Abstract
Abstract. The strong phosphorus (P) sorption capacity of iron (Fe) and aluminum (Al) minerals in highly weathered, acidic soils of humid tropical forests is generally assumed to be an important driver of P limitation to plants and microbial activity in these ecosystems. Humid tropical forest soils often experience fluctuating redox conditions that reduce Fe and raise pH. It is commonly thought that Fe reduction generally decreases the capacity and strength of P sorption. Here we examined the effects of 14 d oxic and anoxic incubations on soil P sorption dynamics in humid tropical forest soils from Puerto Rico. Contrary to the conventional belief, soil P sorption capacity did not decrease under anoxic conditions, suggesting that soil minerals remain strong P sinks even under reducing conditions. Sorption of P occurred very rapidly in these soils, with at least 60 % of the added P disappearing from the solution within 6 h. Estimated P sorption capacities were much higher, often by an order of magnitude, than the soil total P contents. However, the strength of P sorption under reducing conditions was weaker, as indicated by the increased solubility of sorbed P in NaHCO3 solution. Our results show that highly weathered soil minerals can retain P even under anoxic conditions, where it might otherwise be susceptible to leaching. Anoxic events can also potentially increase P bioavailability by decreasing the strength, rather than the capacity, of P sorption. These results improve our understanding of the redox effects on biogeochemical cycling in tropical forests.
Highlights
Phosphorus (P) is often thought to limit net primary productivity and organic-matter decomposition in humid tropical forests that grow on strongly weathered soils (Vitousek and Sanford Jr., 1986; Cleveland et al, 2011; Camenzind et al, 2017)
Soils were collected from humid tropical forests in the Luquillo Experimental Forest (LEF) in Puerto Rico, part of the NSF-sponsored Long Term Ecological Research (LTER) and Critical Zone Observatory (CZO) networks and the DOE-sponsored NGEE-Tropics program
We found that anoxic conditions did not affect hydrochloric acid (HCl)-Fe(III) concentrations in soils from Rio Icacos and that changes in HCl-Fe(III) and Ammonium oxalate (AO)-Fe were smaller in magnitude compared to the increases in HCl-Fe(II) across all soil types
Summary
Phosphorus (P) is often thought to limit net primary productivity and organic-matter decomposition in humid tropical forests that grow on strongly weathered soils (Vitousek and Sanford Jr., 1986; Cleveland et al, 2011; Camenzind et al, 2017) In these soils, geochemical reactions of adsorption and precipitation, known as sorption, directly compete with plant roots and microorganisms for phosphate (Thompson and Goyne, 2012). One of the commonly hypothesized, yet rarely tested, mechanisms is that reducing events can decrease the effectiveness of soil minerals in sorbing P (Chacón et al, 2006; Lin et al, 2018).
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