Nitrogen, Volume 3, Issue 4 (December 2022) – 8 articles

Numerous biological and chemical methods have been proposed over the years for estimating the nitrogen (N) mineralization capacity of soils; however, none of them has found general use in soil fertility testing. The efficacy of a recently proposed alkaline hydrolysis method for assessing N availability in soils compared with the standard long-term incubation technique for determining potentially available N was evaluated. The nitrogen mineralization of 12 surface soils incubated under aerobic conditions at 25 °C for 26 weeks was determined. Field-moist soils were direct-steam distilled with 1 M KOH or 1 M NaOH; the NH₃ released was trapped in boric acid, and its concentration was determined successively every 5 min for 40 min. The cumulative N mineralized or hydrolyzed was fitted to the first-order exponential equation to determine the potentially mineralizable N (N_o) and an analogous “potentially hydrolyzable N (N_max)” for the soils. The flush of CO₂ (fCO₂) following the rewetting and incubation of air-dried soils under aerobic conditions for 3 days was also determined. The results showed that the N_max values differed considerably among the soils, indicating differences in the chemical nature and reactivity of the organic N content of the soils, and were significantly correlated with N_o and fCO₂ values. The estimated N_max and N_o values ranged from 105 to 371 mg N kg⁻¹ and 121 to 292 mg kg⁻¹, respectively. Based on the simple and inexpensive nature of the alkaline hydrolysis procedure, the reduction in the incubation time required to obtain N_o (months to minutes), and the strong association between N_max and N_o, we concluded that N_max is a good predictor of the biologically discrete and quantifiable labile pool of mineralizable soil organic N (ON), and the use of the alkaline hydrolyzable ON as a predictor of N_o merits consideration for routine use in soil testing laboratories for estimating the N-supplying capacity of soils. Full article

Nitrate concentration is variable in soils, so the absorbed N from roots in a high-nitrate site is recycled from shoots to the root parts in N-poor niche. In this report, the absorption, transport, and recycling of N derived from ¹⁵N-labeled nitrate were investigated with split-root systems of nodulated soybean. The NO₃⁻ accumulated in the root in 5 mM NO₃⁻ solution; however, it was not detected in the roots and nodules in an N-free pot, indicating that NO₃⁻ itself is not recycled from leaves to underground parts. The total amount of ¹⁵NO₃⁻ absorption from 2 to 4 days of the plant with the N-free opposite half-root accelerated by 40% compared with both half-roots that received NO₃⁻. This result might be due to the compensation for the N demand under one half-root could absorb NO₃⁻. About 2–3% of the absorbed ¹⁵N was recycled to the opposite half-root, irrespective of N-free or NO₃⁻ solution, suggesting that N recycling from leaves to the roots was not affected by the presence or absence of NO₃⁻. Concentrations of asparagine increased in the half-roots supplied with NO₃⁻ but not in N-free half-roots, suggesting that asparagine may not be a systemic signal for N status. Full article

In a split N-application system, the objective was to quantify N/¹⁵N in gluten and non-gluten proteins after the late application of 30 or 60 kg N, whereby 10% of the third split was applied as ¹⁵N. This fertilization was combined with a reduced water supply for 21 days (well-watered (ww); water deficit (wd)). German spring wheat cultivars, Elite wheat Taifun, Quality wheat Monsun and cultivars from the Mediterranean territory, Golia, Gönen, were examined. The protein content in gluten was for 30 kg N, ww, similar for Taifun, Golia, and Gönen, but markedly lower in Monsun (231, 245, 247, 194 mg protein/g DM). The water deficit increased the protein content in the gluten of Golia and Gönen and was higher than that of Taifun and Monsun (297, 257, 249, 202 mg protein/g DM). Fertilization of 60 kg N, ww, did not result in any change in the protein content in gluten and differences between the cultivars were not detectable. The ¹⁵N protein in gluten was for 30 kg N, ww, markedly higher in Gönen (2.32 mg ¹⁵N protein/g DM), compared to Golia and Monsun (1.93, 1.50 mg ¹⁵N protein/g DM), and similar in Taifun (1.64 mg ¹⁵N protein/g DM). ¹⁵N fertilizer uptake into gluten was stimulated by water deficit for 30 and 60 kg N, leading to significantly increased ¹⁵N protein in Golia and Gönen, (2.38, 2.99, 4.34, 5.87 mg ¹⁵N protein/g DM). Fertilization of 60 kg N led to a proportional two-time increase in the ¹⁵N gluten protein of the four cultivars, in ww and wd plants. Assessed on the basis of ¹⁵N fertilizer allocation under wd conditions into gluten proteins, Golia and Gönen have a stronger sink activity, compared to Taifun and Monsun. Full article

Ice–rich Pleistocene permafrost deposits (Yedoma) store large amounts of nitrogen (N) and are susceptible to rapid thaw. In this study, we assess whether eroding Yedoma deposits are potential sources of N and gaseous carbon (C) losses. Therefore, we determined aerobic net ammonification and nitrification, as well as anaerobic production of nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄) in laboratory incubations. Samples were collected from non-vegetated and revegetated slump floor (SF) and thaw mound (TM) soils of a retrogressive thaw slump in the Lena River Delta of Eastern Siberia. We found high nitrate concentrations (up to 110 µg N (g DW)⁻¹) within the growing season, a faster transformation of organic N to nitrate, and high N₂O production (up to 217 ng N₂O-N (g DW)⁻¹ day⁻¹) in revegetated thaw mounds. The slump floor was low in nitrate and did not produce N₂O under anaerobic conditions, but produced the most CO₂ (up to 7 µg CO₂-C (g DW)⁻¹ day⁻¹) and CH₄ (up to 65 ng CH₄-C (g DW)⁻¹ day⁻¹). Nitrate additions showed that denitrification was substrate limited in the slump floor. Nitrate limitation was rather caused by field conditions (moisture, pH) than by microbial functional limitation since nitrification rates were positive under laboratory conditions. Our results emphasize the relevance of considering landscape processes, geomorphology, and soil origin in order to identify hotspots of high N availability, as well as C and N losses. High N availability is likely to have an impact on carbon cycling, but to what extent needs further investigation. Full article

Current understanding of nitrogen (N) mineralization from organic soil inputs considers three alternative processes: immediate net mineralization of N, net immobilization followed by net mineralization, or exclusively net immobilization. The three processes are compatible and linked with the C:N ratio rule. However, research evidence from a number of incubation studies incorporating processed materials like manures, composts, manure composts, or already decomposed plant residues suggest the presence of a second N immobilization phase. The mechanisms and conditions of this process, which is against the prevailing theory of soil N cycling, have not been ascertained, but they should most likely be attributed to impeded dead microbial biomass turnover. The transfer of mineral forms of N to the organic N pool may reasonably be explained by the chemical stabilization of nitrogenous compounds with secondary products of lignin degradation, which occurs late after incorporation of an organic input in soil. Secondary immobilization questions the reliability of the C:N ratio and most likely of other quality indices if proved to be real, even to some extent, while it may also have significant consequences on the management of soil organic additives applied as fertilizers. Full article

Legume cover crops in temperate cropping systems can fix substantial amounts of nitrogen (N) and reduce N fertiliser requirements for subsequent crops. However, little is known about potential biological N₂ fixation by summer cover crop legumes in the short summer fallow in Mediterranean-type cropping systems. Six legume species (balansa clover, barrel medic, mung bean, sunn hemp, lablab and cowpea) were grown for 8–9 weeks in the field in semi-arid southern Australia during the summer fallow, and in a glasshouse experiment, to estimate N₂ fixation using the ¹⁵N natural abundance method. Cowpea, sunn hemp and lablab produced 1.2–3.0 t ha⁻¹ biomass in the field while balansa clover and barrel medic produced < 1.0 t ha⁻¹. The percent of N derived from the atmosphere (%Ndfa) in the field ranged from 39% in barrel medic to 73% in sunn hemp, but only 15% (balansa clover) to 33% (sunn hemp) in the glasshouse experiment, likely due to higher soil mineral N availability in the glasshouse study. Biological N₂ fixation of cowpea and sunn hemp in the field was 46–55 kg N ha⁻¹, while N₂ fixation in lablab and mung bean was lower (around 26 kg N ha⁻¹). The N₂ fixation in cowpea and sunn hemp of around 50 kg N ha⁻¹ with supplementary irrigation in the field trial likely represents the upper limit of N contributions in the field in typically hot, dry summer conditions in Mediterranean-type climates. Given that any increase in summer cover crop biomass will have implications for water balances and subsequent cash crop growth, maximising N benefits of legume cover crops will rely on increasing the %Ndfa through improved rhizobium strains or inoculation technologies. This study provides the first known estimates of biological N₂ fixation by legume cover crops in the summer fallow period in cropping systems in Mediterranean-type environments, providing a benchmark for further studies. Full article

The San Francisco Bay Delta has been an estuary of low productivity, with causes hypothesized to relate to light limitation, grazing by invasive clams, and polluting levels of NH₄⁺ discharge from a wastewater treatment plant. Suppression of phytoplankton NO₃⁻ uptake by NH₄⁺ has been well documented, and thus this estuary may have experienced the counterintuitive effect of depressed productivity due to wastewater NH₄⁺ enrichment. In 2021, a new wastewater treatment plant came online, with a ~75% reduction in nitrogen load, and within-plant nitrification, converting the discharge to NO₃⁻. The expectation was that this change in nitrogen loading would support healthier phytoplankton production, particularly of diatoms. Here, responses of the post-upgrade Bay Delta phytoplankton were compared to five years of data collected pre-upgrade during the fall season. Indeed, increased chlorophyll a accumulation in the estuary was documented after the implementation of the upgraded wastewater treatment and photophysiological responses indicated comparatively less stress. Major differences in river flow were also observed due to drought conditions during the decade covered by this study. While short-term favorable effects were observed, understanding longer-term ecological feedback interactions that may follow from this major nutrient change under variable flow conditions will require more years of observations. Full article

Maize (Zea mays L.) is a crop widely cultivated in the state of São Paulo, and the sustainable management of nitrogen (N) nutrition is crucial to improving productivity and the environment, which calls for a reliable means of predicting potentially available soil N. A study was undertaken to evaluate and compare biological and chemical indices of potential N availability for a diverse set of 17 soils collected in the northwest region of São Paulo state. For this purpose, mineralization assays were performed at three distinct temperatures, and chemical assessments were carried out using the Illinois Soil Nitrogen Test (ISNT) and by fractionation of hydrolysable soil N. In addition, a greenhouse experiment was conducted to determine dry matter and N accumulation in the aboveground parts of maize plants. Potentially available N estimated by the incubation methods increased with increasing temperature and was strongly correlated with N uptake (r = 0.90). Hydrolysable N fractions varied widely among the soils studied and were more variable for amino sugar N than for other fractions. Potentially available N estimated by the ISNT was highly correlated with hydrolysable amino acid N and amino sugar N (r = 0.95–0.96) and also with plant dry matter accumulation (r = 0.82) and N uptake (r = 0.93). The ISNT has potential to improve fertilizer N recommendations for maize production in Brazil, provided that the test values are interpreted relative to an appropriate calibration database, planting density, and other factors affecting crop N requirement. Full article