Chemical processes in receiving soils accelerate solubilisation of phosphorus from desert dust and fire ash

Abstract

Atmospheric deposition of desert dust and ash from wildfires are recognised as major suppliers of phosphorus (P) to soils in many terrestrial ecosystems. The biological effects of desert dust and fire ash P depend on their interactions with the receiving soils. However, the chemical processes and environmental conditions that control dust and ash P bioavailability post deposition are unknown. Soil and rhizospheric pH and organic exudates are acknowledged as central parameters that control soil P bioavailability. Here we performed a series of in‐vitro incubation experiments to study their effects on P solubility of various desert dust and fire ash samples. Then, we tested the dust and ash P solubility that were added to acidic and alkaline soils. Our results show that dust P solubility gradually increases by a few orders of magnitude as pH decreases from 7 to 2.5 and also with increased concentrations of oxalic and malic acids, two ubiquitous P solubilising organic exudates in soils. In both cases, the increase in dust P solubility occurs mainly by dissolving P from insoluble Ca‐P minerals such as apatite, the major P bearing mineral in desert dust. P solubility of fire ash was dramatically higher than that of desert dust but showed the same pattern of a gradual increase with decreasing pH or with increasing concentrations of oxalic and malic acids. When the dust was suspended with soils, extractable P level was ~10 times higher in alkaline soil than acidic soil despite the lower pH of the later, probably due to the strong P sorption capacity of the acidic soils. On the contrary, when fire ash was suspended, extractable P in the acidic soil was higher than that of the alkaline soil indicating substantial variation in P forms. Overall, our results indicate that the bioavailability of P from desert dust and fire ash is largely controlled by environmental and chemical factors in the receiving soils. Highlights Atmospheric deposition from desert dust and fire ash is a major phosphorus source to soils. However, the actual bioavailability of dust and ash P post deposition is unknown. Soil pH, organic acids concentrations and P sorption capacity controls dust and ash bioavailability. The bioavailability of atmospheric P is controlled by environmental factors in the receiving soils.

Direct foliar phosphorus uptake from wildfire ash

“…The ash was processed through a set of sieves to achieve a particle size smaller than 63 µm, the size subject to wind dispersion (Guieu et al, 2010). The chemical and mineralogical properties, as well as P concentrations of our fire ash samples, resemble reported values in the literature (Bigio and Angert, 2019;Tan and Lagerkvist, 2011;Tiwari et al, 2022).…”

“…Also, the fractionation of P in the ashes aligns with fractionation reported in other studies (Wu et al, 2023). The results demonstrate fire ash exhibit a higher concentration of total P in comparison to soils, mineral dust, or volcanic ash (Ranatunga et al, 2009;Tiwari et al, 2022;Starr et al, 2023), with a significant percentage being in soluble form (18 %-39 %). Yet, the majority of fire ash P is not readily soluble and is found in biologically unavailable form.…”

“…The nutritional impact of fire ash may be even higher than that of dust and volcanic ash, as the concentration of P in fire ash particles can reach up to 10 000 ppm, which is 5-10 times larger than in desert dust (Tan and Lagerkvist, 2011;Tiwari et al, 2022;Barkley et al, 2019). The knowledge regarding the bioavailability and chemical speciation of P in fire ash is still poor; yet, solubility is supposedly significantly higher than that of desert dust and volcanic ash, owing to P-releasing chemical reactions that occur in high temperatures during biomass burning such as the high-melting crystalline phases (calcium/magnesium/potassium phosphates) during combustion (Tan and Lagerkvist, 2011;Tiwari et al, 2022;Myriokefalitakis et al, 2016;Anderson et al, 2010).…”

Dust storms increase the tolerance of phytoplankton to thermal and pH changes

“…as well as dust enriched with ashes from wildfires (0.77-0.84%; see Tiwari et al, 2022), or dust collected near urban areas in Pakistan (1.6%; Shafqat et al, 2016). Furthermore, the fraction of bioavailable phosphorus over the total phosphorus, on average, was 30% in our dust, compared to 15% in North African aerosols (Dam et al, 2021) and 54% in European aerosols (Longo et al, 2014).…”

“…On the other hand, it is important to note that composition of the dust sample used in our experiment and its effects on phytoplankton growth are representative of events in other parts of the planet. The phosphorus content in our sample (1113.86 mg P kg −1 ; 0.11% of P relative to the dust) falls within the intermediate range of Sahara Desert dust samples (0.022%–0.6%; see Gross et al., 2015, 2016, 2021; Guieu et al., 2002), similar to Colorado dust (0.055–0.091%; see Lawrence et al., 2010; Zhang et al., 2018), and very close to the mean concentration determined by Lawrence and Neff (2009) for 10 samples of dust deposited in different regions of the planet (1086.6 mg P kg −1 ), or even lower than Negev Desert dust (0.41–1.2%; see Gross et al., 2021; Uni & Katra, 2017), as well as dust enriched with ashes from wildfires (0.77–0.84%; see Tiwari et al., 2022), or dust collected near urban areas in Pakistan (1.6%; Shafqat et al., 2016). Furthermore, the fraction of bioavailable phosphorus over the total phosphorus, on average, was 30% in our dust, compared to 15% in North African aerosols (Dam et al., 2021) and 54% in European aerosols (Longo et al., 2014).…”