Answers to Review Questions

1. The transformation of minerals in rock involves both the physical breakdown (mechanical weathering) and chemical alteration (chemical weathering) of the minerals. Mechanical weathering occurs through expansion and contraction of solid materials due to freezing-thawing or drying-rewetting cycles, gravitational forces such as landslides, and pressure exerted by plant roots.

2. The original source of nitrogen for plants is dinitrogen gas (N₂) in the atmosphere, but they cannot use it unless it is converted to other forms by the process of nitrogen fixation. Only bacteria can carry out nitrogen fixation, which is an energetically expensive process. Some plants, such as legumes, have symbiotic relationships with nitrogen-fixing bacteria. As ecosystems develop, nitrogen builds up in the pool of detritus and is converted into soluble organic and inorganic forms through decomposition. Some of the nitrogen released by decomposition is consumed by microorganisms, lowering the supply available to plants.

3. While both primary production and decomposition influence the buildup of organic matter and associated nutrients in the soil, decomposition is more sensitive to climatic controls than is primary production. The mean residence time of nutrients is therefore more strongly controlled by decomposition. Low soil temperatures in boreal forests result in very long mean residence times. High rates of decomposition limit the buildup of soil organic matter in tropical forests, and the mean residence times of nutrients such as nitrogen and phosphorus are two orders of magnitude lower than those in boreal forests.

4. Nutrient transfers between trophic levels are efficient in both tropical and temperate-zone lakes, but organic matter is progressively lost from the surface layers in both systems, falling into the sediments in the benthic zone, where oxygen concentrations, and thus decomposition rates, are low. In the temperate zone, some of these nutrient-rich sediments are brought back to the surface layers during seasonal turnover of water, where they decompose, providing nutrients to support production. Turnover is largely absent in tropical lakes, which are therefore more dependent on external inputs of nutrients from streams and terrestrial ecosystems.