http://utsc.utoronto.ca/%7Ebritto/publications/amtox.pdf

NH4+ toxicity in higher plants: a critical review

MORE than half of all tropical soils are highly weathered, leached and impoverished, requiring the ecosystem to develop nutrient-conserving mechanisms1,2. Nutrient retention and withdrawal mechanisms are most effective in nutrient-poor systems3,4. Thus, although dry tropical forests and savanna have the potential capacity to grow at high rates5,6, this capacity is strictly limited by climate and nutrients. Our studies on these nutrient-poor ecosystems show that a reciprocal relationship exists between microbial biomass and plant growth rate. This suggests that microbial immobilization may be a main source of nutrients for the plants and may lead to nutrient conservation.

Microbial biomass acts as a source of plant nutrients in dry tropical forest and savanna

(I) Inorganic components . . . . . . . (a) Nitrate . . . . . . . . . (b) Cation-anionuptake . . . . . . (2) Organic compounds . . . . . . . (a) Amino acids and other organic nitrogenous compounds (b) Organic acids . . . . . . . . (3) Effects on flowering . . . . . . . (4) Ammonium plus nitrate as a nitrogen source . . . ( 5 ) Metabolic changes characteristic of ammonium toxicity . VII . Summary . . . . . . . . . . VI . Physiological effects of nitrogen sources . . . .

Ammonium and nitrate nutrition of plants

The availability of nitrogen (N) limits production in many forest ecosystems, and many methods are available for estimating N availability (Keeney, 1980; Binkley 1986; Mahendrappa et al., 1986; Binkley and Vitousek, 1989). The concept of soil availability may represent the rate at which N is converted from unavailable to available forms within the rooting zone (Scarsbrook, 1965). Alternatively, it may refer to the extent to which plant production is constrained by a limited supply of available N. These two aspects of N availability were termed N supply rate and N limitation by Chapin et al. (1986). In agroecosystems, N supply rate and N limitation are often closely linked. In forest ecosystems, differences in species composition, stand age, and soil moisture may uncouple low N supply from N limitation (Chapin et al., 1986). In addition, the nonuniform rooting distribution of trees and the presence of forest floors add spatial complexities to forest N cycles that make it more difficult to estimate N availability in forests than in agroecosystems.

The Components of Nitrogen Availability Assessments in Forest Soils

Soil architecture is the dominant control over microbially mediated decomposition processes in terrestrial ecosystems. Organic matter is physically protected in soil so that large amounts of well-decomposable compounds can be found in the vicinity of largely starving microbial populations. Among the mechanisms proposed to explain the phenomena of physical protection in soil are adsorption of organics on inorganic clay surfaces and entrapment of materials in aggregates or in places inaccessible to microbes. Indirect evidence for the existence of physical protection in soil is provided by the occurrence of a burst of microbial activity and related increased decomposition rates following disruption of soil structures, either by natural processes such as the remoistening of a dried soil or by human activities such as ploughing. In contrast, soil compaction has only little effect on the transformation of 14C-glucose. Another mechanism of control by soil structure and texture on decomposition in terrestrial ecosystems is through their impact on microbial turnover processes. The microbial population is not only the main biological agent of decomposition in soil, it is also an important, albeit small, pool through which most of the organic matter in soil passes. Estimates on the relative importance of different mechanisms controlling decomposition in soil could be derived from results of combined tracer and modelling studies. However, suitable methodology to quantify the relation between soil structure and biological processes as a function of different types and conditions of soils is still lacking.

Soil structural aspects of decomposition of organic matter by micro-organisms

Nitrogen nutrition has great influences not only on the yields, but also on the chemical components of the plants. Nitrate nitrogen is generally found to be superior to ammonium nitrogen as the nitrogen source for higher plants (1). As reported in the previous paper (2), young plants of sugar beet grown with the culture solution containing a high level of ammonium nitrogen developed the symptom of ammonia toxicity. To make clear the mechanism of ammonia toxicity, comparative studies on the absorption and the assimilation of ammonium- and nitrate-nitrogen in higher plants at young stages of growth were carried out.

Effect of nitrogen sources on the chemical components in young plants

Usually, immobilization and release of nitrogen in soil are going on continuously and concurrently, whereby the nitrogen of the decomposing system is transformed steadily from inorganic to organic state by immobilization and back from organic to inorganic state by decay and mineralization. The driving agents of this turnover are soil microorganisms, C/N ratio, temperature, moisture, pH etc., and the energy needed to keep this cycle running derives from decomposable organic compounds added to soil in the forms of plant residues and excretions of roots and stored in the form of soil organic matter.

https://www.tandfonline.com/doi/pdf/10.1080/00380768.1969.10432803

FACTORS AFFECTING IMMOBILIZATION AND RELEASE OF NITROGEN IN SOIL AND CHEMICAL CHARACTERISTICS OF THE NITROGEN NEWLY IMMOBILIZED : I. Effect of Temperature on Immobilization and Release of Nitrogen in Soil

The immobilization and release process of nitrogen and the chemical characteristics of newly immobilized-, subsequently released-, and residual-N were studied in soils receiving KNO3 labelled with 15N together with glucose, straw, and cellulose. Immobilization of nitrogen proceeded rapidly and reached its maximum at incubation periods of 3 days to 8 weeks or more, varying with the kind of carbon sources added. Following the maximum tie-up of nitrogen a rapid release began. After 20 weeks the immobilized nitrogen was released at a considerable rate for each carbon source. At the period of maximum immobilization of nitrogen added to soil there was a clear difference in the percentage distribution of various forms of organic nitrogen between applied-N and native-N. The former was higher in amino acid N and unidentified N and lower in nonhydrolyzable N than the latter, and almost the same in hexosamine N and ammonium N as the latter. With regard to all carbon sources the principal form of organic nit...

https://www.tandfonline.com/doi/pdf/10.1080/00380768.1973.10432597

Factors affecting immobilization and release of nitrogen in soil and chemical characteristics of the nitrogen newly immobilized III. Transformation of the nitrogen immobilized in soil and its chemical characteristics

1) The mineralization of several kinds of microbial cells added to soil were accelerated considerably by the drying effect. 2) When microbial cells were roughly divided by mechanical procedure into two parts, i.e., cytoplasmic and cell wall substances, and separately added to soil with or without drying previously, the former was mineralized very quickly both with and without drying previously and its mineralization was not accelerated by the drying effect. The latter without drying previously was mineralized rather slowly, and the latter with drying previously was mineralized very quickly and remarkably. Furthermore, the former with and without drying previously left hardly any residual matter in soil, but the latter without drying previously left considerable residual matter because of making a complex resistant to microbial decomposition with colloid materials such as clay minerals and humus, and mineralization of the residual matter was remarkably accelerated by the drying effect. 3) From the...

http://petit.lib.yamaguchi-u.ac.jp/G0000006y2j2/file/802/20100928160815/2007020028.pdf

Drying effect on mineralizations of microbial cells and their cell walls in soil and contribution of microbial cell walls as a source of decomposable soil organic matter due to drying

It is well known that the loss of nitrogen from the cultivated field principally occurs in the following two ways: (a) denitrification of nitrate, (b) leaching down of nitrate by percolating water. The problem of nitrification in soil is very important in connection with the efficient use of nitrogenous fertilizers applied to both the paddy- and the upland-field.

https://www.tandfonline.com/doi/pdf/10.1080/00380768.1968.10432003

Togoro Harada

Papers

Effect of nitrogen sources on the chemical components in young plants

FACTORS AFFECTING IMMOBILIZATION AND RELEASE OF NITROGEN IN SOIL AND CHEMICAL CHARACTERISTICS OF THE NITROGEN NEWLY IMMOBILIZED : I. Effect of Temperature on Immobilization and Release of Nitrogen in Soil

Factors affecting immobilization and release of nitrogen in soil and chemical characteristics of the nitrogen newly immobilized III. Transformation of the nitrogen immobilized in soil and its chemical characteristics

Drying effect on mineralizations of microbial cells and their cell walls in soil and contribution of microbial cell walls as a source of decomposable soil organic matter due to drying

Studies on the environmental conditions controlling nitrification in soil