Some thoughts on the biogeochemical cycling of potassium in terrestrial ecosystems
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Some thoughts on the biogeochemical cycling of potassium in terrestrial ecosystems William H. Schlesinger
Received: 17 July 2020 / Accepted: 18 September 2020 Ó Springer Nature Switzerland AG 2020
Abstract Potassium presents a conundrum for biogeochemists. Potassium is cycled wastefully at the plant level, but it appears to be conserved in the nutrient budgets of entire ecosystems, where it sometimes limits net primary productivity. An increasing demand for K fertilizer may accompany the expansion of agriculture into highly weathered tropical soils, where limited supplies of K may control the distribution and productivity of natural vegetation. However, the molar ratios of 4.6 for N/K in herbaceous plants, 17.3 in current global fertilizer applications, and 0.33 in the commodity price of fertilizer suggest that N is vastly overused and K is vastly over-priced in modern agriculture. Keywords Agriculture Biogeochemistry Potassium limitation Stoichiometry Weathering
Responsible Editor: Kate Lajtha This paper is an invited contribution to the 35th Anniversary Special Issue, edited by Sujay Kaushal, Robert Howarth, and Kate Lajtha. W. H. Schlesinger (&) Cary Institute of Ecosystem Studies, Millbrook, NY 12545, USA e-mail: [email protected]
Introduction Every year nearly 32.2 9 1012 g of potassium (K) is mined from the Earth’s crust, where economic deposits of K are largely found in evaporite minerals, especially sylvite and halite (USGS 2020). An essential nutrient for plants, nearly all of this K is applied to agricultural crops worldwide. In both plants and animals, K is involved in osmotic and ionic balance. Movements of K are the major determinant of stomatal function in plants. In humans, K controls the electrolyte balance of blood, and a cellular sodium/ potassium pump controls the contraction of muscles. Reiners (1986) calculated a C/K ratio of 29 in mammals and 46 for herbaceous plants. While not overlooked by agronomists, K has garnered relatively little interest among biogeochemists (Sardans and Penuelas 2015). Together with nitrogen (N) and phosphorus (P), potassium may represent a major, but unappreciated, control on terrestrial plant productivity. Applications of K may be needed as agriculture expands into highly weathered soils of the humid tropics.
Potassium biogeochemistry Potassium is contained in a variety of primary minerals, especially orthoclase (feldspar), biotite, and muscovite, from which it is released by chemical
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Biogeochemistry
weathering. Some plants and soil bacteria actively access K from primary minerals via the release of organic acids (Boyle and Voigt 1973; Sarikhani et al. 2018). As K is lost from terrestrial ecosystems in runoff, it is eventually delivered to the sea, where it accumulates in evaporite deposits and secondary clay minerals, including illite (Siever 1974; Michalopoulos and Aller 1995; Berner and Berner 2012, p. 375). The formation of authigenic minerals fractionates the isotopes of K, with a
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