Nitrogen—Inorganic Forms

Most soils contain inorganic nitrogen (N) in the form of ammonium (NHt) and nitrate (NO)"). Nitrite (NOz) also may be present, but the amount is usually too small to warrant its determination, except in cases where NHt or NHt-forming fertilizers are applied to neutral or alkaline soils. Several other forms of inorganic N have been proposed as intermediates during microbial transformations of N in soils, including hydroxylamine (NH20H), hyponitrous acid (H2N20 2), and nitramide (NH2N02), but these compounds are thermodynamically unstable and have not been detected in soil. Until the 1950s, inorganic N was believed to account for <2% of total soil N, on the assumption that NHt and NO)" are completely recovered by extracting soil with a neutral salt solution. The validity of this assumption was challenged by the finding that some soils contain NHt in a form that is not extracted by exchange with other cations (e.g., Rodrigues, 1954; Dhariwal & Stevenson, 1958; Stevenson & Dhariwal, 1959; Bremner & Harada, 1959; Bremner, 1959; Schachtschabel, 1960, 1961; Young, 1962), and by estimates that the proportion of soil N in this form can exceed 50% for some subsurface soils (Stevenson & Dhariwal, 1959; Young, 1962). In such cases, NHt is said to be fixed, and fixed NHt has subsequently been defined as the NHt in soil that cannot be replaced by a neutral potassium salt solution (SSSA, 1987), such as 1 or 2 M KCI or 0.5 M K2S04, in contrast to exchangeable NHt, which is extractable at room temperature with such a solution. Existing information indicates that fixed NHt occurs largely, if not entirely, between the layers of 2: I-type clay minerals, particularly vermiculite and illite (hydrous mica), and that fixation results from entrapment of NHt in ditrigonal voids in the exposed surfaces upon contraction of the clay lattice (Nommik & Vahtras, 1982). The term, nonexchangeable NHt, has been used by Bremner (1965) and Keeney and Nelson (1982) in previous editions of this publication as a more precise alternative to fixed NHt. The same term is used in the present treatment, with specific reference to NHt determined by the method described in "Determination of Nonexchangeable Ammonium," which involves digestion with an HF-HCI solution following treatment of the soil with alkaline KOBr to remove exchangeable NHt and labile organic-N compounds.

Nitrogen - inorganic forms.

Currently available microbiological techniques are not designed to deal with very slowly growing microorganisms. The enrichment and study of such organisms demands a novel experimental approach. In the present investigation, the sequencing batch reactor (SBR) was applied and optimized for the enrichment and quantitative study of a very slowly growing microbial community which oxidizes ammonium anaerobically. The SBR was shown to be a powerful experimental set-up with the following strong points: (1) efficient biomass retention, (2) a homogeneous distribution of substrates, products and biomass aggregates over the reactor, (3) reliable operation for more than 1 year, and (4) stable conditions under substrate-limiting conditions. Together, these points made possible for the first time the determination of several important physiological parameters such as the biomass yield (0.066 ± 0.01 C-mol/mol ammonium), the maximum specific ammonium consumption rate (45 ± 5 nmol/mg protein/min) and the maximum specific growth rate (0.0027 · h−1, doubling time 11 days). In addition, the persisting stable and strongly selective conditions of the SBR led to a high degree of enrichment (74% of the desired microorganism). This study has demonstrated that the SBR is a powerful tool compared to other techniques used in the past. We suggest that the SBR could be used for the enrichment and quantitative study of a large number of slowly growing microorganisms that are currently out of reach for microbiological research.

The sequencing batch reactor as a powerful tool for the study of slowly growing anaerobic ammonium-oxidizing microorganisms

Changes in specific leaf area (SLA, projected leaf area per unit leaf dry mass) and nitrogen partitioning between proteins within leaves occur during the acclimation of plants to their growth irradiance. In this paper, the relative importance of both of these changes in maximizing carbon gain is quantified. Photosynthesis, SLA and nitrogen partitioning within leaves was determined from 10 dicotyledonous C 3 species grown in photon irradiances of 200 and 1000 μ mol m - 2 s - 1 . Photosynthetic rate per unit leaf area measured under the growth irradiance was, on average, three times higher for high-light-grown plants than for those grown under low light, and two times higher when measured near light saturation. However, light-saturated photosynthetic rate per unit leaf dry mass was unaltered by growth irradiance because low-light plants had double the SLA. Nitrogen concentrations per unit leaf mass were constant between the two light treatments, but plants grown in low light partitioned a larger fraction of leaf nitrogen into light harvesting. Leaf absorptance was curvilinearly related to chlorophyll content and independent of SLA. Daily photosynthesis per unit leaf dry mass under low-light conditions was much more responsive to changes in SLA than to nitrogen partitioning. Under high light, sensitivity to nitrogen partitioning increased, but changes in SLA were still more important.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.2001.00724.x

Photosynthetic acclimation of plants to growth irradiance: the relative importance of specific leaf area and nitrogen partitioning in maximizing carbon gain

Plant analysis (sometimes referred to as leaf analysis) is the determination of the total elemental content of a specified plant part. The emphasis in this chapter will be on the determination of those elements required for plant growth. Interpretation is normally based on the use of a "critical value" or "sufficiency range" (Smith, 1962) comparison between the elemental concentration found and a known norm (Goodall & Gregory, 1947; Chapman, 1966; Reuter & Robinson, 1986; Adriano, 1986; Martin-Prevel et al., 1987). An alternative method of interpretation is Diagnosis and Recommendation Integrated System (ORIS), which interprets the ratios of elements (N/P, K/Ca, and K/Mg) as indicators of elemental status (Beaufils, 1973; Sumner, 1977, 1982). Most growers primarily use a plant analysis for diagnosing suspected elemental insufficiencies, while its most significant, yet little used application, is for evaluating the soil/plant elemental status. This is partially reflected in the relatively few plant tissue samples assayed for growers, about 500 000, in the USA each year (Jones, 1985). Tissue testing, an elemental assay of extracted cell sap by means of quick chemical tests in the field, seems to be gaining an interest at levels equal to that observed several decades ago. A plant analysis is carried out in a series of steps as shown in Fig. 15-1. The results obtained are no better than the care taken in collecting, handling, preparing, and analyzing the collected tissue. An error made in one of these steps can result in an erroneous interpretation leading to recommendations that may be either unnecessary, costly, or even damaging to the crop. Therefore, it is important for those employing either a plant analysis or tissue test to follow the proper sampling, preparation, and analysis procedures. This chapter deals with the procedures required to successfully conduct a plant analysis or tissue test.

Sampling, handling and analyzing plant tissue samples.

An analysis is described for the rapid determination of nitrate‐N in plant extracts. The complex formed by nitration of salicylic acid under highly acidic conditions absorbs maximally at 410 nm in basic (pH>12) solutions. Absorbance of the chromophore is directly proportional to the amount of nitrate‐N present. Ammonium, nitrite, and chloride ions do not interfere.

L. E. Schrader

Papers

Rapid colorimetric determination of nitrate in plant tissue by nitration of salicylic acid

Analysis by Digestion and Colorimetric Assay of Total Nitrogen in Plant Tissues High in Nitrate1