Nitrogen, Volume 1, Issue 2 (December 2020) – 7 articles

Nitrogen (N) cycling in mangroves is complex, with rapid turnover of low dissolved N concentrations, but slow turnover of particulate N. Most N is stored in soils. The largest sources of N are nearly equal amounts of mangrove and benthic microalgal primary production. Dissolved N fluxes between the forests and tidal waters show net uptake, indicating N conservation. N₂-fixation is underestimated as rapid rates measured on tree stems, aboveground roots and cyanobacterial mats cannot currently be accounted for at the whole-forest scale due to their extreme patchiness and the inability to extrapolate beyond a localized area. Net immobilization of NH₄⁺ is the largest ecosystem flux, indicating N retention. Denitrification is the largest loss of N, equating to 35% of total N input. Burial equates to about 29% of total inputs and is the second largest loss of N. Total inputs slightly exceed total outputs, currently suggesting net N balance in mangroves. Mangrove PON export equates to ≈95% of PON export from the world’s tropical rivers, but only 1.5% of the entire world’s river discharge. Mangrove N₂O emissions, denitrification, and burial contribute 0.4%, 0.5–2.0% and 6%, respectively, to the global coastal ocean, which are disproportionate to their small worldwide area. Full article

Upland rice (Oryza sativa L.) production systems in sub-Saharan Africa are faced with challenges of water stress and nitrogen (N) deficiency, which reduce grain yield, water use efficiency (WUE), and nitrogen use efficiency (NUE). The objective of the study was to determine the response of upland rice to N fertilizer rates under well-watered conditions and to clarify the relationships between WUE and NUE. Upland rice variety Nerica 10 was grown under well-watered conditions and varying N rates in 2014/2015 (Y₁) and 2015/2016 (Y₂) on the same field at the University of Pretoria’s Hatfield Experimental Farm, South Africa. Yields at harvest increased with increasing N rates, linearly in Y₁, and following a quadratic trend in Y₂. Mean grain yield was highest (4.5 t ha⁻¹) at 120 kg N ha⁻¹ and lowest (2.4 t ha⁻¹) at 0 kg N ha⁻¹. Agronomic NUE was lowest for 160 kg N ha⁻¹ (11.7 kg kg⁻¹ N), while WUE was highest for the 120 kg N ha⁻¹ (7.58 ± 1.7 kg mm⁻¹) and lowest for 0 kg N ha⁻¹ (4.1 ± 0.9 kg mm⁻¹). Findings revealed that at high N levels, compensative N uptake during tillering can reduce harvest index, WUE, and to a lesser extent, grain N concentration. Full article

Vegetable crop production, which is expanding worldwide, is managed extremely intensively and is therefore raising concerns about soil degradation. The objective of this study was to analyze the impact of using rye mulch as a conservation practice on nutrient availability for lettuce grown in histosols. The rye cover crop was established in the fall of 2018 at two cultivated peatland sites. The following summer, lettuce crops were planted at both sites on the rye mulch cover and on control plots. Lysimeters were used to extract the soil solution once a week during lettuce growth. Various soil properties were analyzed in the soil sampled at the end of the lettuce growing season. The rye yield was higher at site 1 than at site 2 and the lettuce growth was reduced at site 1 under the rye mulch treatment. The rye mulch reduced mineral N and dissolved organic N availability at both sites. The N dynamics in histosols might be fast enough to supply the lettuce needs; however, the implantation difficulties must first be overcome to confirm that hypothesis. At the end of the lettuce growth period, soil total and active C pools and soluble organic soil N in the rye mulch treatment sample were significantly higher at site 1 than at site 2. The presence of rye mulch improved the carbon pool over a single growing season. The use of rye mulch as a soil conservation practice for vegetable crop production appears promising for histosols; however, more work is needed to gain a better understanding on the long-term effects of decomposing rye mulch and roots on soil nutrient availability, soil health and C sequestration, and on the nitrogen uptake pathways and growth of cash crops. Future works which would include consecutive years of study at multiple sites are also needed to be able to confirm and generalize the observations found in the present work. Full article

Layered double hydroxides (LDH) are anionic clays that have potential as slow-release fertilizers; however, their formulation as powders makes them difficult to apply, and their slow-release properties are impaired due to instability under acidic conditions. In the work reported, Zn-Al LDH containing interlayered ¹⁵NO₃⁻ was synthesized for use as powder (LDH-N) or for encapsulation in alginate beads (LDH-AN), and then authenticated by X-ray diffraction, attenuated total reflectance-Fourier transform infrared spectroscopy, and elemental analyses. The two LDHs were compared to K¹⁵NO₃ for evaluating their slow-release properties through (i) a kinetic study of NO₃⁻ release in water under dynamic conditions, and (ii) a growth chamber experiment designed to estimate fertilizer N uptake efficiency (FNUE) by growing pearl millet (Pennisetum glaucum L.) on an acidic Oxisol in the absence of N losses. Both LDH materials exhibited slow-release properties in the kinetic studies, and NO₃⁻ release was reduced for LDH-AN as compared to LDH-N. Because of these properties, FNUE measurements in the growth chamber experiment should have been lower with the LDHs than with K¹⁵NO₃, but this was not the case for LDH-N, which was attributed to the structural instability of powdered LDH in the presence of soil acidity and to the exchange of NO₃⁻ by more competitive anions such as CO₃²⁻. A significant decrease in FNUE was observed for LDH-AN, demonstrating retention of slow-release behavior that most likely resulted from the presence of a physicochemical barrier having high cation-exchange and buffering capacities while limiting exposure to soil acidity and anion exchange. Alginate encapsulation expands the practical potential of LDH for slow-release NO₃⁻ fertilization. Full article

Plants that enter symbiotic relationships with nitrogen (N)-fixing microbes contribute some of their N to the community through leaf litter decomposition and mineralization processes. The speed of these processes varies greatly among tree species. Mesocosm methods were used to determine the speed of N and carbon (C) release from Cycas micronesica, Intsia bijuga, and Serianthes nelsonii leaf litter. Microcosm methods were used to determine soil respiration traits in soils containing the leaf litter. The speed of leaf litter N and C release during decomposition occurred in the order C. micronesica < I. bijuga < S. nelsonii. Soil carbon dioxide efflux was increased by adding leaf litter to incubation soils, and the increase was greatest for S. nelsonii and least for C. micronesica litter. Ammonium, nitrate, total N, organic C, and total C were increased by adding litter to incubation soils, and the differences among the species converged with incubation duration. The rate of increases in available N and decreases in organic C were greatest for S. nelsonii and least for C. micronesica litter. These findings indicate that S. nelsonii litter released N and C rapidly, C. micronesica litter released N and C slowly, and the leaf economic spectrum accurately predicted the differences. Full article

In no-tillage systems, there is an accumulation of crop residues (CR), which plays an essential role in the availability of soil-N. A study was set up to provide information regarding the N credit and the influence of N mineral fertilizer. There was the addition of a similar rate of residue (10 Mg ha⁻¹; sugarcane, soybean, and brachiaria) and N mineral fertilizer (urea; 120 kg N ha⁻¹) in loam soil. After the stabilization of biological activity (73 days), soil and remaining residues were collected, and C and N monitored. The results showed that the N credit was positive with the application of soybean, sugarcane, and brachiaria. There was a positive balance of the soybean N credit in soil with a reduction from 2.49 to 0.90 g kg⁻¹ of N in remaining residue, and a direct increase of 90% of soil-N. There is no need of N fertilizer to potentialize the soybean N credit, but it is required to potentialize N credit of brachiaria and sugarcane. The urea demonstrated to be an excellent enhancer of brachiaria N credit, but it was not adequate for sugarcane residues. Based on our result, the accumulation and incorporation of CR can be considered as N credit with a positive contribution in soil-N. Full article

Rhizobia have two major life styles, one as free-living bacteria in the soil, and the other as bacteroids within the root/stem nodules of host legumes where they convert atmospheric nitrogen into ammonia. In the soil, rhizobia have to cope with changing and sometimes stressful environmental conditions, such as nitrogen limitation. In the beta-rhizobial strain Paraburkholderia phymatum STM815, the alternative sigma factor σ⁵⁴ (or RpoN) has recently been shown to control nitrogenase activity during symbiosis with Phaseolus vulgaris. In this study, we determined P. phymatum’s σ⁵⁴ regulon under nitrogen-limited free-living conditions. Among the genes significantly downregulated in the absence of σ⁵⁴, we found a C₄-dicarboxylate carrier protein (Bphy_0225), a flagellar biosynthesis cluster (Bphy_2926-64), and one of the two type VI secretion systems (T6SS-b) present in the P. phymatum STM815 genome (Bphy_5978-97). A defined σ⁵⁴ mutant was unable to grow on C₄ dicarboxylates as sole carbon source and was less motile compared to the wild-type strain. Both defects could be complemented by introducing rpoNin trans. Using promoter reporter gene fusions, we also confirmed that the expression of the T6SS-b cluster is regulated by σ⁵⁴. Accordingly, we show that σ⁵⁴ affects in vitro competitiveness of P. phymatum STM815 against Paraburkholderia diazotrophica. Full article