen (N) partitioning between plant and soil pools is closely related to biomass accumulation and allocation, and is of great importance for quantifying the biomass dynamics and N fluxes of ecosystems, especially in low N-availability desert ecosystems. However, partitioning can differ among species even when growing in the same habitat. To better understand the variation of plant biomass allocation and N retention within ephemeral and annual species we studied the responses of Malcolmia Africana (an ephemeral) and Salsola affinis (an annual) to N addition, including plant growth, N retention by the plant and soil, and N lost to the environment using 15N (double-labeled 15NH415NO3 (5.16% abundance) added at 0, 0.8, 1.6, 3.2, and 6.4 g pot-1, equivalent to 0, 15, 30, 60, and 120 kg N ha-1) in a pot experiment. Higher N addition (N120) inhibited plant growth and biomass accumulation of the ephemeral but not the annual. In addition, the aboveground:belowground partitioning of N (the R:S ratio) of the ephemeral decreased with increasing N addition, but that of the annual increased. The N input corresponding to maximum biomass and 15N retention of the ephemeral was significantly less than that of the annual. The aboveground and belowground retention of N in the ephemeral were significantly less than those of the annual, except at low N rates. The average plant–soil system recovery of added 15N by the ephemeral was 70%, significantly higher than that of the annual with an average of 50%. Although the whole plant–soil 15N recovery of this desert ecosystem decreased with increasing N deposition, our results suggested that it may vary with species composition and community change under future climate and elevated N deposition.

Ecosystem nitrogen retention is regulated by plant community trait interactions with nutrient status in an alpine meadow

Abstract Biotic nitrogen (N) retention is an important ecosystem function in the context of ongoing land‐use intensification, N deposition and global warming. However, a paucity of experimental evidence limits understanding of how different plant community components influence N retention in terrestrial ecosystems. In this investigation, we conducted a 15N labelling experiment to test how plant community properties, including plant species richness/diversity, dominance and functional traits, influence plant N uptake and retention under different nutrient availabilities. A 3‐year experiment examined the effects of adding N (10 g N m−2 year−1) and phosphorus (P) (5 g P m−2 year−1) to an alpine meadow on the Qinghai‐Tibetan Plateau. Results show that 15N retention increased with the addition of N and P; the addition of P produced the largest increase of 15N retention in plant and soil N pools. Changes in soil nutrient conditions also facilitated different plant community controls on ecosystem N retention. Ecosystem 15N retention was influenced by species richness and root biomass in the control plots; whereas the N addition treatment showed an important effect of community‐weighted means (CWM) of specific leaf area, and plots with additional P recorded lower CWM of root nitrogen content (root N) and larger CWM root:shoot ratios as important determinants. Synthesis. Ecosystem N retention was influenced by conservative and exploitative plant species and/or their traits under N deficient and abundant conditions, respectively, whereas species richness and community plant biomass were most influential under middle condition. The discovery of an interaction between plant community traits and nutrient biogeochemistry as a mechanism for ecosystem N retention offers a means to predict how vegetation in alpine meadow ecosystems will respond to expected global change.

Nitrate retention capacity of milldam-impacted legacy sediments and relict A horizon soils

Abstract. While eutrophication is often attributed to contemporary nutrient pollution, there is growing evidence that past practices, like the accumulation of legacy sediment behind historic milldams, are also important. Given their prevalence, there is a critical need to understand how N flows through, and is retained in, legacy sediments to improve predictions and management of N transport from uplands to streams in the context of climatic variability and land-use change. Our goal was to determine how nitrate (NO3−) is cycled through the soil of a legacy-sediment-strewn stream before and after soil drying. We extracted 10.16 cm radius intact soil columns that extended 30 cm into each of the three significant soil horizons at Big Spring Run (BSR) in Lancaster, Pennsylvania: surface legacy sediment characterized by a newly developing mineral A horizon soil, mid-layer legacy sediment consisting of mineral B horizon soil and a dark, organic-rich, buried relict A horizon soil. Columns were first preincubated at field capacity and then isotopically labeled nitrate (15NO3−) was added and allowed to drain to estimate retention. The columns were then air-dried and subsequently rewet with N-free water and allowed to drain to quantify the drought-induced loss of 15NO3− from the different horizons. We found the highest initial 15N retention in the mid-layer legacy sediment (17 ± 4 %) and buried relict A soil (14 ± 3 %) horizons, with significantly lower retention in the surface legacy sediment (6 ± 1 %) horizon. As expected, rewetting dry soil resulted in 15N losses in all horizons, with the greatest losses in the buried relict A horizon soil, followed by the mid-layer legacy sediment and surface legacy sediment horizons. The 15N remaining in the soil following the post-drought leaching was highest in the mid-layer legacy sediment, intermediate in the surface legacy sediment, and lowest in the buried relict A horizon soil. Fluctuations in the water table at BSR which affect saturation of the buried relict A horizon soil could lead to great loses of NO3− from the soil, while vertical flow through the legacy-sediment-rich soil profile that originates in the surface has the potential to retain more NO3−. Restoration that seeks to reconnect the groundwater and surface water, which will decrease the number of drying–rewetting events imposed on the relict A horizon soils, could initially lead to increased losses of NO3− to nearby stream waters.

Nitrogen use efficiencies to grow, feed, and recycle manure from the major diet components fed to dairy cows in the USA

Crops and livestock transform a general range of 20% to 50% of applied nitrogen (N) into product N (e.g., grain, forage, meat, milk). Most N not incorporated into agricultural products is lost to the environment. The objective of this study was to quantify soil N input (fertilizerN, biologically fixed-N) incorporation into the major diet components fed to dairy cows, diet component N secretions in milk N and excretion in manure N constituents, and manure N recycled back through the feed supply. Alfalfa for silage (AS), corn for silage (CS), corn grain (CG) and soybeans (SB, eventually solvent-extracted into soybean meal, SBM) were enriched in the field with fertilizers containing the stable isotope 15N. Each 15N-labeled diet component was fed individually to twelve mid-lactation cows (3 cows per 15N-enriched diet component) as part of a total mixed ration (TMR). Proportions of each component’s 15N intake (15NI) recovered in milk, feces, fecal undigested dietary N, urine, urinary urea and retained by cows were determined during a 4-day 15N feeding period and 4days thereafter. Diet component 15N manure was applied to field plots and 15N uptake by corn for silage was determined over two succeeding years. The wide range in total 15N recoveries (% 15NI), greatest from cows fed AS (67) followed by CS (61), SBM (61) and CG (54) indicate significant differences in diet component 15N retention by cows. Relative 15N recoveries (% of total 15N recovered) in milk were greater (and statistically similar) from cows fed CG and SBM (average of 29.2) than from cows fed AS and CS (also statistically similar, average of 18.4). Relative 15N recoveries in feces were greater (and statistically similar) from cows fed AS and CS (average of 42.2) than from cows fed CG and SBM (also statistically similar, average of 30.7), and 15N recoveries as fecal undigested dietary N were greatest from cows fed CG (2.5) followed by AS and CS (average of 2.2) and SBM (<1). Relative 15N recoveries in urine (average of 39.7) and urinary urea (average 34.0) were similar across all diet components. Over the 2-year field study period, greatest manure 15NUE (% of applied manure 15N recovered as corn silage 15N) was obtained in plots amended with manure 15N derived from SBM (38.2) and lowest from CS (30.5). The greater total N use efficiency (percent N inputs incorporated into milk N plus corn silage N) for SBM (68.3) and AS (51.5) than for CG (47.4) and CS (40.6) can be attributed mostly to differences in N use efficiencies of the biologically-fixed-N and fertilizer N to grow diet components. A balance between corn, alfalfa and soybeans in dairy cropping systems should be encouraged to not only enhance N use efficiency in feed and milk production and manure N recycling, but also to capture many of the long-term benefits associated with corn-legume rotations.

Increased retention of soil nitrogen over winter by biochar application: Implications of biochar pyrolysis temperature for plant nitrogen availability

While soil freeze-thaw cycles can decrease soil nutrient retention over winter by increasing leaching losses and greenhouse gas emissions, biochar as a soil amendment could mitigate these effects. Nevertheless, there is often variation in the effectiveness of different biochar formulations with respect to soil nutrient retention. We added 15N tracer to soil mesocosms to examine the effects of biochar produced under a series of pyrolysis temperatures (250–600°C) on soil nitrogen retention in response to variation in soil freeze-thaw cycle intensity (−10°C vs. 0°C following spring melt). We also examined the subsequent effects on plant nitrogen uptake, soil nitrous oxide (N2O) emissions and nitrogen leaching losses. As we predicted, increased soil freezing increased inorganic nitrogen losses through leaching and decreased the biomass of the test crop Arugula (Eruca sativa) the following growing season. Biochar amendment increased both soil 15N retention over winter and the subsequent plant 15N uptake, with the biochar generated at the highest temperature exhibiting the strongest effects on plant 15N uptake. Biochar addition also significantly mitigated the negative soil freezing effect on subsequent plant biomass. Nevertheless, biochar addition combined with freezing increased N2O emissions. Overall, our results confirm that biochar application can mitigate soil nitrogen losses over winter, although it may also interact with soil freezing to increase emissions of the greenhouse gas N2O.

Effect of long-term fertilization on 15N uptake and retention in soil

ABSTRACTWe did a pot experiment with three different fertilized soils (no fertilizer (No-F), inorganic fertilizer nitrogen, phosphorus and potassium (NPK), manure plus inorganic fertilizer (MNPK)) from a 19-year fertilizer trial. Three N treatments, (1) no N, (2) 100 mg/kg urea-15N (N), (3) 50 mg/kg urea-15N + 50 mg/kg corn straw-N (1/2N + 1/2S), were applied to each soil. The residual soil from the same treatments was used to grow second wheat crop. The MNPK soil had significantly higher nitrogen use efficiency (NUE) in the first growing season, and lower N loss than the NPK, and No-F soils. The 1/2N + 1/2S treatment decreased NUE on each soil, even though the MNPK soil still had highest NUE and lowest N loss. The residual 15N use efficiency (RNUE) in 1/2N + 1/2S treatment of MNPK soil was higher than NPK and No-F soils. We concluded that long-term application of manure plus inorganic fertilizer increased NUE and decreased N loss.

Uptake and fate of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in coastal marine biota determined using a stable isotopic tracer, 15N – [RDX

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is globally one of the most commonly used military explosives and environmental contaminant. 15N labeled RDX was added into a mesocosm containing 9 different coastal marine species in a time series experiment to quantify the uptake of RDX and assess the RDX derived 15N retention into biota tissue. The 15N attributed to munitions compounds reached steady state concentrations ranging from 0.04 to 0.67 μg 15N g dw−1, the bulk 15N tissue concentration for all species was 1–2 orders of magnitude higher suggesting a common mechanism or pathway of RDX biotransformation and retention of 15N. A toxicokinetic model was created that described the 15N uptake, elimination, and transformation rates. While modeled uptake rates were within previous published values, elimination rates were several orders of magnitude smaller than previous studies ranging from 0.05 to 0.7 days−1. These small elimination rates were offset by high rates of retention of 15N previously not measured. Bioconcentration factors and related aqueous:organism ratios of compounds and tracer calculated using different tracer and non-tracer methods yielded a broad range of values (0.35–101.6 mL g−1) that were largely method dependent. Despite the method-derived variability, all values were generally low and consistent with little bioaccumulation potential. The use of 15N labeled RDX in this study indicates four possible explanations for the observed distribution of compounds and tracer; each with unique potential implications for possible toxicological impacts in the coastal marine environment.

Metabolism of mineral‐sorbed organic matter and microbial lifestyles in fluvial ecosystems

AbstractIn fluvial ecosystems mineral erosion, carbon (C), and nitrogen (N) fluxes are linked via organomineral complexation, where dissolved organic molecules bind to mineral surfaces. Biofilms and suspended aggregates represent major aquatic microbial lifestyles whose relative importance changes predictably through fluvial networks. We tested how organomineral sorption affects aquatic microbial metabolism, using organomineral particles containing a mix of 13C, 15N‐labeled amino acids. We traced 13C and 15N retention within biofilm and suspended aggregate biomass and its mineralization. Organomineral complexation restricted C and N retention within biofilms and aggregates and also their mineralization. This reduced the efficiency with which biofilms mineralize C and N by 30% and 6%. By contrast, organominerals reduced the C and N mineralization efficiency of suspended aggregates by 41% and 93%. Our findings show how organomineral complexation affects microbial C:N stoichiometry, potentially altering the biogeochemical fate of C and N within fluvial ecosystems.

The fate of &amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N-nitrate in mesocosms from five European peatlands differing in long-term nitrogen deposition rate

Abstract. Elevated nitrogen (N) deposition changes the retention, transformation, and fluxes of N in ombrotrophic peatlands. To evaluate such effects we applied a 15N tracer (NH4 15NO3) at a rate of 2.3 g N m−2 yr−1 to mesocosms of five European peatlands with differing long-term N deposition rates for a period of 76 days of dry and 90 days of wet conditions. We determined background N content and moss length growth, and recovered the 15N tracer from the mosses, graminoids, shrubs, the peat, and dissolved N. Background N contents in Sphagnum mosses increased from 5.5 (Degerö Stormyr, deposition &lt; 0.2 g N m−2 yr−1) up to 12.2 mg g−1 (Frölichshaier Sattelmoor, 4.7–6.0 g N m−2 yr−1). In peat from Degerö, nitrate and ammonium concentrations were below 3 mg L−1, whereas up to 30 (nitrate) and 11 mg L−1 (ammonium) was found in peat from Frölichshaier Sattelmoor. Sphagnum mosses (down to 5 cm below surface) generally intercepted large amounts of 15N (0.2–0.35 mg g−1) and retained the tracer most effectively relative to their biomass. Similar quantities of the 15N were recovered from the peat, followed by shrubs, graminoids, and the dissolved pool. At the most polluted sites we recovered more 15N from shrubs (up to 12.4 %) and from nitrate and ammonium (up to 0.7 %). However, no impact of N deposition on 15N retention by Sphagnum could be identified and their length growth was highest under high N background deposition. Our experiment suggests that the decline in N retention at levels above ca. 1.5 g m−2 yr−1, as expressed by elevated near-surface peat N content and increased dissolved N concentrations, is likely more modest than previously thought. This conclusion is related to the finding that Sphagnum species can apparently thrive at elevated long-term N deposition rates in European peatlands.

Soil processes drive seasonal variation in retention of 15N tracers in a deciduous forest catchment.

Seasonal patterns of stream nitrate concentration have long been interpreted as demonstrating the central role of plant uptake in regulating stream nitrogen loss from forested catchments. Soil processes are rarely considered as important drivers of these patterns. We examined seasonal variation in N retention in a deciduous forest using three whole-ecosystem 15N tracer additions: in late April (post-snowmelt, pre-leaf-out), late July (mid-growing- season), and late October (end of leaf-fall). We expected that plant 15N uptake would peak in late spring and midsummer, that immobilization in surface litter and soil would peak the following autumn leaf-fall, and that leaching losses would vary inversely with 15N retention. Similar to most other 15N tracer studies, we found that litter and soils dominated ecosystem retention of added 15N. However, 15N recovery in detrital pools varied tremendously by season, with > 90% retention in spring and autumn and sharply reduced 15N retention in late summer. During spring, over half of the 15N retained in soil occurred within one day in the heavy (mineral-associated) soil fraction. During summer, a large decrease in 15N retention one week after addition coincided with increased losses of 15NO3- to soil leachate and seasonal increases in soil and stream NO3- concentrations, although leaching accounted for only a small fraction of the lost 15N (< 0.2%). Uptake of 15N into roots did not vary by season and accounted for < 4% of each tracer addition. Denitrification or other processes that lead to N gas loss may have consumed the rest. These measurements of 15N movement provide strong evidence for the dominant role of soil processes in regulating seasonal N retention and losses in this catchment and perhaps others with similar soils.

NanoSIMS investigation of glycine-derived C and N retention with soil organo-mineral associations

While microbial-mineral-organic matter interactions are key features controlling the fates of low molecular-weight compounds in soils, direct investigations of how they control their fine-scale spatial distribution are scant. Here, we addressed how microbial transformations affect the retention of 13C/15N-labeled glycine in a forest topsoil 8 h after application. We assessed the contribution of soil microorganisms to glycine-derived 13C and 15N retention using γ-irradiated and non-irradiated soils. We tracked down the glycine-derived 13C and 15N at the surface of particles randomly isolated from soil density fractions using nano-scale secondary ions mass spectrometry (NanoSIMS) imaging. Eight hours after addition, 7 % of the glycine-derived 13C and 15N initially applied was recovered among soil density fractions, mainly via the activity of soil microorganisms (>85 % of total retention). Glycine-derived 13C and 15N distribution among density fractions was correlated with that of soil organic matter (SOM) determined by NanoSIMS (R ≥ 0.85), suggesting that the spatial patterns of the mineral-attached SOM controls the spatial distribution of the glycine-derived 13C and 15N. NanoSIMS images showed largely decoupled glycine-derived 13C and 15N spots preferentially attached to aggregated particles. We speculate that the glycine-derived 13C was principally found within or in the vicinity of microbial cells, whereas the glycine-derived 15N was mostly found as NH4 + and/or exoenzymes spread across soil surfaces. The C:N ratios determined by NanoSIMS suggest that local chemical properties of mineral-attached SOM drive glycine-derived 15N attachment, with the preferential attachment to mineral-attached SOM rich in N (C:N ratios mostly 2.65 g cm−3).

Sinks for nitrogen inputs in terrestrial ecosystems: a meta‐analysis of 15N tracer field studies

Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched 15N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem 15N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem 15N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (< 1 week after 15N tracer application), total ecosystem 15N recovery was negatively correlated with fine-root and soil 15N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term (3-18 months after 15N tracer application), total ecosystem 15N retention was negatively correlated with foliar natural-abundance 15N but was positively correlated with mineral soil C and N concentration and C:N, showing that plant and soil natural-abundance 15N and soil C:N are good indicators of total ecosystem N retention. Foliar N concentration was not significantly related to ecosystem 15N tracer recovery, suggesting that plant N status is not a good predictor of total ecosystem N retention. Because the largest ecosystem sinks for 15N tracer were below ground in forests, shrublands, and grasslands, we conclude that growth enhancement and potential for increased C storage in aboveground biomass from atmospheric N deposition is likely to be modest in these ecosystems. Total ecosystem 15N recovery decreased with N fertilization, with an apparent threshold fertilization rate of 46 kg N x ha(-1) x yr(-1) above which most ecosystems showed net losses of applied 15N tracer in response to N fertilizer addition.

Linking Carbon Saturation Concepts to Nitrogen Saturation and Retention

Recent advances in soil C saturation concepts have increased our understanding of soil C storage and mineralization without explicit links to N retention and saturation theories. Here, we exploit soil texture and organic matter (OM) gradients in a Maryland, USA hardwood forest to test hypotheses that link soil organic C saturation with soil 15N retention and nitrification. At our site, mineral-associated OM (MAOM) N concentrations in the silt + clay particle fraction (g MAOM-N g silt + clay−1) were negatively correlated with the fraction of NH4-N transferred to MAOM during a 3-day in situ incubation (R = −0.85), but positively correlated with potential net nitrification (R = 0.76). Moreover, the fraction of NH4-N transferred to MAOM was negatively correlated with potential net nitrification (R = −0.76). Due to physico-chemical stabilization mechanisms, MAOM is considered to be resistant to mineralization. Carbon saturation theory suggests that the proportion of new C inputs that can be stabilized in MAOM decreases in proportion to the amount of C already present in the fraction; C inputs not stabilized in MAOM are susceptible to rapid mineralization. We demonstrate that NH4-N stabilization in MAOM is similar to C stabilization in MAOM and associated with nitrification, thereby extending soil C saturation theory to mineral N and linking it with N retention and saturation theories. These data and concepts complement N saturation models that emphasize vegetation type, N input levels, and microbial turnover. Incorporating the OM retention capacity of fine mineral particles into N saturation theory can improve predictions of N saturation rates and resolve inconsistent relationships between soil organic matter, texture, N mineralization, and N retention.

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.

Abiotic Nitrate Retention in Agroecosystems and a Forest Soil

Agricultural ecosystems are the primary source of nitrate (NO3−) pollution in many watersheds. We used a laboratory 15N tracer study with live and gamma irradiated soils to examine the role of land management in abiotic retention of NO3− in three agroecosytems and a forest soil. Nitrogen retention was defined as the quantity of 15N that remained in soil following salt extractions. Land management only affected N retention after 15 min of incubation, with forested soils retaining &lt;4% of the added 15N and agricultural soils retaining &lt;1%. In all four ecosystems, abiotic retention (retention in gamma irradiated soils) occurred after 1 d (15–20% of the 15N) and 21 d (up to 60% of 15N) of incubation. After 1 and 21 d, 15N retention in irradiated soils was larger than in live soils, revealing a methodological artifact associated with gamma irradiation. While our results show that abiotic reactions can occur in the laboratory, it is unlikely that such high rates of abiotic 15N retention occur in the field. Gamma irradiation appears to impart methodological artifacts that limit its use in partitioning microbial and abiotic N retention in soils.

Tillage Effects on Microbial and Carbon Dynamics during Plant Residue Decomposition

One goal of soil C sequestration is to increase the mass of C stored in agricultural soils. Reducing soil disturbance, e.g., no‐till management, facilitates soil fungal growth and results in higher C sequestration rates; however, the specific mechanisms associated with short‐term plant residue C and N retention are less clear. We applied 13C‐ and 15N‐enriched grain sorghum [Sorghum bicolor (L.) Moench] residue to no‐till (NT) and conventional tillage (CT) soils, and measured the 13C and 15N retention in the soil and in aggregate fractions, along with soil microbial dynamics, during a growing season. The added plant residue mineralized rapidly in both tillage systems, with similar decomposition kinetics, as indicated by 13C data. Mass balance calculations indicated that approximately 70% of the added 13C was mineralized to CO2 by 40 d. Total Gram positive and Gram negative bacteria and fungal phospholipid fatty acids were higher under NT 0 to 5 cm during the most active period of residue mineralization compared with the CT 0‐ to 5‐ or 5‐ to 15‐cm depths. No changes were observed in the NT 5‐ to 15‐cm depth. The &gt;1000‐μm aggregate size class retained the most 13C, regardless of tillage. The NT &gt;1000‐μm aggregates retained more 15N at the end of the experiment than other NT and CT aggregates size classes. Data obtained indicate higher biological activity associated with NT soils than under CT, and increased retention of plant residue C and N in macroaggregates.

Dietary protein influences the rate of 15N incorporation in blood cells and plasma of Yellow-vented bulbuls (Pycnonotus xanthopygos)

The rate at which an animal's tissues incorporate the isotopic composition of food determines the time window during which ecologists can discern diet changes. We investigated the effect of protein content in the diet on the incorporation rate of (15)N into the plasma proteins and blood cells of Yellow-vented bulbuls (Pycnonotus xanthopygos). Using model comparison analyses, we found that one-compartment models described incorporation data better than two-compartment models. Dietary protein content had a significant effect on the residence time of (15)N in plasma proteins and blood cells. The diet with the highest protein content led to a (15)N retention time of 21 and 5 days for cells and plasma, respectively. In contrast, average (15)N retention time in the cells and plasma of birds fed on the diet with the lowest protein was 31 and 7 days, respectively. The isotopic discrimination factor Delta(15)N=delta(15)N(tissues)-delta(15)N(diet) was also dependent on dietary protein content, and was lowest in birds fed the diet with the highest protein content. Blood, plasma and excreta were enriched in (15)N relative to diet. In contrast, ureteral urine was either significantly depleted of (15)N in birds fed the diet with the lowest protein content or did not differ in delta(15)N from the diets with the intermediate and high protein content. Thus, isotopic incorporation rates and tissue-to-diet discrimination factors cannot be considered fixed, as they depend on diet composition.

Estimation of Gross Nitrogen Transformations and Nitrogen Retention in Grassland Soils Using FLUAZ

The flux of N through mineralization–immobilization turnover (MIT) in grassland soils is a major determinant for plant N uptake and for N loss processes. In this study we investigated the dynamics of gross N transformation rates and potential N retention on mineral fertilizer addition in three permanent grassland soils of varying texture (loamy sand, loam, and clay loam). Gross N transformation rates were calculated with the 15N‐tracing model FLUAZ. Differentially 15N‐labeled NH4NO3 (at a rate of 100 mg N kg−1 soil) was added to the soils in paired laboratory incubation experiments. Size and 15N enrichment of the NH4+, NO3−, and soil organic N pools were measured at 0, 1, 3, 7, 14, and 30 d after NH4NO3 addition. The accuracy of the simulations of the data using FLUAZ were robust, but tended to decrease (i) with increasing incubation times, (ii) with increasing duration of the time intervals considered, and (iii) with increasing experimental variability. The proportion of the initial N content mineralized on incubation was largest in the loamy sand soil (2.5%), followed by the clay loam soil (1.2%) and the loam soil (0.8%). The actual gross nitrification and N immobilization activity followed the same trend. The loam soil showed the lowest relative N retention (ratio N immobilization over [gross N mineralization + gross nitrification]), which was attributed to its low C availability. In general, the 15N retention after addition of 15NH414NO3 was approximately five times larger than after addition of 14NH415NO3

NITROGEN RETENTION ACROSS A GRADIENT OF15N ADDITIONS TO AN UNPOLLUTED TEMPERATE FOREST SOIL IN CHILE

Accelerated nitrogen (N) inputs can drive nonlinear changes in N cycling, retention, and loss in forest ecosystems. Nitrogen processing in soils is critical to understanding these changes, since soils typically are the largest N sink in forests. To elucidate soil mechanisms that underlie shifts in N cycling across a wide gradient of N supply, we added 15NH415NO3 at nine treatment levels ranging in geometric sequence from 0.2 kg to 640 kg N·ha−1·yr−1 to an unpolluted old-growth temperate forest in southern Chile. We recovered roughly half of 15N tracers in 0–25 cm of soil, primarily in the surface 10 cm. Low to moderate rates of N supply failed to stimulate N leaching, which suggests that most unrecovered 15N was transferred from soils to unmeasured sinks above ground. However, soil solution losses of nitrate increased sharply at inputs >160 kg N·ha−1·yr−1, corresponding to a threshold of elevated soil N availability and declining 15N retention in soil. Soil organic matter (<5.6 mm) dominated tracer retention at low rates of N input, but coarse roots and particulate organic matter became increasingly important at higher N supply. Coarse roots and particulate organic matter together accounted for 38% of recovered 15N in soils at the highest N inputs and may explain a substantial fraction of the “missing N” often reported in studies of fates of N inputs to forests. Contrary to expectations, N additions did not stimulate gross N cycling, potential nitrification, or ammonium oxidizer populations. Our results indicate that the nonlinearity in N retention and loss resulted directly from excessive N supply relative to sinks, independent of plant–soil–microbial feedbacks. However, N additions did induce a sharp decrease in microbial biomass C:N that is predicted by N saturation theory, and which could increase long-term N storage in soil organic matter by lowering the critical C:N ratio for net N mineralization. All measured sinks accumulated 15N tracers across the full gradient of N supply, suggesting that short-term nonlinearity in N retention resulted from saturation of uptake kinetics, not uptake capacity, in plant, soil, and microbial pools.

N uptake, soil retention and loss of soil-applied 15NH415NO3 in young Fuji/M.26 apple trees with different N status

SummaryNitrogen (N) uptake, soil retention and loss of soil-applied N were studied in young apple trees with different N backgrounds. Bench-graft potted Fuji/M.26 (Malus domestica Borkh) trees were fertigated with 5, 10 or 20 mM N twice a week from June to August, and the trees were removed from soil and bare-root stored in a 2°C cold room in December of the first season. In April of the second season, the trees were washed and replanted in containers with a N-free medium (perlite:vermiculite=l:l v), and received 500 ml Hoagland’s nutrient solution without N weekly through the experiment. The trees received 15NH415NO3 at 1g per plant or no N on June 21. Four trees from each treatment were harvested at one, two and four weeks after 15N application, and 15N and total N in plant tissues and soil were analysed. N fertigation rates during the first growing season increased tree growth and N reserve levels, and N content in trees in the second year. New shoot and leaf growth in the following season was positively related to reserve N. 15N uptake increased during the four weeks after 15N application while soil 15N retention decreased. There was no significant difference in the total 15N uptake per tree. However, trees with the lowest N contents at the end of the first growing season had the highest rate of 15N uptake per unit root dry weight. Four weeks after application of 15N, tree uptake of N accounted for about 60% of applied 15N, while about 20% of the N still remained in the soil, and another 20% of the N was lost. Our results suggested that trees with lower N status are more efficient in N uptake from soil.