Key Concepts in Ecology: Movement of elements in ecosystems

/ February 6, 2024

This blog post on ‘Movement of elements in ecosystems’ is part of the BES ‘Key Concepts in Ecology’ series, designed to help ecologists in learning the key topics in ecology! Take a look at the full blog series for a list of key topics you might typically find in an ecology textbook, each providing a quick introduction to the topic, and a list of suggested papers for students to refer to.

Ecological studies serve as an important cornerstone for understanding the movement of elements such as carbon (C), nitrogen (N) and phosphorus (P) within ecosystems. The transformation of C, N, and P moving from inorganic to organic forms creates the biochemical basis for energy flow and metabolism in the range of organisms that are present in all ecosystems from the open ocean to tropical rainforests (Schlesinger & Bernhardt 2013).  Biogeochemical cycles are important motors for ecosystem functioning, determining the circulation of essential elements between biotic and abiotic compartments. The C cycle, essential for energy flow in all ecosystems, involves the exchange of carbon between the atmosphere, oceans, soil, and living organisms, with processes such as photosynthesis and respiration. In order to be accessible to the biota, nitrogen from the atmosphere must be initially converted to organic forms via nitrogen fixation and then undergoes a complex cycle of transformations through soil and living organisms in water and terrestrial environments. Phosphorus, crucial for energy transfer in living organisms also cycles through rocks, soil, water, and organisms, with weathering and biological processes contributing principally to its bioavailability.  Most interesting, it is not only these elemental cycles operating independently but their interaction or relative availability in terms of ratios, called elemental stoichiometry, which is important for determining nutrient limitation and the structure of food webs. These interconnected cycles not only sustain life but also influence global climate patterns and nutrient availability, underscoring their significance in maintaining ecosystem functioning.

Human activities have significantly disrupted the flows and pools of C, N, and P with important consequences for ecology. The burning of fossil fuels and deforestation have led to an excess release of carbon dioxide (CO2) into the atmosphere, contributing to global warming and other profound climate changes (IPCC 2023). In the nitrogen cycle, industrial activities and agricultural practices have intensified nitrogen fixation, causing an overabundance of reactive nitrogen compounds that lead to issues like air and water pollution, as well as disruptions in the elemental stoichiometry of many ecosystems. Similarly, the widespread use of phosphorus-based fertilizers in agriculture has altered the phosphorus cycle, causing nutrient runoff into water bodies, triggering eutrophication, and negatively affecting aquatic ecosystems. Human activities have thus become a dominant force shaping these biogeochemical cycles, with consequences for global climate, biodiversity, and ecosystem health. Ecology serves as a foundation for understanding and mitigating these profound changes that humans are having in aquatic and terrestrial environments.

There are many examples of how plants, animals and the abiotic environment interact to determine elemental cycles, but a few illustrations of how important ecological interactions are can be found in these papers published in the BES journals. While plants are the basis of the food web through their ability to transform solar energy and CO2 into these organic polymers, the crucial role of microorganisms in this cycle cannot be understated, which largely determine soil carbon stocks in terrestrial environments, with transformations possible through an array of soil microbial enzymes (García-Palacios & Chen 2022).  Additionally, nitrogen cycling in terrestrial environments is clearly affected by guilds of microorganisms in the soil, which link to plant traits and their acquisition of different nitrogen forms (Moreau et al. 2019).  Larger organisms also demonstrate their versatility in modulating the C cycle: depending on the microbial abundance on the sponges in cold waters, sponges functioned at both at the bottom and the top of the benthic food chain, utilizing resources and efficiently recycling carbon and nutrients that were unavailable to other organisms (Hanz et al. 2022).  For P, plant cover and the ingenious cryptogramic crusts are largely responsible for maintain labile P cycling with the ecosystems, with their presence largely determined by macroclimatic factors of rainfall (García-Velázquez et al. 2020).

Interactions across and within trophic levels and nutrients can have important consequences for biogeochemical cycles, but the intricate nature and complexity of these effects often produces more questions and future areas of investigation. For example, trying to understand the impact of plant traits on carbon cycling in arctic ecosystems, average plant height, emerged as a robust predictor in modulating the tundra C cycle (Happonen et al. 2022). Nevertheless, within-community trait variability, particularly root traits did not. This highlights the need for further exploration into particularly below-ground mechanisms as predictors of carbon pools in tundra ecosystems (Happonen et al. 2022). The identify of the plants is key for determining biogeochemical impacts, demonstrated in a controlled-condition study that species-specific responses to elevated CO2 and P influenced growth, photosynthesis and foliar nutrients of tropical tree species, often in different ways.  These results suggest that we may be masking the complexity of community-wide responses to alterations of C and P cycles, and that individual species’ must be considered in assessing the impacts of global change on biogeochemistry in tropical forests (Thompson et al. 2019). Finally, it is well known that the movement of animals in the landscape can affect biogeochemical cycles. A global analysis of seabirds confirmed their importance as vectors for nutrient transport and concentration between marine and terrestrial environments via guano deposition, demonstrating that inputs can substantially enhance soil fertility (Grant et al. 2022). At the same time, there may be too much of a good thing with guanotrophication which results in highly concentrated nutrient levels and toxicity from pollutant transport, with different, and sometimes negative effects on plant community composition.

Taken together, human impact on all these interactions, from land-use change, elevated CO2 concentrations or perhaps most importantly impacts on plant or animal populations could all have important consequences on the balances of elements within these ecosystems. 

Introduction written by Amy Austin (Senior Editor, Journal of Ecology). Reading list curated by the BES journal Editors.

References and suggested reading

  • IPCC (2023) Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, H. Lee and J. Romero (eds.)]. (ed. I.P.o.C.C. (IPCC)), pp. 35-115. Geneva, Switzerland.

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