15N Recovery Research Articles

Investigating the effects of sex of growing Nellore cattle and crude protein intake on the utilization of recycled N for microbial protein synthesis in the rumen by using intravenous 15N15N-urea infusion

The aim of this study was to investigate the effect of sex and levels of dietary crude protein (CP) on the recovery of 15N in microbial protein as well as to evaluate nitrogen balance compounds and variables related to urea renal handling in Nellore cattle. Four Nellore bulls and four Nellore heifers, fitted with rumen cannulas, were distributed in a two 4×4 Latin square design. The four experimental diets differed in levels of CP: 90.0, 110.0, 130.0, and 150.0g/kg on a dry matter basis. Continuous intravenous infusion of 15N15N-urea solution was provided at a rate of 0.30mmol urea/h followed by determination of 15N in ruminal bacteria, ammonia (NH3), and feces to determine the incorporation of recycled N in rumen bacteria. The increased dietary CP revealed a linear increase (P<0.05) in total digestible nutrients (TDN), total digestibility of CP and neutral detergent fiber (NDF), retained nitrogen, urinary excretion of urea-N, amount of filtered and absorbed urea (g/day), and plasmatic concentrations of urea-N. Fecal excretion of N presented a linear increase (P<0.05) in relation to dietary CP. Rumen NH3 concentration increased in a linear fashion (P<0.05), and rumen 15N-NH3 contents and fecal 15N contents decreased in a linear fashion (P<0.05) with increasing dietary CP. The regressions NH3=23.6183×e(0.0104×NI) and 15N-NH3=−0.00062×NI+0.1219 were obtained in order to calculate rumen NH3 concentrations (mg/l) and 15N-NH3 (atom percent excess), respectively, as a function of N intake (NI, g/day). The contents of total-N and 15N did not differ between liquid-associated bacteria (LAB) and particle-associated bacteria (PAB). There was no effect of sex (P>0.05) on the percentage of infused 15N that was recovered from feces and bacteria. There was no effect of dietary CP level on the percentage of 15N infused in plasma that was recovered from feces. However, there was a decreasing linear effect of the level of dietary CP (P<0.05) on the percentage of 15N infused in the jugular vein recovered from bacteria. We concluded that microbial protein synthesis was not affected by sex. The LAB and PAB had the same total-N content and recovery of infused 15N-urea. The greater recovery of infused intravenous 15N-urea on microbial protein synthesis in a low CP diet suggests greater urea recycling in association with this diet.

Chinese Milk Vetch Improves Plant Growth, Development and 15N Recovery in the Rice-Based Rotation System of South China

Chinese milk vetch (CMV) is vital for agriculture and environment in China. A pot experiment combined with 15N labeling (including three treatments: control, no fertilizer N and CMV; 15N-labeled urea alone, 15NU; substituting partial 15NU with CMV, 15NU-M) was conducted to evaluate the impact of CMV on plant growth, development and 15NU recovery in rice-based rotation system. The 15NU-M mitigated oxidative damage by increasing antioxidant enzymes activities and chlorophyll content while decreased malondialdehyde content in rice root and shoot, increased the biomass, total N and 15N uptake of plant shoots by 8%, 12% and 39% respectively, thus inducing a noticeable increase of annual 15N recovery by 77% versus 15NU alone. Remarkable increases in soil NH4+ and populations of bacteria, actinomycetes and azotobacter were obtained in legume-rice rotation system while an adverse result was observed in soil NO3− content versus fallow-rice. CMV as green manure significantly increased the fungal population which was decreased with cultivating CMV as cover crop. Therefore, including legume cover crop in rice-based rotation system improves plant growth and development, annual N conservation and recovery probably by altering soil nitrogen forms plus ameliorating soil microbial communities and antioxidant system which alleviates oxidative damages in plants.

Cotton growth and the fate of N fertilizer as affected by saline water irrigation and N fertigation in a drip-irrigated field

ABSTRACTThe objective of this two-year field experiment was to study the effects of irrigation amount, N rate, and irrigation water salinity on cotton growth and the fate of N fertilizer. The movement of N through the plant-soil system was traced using 15N-labeled urea. The study consisted of twelve treatments, including two irrigation amounts (405 and 540 mm, I405 and I540, respectively); two N application rates (240 and 360 kg N/ha, N240 and N360, respectively); and three irrigation water salinity levels [0.35, 4.61 and 8.04 dS/m, representing fresh water (FW), brackish water (BW), and saline water (SW), respectively]. A randomized complete block design was used with three replications. The results showed that cotton biomass, N uptake, and yield increased as irrigation amount and N amount increased; however, all three variables were significantly less in SW than in FW and BW. Plant 15N recovery rates were greater (i) in the I540 treatments than in the I405 treatments and (ii) in the N360 treatments than in the N240 treatments. Plant 15N recovery rates in BW were 7.98% and 30.01% greater than those in FW and SW, respectively. Residual soil 15N increased as N fertilizer amount increased but declined as irrigation amount increased. Residual soil 15N in BW and SW was 6.02% and 21.44% greater, respectively, than in FW. Total 15N recovery was significant greater in BW than in FW and SW. The 15N leaching losses increased significantly with increases in irrigation amount, irrigation water salinity, and N rate. Our study suggests that if appropriate amounts of irrigation water and N fertilizer are used, then brackish irrigation water (4.61 dS/m) will not affect cotton growth, yield and N recovery. In contrast, saline irrigation water (EC > 8 dS/m) reduces cotton growth, yield, and N use efficiency.

Responses of Degraded Tibetan Kobresia Pastures to N Addition

AbstractKobresia pastures on the Tibetan Plateau are the largest alpine pastoral ecosystems. Kobresia pastures have experienced severe degradation in recent decades, inducing large nitrogen (N) losses from these ecosystems. This is particularly problematic, as it intensifies prevailing N limitation in these regions. Simultaneously, anthropogenic N deposition has increased across these ecosystems, but the fate of added N on variously degraded Kobresia pastures remains unclear. Kobresia pastures of three degradation stages were investigated: living, dying and dead root mats. High and very low (as a tracer) amounts of 15N‐labelled ammonium nitrate (NH4NO3) were applied to root mats under controlled conditions. Leaching was simulated over 3 months, and 15N recovery was measured in the plant–soil system. N addition promoted aboveground biomass and foliar N content of Kobresia during the early growth period, indicating a short‐term offset of N limitation. After 7–8 weeks, plant growth and 15N uptake were reduced in plants with initial N addition, reflecting a transition to N limitation induced by N uptake and leaching from soil. This limitation was also indicated by the strong decline of NO3− in leachates from living root mats compared with degraded root mats. Leaching N losses from dying and dead root mats increased 2⋅2 and 6⋅3 times, respectively, compared with those of living root mats. We conclude that N addition can facilitate plant growth in living root mats but contributes to N leaching in degraded pastures. This contribution to N leaching may weaken ecosystem recovery, increase NO3− loading of adjacent lower landscape parts and cause eutrophication of aquatic ecosystems. Copyright © 2017 John Wiley &amp; Sons, Ltd.

15N recoveries from ruminant urine patches on three forage types

15Nitrogen balances are useful tools to increase our understanding of soil N processes and the efficiency of plants in taking up animal urine-N. This study aimed to investigate the effect of forage type on the fate of autumn-applied urine-N (700 kg N ha−1). In a 17-month lysimeter study the recoveries of 15N in herbage, leachate, nitrous oxide, and soil + roots (at the end of the trial) were determined for three forage types: perennial ryegrass-white clover (RGWC, Lolium perenne L.-Trifolium repens L.), Italian ryegrass (Italian RG, Lolium multiflorum Lam.), and lucerne (Medicago sativa L.) under simulated autumn grazing. We recovered 81.5–85.5% of the 15N applied in the urine. Italian RG had the greatest amount of urine-15N recovered in the herbage (49.5%) and the least recovered in leachate (16.8%) making it the most efficient of the three forages for utilizing urine-N. For RGWC, herbage and leachate recoveries were 40.1% and 23.7%, respectively. Lucerne was the least efficient with recoveries of 15N in herbage and leachate equaling 18.4% and 52.5%, respectively. This study demonstrates that Italian ryegrass could offer benefits in reducing urine-N leaching losses through its ability to take up more urine-N over winter.

Growth, Water Use, and Nitrate-15N Uptake of Greenhouse Tomato as Influenced by Different Irrigation Patterns, 15N Labeled Depths, and Transplant Times.

Increasing water use efficiency and reducing nitrogen pollutant discharge are important tasks for modern agriculture. To evaluate the effect of alternate partial root-zone irrigation (APRI) on tomato plant growth, water use efficiency and nitrate-15N uptake, an experiment was conducted from June to December in 2014 under greenhouse condition in northern China. The experiment contained two irrigation patterns [APRI and conventional irrigation (CI)], two 15N labeled depths in soil (10 and 50 cm) and two transplant time (early and late summer). Results showed that, compared to CI, APRI did not significantly (p > 0.05) impact the growth and biomass accumulation in aboveground part of tomato, while it enhanced the root, reflecting by greater length density, and more dry mass. APRI produced marginally lower yields, but saved 34.9% of irrigation water, and gave a 37.6–49.9% higher water use efficiency relative to CI. In addition, APRI improved fruit quality, mainly through increasing the contents of soluble solid (by 12.8–21.6%), and vitamin C (2.8–12.7%), and the sugar/acid ratio (3.5–8.5%). The 15N utilization efficiency (15NUE) in APRI was higher than that in CI, which was more evident when 15N was labeled at 50 cm depth. Significant (p < 0.05) 15N recovery increase of 10.2–13.2% and 15N loss decrease of 35.4–54.6% were found for APRI compared to CI. The overall results suggest that APRI under greenhouse could benefit the nitrate-N recovery and increase the water use efficiency in tomato.

Assessment of N2O emissions from a fertilised vegetable cropping soil under different plant residue management strategies using 15N tracing techniques

Combined application of plant residues and N fertilisers strongly affect soil mineral N dynamics and N2O emissions depending on the quality of the plant residues, their application methods and other management strategies. We investigated the effect of combined application of two vegetable plant residues (cauliflower and sweet corn) and 15N fertiliser on N dynamics and N2O emission in a glasshouse pot study. The experiment was conducted under two residue management practices (soil incorporation vs surface mulching) over 98days with growing basil (Ocimum basilicum) plants. We also assessed the efficacy of applying the nitrification inhibitor, 3,4-dimethylpyrazole phosphate (DMPP) to the plant residues, for reducing N loss and mitigating N2O emissions. Application of plant residues, both on the soil surface or into soil, resulted in net N mineralisation and increased cumulative N2O emission compared with the application of N fertiliser alone. Soil surface mulching of sweet corn decreased total and residue-induced cumulative N2O emission compared with the incorporation method, while it showed opposite effect on N2O emissions from cauliflower residue. The application of DMPP with sweet corn residue reduced total, residue- and fertiliser-induced N2O emissions; however its application with cauliflower residue did not show any mitigating effect on the N2O emissions. The residue application methods and the use of DMPP did not significantly affect 15N recovery by the basil plants. In contrast, soil incorporation of these residues doubled the microbial immobilisation of applied 15N into soil organic matter. Linear regression analysis of N2O emission during the experimental period indicated that in the treatments without DMPP application, soil NO3−-N concentration was the most important factor in controlling the magnitude of N2O emissions, while the application of DMPP changed the dominant regulating factor from NO3−-N to NH4+-N concentration.

Distinct fates of atmogenic NH4 + and NO3 − in subtropical, N-saturated forest soils

Subtropical forests receive increasing amounts of atmogenic nitrogen (N), both as ammonium (NH4 +) and nitrate (NO3 −). Previous long-term studies indicate efficient turnover of atmogenic NH4 + to NO3 − in weathered, acidic soils of the subtropics, leading to excessive NO3 − leaching. To clarify the mechanism governing the fate of atmogenic inputs in these soils, we conducted an in situ 15N tracing experiment in the TieShanPing (TSP) forested catchment, SW China. 15NH4NO3, NH 4 15 NO3 and 15N-glutamic acid were applied to an upland hillslope soil and inorganic N, total soil N and nitrous oxide (N2O) were monitored for nine days. Incorporation of 15NO3 − into soil organic N was negligible and 80% of the applied label was lost from the top soil (0–15 cm) primarily by leaching within 9 days. In contrast, 15NH4 + was largely retained in soil organic N. However, instant production of 15NO3 − in the 15NH4 + treatment suggested active nitrification. In both the 15NH4 + and 15N-glutamic acid treatments, the 15N enrichment in the NO3 − pool exceeded that in the NH4 + pool one day after 15N application, suggesting preferential nitrification of added 15NH4 + with subsequent dilution of the NH4 + pool and/or immobilization of 15NH4 + followed by heterotrophic nitrification. The cumulative recovery of 15N in N2O after 9 days ranged from 2.5 to 6.0% in the 15NO3 − treatment, confirming the previously reported significant response of N2O emission to N deposition. Source partitioning of 15N2O demonstrated a measurable contribution of nitrification to N2O emissions, particularly at low soil moistures. Our study emphasizes the role of a fast-cycling organic N pool (including microbial N) for retention and transformation of atmogenic NH4 + in subtropical, acid forest soils. Thus, it explains the near-quantitative leaching of deposited N (as NO3 − and NH4 +) common to subtropical forest soils with chronic, elevated atmogenic N inputs by (i) negligible retention of NO3 − in the soil and (ii) rapid immobilization-mineralization of NH4 + followed by nitrification. Our findings point to a leaky N cycle in N-saturated Chinese subtropical forests with consequences for regional soil acidification, N pollution of fresh waters and N2O emission.

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.

Aged biochar affects gross nitrogen mineralization and recovery: a 15N study in two contrasting soils

AbstractBiochar is a pyrolysed biomass and largely consists of pyrogenic carbon (C), which takes much longer to decompose compared to the biomass it is made from. When applied to soil, it could increase agricultural productivity through nutrient retention and changing soil properties. The biochar‐mediated nutrient retention capacity depends on the biochar properties, which change with time, and on soil properties. Here, we examined the effects of a wood biochar (20 t ha−1), that has aged (21 months) in a grassland field, on gross nitrogen (N) mineralization (GNM) and 15N recovery using a 15N tracer. A field experiment was conducted in two soil types, that is a Tenosol and a Dermosol, and also included a phosphorus (P) addition treatment (1 kg ha−1). Compared to the control, biochar with P addition significantly increased GNM in the Tenosol. Possibly, biochar and P addition enhanced nutrient availability in this nutrient‐limited soil, thereby stimulating microbial activity. In contrast, biochar addition reduced GNM in the Dermosol, possibly by protecting soil organic matter (SOM) from decomposition through sorption onto biochar surfaces and enhanced formation of organo‐mineral complexes in this soil that had a higher clay content (29% vs. 8% in the Tenosol). Compared to the control, biochar significantly increased total 15N recovery in the Tenosol (on average by 12%) and reduced leaching to subsurface soil layers (on average by 52%). Overall, 15N recovery was greater in the Dermosol (83%) than the Tenosol (63%), but was not affected by biochar or P. The increased N recovery with biochar addition in the sandy Tenosol may be due to retention at exchange sites on aged biochar, while such beneficial effects may not be visible in soils with higher clay content. Our results suggest that aged biochar may increase N use efficiency through reduced leaching or gaseous losses in sandy soils.

Effects of grazing on the acquisition of nitrogen by plants and microorganisms in an alpine grassland on the Tibetan plateau

Nitrogen (N) limitation leads to intense competition between plants and soil microorganisms for available N. However, it is unclear how grazing affects the acquisition of N by plants and microorganisms. We conducted short-term 15N tracer experiments during the growing season (June, early growing season; July, mid-growing season; and September, late growing season) in an alpine grassland on the Tibetan Plateau to investigate the effects of grazing on the acquisition of NO3 −-N, NH4 +-N, and glycine-N by plants and soil microorganisms. Three dominant plant species (one graminoid, Kobresia pygmaea, and two forbs, Potentilla bifurca and Potentilla multifida) were selected for the study. As these species represented >90% of the vegetation, the plant recovery of 15N reflected competition at the plant community. Grazing decreased the recovery of 15N by soil microorganisms and plants by 46 and 69%, respectively, indicating that grazing strongly reduced the uptake of 15N by plants and microorganisms and altered the partitioning of 15N between them. Significant interactions were found between grazing, season and the different forms of N. In the absence of grazing, plants acquired relatively more N than soil microorganisms for the three forms of N in July and September, whereas the microorganisms obtained relatively more 15N glycine in July and all three forms of N in September under grazing conditions. Under grazing, the plant root biomass played an important role in controlling plant–microbial N acquisition. Grazing alters the partitioning of inorganic and organic N between plants and soil microorganisms by reducing microbial 15N recovery to a lesser extent than plant 15N recovery. This indicates that heterotrophic microorganisms play an important part in N cycling in N-limited ecosystems.

Late‐Fall, Winter, and Spring Broadcast Applications of Urea to No‐Till Winter Wheat II. Fertilizer N recovery, Yield, and Protein as Affected by NBPT

Core Ideas Fertilizer 15N recovery of urea (with and without NBPT) was directly related to cumulative NH3 loss. Spring applied urea provided better recovery than late‐fall and winter applications. NBPT improved fertilizer 15N recovery, particularly for late‐fall and winter applications that were most susceptible to volatility. Surface application of urea to winter wheat (Triticum aestivum L.) during the late‐fall, winter, or spring is a common practice in the northern Great Plains (NGP). However, surface applications are susceptible to volatilization, and management strategies are needed to enhance fertilizer‐N recovery (FNR). This study was conducted to determine the effect of urea application timing (late‐fall, winter, spring) and N‐(n‐butyl) thiophosphoric triamide (NBPT) on FNR in grain (FNRG) and grain plus straw (FNRT), and grain yield and protein. Replicated small‐plot experiments were conducted in no‐till winter wheat fields over 3 yr in Montana, and FNR was quantified using 15N‐enriched labeled urea (5% atom). Fertilizer‐N recovery was greater (P &lt; 0.05) for spring (FNRG 36.5%, FNRT 46.1%) than late‐fall (FNRG 24.9%, FNRT 31.7%) and winter (FNRG 26.8, FNRT 34.1%) applications. Addition of NBPT to urea improved FNR of all urea timings, but the response was greater for late‐fall and winter compared with spring applications. Fertilizer‐N recovery among the management strategies tested was directly related to cumulative NH3 emissions (r2 = 0.77, P &lt; 0.0001) measured in a companion study, indicating that volatilization represented an important pathway for N loss. Grain yield was not sensitive to application timing or NBPT. However, grain protein increased 6 to 8 g kg–1 from spring applications or NBPT additions. This study found that management strategies to enhance urea FNR in the NGP should consider delaying applications until the spring or addition of NBPT for late‐fall and winter applications that are more susceptible to NH3 volatilization.

Uranium nitride fuels in superheated steam

ABSTRACTUranium mononitride (UN) pellets of different densities were subjected to a superheated steam/argon mixture at atmospheric pressure to evaluate their resistance to hydrolysis. Complete degradation of pure UN pellets was obtained within 1 h in 0.50 bar steam at 500 °C. The identified reaction products were uranium dioxide, ammonia, and hydrogen gas, with no detectable amounts of nitrogen oxides formed. However, the reaction could not be carried to completion, and the presence of uranium sesquinitride and higher uranium oxides or uranium oxynitrides in the solid residue is indicated. Evolution of elemental nitrogen was seen in connection with very high reaction rates. The porosity of the pellets was identified as the most important factor determining reaction rates at 400–425 °C, and it is suggested that in dense pellets, cracking due to internal volume increase initiates a transition from slow surface corrosion to pellet disintegration. The implications for the use of nitride fuels in light water reactors (LWR) are discussed, with some observations concerning hydrolysis as a method for 15N recovery from isotopically enriched spent nitride fuel.

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.

Critical zone properties control the fate of nitrogen during experimental rainfall in montane forests of the Colorado Front Range

Several decades of research in alpine ecosystems have demonstrated links among the critical zone, hydrologic response, and the fate of elevated atmospheric nitrogen (N) deposition. Less research has occurred in mid-elevation forests, which may be important for retaining atmospheric N deposition. To explore the fate of N in the montane zone, we conducted plot-scale experimental rainfall events across a north–south transect within a catchment of the Boulder Creek Critical Zone Observatory. Rainfall events mimicked relatively common storms (20–50% annual exceedance probability) and were labeled with 15N-nitrate ( $$ {\text{NO}}_{3}^{ - } $$ ) and lithium bromide tracers. For 4 weeks, we measured soil–water and leachate concentrations of Br−, $$ {}^{15}{\text{NO}}_{3}^{ - } , $$ and $$ {\text{NO}}_{3}^{ - } $$ daily, followed by recoveries of 15N species in bulk soils and microbial biomass. Tracers moved immediately into the subsurface of north-facing slope plots, exhibiting breakthrough at 10 and 30 cm over 22 days. Conversely, little transport of Br− or $$ {}^{15}{\text{NO}}_{3}^{ - } $$ occurred in south-facing slope plots; tracers remained in soil or were lost via pathways not measured. Hillslope position was a significant determinant of soil 15N- $$ {\text{NO}}_{3}^{ - } $$ recoveries, while soil depth and time were significant determinants of 15N recovery in microbial biomass. Overall, 15N recovery in microbial biomass and leachate was greater in upper north-facing slope plots than lower north-facing (toeslope) and both south-facing slope plots in August; by October, 15N recovery in microbial N biomass within south-facing slope plots had increased substantially. Our results point to the importance of soil properties in controlling the fate of N in mid-elevation forests during the summer season.

The fate of 15N-labelled urea in an alkaline calcareous soil under different N application rates and N splits

We aimed to quantitatively investigate the effects of rate and timing of nitrogen (N) application on fate of 15N-labelled urea in an alkaline calcareous soil during a winter wheat (WW) and summer maize (SM) seasons. The treatments consisted of conventional N application (i.e., WN300-2T or MN240-2T, 300 or 240 kg N ha−1 with two N splits to WW or SM), reduced N application (i.e., WN210-2T or MN168-2T, 210 or 168 kg N ha−1 with two N splits to WW or SM), recommended N application (i.e., WN210-3T or MN168-3T, 210 or 168 kg N ha−1 with three N splits to WW or SM), and control (N0). The result showed that the fate of 15N fertilizer was significantly influenced by rate and timing of the applied N. Compared with the conventional N treatment, crop 15N recovery in the recommended N treatment increased significantly by 16.7 % for WW and 17.2 % for SM, but total 15N losses reduced significantly by 12.3 and 13.5 %, respectively. Residual 15N in 100–200 cm soil layer was the lowest in recommended N treatment, preventing leaching of much 15NO3 −–N to deeper soil layers. Our results indicated that the recommended N treatment at rate of 210 or 168 kg N ha−1 with three N splits to WW or SM would maintain crop yields but significantly increase N recovery efficiency and reduce the risk of environmental pollution caused by N losses.

Delayed permanent water rice production systems do not improve the recovery of 15 N-urea compared to continuously flooded systems

Crop recovery of nitrogen (N) fertiliser in flooded rice systems is low relative to fertiliser N recoveries in aerobic crops, and the N losses have environmental consequences. Recent water shortages across the globe have seen a move towards alternative water management strategies such as delayed permanent water (DPW, also known as delayed flood). To investigate whether N fertiliser regimes used in DPW systems result in greater recovery of N fertiliser than traditional continuously flooded (CF) rice systems, we conducted a multi-N rate field trial using 15N-labelled urea. Around 27% of the 15N-labelled fertiliser was recovered in aboveground biomass at maturity, regardless of water regime or N fertiliser rate, and approximately 20% recovered in the soil to 300 mm depth. Plants in the CF system accumulated more total N at each rate of applied N fertiliser than plants in the DPW system due to greater exploitation of native soil N reserves, presumably because the earlier application of N fertiliser in the CF systems led to greater early growth and higher crop N demand. The greater crop biomass production as a result of higher N uptake in the CF system did not increase grain yields above those observed in the DPW system, likely due to cold weather damage. In the following season at the same site, a single N rate (150 kg N ha−1) trial found no significant differences in crop N uptake, biomass yields, grain yields or 15N-labelled urea recovery in DPW, CF and drill sown-CF (DS-CF) treatments. However, owing to higher 15N fertiliser recovery in the 0–100 mm soil horizon, total plant + soil recovery of 15N was significantly higher in the CF treatment (63%) than the DS-CF and DPW treatments (around 50% recoveries). The loss of 40–50% of the applied N (presumably as NH3 or N2) in both seasons regardless of watering regime suggests that new fertiliser N management strategies beyond optimising the rate and timing of urea application are needed, particularly in light of increasing N fertiliser prices.

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.

Fate of Organic and Inorganic Nitrogen in Crusted and Non‐Crusted Kobresia Grasslands

AbstractA widespread pattern of the Tibetan plateau is mosaics of grasslands of Cyperaceae and grasses with forbs, interspersed with patches covered by lichen crusts induced by overgrazing. However, the fate of inorganic and organic N in non‐crusted and crusted patches in Kobresia grasslands remains unknown. We reported on a field 15N‐labeling experiment in two contrasting patches to compare retention of organic and inorganic N over a period of 29 days. 15N as KNO3, (NH4)2SO4 or glycine was sprayed onto soil surface. Crusted patches decreased plant and soil N stocks. More 15N from three N forms was recovered in soil than plants in both patches 29 days after the labeling. In non‐crusted patches, 15N recovery by the living roots was about two times higher than in crusted ones, mainly because of higher root biomass. Microorganisms in non‐crusted patches were N‐limited because of more living roots and competed strongly for N with roots. Inorganic N input to non‐crusted patches could alleviate N limitation to plants and microorganisms, and leads to higher total 15N recovery (plant + soil) for inorganic N forms. Compared to non‐crusted patches, microorganisms in crusted patches were more C‐limited because of depletion of available C caused by less root exudation. Added glycine could activate microorganisms, together with the hydrophobicity of glycine and crusts, leading to higher 15N‐glycine than inorganic N. We conclude that overgrazing‐induced crusts in Kobresia grasslands changed the fate of inorganic and organic N, and lead to lower total recovery from inorganic N but higher from organic N. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Delaying nitrogen fertiliser application improves wheat 15N recovery from high rainfall cropping soils in south eastern Australia

Improving nitrogen (N) fertiliser uptake of crops growing in soils susceptible to waterlogging could potentially reduce fertiliser input costs and harmful losses of N to the surrounding environment. The fate of 15N labelled urea applied to wheat cv. Bolac was studied on brown chromosol soils at Hamilton and Tarrington, in the high rainfall zone of south western Victoria, in south eastern Australia. Wheat was fertilised with 15N-urea solution, either deep banded 0.1 m below the seed at sowing or top-dressed with or without the nitrification inhibitor DMPP (3,4-dimethylpyrazole phosphate or ENTEC®) later in the crops development. Daily temporal topsoil (0–0.1 m) water was monitored, throughout the growing season, and at physiological maturity the recovery of 15N-urea in straw, grain and soil (to 0.4 or 0.6 m depth) was measured. Delaying untreated 15N-urea application until top-dressing at mid-tillering to first node stage of growth resulted in significantly (P 2.8 %) and the potential for large rates of mineralisation during the growing season.