Influence of Soil Mineralogy on Phosphorus Sorption, Partitioning, and Bioavailability in Contrasting Tropical Soils of India

Phosphorus (P) is a bio-critical and non-substitutable element, essential for life, forming the backbone of DNA, RNA, and ATP, playing a vital role in agricultural productivity. Unlike nitrogen, P lacks an atmospheric cycle, relying solely on slow rock weathering or finite geological reserves for replenishment. The challenges surrounding phosphorus are less about its geological availability and more about socio-economic factors, such as limited access to fertilizers, and environmental concerns, including water pollution. These challenges emphasize the importance of adopting sustainable agricultural practices to optimize phosphorus use and reduce environmental impact. The instability of the phosphorus market, as demonstrated during the 2007-2008 global food crisis and the recent 2020-2022 and ongoing price surges, further underscores the need for effective phosphorus management, particularly in countries like India, which relies heavily on imports to sustain agricultural productivity ^[1][2].

We examined two contrasting soil types, ultisols and vertisols, collected from the Western Ghats, India. These soils were characterized physiochemically, geochemically and mineralogically. Ultisols, with slightly acidic pH, are enriched in iron and aluminium oxides, oxyhydroxides, and 1:1 type clay minerals. In contrast, vertisols, which are alkaline, are dominated by primary basaltic minerals, 2:1 and 1:1 type clay, with minor amounts of iron oxides and hydroxides. We performed sorption isotherm, bioavailability, and fractionation experiments on representative samples of each soil type. Sorption experiments were fitted using Langmuir and Freundlich isotherm models, revealing significantly higher adsorption maxima for phosphorus in ultisols than vertisols. Bioavailability tests reveal greater phosphorus availability in vertisols compared to ultisols, both pre-and post-fertilizer application. Hedley fractionation revealed that phosphorus in ultisols is mainly partitioned in moderately available fractions, while in vertisols, it is predominantly in readily available fractions, explaining the higher phosphorus bioavailability in vertisols than in ultisols. This difference is linked to the mineralogical composition of the soils; ultisols, enriched with iron and aluminium oxides, oxyhydroxides, bind phosphorus to high-energy sites associated with Fe and Al, thereby restricting its availability. In contrast, the near absence of these minerals in vertisols allows for greater phosphorus bioavailability. These findings underscore the importance of considering soil mineralogy in developing efficient and sustainable fertilizer application strategies. Currently, we are investigating the interactions between individual minerals prevalent in these soils and bacteria isolated from the same soils to understand the role of microbes in phosphorus dynamics.

References:

[1] Cordell, D., Drangert, J. O., & White, S. (2009). The story of phosphorus: global food security and food for thought. Global environmental change, 19(2), 292-305.

[2] Brownlie WJ, Sutton MA, Cordell D, Reay DS, Heal KV, Withers PJA, Vanderbeck I and Spears BM (2023) Phosphorus price spikes: A wake-up call for phosphorus resilience. Front. Sustain. Food Syst. 7:1088776. doi: 10.3389/fsufs.2023.1088776