Soil Litter - Nutrient Cycle

Nutrient Cycle

During leaf senescence, a portion of the plant’s nutrients are reabsorbed into the leaves. The nutrient concentrations in litterfall differ from the nutrient concentrations in the mature foliage by the reabsorption of constituents during leaf senescence. Plants that grow in areas with low nutrient availability tend to produce litter with low nutrient concentrations, but a larger proportion of the available nutrients is reabsorbed. After senescence, the nutrient-enriched leaves become litterfall and settle on the soil below.

Litterfall is the dominant pathway for nutrient return to the soil, especially for nitrogen (N) and phosphorus (P). The accumulation of these nutrients in the top layer of soil is known as soil immobilization. Once the litterfall has settled, decomposition of the litter layer, accomplished through the leaching of nutrients by rainfall and throughfall and by the efforts of detritivores, releases the breakdown products into the soil below and therefore contributes to the cation exchange capacity of the soil. This holds especially true for highly weathered tropical soils.

Leaching is the process by which cations such as iron (Fe) and aluminum (Al), as well as organic matter are removed from the litterfall and transported downward into the soil below. This process is known as podzolization and is particularly intense in boreal and cool temperate forests that are mainly constituted by coniferous pines whose litterfall is rich in phenolic compounds and fulvic acid.

By the process of biological decomposition by microfauna, bacteria, and fungi, CO₂ and H₂O, nutrient elements, and an exceedingly resistant organic compound called humus are released. Humus composes the bulk of organic matter in the lower soil profile.

The decline of nutrient ratios is also a function of decomposition of litterfall (i.e. as litterfall decomposes, more nutrients enter the soil below and the litter will have a lower nutrient ratio). Litterfall containing high nutrient concentrations will decompose more rapidly and asymptote as those nutrients decrease. Knowing this, ecologists have been able to use nutrient concentrations as measured by remote sensing as an index of a potential rate of decomposition for any given area. Globally, data from various forest ecosystems shows an inverse relationship in the decline in nutrient ratios to the apparent nutrition availability of the forest.

Once nutrients have re-entered the soil, the plants can then reabsorb them through their roots. Therefore, nutrient reabsorption during senescence presents an opportunity for a plant’s future net primary production use. A relationship between nutrient stores can also be defined as:

annual storage of nutrients in plant tissues + replacement of losses from litterfall and leaching = the amount of uptake in an ecosystem