Explore the vast range of titles available.

Soil organic nitrogen (N) is a critical resource for plants and microbes, but the processes that govern its cycle are not well-described. To promote a holistic understanding of soil N dynamics, we need an integrated model that links soil organic matter (SOM) cycling to bioavailable N in both unmanaged and managed landscapes, including agroe-cosystems. We present a framework that unifies recent conceptual advances in our understanding of three critical steps in bioavailable N cycling: organic N (ON) depolymerization and solubilization; bioavailable N sorption and desorption on mineral surfaces; and microbial ON turnover including assimilation, mineralization, and the recycling of microbial products. Consideration of the balance between these processes provides insight into the sources, sinks, and flux rates of bioavailable N. By accounting for interactions among the biological, physical, and chemical controls over ON and its availability to plants and microbes, our conceptual model unifies complex mechanisms of ON transformation in a concrete conceptual framework that is amenable to experimental testing and translates into ideas for new management practices. This framework will allow researchers and practitioners to use common measurements of particulate organic matter (POM) and mineral-associated organic matter (MAOM) to design strategic organic N-cycle interventions that optimize ecosystem productivity and minimize environmental N loss.

Nitrogen is an essential nutrient for plant growth. World-wide, large quantities of nitrogenous fertilizer are applied to ensure maximum crop productivity. However, nitrogen fertilizer application is expensive and negatively affects the environment, and subsequently human health. A strategy to address this problem is the development of crops that are efficient in acquiring and using nitrogen and that can achieve high seed yields with reduced nitrogen input. This review integrates the current knowledge regarding inorganic and organic nitrogen management at the whole-plant level, spanning from nitrogen uptake to remobilization and utilization in source and sink organs. Plant partitioning and transient storage of inorganic and organic nitrogen forms are evaluated, as is how they affect nitrogen availability, metabolism and mobilization. Essential functions of nitrogen transporters in source and sink organs and their importance in regulating nitrogen movement in support of metabolism, and vegetative and reproductive growth are assessed. Finally, we discuss recent advances in plant engineering, demonstrating that nitrogen transporters are effective targets to improve crop productivity and nitrogen use efficiency. While inorganic and organic nitrogen transporters were examined separately in these studies, they provide valuable clues about how to successfully combine approaches for future crop engineering.

AimsThe transformation of the various fractions of soil organic nitrogen (N) plays a pivotal role in soil N retention and in supplying N for crop growth. Soil texture essentially determines the soil’s physical, chemical and biological properties, which affect the soil organic N content and distribution. However, little is known about the incorporation and allocation of fertilizer N among different soil organic N fractions and the fate of fertilizer N in soils with different textures.MethodsThis study was conducted in a long-term experiment (began in 1990) including three soils of different texture (sandy soil, sandy clay loam and loamy clay) in a winter wheat-summer maize cropping system. A 15N-labelling microplot field experiment was carried out to investigate the distribution and seasonal dynamics of fertilizer N among different soil organic N fractions from October 2017 to May 2018.ResultsThe residual amount of fertilizer N in different textural soils were loamy clay > sandy clay loam > sandy soil. More than 74% of the residual fertilizer N existed in the form of acid hydrolyzable nitrogen (AHN). The levels of hydrolyzable ammonium N (HAN) and hydrolyzable unknown N (UHN) derived from fertilizer decreased gradually during the wheat growing season, and the amino acid-15N (AAN-15N) and amino sugar-15N (ASN-15N) levels decreased first and then increased. The average value of ASN-15N in loamy clay and sandy clay loam was 119% and 58% higher than that in sandy soil; the concentrations of AAN-15N and HAN-15N and the N use efficiency were found to be highest in sandy clay loam. On average, the content of acid insoluble N (NHN) derived from fertilizer in loamy clay was 2.4 and 1.9 times those in sandy soil and sandy clay loam, respectively. The structural equation modeling (SEM) revealed that the different concentrations of dissolved organic nitrogen in the fluvo-aquic soils with different textures directly induced changes in the distribution and seasonal dynamics of fertilizer N in the HAN, ASN and UHN pools, further influencing fertilizer N storage in the soil and supply for wheat.ConclusionsLoamy clay had the highest N fertilizer retention capacity of all tested soil types. Sandy clay loam provided more labile fertilizer-derived organic nitrogen and a better environment for roots, thereby increasing the N use efficiency of wheat. The fertilizer N residue in the soils and the fertilizer N supply for wheat were closely related to the concentrations of HAN-15N, ASN-15N and UHN-15N and were affected by the dissolved organic N contents of the soils with different textures.

Stormwater runoff is a leading cause of nitrogen (N) transport to water bodies and hence one means of water quality deterioration. Stormwater runoff was monitored in an urban residential catchment (drainage area: 3.89 hectares) in Florida, United States to investigate the concentrations, forms, and sources of N. Runoff samples were collected over 22 storm events (May to September 2016) at the end of a stormwater pipe that delivers runoff from the catchment to the stormwater pond. Various N forms such as ammonium (NH4-N), nitrate (NOx-N), dissolved organic nitrogen (DON), and particulate organic nitrogen (PON) were determined and isotopic characterization tools were used to infer sources of NO3-N and PON in collected runoff samples. The DON was the dominant N form in runoff (47%) followed by PON (22%), NOx-N (17%), and NH4-N (14%). Three N forms (NOx-N, NH4-N, and PON) were positively correlated with total rainfall and antecedent dry period, suggesting longer dry periods and higher rainfall amounts are significant drivers for transport of these N forms. Whereas DON was positively correlated to only rainfall intensity indicating that higher intensity rain may flush out DON from soils and cause leaching of DON from particulates present in the residential catchment. We discovered, using stable isotopes of NO3-, a shifting pattern of NO3- sources from atmospheric deposition to inorganic N fertilizers in events with higher and longer duration of rainfall. The stable isotopes of PON confirmed that plant material (oak detritus, grass clippings) were the primary sources of PON in stormwater runoff. Our results demonstrate that practices targeting both inorganic and organic N are needed to control N transport from residential catchments to receiving waters.

Background For more than a century, crop N nutrition research has primarily focused on inorganic N (IN) dynamics, building the traditional model that agricultural plants predominantly take up N in the form of NO 3 − and NH 4 + . However, results reported in the ecological and agricultural literature suggest that the traditional model of plant N nutrition is oversimplified. Scope We examine the role of organic N (ON) in plant N nutrition, first by reviewing the historical discoveries by ecologists of plant ON uptake, then by discussing the advancements of key analytical techniques that have furthered the cause (stable isotope and microdialysis techniques). The current state of knowledge on soil ON dynamics is analyzed concurrently with recent developments that show ON uptake and assimilation by agricultural plant species. Lastly, we consider the relationship between ON uptake and nitrogen use efficiency (NUE) in an agricultural context. Conclusions We propose several mechanisms by which ON uptake and assimilation may increase crop NUE, such as by reducing N assimilation costs, promoting root biomass growth, shaping N cycling microbial communities, recapturing exuded N compounds, and aligning the root uptake capacity to the soil N supply in highly fertilized systems. These hypothetical mechanisms should direct future research on the topic. Although the quantitative role remains unknown, ON compounds should be considered as significant contributors to plant N nutrition.

Aims High-yielding maize-based crop systems require maize to take up large quantities of nitrogen over short periods of time. Nitrogen management in conventional crop systems assumes that soil N mineralization alone cannot meet rapid rates of crop N uptake, and thus large pools of inorganic N, typically supplied as fertilizer, are required to meet crop N demand. Net soil N mineralization data support this assumption; net N mineralization rates are typically lower than maize N uptake rates. However, net N mineralization does not fully capture the flux of N from organic to inorganic forms. Gross ammonification may better represent the absolute flux of inorganic N produced by soil N mineralization. Methods Here we utilize a long-term cropping systems experiment in Iowa, USA to compare the peak rate of N accumulation in maize biomass to the rate of inorganic N production through gross ammonification of soil organic N. Results Peak maize N uptake rates averaged 4.4 kg N ha⁻¹ d⁻¹, while gross ammonification rates over the 0–80 cm depth averaged 23 kg N ha⁻¹ d⁻¹. Gross ammonification was highly stratified, with 63% occurring in the 0–20 cm depth and 37% in the 20–80 cm depth. Neither peak maize N uptake rate nor gross ammonification rate differed significantly among three cropping systems with varied rotation lengths and fertilizer inputs. Conclusions Gross ammonification rate was 3.4 to 4.5 times greater than peak maize N uptake across the cropping systems, indicating that inorganic N mineralized from soil organic matter may be able to satisfy a large portion of crop N demand, and that explicit consideration of gross N mineralization may contribute to development of strategies that reduce crop reliance on large soil inorganic N pools that are easily lost to the environment.

Aims The degradation and transformation of soil organic nitrogen (SON) in semi-arid steppe are regulated by a series of enzymes involved in nitrogen(N) hydrolysis, the influence of N and carbon (C) additions on the soil N reserves, activities of N-hydrolyzing enzymes, and their relationships remain unclear. Methods In the Inner Mongolia prairie of China, a field experiment was conducted to study the effects of N (0, 25, 50, 100, 200 kg N ha −1 yr −1 ) and C (0, 250, 500 kg C ha −1 yr −1 ) additions on SON fractions and their relationships with N-hydrolyzing enzymes. Results Our results indicated that N addition significantly increased active-SON and N-acetyl-β-D-glucosaminidase (NAG) activities and decreased urease activities. C addition significantly increased microbial biomass carbon (MBC), NAG, and urease activities, and decreased protease activity and hydrolyzable unknown-N. N and C additions interacted affected the microbial biomass nitrogen (MBN), MBC: MBN, protease, and amidase activities. Structural equation modeling suggested that N addition had a direct positive effect on hydrolyzable NH 4 + -N and amino acid-N. Furthermore, N addition indirectly affected amino sugar-N through MBN and the activities of NAG and protease. C addition directly affected urease activity. Conclusion Our findings suggest that active-SON responded significantly to N addition, whereas stable-SON did not. Moreover, N-hydrolysis enzymes, especially NAG and proteases, play a fundamental role in the N turnover under N and C additions in semi-arid steppe soils. As such, our work provides useful information for the development of sustainable steppe farming practices.

Aims Soil organic nitrogen (N) turnover is significantly influenced by elevated N deposition, precipitation and human-caused disturbances, but the underlying mechanism remains unclear. Identifying the relationships among the soil organic N fractions and N-mineralizing enzymes activities may advance our knowledge of the dynamics of soil organic N. Methods A field experiment was conducted in a semi-arid steppe and an abandoned cropland in northern China to investigate the effects of elevated N deposition and precipitation on soil organic N fractions and their relationships with N-mineralizing enzymes, i.e., protease, amidase, urease and N-acetyl-β-D-glucosaminidase (NAG) activities. Results The concentrations of N in various fractions were consistently lower in the abandoned cropland compared with the steppe. Nitrogen addition consistently decreased amino acid N content and activities of urease, protease and amidase in both sites but increased amino sugar N content and NAG activity in the steppe. Water addition decreased hydrolysable ammonium N content but increased amino sugar N content and activities of protease and NAG in both sites. Furthermore, urease and NAG activities were significantly positively correlated with the proportions of amino acid N and amino sugar N and, explained significant proportions of the variations in soil organic N fractions in the steppe. However, soil organic carbon (C), rather than N-mineralizing enzymes, explained greatest proportion of the variations in soil organic N fractions in the abandoned cropland. Conclusions The concurrent increase of N deposition and precipitation could promote the recovery of soil N (and C) losses in the abandoned cropland resulting from previous agriculture. Furthermore, in the steppe where NH4+ was available at relative high concentrations, enzymatic mineralization was the dominant route involved in potential soil organic N turnover. However, the direct route may be favored over the enzymatic mineralization route with decreasing availability of C relative to N in the abandoned cropland, which is driven by the need for C. These findings confirmed that the forms of N available, and the relative availability of C and N determine N uptake pathways both through enzymatic mineralization route and direct uptake route in the semi-arid grasslands.

Heterotrophic nitrification is regarded as an eternal mystery in the nitrogen (N) cycle, although it was first reported more than 100 years ago. In this review, we discuss microbial mechanisms driving heterotrophic nitrification of organic N (O HORG ) in soil and their modulations by pH, and carbon (C) and N contents. In acidic and oligotrophic soils, O HORG may occur as endogenous respiration or during oxidation of recalcitrant organics and cell lysis. Likely in soils with a low C:N ratio and a high pH, C limitation of N immobilization creates conditions for the transformation of dissolved organic N to nitrate (NO 3 − ). Fungi with a deficiency in ammonia mono-oxygenase can drive O HORG . Heterotrophic nitrifiers include not only acid-tolerant, nitrophobic species, but also acid-sensitive, nitrotolerant species. There is concern about O HORG in soil because of its contribution to NO 3 − production and nitrous oxide (N 2 O) emissions. Therefore, reliable measurements of soil heterotrophic nitrifying activity are needed. We propose more biochemical surveys to understand the pathway for O HORG in ecologically relevant species, considering the challenges we are facing in managing heterotrophically derived nitrification to prevent N losses from cropland, forest, and grassland soils.

Arctic river watersheds are important components of the global climate system and show an amplified response to climate change. Here, we characterize origins and transformations of dissolved organic matter (DOM) in five major Arctic rivers (Kolyma, Lena, Yenisei, Ob, Mackenzie) over 3 years with seasonal sampling periods using measurements of carbohydrates, amino acids, bacterial biomarkers (D-amino acids), and plant protein biomarkers (hydroxyproline). A strong seasonal cycle of bioavailable DOM export was observed that correlated with discharge, vegetation, river morphology and water residence time. The chemical composition of bioavailable DOM was different among rivers reflecting unique characteristics of Arctic river watersheds. Trends in specific bacterial biomarkers were synchronous to changes in bacterial community compositions demonstrating that bacterial communities responded to the seasonal shifts in organic matter quality and chemical composition. Extensive heterotrophic processing of plant and soil-derived DOM resulted in major inputs of bacterial detritus, and bacterial organic matter accounted for 21–42% of DOC in all watersheds. Dissolved organic nitrogen sources were dominated by bacterially-derived nitrogen and important contributions of soluble plant protein during the Spring freshet. Overall, our results demonstrated the importance of watershed characteristics and bacterial metabolism in regulating DOM composition, reactivity and carbon fluxes in Arctic river watersheds.