15N Recovery In Soil Research Articles

Nitrogen dynamics after slurry application as affected by anaerobic digestion, biochar and a nitrification inhibitor

AbstractAnimal manures are valuable multi‐nutrient fertilizers, but their short‐term nitrogen (N) use efficiency (NUE) by plants is low, bearing the potential of harmful N losses to the environment, such as nitrate () leaching. To develop strategies to increase the NUE of cattle slurry, a comprehensive understanding of slurry N dynamics in the soil–plant system is needed. In a 57‐day microcosm experiment in the greenhouse, we assessed the effect of different slurry treatments on slurry N turnover in the soil and its uptake by ryegrass (Lolium multiflorum var. Westerwoldicum). Employing a two‐factorial design, 15N cattle slurry (SLU), 15N anaerobically digested cattle slurry (SLA), and 15N anaerobically digested cattle slurry plus biochar (SLA+) were combined with and without the nitrification inhibitor 3,4‐dimethyl‐1H‐pyrazole monophosphate (DMPP). As references, a mineral fertilizer (MIN) and an unfertilised treatment (N0) were included. The 15N recovery, hence NUE, in plant biomass was higher for SLA than for SLU, while recovery in soil at 55 days after set‐up showed an opposite trend, with over 45% of N from SLU still being recovered in soil. DMPP and biochar only marginally affected NUE and fertilizer N recovery in soil. Although 15N recovery in soil was highest for SLU, residual N leaching from SLU was low (<1% of added N). We attribute this to the limited presence of slurry N in mineral forms at this point of time, with the majority being stored in the non‐microbial organic soil N pool. Leaching of residual N from MIN was significantly higher for MIN than for SLU, while SLA and SLA+ ranged in between. Overall, anaerobic digestion appeared suitable for increasing NUE of cattle slurry, but further investigations under field conditions are necessary in order to assess its potential to reduce nitrate leaching in the long‐term.

Moderate nitrogen retention in temperate heath ecosystem after elevated CO2, drought and warming through 7 years

AbstractNitrogen (N) dynamic is one of the main controlling factors of responses to climate change in N‐limited terrestrial ecosystems, which rely on nutrient recycling and retention. In this study we investigate the N partitioning in ecosystem compartments of a grassland heath, and the impact of multiple climate change factors on long‐term N retention after 15N pulse labelling. The impacts of elevated carbon dioxide (eCO2), warming and drought and the treatments in combination on ecosystem N retention were investigated in a field scale manipulation experiment. A 6‐year time‐course was assessed by pulse‐labelling with the stable N isotope 15N and by sampling after 1 day, 1 year and 6 years. After 6 years we observed that the total ecosystem retained 42% of the amended 15N across treatments (recovery of the amended 15N in the pool). The fate of the applied 15N was mainly stabilization in soil, with 36% 15N recovery in soil, while the plant compartment and microbial biomass each retained only 1%–2% of the added 15N. This suggests a moderate retention of N, for all treatments, as compared to similar long‐term studies of forest ecosystems. A decreased ammonium and vegetation N pool combined with higher 15N retention in the soil under eCO2 treatments suggests that eCO2 promoted processes that immobilize N in soil, while warming counteracted this when combined with eCO2. Drought treatments contrastingly increased the vegetation N pool. We conclude that as the organic soil layer has the main capacity for N storage in a temperate heathland‐grassland, it is important for buffering nutrient availability and maintaining a resilient ecosystem. However, the full treatment combination of drought, warming and eCO2 did not differ in 15N recovery from the control, suggesting unchanged long‐term consequences of climate change on retention of pulse added N in this ecosystem.

Fate of fertiliser nitrogen in a ryegrass–kikuyu dairy pasture system

Context Dairy pasture production is reliant on fertiliser to supply nitrogen (N); however, fertiliser N-use efficiency (FNUE) is low and N can be lost to the environment. Aims The aim of this study was to track the fate of N fertiliser applied in a pasture system of ryegrass (Lolium multiflorum, temperate grass) oversown into kikuyu (Pennisetum clandestinum, tropical grass). Methods We used 15N-labelled urea to track the residual plant uptake of a one-off application of 15N over three pasture cuts subsequent to the first cut in the kikuyu growing season from February 2018 to April 2018 (Experiment 1), followed by total soil and plant recoveries of 15N over a 12-month period (Experiment 2). Total N treatment rates were 0, 120, 240 and 480 kg N ha−1 year−1, consisting of application events of 40 kg N ha−1. In Experiment 1, 15N was applied only at the first fertilisation, whereas in Experiment 2, 15N-labelled urea was applied at each fertilisation event. Key results In Experiment 1, uptake of residual 15N fertiliser in the pasture biomass was <6%. In Experiment 2, FNUE was 29–32% and unaccounted 15N fertiliser was 22–142 kg N ha−1, across the 120, 240 and 480 kg N ha−1 year−1 treatments. Conclusions Recovery of 15N residual fertiliser did not increase with N rate and was attributed to the mass increase in soil 15N recovery. FNUE in the pasture did not decrease with N rate. Unaccounted 15N increased with N rate. Implications Existing and alternative N and pasture management strategies such as clover and multi-species pasture need to be implemented and explored to reduce the amount of unaccounted N in dairy pasture production.

Growth and nitrogen status of cotton (Gossypium hirsutum L.) under salt stress revealed using 15N-labeled fertilizer

Abstract Salt stress is a vital factor limiting nitrogen uptake and cotton growth in arid regions. The mechanisms underlying salt stress tolerance in cotton plants under high soil salinity have not been fully elucidated. Therefore, the aim of this study was to examine the proportion and mechanism of cotton nitrogen uptake under salt stress using the 15N isotope labeling technique. Cotton plants were grown in four undisturbed saline soils (1, 3, 6 and 9 dS m−1), and the experiment was designed using the ENVIRO-GRO (E-G) model. The results showed that the dry matter of roots, stems and leaves of the cotton parts in slightly saline soil (C2, 3 dS m−1) was not significant compared with the non-saline soil (C1, 1 dS m−1). The cotton fruit grown in low-salinity soil (C2, 3 dS m−1) had significantly higher dry matter than that grown in the other treatments, implying that cotton plants grown in 3 dS m−1 soil have the best nitrogen uptake and salt tolerance. Cotton plants grown in weakly (C3, 6 dS m−1) and moderately (C4, 9 dS m−1) saline soils exhibited premature senescence. The distribution of total nitrogen and nitrate content in cotton was the best explanatory variable of total 15N recovery, of which cotton 15N recovery was between 26.1% and 47.2%, and soil 15N recovery was between 7.7% and 14.9%. Our findings provide guidance for further exploitation and utilization of saline soil resources and sustainable development of the agricultural soil ecosystem in arid regions.

Fire increases soil nitrogen retention and alters nitrogen uptake patterns among dominant shrub species in an Arctic dry heath tundra

Climate change increases the frequency and severity of fire in the Arctic tundra regions. We assessed effects of fire in combination with summer warming on soil biogeochemical N- and P cycles with a focus on mineral N over two years following an experimental fire in a dry heath tundra, West Greenland. We applied stable isotopes (15NH4+-N and 15NO3−-N) to trace the post-fire mineral N pools. The partitioning of 15N in the bulk soils, soil dissolved organic N (TDN), microbes and plants (roots and leaves) was established. The fire tended to increase microbial P pools by four-fold at both one and two years after the fire. Two years after the fire, the bulk soil 15N recovery has decreased to 10.4% in unburned plots while relatively high recovery was maintained (30%) in burned plots, suggesting an increase in soil N retention after the fire. The contribution of microbial 15N recovery to bulk soil 15N recovery increased from 11.2% at 21 days to 31.5% two years after the fire, suggesting that higher post-fire N retention was due largely to the increased incorporation of N into microbial biomass. Fire also increased 15N recovery in bulk roots after one and two years, but only under summer warming. This suggests that higher retention of post-fire N can strongly increase the potential for N uptake of recovering plants under a future warmer climate. There was significantly lower 15N enrichment of Betula nana leaves while higher 15N enrichment of Vaccinium uliginosum leaves (after three years) in burned than control plots. This shows that fire can alter the N uptake differently among dominant shrub species in this tundra ecosystem, and implies that wildfires may change plant species composition in the longer term.

Impact of manure and biochar additions on annual crop growth, nutrient uptake, and fate of 15N-labelled fertilizer in two contrasting temperate prairie soils after four years

Liquid hog manure (LHM) and solid cattle manure (SCM) co-applied with biochar could beneficially influence soil nitrogen (N) and phosphorus (P) dynamics. A split-plot design was used at two sites (Brown and Black) in Saskatchewan to assess LHM and SCM (100 kg N·ha−1) applied alone or in combination with biochars (8 Mg carbon·ha−1) produced using slow or fast pyrolysis. Crop growth and nutrient uptake, along with fertilizer 15N recovery, were followed over four years in a cereal–oilseed rotation. Crop growth on the Brown soil was more responsive to the treatments than the Black soil, reflecting lower fertility of the Brown soil. The manure and biochar, applied alone or in combination, had little impact on available soil P. Manure and biochar effects on crop growth and nutrient uptake were attributed to temporal effects on soil N immobilization–mineralization influencing plant-available soil N. A negative impact of the fast pyrolysis biochar on growth and plant uptake was observed and attributable to its greater labile-carbon content, which likely promoted soil N immobilization. Synergism observed between SCM and the slow pyrolysis biochar may reflect enhanced net SCM-N mineralization and increased water-holding capacity. The majority (55%–80%) of plant 15N recovery occurred during the first year, with 86% of fertilizer 15N conserved within the soil–plant system after four years. Greater (40%) plant 15N recovery without biochar addition, coupled with increased (38%) soil 15N recovery with added biochar, suggests biochar-related immobilization and (or) sorption in the biochar-amended soils.

Deepened snow cover alters biotic and abiotic controls on nitrogen loss during non-growing season in temperate grasslands

Despite the fact that winter lasts for a third of the year in the temperate grasslands, winter processes in these ecosystems have been inadequately represented in global climate change studies. While climate change increases the snow depth in the Mongolian Plateau, grasslands in this region are also simultaneously facing high pressure from land use changes, such as grazing, mowing, and agricultural cultivation. To elucidate how these changes affect the grasslands’ winter nitrogen (N) budget, we manipulated snow depth under different land use practices and conducted a 15NH415NO3-labeling experiment. The change in 15N recovery during winter time was assessed by measuring the 15N/14N ratio of root, litter, and soils (0–5 cm and 5–20 cm). Soil microbial biomass carbon and N as well as N2O emission were also measured. Compared with ambient snow, the deepened snow treatment reduced total 15N recovery on average by 21.7% and 19.2% during the first and second winter, respectively. The decrease in 15N recovery was primarily attributed to deepened snow increasing the soil temperature and thus microbial biomass. The higher microbial activity under deepened snow then subsequently resulted in higher gaseous N loss. The N2O emission under deepened snow (0.144 kg N ha−1) was 6.26 times than that of under ambient snow (0.023 kg N ha−1) during the period of snow cover and spring thaw. Although deepened snow reduced soil 15N recovery, the surface soil N concentration remained unchanged after five years’ deepened snow treatment because deepened snow reduced soil N loss via wind erosion by 86%.

Differences in Root Nitrogen Uptake Between Tropical Lowland Rainforests and Oil Palm Plantations.

Conversion of lowland tropical rainforests to intensely fertilized agricultural land-use systems such as oil palm (Elaeis guineensis) plantations leads to changes in nitrogen (N) cycling. Although soil microbial-driven N dynamics has been largely studied, the role of the plant as a major component in N uptake has rarely been considered. We address this gap by comparing the root N contents and uptake in lowland rainforests with that in oil palm plantations on Sumatra, Indonesia. To this aim, we applied 15N-labeled ammonium to intact soil, measured the 15N recovery in soil and roots, and calculated the root relative N uptake efficiency for 10 days after label application. We found that root N contents were by one third higher in the rainforest than oil palm plantations. However, 15N uptake efficiency was similar in the two systems. This finding suggests that lower N contents in oil palm roots were likely caused by plant internal utilization of the absorbed N (e.g., N export to fruit bunches) than by lower ability to take up N from the soil. 15N recovery in roots was primarily driven by the amount of root biomass, which was higher in oil palm plantation than rainforest. The oil palms unveiled a high capacity to acquire N, offering the possibility of enhancing sustainable plantation management by reducing N fertilizer application.

Phosphorus availability and plants alter soil nitrogen retention and loss

Availability of phosphorus (P) can directly and/or indirectly affect nitrogen (N) retention and loss from soil by stimulating microbial and plant root activities. However, it is not clear how P availability and plant presence interact on nitrous oxide (N2O) emission and nitrate (NO3−) leaching in soil. A mesocosm experiment was conducted to investigate the effect of P addition (0, 10 and 20 mg P kg−1) with and without plant presence (Phalaris aquatica, C3 grass) on N2O emission, NO3− leaching and 15N recovery. Our results showed large variation in N2O emission with significant increases after leaching events. We observed that initially low but later (after 53 days of sowing) high levels of P addition increased N2O emission rates, possibly by stimulating nitrifiers and/or denitrifiers in soil. Plant presence decreased N2O emission at times when plants reduced water and NO3− in the soil, but increased N2O emission at times when both water and NO3− in the soil were abundant, and where plants may have stimulated denitrification through supply of labile organic C. Furthermore, an increase in net N mineralization, possibly due to increased decomposition stimulated by root derived C, may also have contributed to the higher cumulative N2O emission with plant presence. P addition increased 15N recovery in soil, but reduced it in leachates, suggesting increased 15N fixation in microbial biomass. Our results showed that both P addition and plant presence stimulated N loss as N2O, but also increased N retention in the soil-plant system and thus reduced N loss through leaching.

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.

Effects of carbon and phosphorus addition on microbial respiration, N2O emission, and gross nitrogen mineralization in a phosphorus-limited grassland soil

Soil microbes are frequently limited by carbon (C), but also have a high phosphorus (P) requirement. Little is known about the effect of P availability relative to the availability of C on soil microbial activity. In two separate experiments, we assessed the effect of P addition (20 mg P kg−1 soil) with and without glucose addition (500 mg C kg−1 soil) on gross nitrogen (N) mineralization (15N pool dilution method), microbial respiration, and nitrous oxide (N2O) emission in a grassland soil. In the first experiment, soils were incubated for 13 days at 90% water holding capacity (WHC) with addition of NO3− (99 mg N kg−1 soil) to support denitrification. Addition of C and P had no effect on gross N mineralization. Initially, N2O emission significantly increased with glucose, but it decreased at later stages of the incubation, suggesting a shift from C to NO3− limitation of denitrifiers. P addition increased the N2O/CO2 ratio without glucose but decreased it with glucose addition. Furthermore, the 15N recovery was lowest with glucose and without P addition, suggesting a glucose by P interaction on the denitrifying community. In the second experiment, soils were incubated for 2 days at 75% WHC without N addition. Glucose addition increased soil 15N recovery, but had no effect on gross N mineralization. Possibly, glucose addition increased short-term microbial N immobilization, thereby reducing N-substrates for nitrification and denitrification under more aerobic conditions. Our results indicate that both C and P affect N transformations in this grassland soil.

Nitrogen fertilizer fate after introducing maize and upland-rice into continuous paddy rice cropping systems

Water scarcity and economic incentives favor the introduction of upland crops into permanent paddy rice systems during dry seasons. However, introducing upland crops into permanently flooded cropping systems temporarily changes soil conditions from anaerobic to aerobic, affecting nitrogen (N) dynamics profoundly. We hypothesized that under maize and dry rice, total fertilizer 15N recovery in soil as well as the immobilization of fertilizer 15N in microbial residues is reduced compared with continuous paddy rice cropping. Furthermore, we expected enhanced emissions of fertilizer 15N in form of nitrous oxide (N2O) under maize and dry rice. To test these hypotheses, we traced the fate of a 15N-urea pulse in a field experiment in the Philippines with three different crop rotations: continuous paddy rice, paddy rice – dry rice, and paddy rice – maize for two years. Indeed, the 15N recovery in the first 5 cm of bulk soil was lowest in the paddy rice – maize rotation (arithmetic mean with standard error: 19.2 ± 1.8% of applied 15N), while twice as much was recovered in the first 5 cm of bulk soil of the continuous paddy rice cropping systems (37.8 ± 2.2% of applied 15N) during the first dry season. The 15N recovery in the plant biomass (shoots and roots) in the continuous paddy rice cropping was 13% larger than in the dry rice plant biomass and 5% larger than in the maize plant biomass during the first dry season. Fertilizer 15N remained longest in paddy rice – maize (mean residence time = 90 ± 25 days) and in continuous paddy rice (mean residence time = 77 ± 30 days), compared with dry rice – paddy rice rotation (mean residence time = 16 ± 5 days). After 2 years, 10% (paddy rice – dry rice, paddy rice – maize) to 23% (continuous paddy rice) of the applied fertilizer 15N were still stored in soil. The largest fraction of this 15N was immobilized by soil microbes, which stored 3–4% of applied 15N in the form of amino sugars as specific cell wall constituents, in all cropping systems. Nevertheless, introducing upland crops into continuous paddy rice systems likely increased N leaching losses and resulted in initial losses of urea- 15N to N2O, which thus has to be considered in climate smart mitigation strategies.

The dynamics of nitrogen derived from a chemical nitrogen fertilizer with treated swine slurry in paddy soil-plant systems

A well-managed chemical nitrogen (N) fertilization practice combined with treated swine slurry (TSS) is necessary to improve sustainability and N use efficiency in rice farming. However, little is known about the fate of N derived from chemical N fertilizer with and without TSS in paddy soil-plant systems. The objectives of this study were (1) to estimate the contribution of applied N fertilizer to N turnover in rice paddy soil with different N fertilization practices that were manipulated by the quantity of treated swine slurry and chemical N fertilizer (i.e., HTSS+LAS, a high amount of TSS with a low amount of ammonium sulfate; LTSS+HAS, a low amount of TSS with a high amount of ammonium sulfate; AS, ammonium sulfate with phosphorus and potassium; C, the control) and (2) to compare the rice response to applied N derived from each N fertilization practice. Rice biomass yield, 15N recovery in both rice grain and stems, soil total N (TN), soil inorganic N, and soil 15N recovery were analyzed. Similar amounts of 15N uptake by rice in the TSS+AS plots were obtained, indicating that the effects of the different quantities of TSS on chemical fertilizer N recovery in rice during the experimental period were not significant. The soil 15N recoveries of HTSS+LAS, LTSS+HAS, and AS in each soil layer were not significantly different. For the HTSS+LAS, LTSS+HAS and AS applications, total 15N recoveries were 42%, 43% and 54%, respectively. Because the effects of reducing the use of chemical N fertilizer were attributed to enhancing soil quality and cost-effectiveness, HTSS+LAS could be an appropriate N fertilization practice for improving the long-term sustainability of paddy soil-plant systems. However, N losses, especially through the coupled nitrification-denitrification process, can diminish the benefits that HTSS+LAS offers.

Denitrification and associated N2O emissions are limited by phosphorus availability in a grassland soil

Effects of phosphorus (P) availability on nitrogen (N) loss through microbial nitrification and denitrification processes remain poorly understood. We conducted an incubation experiment to study the responses of N2O production after P (0 and 3mgPkg−1 soil) addition with and without NH4+ or NO3− (0 and 15mgNkg−1 soil) in a high P-fixing grassland soil. The N addition included a 15N tracer (1mg15Nkg−1). In a separate experiment, we also investigated the effect of P availability on potential nitrification and denitrification. We hypothesised that the addition of P in the soil would increase N2O emission from microbial nitrification and denitrification and thereby less soil 15N-NH4+ and 15N-NO3− would be recovered. We found that P addition only significantly increased cumulative N2O emission and respiration in the NO3− treatment, suggesting stimulated activities of denitrifying microorganisms due to their relief from P limitation. However, a decrease in soil 15N recovery after P addition in the NO3− treatment was not detected because of the very small loss of N as N2O (<0.3% of the applied 15N tracer). Potential denitrification marginally increased with P addition, while potential nitrification was not affected by P addition. We conclude that P addition may increase gaseous loss of N as N2O in P-poor soils through microbial denitrification rather than nitrification when inorganic N is abundant.

The fate of cumulative applications of 15N-labelled fertiliser in perennial and annual bioenergy crops

The fate of nitrogen (N) fertiliser applied to bioenergy crops is a key issue to allow high biomass production while minimising environmental impacts due to N losses. The aim of this study was to follow the fate in the soil-plant system of N fertiliser applied to perennial (Miscanthus×giganteus and switchgrass), “semi-perennial” (fescue and alfalfa) and annual (sorghum and triticale) bioenergy crops. Crops received 15N-labelled fertiliser (urea ammonium nitrate solution) during 4 or 5 successive years on the same subplots, at a rate varying from 24 to 120kgNha−1yr−1. Biomass production, N and 15N removal at harvest were measured each year. The 15N recovery in crop residues, non-harvested crop parts and soil was measured at the end of the 15N-labelling period. Perennial crops had higher biomass production but generally lower 15N recovery in harvested biomass than other crops, particularly when harvested late (end of winter). At the end of the 4 or 5-year period, the proportion of 15N recovered in harvested biomass was 13–34% for perennials, 23–38% for semi-perennials and 34–39% for annual crops. Perennial crops stored large amounts of N in their belowground organs; the mean 15N recovery in these organs was 12%, corresponding to a N storage flux of 14kgNha−1 yr−1. The 15N recovery in soil (including crop residues) was higher for perennials (average 36%) than semi-perennials (28%) and annual crops (19%), corresponding to a N immobilisation rate of 43, 15 and 12kgNha−1yr−1 respectively. The mean overall 15N recovery in the soil-plant system was 69% in perennials, 61% in semi-perennials to 56% in annual crops, suggesting that important fertiliser losses occurred through volatilisation and denitrification. Perennial bioenergy crops had the better efficiency by storing fertiliser-N in soil organic matter and living belowground biomass used as N reserves for succeeding years.

Nitrogen recovery in planted seedlings, competing vegetation, and soil in response to fertilization on a boreal mine reclamation site

Field fertilization during reforestation often yields variable results, particularly on harsh restoration sites. An improved understanding of the recovery of applied nitrogen (N) under different fertilization practices should aid in developing more effective fertilizer prescriptions. We evaluated field establishment of white spruce (Picea glauca (Moench) Voss) and trembling aspen (Populus tremuloides Michx.) seedlings as well as N recovery within planted seedlings, soil, and competing vegetation on a mine reclamation site in the oil sands region of northern Alberta in response to immediately available fertilizer (IAF) and polymer-coated controlled-release fertilizer (CRF) applications. 15N-enriched urea was applied as IAF and as a polymer-coated CRF (20gNseedling−1 and 4gNseedling−1, respectively) to each species. Seedling survival, growth, and nutritional status, along with occurrence of competing vegetation and plant and soil 15N recovery were quantified after the first field season. Seedlings receiving CRF exhibited increased diameter and organ dry mass relative to the IAF and control treatments. Both IAF and CRF promoted comparable increases in seedling N status, and fertilizer type did not influence within-seedling 15N allocation. Neither IAF nor CRF affected vegetation cover or dry mass. Recovery of fertilizer-derived 15N was low, with much of the recovered 15N remaining in soils and only small amounts observed in seedlings and competing vegetation for both fertilizer treatments. Findings indicate that directed root zone application of CRF promotes first-year seedling growth and nutritional responses similar to or better than those induced by broadcast IAF applications, but at substantially lower N application rates. Our results suggest that a shift from broadcast IAF to targeted soil applications of CRF may produce similar or improved early seedling growth and nutrient uptake on reclamation sites, while greatly reducing overall quantities of N applied during the regeneration phase, much of which appears to be lost from the site of application regardless of fertilizer type.

Uptake and partitioning of simulated atmospheric N inputs inPopulus tremuloides – Pinus strobusforest mesocosms

Canopies of forest ecosystems frequently intercept and retain &gt;50% of atmospheric nitrogen (N) deposition. Atmospheric N reaching the forest floor often is retained in litter and soil pools, but if atmospheric N retained by the canopy is assimilated through foliar uptake, then N inputs may have greater effects on tree N requirements and growth than suggested by N additions to forest floors alone. To quantify foliar and root uptake of N deposition by trees and to determine patterns of within-tree N allocation, we performed a tightly controlled15N tracer experiment using constructed, seminatural forest mesocosms. Mesocosms contained seedling trees, which assimilated modest amounts of the aqueous15NH4Cl applied to their canopies and surrounding soils (10% and 14%, respectively). The seedlings allocated most of the15N that they took up to leaf biomass (40%–50%), regardless of uptake pathway. Soil pools retained &gt;80% of the15N applied to mesocosm forest floors, with soil organic matter concentration explaining 79% of the variation in soil15N recovery across sampling depths. The low value for15N movement into biomass and its retention in soil pools suggests that atmospheric N deposition measured at a nearby forest site has no short-term effects on tree N content or growth.

Transport of Carbon and Nitrogen Between Litter and Soil Organic Matter in a Northern Hardwood Forest

We used sugar maple litter double-labeled with 13C and 15N to quantify fluxes of carbon (C) and nitrogen (N) between litter and soil in a northern hardwood forest and the retention of litter C and N in soil. Two cohorts of litter were compared, one in which the label was preferentially incorporated into non-structural tissue and the other structural tissue. Loss of 13C from this litter generally followed dry mass and total C loss whereas loss of 15N (20–30% in 1 year) was accompanied by large increases of total N content of this decaying litter (26–32%). Enrichment of 13C and 15N was detected in soil down to 10–15 cm depth. After 6 months of decay (November–May) 36–43% of the 13C released from the litter was recovered in the soil, with no differences between the structural and non-structural labeled litter. By October the percentage recovery of litter 13C in soil was much lower (16%). The C released from litter and remaining in soil organic matter (SOM) after 1 year represented over 30 g C m−2 y−1 of SOM accumulation. Recovery of litter 15N in soil was much higher than for C (over 90%) and in May 15N was mostly in organic horizons whereas by October it was mostly in 0–10 cm mineral soil. A small proportion of this N was recovered as inorganic N (2–6%). Recovery of 15N in microbial biomass was higher in May (13–15%) than in October (about 5%). The C:N ratio of the SOM and microbial biomass derived from the labeled litter was much higher for the structural than the non-structural litter and for the forest floor than mineral SOM, illustrating the interactive role of substrates and microbial activity in regulating the C:N stoichiometry of forest SOM formation. These results for a forest ecosystem long exposed to chronically high atmospheric N deposition (ca. 10 kg N ha−1 y−1) suggest possible mechanisms of N retention in soil: increased organic N leaching from fresh litter and reduced fungal transport of N from soil to decaying litter may promote N stabilization in mineral SOM even at a relatively low C:N ratio.

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.

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.