Plant decay as a major control of river dissolved potassium: A first estimate

Living plants, which acquire significant amounts of K primarily from weathered silicate materials, may be considered as an intermediate storage for part of K that is ultimately of the origin of silicate mineral weathering. Part of this plant K is transferred to streams and rivers, as dead vegetation on the ground surface and within the soil beneath the surface becomes decomposed and leached by surface and ground waters, with the remaining K being recycled to new growth of plants. The plant K transferred to rivers was never included in any previous budget model that assessed the sources and pathways of K in global river. A popularly current view claims that nearly 87% of the total K in global river water comes from weathered silicate source, relative to 5% from evaporite deposits, 1% from atmospheric salt cycling, and 7% from fertilizers. In fact, some fraction of this silicate-material derived K in global river should belong to plant source K derived from leaching of dead vegetation on the surface and in the soils by surface and ground waters. The importance of decay of vegetation on the surface and in soils as being a source of K in global river was assessed by using a mass-balance equation that expresses the K/Rb weight ratio of global river in terms of fractional contributions of K from various sources to global river water and the K/Rb ratios of the contributing sources. ering a wide range of values for the K/Rb ratio of each source of K in global river water, calculations have shown that of the proposed 87% silicate-derived K in global river water, nearly 70 to 30% can be attributed to leaching of dead vegetation on the surface and in soils by surface and ground waters, whereas the remaining 17 to 57% can be assigned to leaching of the weathered silicate materials by surface and ground waters. A more realistic estimate can be based on taking into account the average crustal silicate K/Rb ratio of about 250–300. By this measure, the K derived from leaching of decayed plant by surface and ground waters is at least as important as the K derived from leaching of weathered silicate materials by surface and ground waters (43–44% each), and may be even more important than the K derived from leaching of the weathered silicate materials (70–72% for the plant source K, in contrast to 15–17% for the silicate source K), as a control of the dissolved K in global river water. We have also estimated that nearly 97–98% of K released from dead vegetation on the surface and in the soils is cycled back each year to newly grown plants.