15N-labelled Urea Research Articles

Effect of urea fertilization on growth of broad bean (Vicia faba L.) under various nickel (Ni) levels with or without acetic acid addition, using 15N-labeled fertilizer.

Although nickel (Ni) has direct relationship with nitrogen metabolism of plants, the high dose of Ni fertilizer in broad bean plants may affect the nitrogen use efficiency (NUE), impair plant development and even cause Ni pollution in soil. Thus, a pot experiment was set up to study the effect of urea fertilization on N-uptake, root and shoots' Ni content as well as growth of broad bean plants under different levels of Ni, using 15N tracer technique. 15N-labeled urea (5% 15N atom excess) was added at three doses (0, 30 and 60mgN kg-1 soil). Nickel sulfate (NiSO4) was also applied at three levels (0, 50 and 100mgNi kg-1 soil). The experiment was laid out with or without acetic acid in randomized complete block design in three replicates. Treatment with the addition of 60mg N + 50mg Ni showed the highest values in dry weights of root and shoots, N-uptake by shoots, nitrogen derived from fertilizer (Ndff %) and NUE % by shoots in both with or without acetic acid solution. Higher rate of Ni addition can decrease shoot and root biomass by inhibiting the ability of the plant to uptake the nitrogen efficiently. However, addition of acetic acid solution induced the improvement of NUE % and Ndff % by shoot and root of broad bean plants. This study provides insight into how to improve plant yield without damaging the soil health and will be helpful to create a better world with sustainable agriculture.

Manure application increased crop yields by promoting nitrogen use efficiency in the soils of 40-year soybean-maize rotation

It is great of importance to better understand the effects of the long-term fertilization on crop yields, soil properties and nitrogen (N) use efficiency in a rotation cropping cultivation system under the conditions of frequent soil disturbance. Therefore, a long-term field experiment of 40 years under soybean-maize rotation was performed in a brown soil to investigate the effects of inorganic and organic fertilizers on crop yields, soil properties and nitrogen use efficiency. Equal amounts of 15N-labelled urea with 20.8% of atom were used and uniformly applied into the micro-plots of the treatments with N, NPK, M1NPK, M2NPK before soybean sowing, respectively. Analyses showed that a total of 18.3–32.5% of applied N fertilizer was taken up by crops in the first soybean growing season, and that the application of manure combining with chemical fertilizer M2NPK demonstrated the highest rate of 15N recovery and increased soil organic matter (SOM) and Olsen phosphorus (Olsen P), thereby sustaining a higher crop yield and alleviating soil acidification. Data also showed that no significant difference was observed in the 15N recovery from residue N in the second maize season plant despite of showing a lower 15N recovery compared with the first soybean season. The recovery rates of 15N in soils were ranged from 38.2 to 49.7% by the end of the second cropping season, and the residuals of 15N distribution in soil layers revealed significant differences. The M2NPK treatment demonstrated the highest residual amounts of 15N, and a total of 50% residual 15N were distributed in a soil layer of 0–20 cm. Our results showed that long-term application of organic fertilizers could effectively promote N use efficiency by increasing SOM and improving soil fertility, and thus leading to an increase in crop yields. This study will provide a scientific reference and guidance for improving soil sustainable productivity by manure application.

Improved Root Growth by Liming Aluminum-Sensitive Rice Cultivar or Cultivating an Aluminum-Tolerant One Does Not Enhance Fertilizer Nitrogen Recovery Efficiency in an Acid Paddy Soil.

The root is the main site of nitrogen (N) acquisition and aluminum (Al) toxicity. The objective of this study is to investigate whether liming and cultivation of an Al-tolerant rice (Oryza sativa L.) cultivar can improve root growth, thereby increasing N acquisition by rice plants in acid paddy soil. Two rice cultivars (‘B690’, Al-sensitive, and ‘Yugeng5’, Al-tolerant) were cultivated with 15N-labeled urea, and with or without lime in an acid paddy soil (pH 4.9) in pots. We examined root and shoot growth, soil pH, soil exchangeable Al, N uptake, 15N distribution in plant-soil system, and fertilizer N recovery efficiency. Results showed that liming improved the root growth of ‘B690’ by decreasing soil exchangeable Al concentrations, in both N-limited and N-fertilized soils. Liming enhanced the N uptake of ‘B690’ only in the absence of N fertilizer. The root weight of ‘Yugeng5’ was greater than that of ‘B690’ without lime, but the two cultivars showed similar N uptake. The fertilizer N recovery efficiency and N loss did not differ significantly between limed and non-limed conditions, or between the two rice cultivars. Thus, liming an Al-sensitive rice cultivar and cultivating an Al-tolerant one improves root growth, but does not enhance fertilizer N recovery efficiency in the present acid paddy soil.

Effects of nitrogen supply methods on fate of nitrogen in maize under alternate partial root-zone irrigation

Partial root-zone irrigation (PRI) has been practiced worldwide, but little information is available on nitrogen (N) supply methods influence on fate of applied N fertilizer for crop production under PRI. A field experiment was conducted to investigate effect of N supply methods on the uptake, residual, and loss of applied N fertilizer in maize (Zea mays L.) under alternate PRI at Wuwei, northwest China in 2014. 15N-labeled urea was used as N fertilizer. Two irrigation methods included alternate furrow irrigation (AI) and conventional furrow irrigation (CI). Two N fertilizer supply methods included conventional N supply (CN) and alternate N supply (AN), were applied in combination with each irrigation method. Grain yield, root length density (RLD), N uptake by maize at the maturity stage, and atom % of 15N excess, residual 15N and residual NO3-N in the 0-100 cm soil layer after maize harvest were determined. Results shown that compared to CI coupled with CN, AI coupled with AN or CN significantly increased the grain yield, harvest index, RLD, N uptake by maize, 15N accumulation in grain, atom % of 15N excess in the 0-60 cm soil layer, the residual 15N and 15N uptake rates; but significantly decreased the residual NO3-N in the 0-100 cm soil layers and 15N loss rate. Moreover, the synchronized rather than separation supply of N fertilizer and water enhanced the most above parameters under AI. 15N uptake rate was positively correlated with RLD in the 0-40 cm soil layer, suggesting that the enhanced RLD contributed to the improved 15N uptake rate. Therefore, alternate furrow irrigation coupled with conventional or alternate nitrogen supply (synchronized supply of N fertilizer and water) could help improve 15N uptake rate and reduce the 15N loss rate. Keywords: 15N-labeled technology, root length density, nitrogen fertilizer fate, nitrogen management, residual nitrogen DOI: 10.25165/j.ijabe.20201303.5287 Citation: Qi D L, Hu T T, Song X. Effects of nitrogen supply methods on fate of nitrogen in maize under alternate partial root-zone irrigation. Int J Agric & Biol Eng, 2020; 13(3): 129–135.

Fate of a 15N-labeled Urea Pulse in Heavily Fertilized Banana Crops

Banana crops in the Caribbean are characterized by the use of high rates of nitrogen (N) fertilization which causes severe environmental damages. The aim of this study was to assess the fertilizer N use efficiency (NUE) of banana crops in the field. To do so, a field trial was carried out during the first (GS1) and the fourth (GS4) growing seasons of banana crops, and the fate of a 15N-labeled pulse applied late in the growing season (flowering stage) was determined. At harvest, NUE (average 24% 15N applied) and the total recovery of fertilizer 15N in the soil–plant system (i.e., 40% in GS1 and 62% in GS4) were low. Low NUE resulted mainly from the dilution in a large soil mineral N pool derived from earlier applications of the labeled-N fertilizer applied at flowering, combined with leaching caused by numerous high-intensity rainfall events (>20 mm d−1). Crop residues from previous cycles present at time of fertilizer application in the fourth growing season, promoted fertilizer N immobilization, which in turn favored fertilizer N recovery by decreasing N leaching. The results suggest that N fertilization after the first season could be reduced by 30% (i.e., −90 kg N ha−1) corresponding to the suppression of two applications from flowering to harvest with the current fertilizer management, as available N derived from earlier applications is sufficient to meet plant requirements.

Concurrent carbon and nitrogen transfer between hemiparasite Santalum album and two N2-fixing hosts in a sandalwood plantation

Root hemiparasites feed on host root xylem and prefer N2-fixing hosts. Carbon (C) and nitrogen (N) translocation from hosts to hemiparasites affect growth and development, but detailed C and N economies between hemiparasites and N2-fixing hosts are unknown. In a 6-year-old Santalum album (sandalwood) plantation, concurrent movement of C and N between plants over 14 days were traced among N2-fixing Acacia confusa, N2-fixing Dalbergia odorifera (rosewood) and non-N2-fixing hemi-parasitic sandalwood by adding 13C-labeled and 15N-labeled urea to leaves of one species (donor) and tracing the label into other two species (receivers) within three triple-tree combinations. Haustorial connections and host selectivity were also examined on roots and nodules within adjacent plants. N2 fixation by acacia and rosewood was quantified in the stand by 15N natural abundance. Parasitic haustoria of sandalwood on roots and nodules of N2-fixing acacia and rosewood were more frequent than on other non-N2-fixing herbaceous plants, indicating strong host selectivity and preference. N2 fixation supplied 19.7% and 52.3% of the total N in acacia and rosewood, respectively. During 14 days of labeling, more 13C and 15N was transferred from acacia or rosewood to sandalwood than from sandalwood to these two N2 fixers. Net C and N transfer between tree pairs indicated that hemiparasite sandalwood was the C sink or N sink when associated with N2-fixing acacia or rosewood, while N2-fixing acacia was the C sink or N sink when associated with N2-fixing rosewood. This study indicated that mixed plantations had high rates of interplant nutrient transfer. Our results also showed that the herbaceous plants might acquire some C and N from sandalwood and two N2-fixing plants via both direct and indirect transfers, suggesting a nutrient translocationbetween plants within this community. This mixed plantation of acacia/rosewood/sandalwood could replace low-valued single species or mixed plantations for growing both high-value sandalwood and rosewood heartwoods. Our study quantified nutrient transfer among plants in a sandalwood plantation. These results have improved our understanding of the physiological interactions in resource acquisition between host species and parasitic plants and provided insights into promising mixed-plantation patterns for valuable tropical trees in south China and globally.

Alternate partial root-zone drip irrigation with nitrogen fertigation promoted tomato growth, water and fertilizer-nitrogen use efficiency

Irrigation and fertilization play key roles in crop production. Scarcity of available water resources and low nutrient use efficiency calls for a need to improve water and nitrogen (N) use efficiency. This study aimed to investigate the effects of different irrigation and nitrogen treatments on the growth, yield, irrigation water-use efficiency (IWUE) and fertilizer-N utilization of tomato plants by using pre-buried soil columns (100 cm in depth) in a field. The irrigation treatments included conventional drip irrigation (CDI) and alternate partial root-zone drip irrigation (ADI) with sufficient or deficient water supply. Nitrogen fertilizer was set as high and low N input (8.30 g or 4.15 g 15N-labeled urea per plant) supplied as drip fertigation. Compared with the CDI treatment at the same nitrogen drip fertigation level, ADI promoted tomato root growth, induced more 15N accumulation in the 0−100 cm soil profile, facilitated the absorption of fertilizer-15N as well as soil N by plants, thus increasing plant growth and yield as well as IWUE and fertilizer-N use efficiency, while decreased 15N loss. Low nitrogen supply decreased plant growth, yield, 15N absorption, 15N accumulation in soil profile and IWUE, but increased both the 15N-use efficiency and 15N loss percentage. Thus, ADI coupled with nitrogen fertigation has a potential to save irrigation water, increase use efficiencies of both water and nitrogen fertilizers with reduced loss of N to the environment.

Effects of nitrogen supply methods on fate of nitrogen in maize under alternate partial root-zone irrigation

Partial root-zone irrigation (PRI) has been practiced worldwide, but little information is available on nitrogen (N) supply methods influence on fate of applied N fertilizer for crop production under PRI. A field experiment was conducted to investigate effect of N supply methods on the uptake, residual, and loss of applied N fertilizer in maize (Zea mays L.) under alternate PRI at Wuwei, northwest China in 2014. 15N-labeled urea was used as N fertilizer. Two irrigation methods included alternate furrow irrigation (AI) and conventional furrow irrigation (CI). Two N fertilizer supply methods included conventional N supply (CN) and alternate N supply (AN), were applied in combination with each irrigation method. Grain yield, root length density (RLD), N uptake by maize at the maturity stage, and atom % of 15N excess, residual 15N and residual NO3-N in the 0-100 cm soil layer after maize harvest were determined. Results shown that compared to CI coupled with CN, AI coupled with AN or CN significantly increased the grain yield, harvest index, RLD, N uptake by maize, 15N accumulation in grain, atom % of 15N excess in the 0-60 cm soil layer, the residual 15N and 15N uptake rates; but significantly decreased the residual NO3-N in the 0-100 cm soil layers and 15N loss rate. Moreover, the synchronized rather than separation supply of N fertilizer and water enhanced the most above parameters under AI. 15N uptake rate was positively correlated with RLD in the 0-40 cm soil layer, suggesting that the enhanced RLD contributed to the improved 15N uptake rate. Therefore, alternate furrow irrigation coupled with conventional or alternate nitrogen supply (synchronized supply of N fertilizer and water) could help improve 15N uptake rate and reduce the 15N loss rate. Keywords: 15N-labeled technology, root length density, nitrogen fertilizer fate, nitrogen management, residual nitrogen DOI: 10.25165/j.ijabe.20201303.5287 Citation: Qi D L, Hu T T, Song X. Effects of nitrogen supply methods on fate of nitrogen in maize under alternate partial root-zone irrigation. Int J Agric & Biol Eng, 2020; 13(3): 129–135.

Biological nitrogen fixation and urea-N recovery in 'Coastcross-1' pasture treated with Azospirillum brasilense

Abstract: The objective of this work was to quantify the inoculation effect of Azospirillum brasilense (Ab-V5 and Ab-V6 strains) on the forage yield, biological nitrogen fixation (BNF), and urea-15N recovery of the forage grass 'Coastcross-1'. The experiment was carried out in a 2 (with or without inoculation) × 2 (without N fertilizer and with 100 kg ha-1 N per year as urea) × 7 (cuts) factorial arrangement. The natural 15N abundance method was used to determine BNF; to determine urea-N recovery, 15N-labeled urea was applied in microplots. Forage yield was higher in grasses subjected to inoculation, with 7.4 Mg ha-1 dry matter per year, for the treatment without N fertilizer, and 8.0 Mg ha-1 dry matter per year for the treatment with 100 kg ha-1 N per year, respectively, which shows an additive effect of inoculation and N fertilization. The contribution of BNF was 23.0 and 53.8 kg ha-1 per year for the unfertilized treatment, both in uninoculated and inoculated plants, respectively. Urea-15N recovery was 13.7 and 16.5 kg ha-1 per year for uninoculated and inoculated treatments, respectively, corresponding to 13.7 and 16.5% of the urea-N applied. Inoculation with A. brasilense increases forage yield and the contribution of BNF to grass nutrition with N, as well as urea-N recovery by the forage grass.

Performance of nitrate compared with urea fertilizer in a semiarid climate of the northern Great Plains

Fertilizer NO3-N may represent a benefit over NH4-N containing sources in semiarid regions where rainfall is often not sufficient to leach fertilizer-N out of crop rooting zones, denitrification concerns are not great, and when NH3 volatilization concerns exist. The objective of our study was to contrast plant-N derived from fertilizer-15N (15Ndff), fertilizer-15N recovery (F15NR), total N uptake, grain yield, and protein of wheat (Triticum aestivum L.) from spring-applied NaNO3 relative to urea and urea augmented with urease inhibitor N-(n-butyl)thiophosphoric triamide (NBPT). We established six fertilizer-N field trials widespread within the state of Montana between 2012 and 2017. The trials incorporated different experimental designs and 15N-labeled fertilizer-N sources, including NaNO3, NH4NO3, urea, and urea + NBPT. Overall, F15NR and 15Ndff in mature crop biomass were significantly greater for NaNO3 than urea or urea + NBPT (P < 0.05). Crop 15Ndff averaged 53.8%, 43.9%, and 44.7% across locations for NaNO3, urea, and urea + NBPT, respectively. Likewise, crop F15NR averaged 52.2%, 35.8%, and 38.6% for NaNO3, urea, and urea + NBPT, respectively. Soil 15N recovered in the surface layer (0–15 cm) was lower for NaNO3 compared with urea and urea + NBPT. Wheat grain yield and protein were generally not sensitive to improvements in 15Ndff, F15NR, or total N uptake. Our study hypothesis that NaNO3 would result in similar or better performance than urea or urea + NBPT was confirmed. Use of NO3-N fertilizer might be an alternative strategy to mitigate fertilizer-N induced soil acidity in semiarid regions of the northern Great Plains.

Green manure application improves rice growth and urea nitrogen use efficiency assessed using 15N labeling

ABSTRACT Green manure is an efficient nitrogen (N) source when used as an alternative to chemical fertilizer. However, the N taken up by rice derived from green manure, chemical fertilizers or soil native N in complex nutrient systems is unclear. A pot experiment with partial substitution of urea with Chinese milk vetch (a green manure) implemented with 15N-labeled urea and Chinese milk vetch was set up to study the sources of N in rice and the fate of the fertilizers. The dry weights, N contents, N uptake, and urea N use efficiency were notably higher (by 15–16%, 4–13%, 22–30% and 182%-203%, respectively) in the Chinese milk vetch applied with urea treatment than in the urea alone treatment. The uptake of N from Chinese milk vetch and the use efficiency of Chinese milk vetch N were increased with reductions in the urea input amount. The application of Chinese milk vetch substantially changed the fate of urea: higher amounts of urea N were taken up by rice (approximately 29%) and remained as residue in the soil (approximately 15%) in the related treatments than in the treatment with urea alone (10% and 9%). More urea N than Chinese milk vetch N was taken up by rice (29% vs 20%, respectively) and lost (56% vs 14%, respectively), but less urea N than Chinese milk vetch N remained as residue in the soil (15% vs 66%, respectively). The partial substitution of chemical fertilizer with green manure is an effective method of promoting rice growth by supplying N for rice uptake and promoting more efficient N use.

Nitrogen absorption by field-grown tea plants (Camellia sinensis) in winter dormancy and utilization in spring shoots

The information of nitrogen uptake by subtropical, ever-green broad-leaf plants at cold temperatures of winter is very limited. The present field experiment was conducted to investigate whether 15N is taken up by tea (Camellia sinensis L.) plants in winter dormancy in the absence of active shoot growth and utilization in young spring shoots. We applied 15N-labeled urea to soil at five different times i.e. mid-January, early February, mid-February, and early and mid-March. 15N abundance was determined in fibrous roots, twigs and mature leaves after 3, 7 and 15 days after application and in young shoots the following spring. 15N was taken up by fibrous roots and transported to above-ground tissues within 3 days after application under low winter temperatures. Earlier application significantly increased nitrogen derived from 15N-urea (Ndff) and 15N amount in young spring shoots. Ndff values and 15N amount in young spring shoots were described well by quadratic or linear regressions against soil growing degree days (GDD, T ≥ 8 °C, depth 20 cm) between 15N application and harvesting dates (R2 = 0.58–0.90, p < 0.001). Nitrogen was absorbed and translocated in dormant tea plants in the absence of active root and shoot growth throughout the late winter until early spring. Absorbed N was stored and remobilized to support shoot growth the following spring. Soil GDD between N application and harvesting could predict Ndff and 15N amount in young spring shoots.

Recovery efficiency and loss of 15N-labelled urea in a rice-soil system under water saving irrigation in the Songnen Plain of Northeast China

From 1980 to 2016, the increase in grain production in the Songnen Plain of Northeast China was significant, which was caused by a continuous increase in the use of nitrogen fertilizer; however, the partial fertilizer productivity (PFP) is gradually decreasing, and a large amount of nitrogen fertilizer is lost to the environment, leading to a series of environment problems. This test combining a field plot test with an in-situ micro-zone test, six treatments were set: two irrigation regimens (controlled irrigation and flooded irrigation) and three N rates (85, 110 and 135 kg ha−1). the recovery and loss of nitrogen fertilizer in a rice-soil system under controlled irrigation regimen were studied. Also, the basal, tillering, and panicle fertilizers were marked, and the absorption and accumulation of different periods 15N-labeled urea under different water and nitrogen control regimens as well as the residual rate of 15N-labeled urea in the soil layer of paddy soil in each period were quantitatively analyzed. Each period nitrogen fertilizer recovery and loss in the rice-soil system were quantitative analyzed. The results showed that the loss rate of the rice-soil system of controlled irrigation was reduced by 15.42–28.64% under the three nitrogen application levels compared to conventional flooded irrigation in the black soil region of the Songnen Plain. The system also maintained the original rice yield and greatly reduced the threat of nitrogen fertilizer loss to the environment in the black soil region. The total nitrogen recovery rate in the rice-soil system ranged from 69.18% to 74.19% with controlled irrigation regimen under the different nitrogen application levels, and the nitrogen recovery rate of the basal fertilizer was 41.46%–48.85%. The nitrogen recovery rate of the tillering fertilizer was 84.20–95.85%, whereas the nitrogen recovery rate of the panicle fertilizer was 89.57–93.80%. The total nitrogen recovery rate of the rice plant was 32.05–38.83%, the recovery rate of the basal fertilizer was 11.78–16.44%, and the recovery rate of the tillering fertilizer was 37.49–39.02%. The recovery rate of panicle fertilizer was 54.15–68.38%. Rice-planting under controlled irrigation can effectively reduce N fertilizer losses and also increase the dry matter accumulation of rice plants, which was found to be 13.27–27.52% more than that of the conventional flooded irrigation regimen. The controlled irrigation regimen not only meets the current requirements for grain yield, but also plays a role in saving water, increases production, and reduces environmental pollution.

Residual effects of fertilizer N response to split N applications in semiarid farmland

A better understanding of residual N fate is important for N management in agricultural production. The N application rate and time may dramatically influence N recovery in the plant–soil system, in turn affecting residual N uptake by subsequent crops. A 3-year field experiment was conducted in plastic-mulched maize in semiarid farmland. 15N-labeled urea was applied to microplots with a single application (100% before sowing, N1), two splits (4:6 at sowing and eight-leaf stages, N2), and three splits (4:3:3 at sowing, eight-leaf, and silking stages, N3), and the fate of residual fertilizer N in soils over the following two cropping seasons was examined. Approximately 14.6–18.7% and 5.4–5.8% of labelled fertilizer N were recovered by maize in the second and the third seasons, respectively, with the cumulative recovery efficiency reaching 47.6–60.8% over 3 years. Applying N with three splits significantly increased residual fertilizer N recovery by 24.6% in the second cropping season compared to N1 and N2. About 22.7–32.4% and 15–21% of total labelled N applied was residual in the 0–2 m soil layer after the second- and the third-season harvest, respectively, with the higher residual amount from split N applications. In conclusion, split N applications significantly increased the cumulative fertilizer N recovery in the plant–soil system while decrease the potential losses over 3 years, due to the higher recovery efficiency and the lower N losses from topdressed N.

Environmental Compensation Effect and Synergistic Mechanism of Optimized Nitrogen Management Increasing Nitrogen Use Efficiency in Indica Hybrid Rice

Modern rice cultivation relies heavily upon inorganic nitrogen fertilization. Effective fertilizer management is key to sustainable agricultural development. Field and pot trials were conducted in 2014–2016, including a 15N-labeled urea pot experiment (2014) to investigate mechanism by which optimized nitrogen fertilizer application (OFA) increases nitrogen utilization efficiency (NUE). Results showed that the applied nitrogen recovery efficiencies with OFA were 71.71%, 110.17%, and 51.38% higher than those obtained with traditional nitrogen fertilizer application (TFA) in 2014, 2015, and 2016, respectively. These improvements are attributed mainly to the high recovery efficiency rates derived from spikelet-developing and spikelet-promoting fertilizer applications at the jointing stage and 15–20 d after jointing. Under OFA, the amount of nitrogen fertilizer applied at the early stages was half that used in TFA, which not only promoted the absorption of soil nitrogen, but also reduced nitrogen loss to the environment, as the NUE of basal and tillering fertilizer was only about 22%. Nitrogen applied during the panicle differentiation stage increased the expression of ATM1;1, a NH4+ transporter in roots. This effect significantly improved the uptake of nitrogen derived from fertilizer from jointing to heading stage. Up-regulation of the expression and activity of GS and GOGAT at the panicle differentiation and grain-filling stages promoted nitrogen translocation from vegetative organs to reproductive organs. The uptake of nitrogen derived from fertilizer increased from 22.51% in TFA to 35.58% in OFA. Nevertheless, rice absorbs most of the nitrogen it requires from the soil. The OFA treatment could effectively utilize the environmental compensation effect, promote the absorption and transport of nitrogen, and ultimately lead to improvement in NUE. Future research should aim to understand the soil nitrogen supply capacity in order to apply nitrogenous fertilizer in such a way that it sustains the nitrogen balance.

Urea and legume residues as 15N-N2O sources in a subtropical soil

In this work, we used the 15N labelling technique to identify the sources of N2O emitted by a subtropical soil following application of mineral nitrogen (N) fertiliser (urea) and residues of a legume cover crop (cowpea). For this purpose, a 45-day incubation experiment was conducted by subjecting undisturbed soil cores from a subtropical Acrisol to five different treatments: (1) control (no crop residue or fertiliser-N application); (2) 15N-labelled cowpea residue (200 μg N g–1 soil); (3) 15N-labelled urea (200 μg N g–1 soil); (4) 15N-labelled cowpea residue (100 μg N g–1 soil) + unlabelled urea (100 μg N g–1 soil); and (5) unlabelled cowpea residue (100 μg N g–1 soil) + 15N-labelled urea (100 μg N g–1 soil). Cores were analysed for total N2O formation, δ15N-N2O and δ18O-N2O by continuous flow isotope ratio mass spectrometry, as well as for total NO3–-N and NH4+-N. Legume crop residues and mineral fertiliser increased N2O emissions from soil to 10.5 and 9.7 µg N2O-N cm–2 respectively, which was roughly six times the value for control (1.5 µg N2O-N cm–2). The amount of 15N2O emitted from labelled 15N-urea (0.40–0.45% of 15N applied) was greater than from 15N-cowpea residues (0.013–0.015% of 15N applied). Unlike N-poor crop residues, urea in combination with N-rich residues (cowpea) failed to reduce N2O emissions relative to urea alone. Legume cover crops thus provide an effective mitigation strategy for N2O emissions in relation to mineral N fertilisation in climate-smart agriculture. Judging by our inconclusive results, however, using urea in combination with N-rich residues provides no clear-cut environmental advantage.

Timing and splitting of nitrogen fertilizer supply to increase crop yield and efficiency of nitrogen utilization in a wheat–peanut relay intercropping system in China

Agronomically optimizing the timing and rates of nitrogen (N) fertilizer application can increase crop yield and decrease N loss to the environment. Wheat (Triticum aestivum L.)–peanut (Arachis hypogaea L.) relay intercropping systems are a mainstay of economic and food security in China. We performed a field experiment to investigate the effects of N fertilizer on N recovery efficiency, crop yield, and N loss rate in wheat–peanut relay intercropping systems in the Huang-Huai-Hai Plain, China during 2015–2017. The N was applied on the day before sowing, the jointing stage (G30) or the booting stage (G40) of winter wheat, and the anthesis stage (R1) of peanut in the following percentage splits: 50-50-0-0 (N1), 35-35-0-30 (N2), and 35-0-35-30 (N3), using 300 kg N ha−1, with 0 kg N ha−1 (N0) as control. 15N-labeled (20.14 atom %) urea was used to trace the fate of N in microplots. The yields of wheat and peanut increased by 12.4% and 15.4% under the N2 and N3 treatments, relative to those under the N1 treatment. The 15N recovery efficiencies (15NRE) were 64.9% and 58.1% for treatments N2 and N3, significantly greater than that for the N1 treatment (45.3%). The potential N loss rates for the treatments N2 and N3 were 23.7% and 7.0%, significantly lower than that for treatment N1 (30.1%). Withholding N supply until the booting stage (N3) did not reduce the wheat grain yield; however, it increased the N content derived from 15N-labeled urea in peanuts, promoted the distribution of 15N to pods, and ultimately increased pod yields in comparison with those obtained by topdressing N at jointing stage (N2). In comparison with N2, the N uptake and N recovery efficiency (NRE) of N3 was increased by 12.0% and 24.1%, respectively, while the apparent N loss decreased by 16.7%. In conclusion, applying N fertilizer with three splits and delaying topdressing fertilization until G40 of winter wheat increased total grain yields and NRE and reduced N loss. This practice could be an environment-friendly N management strategy for wheat–peanut relay intercropping systems in China.

Preferential flow pathways in paddy rice soils as hot spots for nutrient cycling

Crop rotations with maize and flooded rice lead to temporally aerobic soil conditions. This promotes the development of desiccation cracks in the soil, which can act as preferential flow pathways for water and solutes. We hypothesized that these cracks are enriched with organic carbon (C), plant nutrients and microbial residues (amino sugars) and that they can thus also serve as hot spots of fertilizer and C cycling. To test this hypothesis, we applied 13C-labelled rice straw and 15N-labelled urea to rice fields of the International Rice Research Institute (Los Baños, Philippines). We then traced the fate of the 13C-labelled rice straw and the 15N-labelled urea in crack and bulk soil and in microbial residues in two approaches, i) in the short term, i.e., 24 h after application of straw and fertilizer jointly with a dye tracer (Brilliant Blue) prior to maize seeding in the paddy - maize cropping system, as well as ii) in the long-term, i.e., during one year and for three different crop rotations (continuous paddy rice, paddy rice – maize, and paddy rice – maize with straw mulching and cover cropping). The short-term analyses of the dye tracer depth profiles showed that flow path areas decreased with increasing depth. A typical impermeable plough pan was not identified. Instead, we observed rapid infiltration of irrigation water down to 60 cm soil depth. The dyed flow paths were enriched in organic C (+12%) and plant nutrients (N: +21%, Ca2+: +59%, K+: +39%, Mg2+: +39%) relative to the bulk soil. The labelled straw and fertilizer quickly reached 60 cm depth with the dye tracer. We could not identify elevated microbial biomass along the flow paths, however, we did find larger microbial activities along the cracks in the long-term experiment than in the surrounding bulk soil. The increased activity fostered microbial uptake of fertilizer 15N along the cracks, which was detected mainly for fungal residues and only in the trials receiving straw (crack soil: 0.6 ± 0.1 mg glucosamine-15N kg soil−1, bulk soil: 0.2 ± 0.1 mg glucosamine-15N kg soil−1). We conclude that analysis of homogenized bulk soil samples can underestimate C and nutrient availability, as well as their microbial processing in paddy rice soils, when crack systems are not considered.

Low recovery efficiency of basal fertilizer-N in plants does not indicate high basal fertilizer-N loss from split-applied N in transplanted rice

Increasing amounts of N fertilizer at panicle initiation or heading, and decreasing amounts at earlier stages, is recommended and commonly practiced in parts of China. The practice is based on the higher recovery efficiency (RE) of fertilizer N from later application than at earlier stages. However, this approach does not consider the amount of fertilizer N remaining in the soil (‘residual soil N’) from each application time, and the fact that a considerable amount of the N uptake by the plants, even in heavily-fertilized crops, comes from the soil N. Therefore, the uptake, residual soil N, loss and recovery efficiency of fertilizer N applied to transplanted rice at different growth stages was investigated in a field experiment conducted in 2013 and 2017. Nitrogen was applied as urea at 210 N kg ha−1 in three equal splits at transplanting (basal), tillering and panicle initiation (PI), and 15N-labeled urea was used to trace the change of fertilizer N concentration in the floodwater and soil and the recovery of fertilizer-N in the plants and soil. The fertilizer N recovery efficiency (RE) in the plants was 21–36%, and 15% of the applied 15N remained within the 0–20 cm soil depth at maturity. The RE of N applied at PI (panicle-RE) (33–58%) was greater than the RE of N applied at transplanting (basal-RE) (20–36%) and at tillering (tiller-RE) (13–29%). However, the residual soil N derived from basal fertilizer accounted for more than 90% of the total fertilizer N remaining in the paddy soil at the time of harvest. At harvest time, 70% of the basal-15N, 29% of the tillering-15N and 46% of the panicle-15N, was recovered in the plants and soil, demonstrating that the low basal-RE in the plants does not indicate high basal fertilizer-N loss. Basal-15N still accounted for more than 1% of the total soil N after heading, and all the fertilizer-15N uptake in plants after heading was derived from basal-N.

Heavy metals in slag affect inorganic N dynamics and soil bacterial community structure and function

Heavy metal contamination of soil in the vicinity of mining sites is a serious environmental problem around the world when mining residue (slag) is dispersed as dust. We conducted an incubation experiment to investigate the effect of a slag containing high levels of Pb and Zn (62.2 and 33.6 g kg−1 slag as PbO and ZnO, respectively, sampled from a site formerly used as a lead and zinc mine) on the nitrogen cycle when mixed with soil (0–0.048 g slag g−1 soil). The nitrogen cycle provides many life supporting-functions. To assess the quality of the soil in terms of the nitrogen cycle we focused on the dynamics of nitrate and ammonium, and bacterial community structure and functions within the soil. After two weeks of pre-incubation, 15N-labeled urea (500 mg N kg−1) was added to the soil. Changes in soil pH, the concentration and 15N ratio of nitrate (NO3−-N) and ammonium, and bacterial relative abundance and community structure were measured. Results indicated that increasing the ratio of slag to soil had a stronger negative effect on nitrification than ammonification, as suggested by slower nitrate accumulation rates as the slag:soil ratio increased. In the treatment with the highest amount of slag, the concentration of NO3−-N was 50% of that in the controls at the end of the incubation. Regarding the bacterial community, Firmicutes had a positive and Planctomycetes a negative correlation with increasing slag concentration. Bacterial community functional analysis showed the proportion of bacterial DNA sequences related to nitrogen metabolism was depressed with increasing slag, from 0.68 to 0.65. We concluded that the slag impacted the soil bacterial community structure, and consequently influenced nitrogen dynamics. This study could form the basis of further investigation into the resistance of the nitrogen cycle to contamination in relation to soil bacterial community.