Moisture-driven controls on the stability of short-range order minerals and phosphorus cycling in tropical volcanic soils

The pedogenic minerals in volcanic soils are predominantly composed of short-range order (SRO) aluminosilicates (e.g. allophane, imogolite) and Fe-hydr(oxides) (e.g. ferrihydrite), which influence the geochemical cycling of phosphorus (P). SRO minerals are metastable and can transform into more stable crystalline phases, a process influenced by environmental conditions like temperature and soil moisture. The present study aimed to analyze the variation in Fe and Al contents associated with SRO and their reactivity toward P across two soil toposequences with different soil moisture conditions on the Irazú Volcano, Costa Rica. Soil samples were collected from various horizons along an East-South (ES) toposequence (1734–2853 m.a.s.l.) with a consistently humid udic regime, and a West-South (WS) toposequence (1724–3178 m.a.s.l.) that transitions from a udic to a drier ustic regime when altitude decreases. Pedogenic forms of Fe, Al, and Si were operationally defined using ammonium oxalate (AO), dithionite-citrate (DC) and sodium pyrophosphate (Py) extractions. Phosphorus pools (P-Olsen, AO-extractable P (P_ox), total P) were also quantified. Phosphorus adsorption was evaluated using batch experiments, and data were interpreted using the Langmuir equation and the mechanistic Charge Distribution (CD) model to estimate P adsorption capacity (Q_max) and reactive surface area (RSA) of the soils. In the humid ES toposequence, AO-extractable Al (Al_ox) and (Fe_ox), Q_max and RSA, increased as altitude decreased. Those trends were attributed to the stable moisture along the altitudinal gradient and the increasing temperatures with decreasing altitude, favoring the formation and persistence of SRO minerals. In contrast, the WS toposequence showed no consistent trend with altitude, probably because the transition to ustic regime at lower altitudes promoted the transformation of SRO minerals into more crystalline phases. The P-Olsen/P_ox ratio was low (<10%) across all samples and significantly lower in the ES toposequence, suggesting that the persistence of SRO minerals under humid conditions severely constrains P availability. An independent dataset of samples (n = 88) from the same study region corroborated the above findings. The udic soils showed a strong negative correlation between altitude and Al_ox (r = -0.80), Fe_ox (r = -0.77), P_ox (r = -0.53), and total C (r = -0.64). In ustic soils, the relationships were not evident and only Fe_ox correlated with altitude (r = -0.63). The results show that soil moisture regime is a key factor regulating the persistence of highly reactive SRO minerals along altitudinal gradients. Thus, in humid regimes, persistent SRO minerals increase the capacity of soils to retain P and stabilize organic C, resulting in direct implications for P availability and cycling in these tropical volcanic landscapes.