Part A. Current views on humus formation have identified two competing interpret
ID: 287963 • Letter: P
Question
Part A. Current views on humus formation have identified two competing interpretations, namely Selective Preservation and Progressive Decomposition. For each of these theories describe the process of humus formation
Selective Preservation:
Progressive Decomposition:
Part B. Describe the Soil Continuum Model, indicating how it varies from the above theories and how it builds on the above ideas to generate a more holistic model.
(HINT: Think of this as a compare/contrast questions – what makes this model similar and different from the theories you have already discussed?)
Explanation / Answer
PART A
(1) The ‘humification’ model is the oldest of the three concepts. In its original definition ‘humification’ assumes a further transformation or synthesis of the initial decomposition products into large, dark-coloured compounds. The resulting macromolecules were thought to be rich in carbon and nitrogen structures specific to ‘humification’, resistant to decomposition and consequently, older than the rest of the soil organic matter. Given the lack of a universally accepted definition of ‘humic substances’ across disciplines and the lack of evidence for their physical existence independent of the alkaline extraction procedure, it is no surprise that there is no agreement on the processes and pathways of ‘humic substance’ formation either . These ‘humic substances’ are variously considered to be ecologically useful (providing cation exchange capacity), chemically reactive (interacting with iron, aluminium and othermetals), and—particularly relevant for biogeochemical models—also inherently ‘stable’ against further decomposition. The suite of hypothetical transformation processes became collectively known as ‘humification’ and is also called the ‘synthesis concept of the genesis of humic substances’ or ‘secondary synthesis.
(2) ‘Selective preservation’, which is also called preferential decomposition, is a newer concept informed by decomposition studies of leaves and visible plant fragments in soils. This concept assumes that organic inputs are composed of both labile and relatively recalcitrant compounds, the latter being used by microorganisms only when the former are exhausted. However, there is now robust evidence that, under suitable conditions, appropriately adapted decomposer organisms have the ability to decompose even presumably persistent materials more quickly than previously anticipated, including polycondensed aromatics, alkanes in soil, fire-derived carbon, crude oil in sea water, and even polyethylene. Also, contrary to previous assumptions, the decomposition of presumably recalcitrant lignin is fastest at the early stages of decomposition, as long as it is easily accessible and small organic molecules are available as a source of energy to help mineralize the lignin.
(3) In the progressive decomposition model (also called ‘biopolymer degradation’; or ‘the degradative concept’, soil organic matter consists of a range of organic fragments and microbial products of all sizes at various stages of decomposition. Several independent lines of evidence revealed alkali-extracted ‘humic substances’ to be a mixture of identifiable compounds such as fragments of plants or microorganisms that are distributed in different locations of micro-aggregates, showing no similarity to the ‘humic’ extract, and having small size. Upon cell death, materials that are synthesized in the course of microbial anabolism are released into the soil, where they are subject to further degradation. Throughout this process, these materials remain on an energetic downhill trajectory, as opposed to the hypothetical ‘humic substances’ , whose ‘secondary synthesis’ would require energy investments for which no thermodynamic rationale has been provided so far
PART B
The soil-plant-atmosphere continuum (SPAC) is the pathway for water moving from soil through plants to the atmosphere. Continuum in the description highlights the continuous nature of water connection through the pathway. The low water potential of the atmosphere, and relatively higher (i.e. less negative) water potential inside leaves, leads to a diffusion gradient across the stomatal pores of leaves, drawing water out of the leaves as vapour. [1] As water vapour transpires out of the leaf, further water molecules evaporate off the surface of mesophyll cells to replace the lost molecules since water in the air inside leaves is maintained at saturation vapour pressure. Water lost at the surface of cells is replaced by water from the xylem, which due to the cohesion-tension properties of water in the xylem of plants pulls additional water molecules through the xylem from the roots toward the leaf.