- Describe why measuring the response of plant productivity (NPP) to independent N and P additions is a better predictor of nutrient limitation than measuring the N:P stoichiometry of leaves ? - Soil minerals have electrostatic charge. The charge is - The ideal C/N ratio of microbe food (plant litter or soil organic matter) is greater than the C/N ratio of microbial biomass. Why? Solution 1.Foliar nutrients, especially nitrogen and phosphorus concentrations ([N] and [P]) and their ratio, have been used widely as indicators of plant nutritional status and have been linked directly to nutrient limitation of NPP. In tropical systems, however, a high number of confounding factors can limit the ability to predict nutrient limitation —as defined mechanistically by NPP responses to fertilization— based on the stoichiometric signal of the plant community. Also.consistent with NPP responses to fertilization, there will be no changes in community-wide foliar [N] and [P].life history traits can become more important controls over tissue nutrient concentrations and the stoichiometric signal of the plant community. 2.Physical adsorption. Electric charges on soil colloids arise from principally three sources: from isomorphous substitution of one ion by another of different valency within the clay mineral structure. This gives rise to mainly negative charges. The charges are permanent and do not change with change in pH of the external solution. from ionisation of OH groups attached to the Al, Si, Fe at the edges of clay minerals. The charges created by this process are negative, zero or positive depending on the pH. from ionisation of - NH2, - OH and - COOH functional groups in the organic matter of soils. This too gives rise to positive, neutral and negative charges depending on the pH. 3.Carbon to Nitrogen ratio (C:N) is a ratio of the mass of carbon to the mass of nitrogen in a substance. Because soil microorganisms burn carbon as a source of energy, not all of the carbon a soil microorganism eats remains in its body; a certain amount is lost as carbon dioxide during respiration. To acquire the carbon and nitrogen a soil microorganism needs to stay alive (body maintenance + energy) it needs a diet with a C:N ratio near 24:1, with 16 parts of carbon used for energy and eight parts for maintenance. Since wheat straw contains a greater proportion of carbon to nitrogen than the 24:1 perfectly balanced diet soil microorganisms require, the microbes will have to find additional nitrogen to go with the excess carbon to consume the wheat straw. This additional nitrogen will have to come from any excess nitrogen available in the soil. As soil microorganisms tie up excess nitrogen (immobilization), this situation could create a deficit of nitrogen in the soil until some of them die, decompose, and release nitrogen (mineralization) contained in their bodies, or some other source of nitrogen becomes available in the soil.up excess nitrogen (immobilization), this si.