Tuesday’s Soil Ecosystem Service – Nutrient cycling

Definition: Soil stores and exchanges macro and micronutrients and other elements with plants. The circle can be described as a transport of nutrients from soil particles to soil water, roots, plant tissues, litter, to soil organic matter and soil clay particles. Besides macronutrients derived from air and water (C, H, and O) primary macronutrients are N, P, and K while secondary macronutrients are Ca, Mg and S. Micronutrients, equally important but needed in small quantities are: Fe, B, Mo, Cu, Mn, Zn, Ni, Na, Se, Cl, Co, Al, Si and some others.

Effects and processes: Nutrients are released from rock minerals during rock/mineral weathering. They are bonded to fine clay particles and organic matter in their ionic form. The ion exchange mechanisms in soil moderate the release of nutrients from clay particles or organic matter to soil liquid where they are available to roots and soil biota. Nutrients are then translocated by plant uptake to different plant tissues. Dead plant material that fells on the ground (litter) is degraded and consumed by soil biota, and partly humificated. Subsequently, in the process of mineralization, nutrients are gradually released back into soluble forms, where they will bound again to clay minerals or they remain in soil liquid, ready for plant uptake – to start a new circle. Nutrient cycling as a process is analogue to carbon and water cycling. Nutrient cycling is enabled by a great diversity of organisms that regulate nutrient exchange among soil compartments. Microorganisms mediate nutrient availability through the decomposition of plant residues and soil organic matter and through nitrogen fixation.

Relations to the entire ecosystem: Nutrients are essential for soil fertility, the most important and complex soil property. Availability of nutrients largely steers soil productivity, soil and above-ground biodiversity. For instance, in the same climate, the natural plant communities, as well as soil biota, varies regarding the availability of nutrients. In terrestrial ecosystems, nutrients are substantially concentrated in living biomass (plants) or in soil organic matter.

Relations to other services: Soil fertility is a fundamental supporting service for biomass production, food, timber, fibre, energy or medicinal plants. The supporting soil ecosystem service nutrient cycle regulation is strongly linked to water budget and biological activity and is crucial for each form of biomass production.

Land use impacts: Nutrient cycling and soil fertility fundamentally define land use (agriculture vs forestry) and steer soil management practices, such as fertilising, liming and soil tillage techniques. Conservation tillage can improve soil fertility by increasing soil organic matter content, and consequently reduce nutrient leaching as well as maintaining them in the root zone, available to crops.

Climate change impacts: In the course of climate change, the nutrient cycle may be affected, as higher temperatures and more extended dry periods, can slow down the nutrient cycling, while pronounced rainfall can lead to leaching – that is a loss of nutrients from the root zone.

Demand aspects: Proper nutrient cycling, i.e. retaining as much as possible nutrients in-situ, careful fertilizing (i.e. replacement of nutrients taken from the soil with plant biomass) and protecting soil biota is a substantial part of the sustainable agricultural soil management practices.

In short:

  • Output: Nutrients available to plants and (soil) biota.
  • Provision: Soil regulates nutrient cycling in general and especially the cycles of essential Nutrient depletion, especially phosphorus which is nowadays becoming a concern, is becoming the main soil threat in some overused and poorly managed agricultural and forest ecosystems. The most important soil properties that enable nutrient cycling are clay, soil organic matter content, soil biota as well as soil pH.
  • Demand: Due to the high demand for food and fodder of the growing world’s population, nutrient cycling and related soil services remain high on the agenda.
  • Threat: Inappropriate soil management threatens the nutrient availability, e.g. by the leaching of nutrients into the groundwater or volatilization.
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