Long-term excessive phosphorus fertilization alters soil phosphorus fractions in the acidic soil of pomelo orchards

The effects of long-term excessive phosphorus (P) fertilization on the P fraction changes and P loss risk in orchard soils remain unclear. This study aimed to assess the concentrations of and relationships among the soil total P (TP), Olsen-P and P fractions in pomelo orchard (PO) soil during different fertilization periods. The PO soils were in a severe P overapplication state (905.4 kg P2O5 ha-1 yr-1), with a high P surplus (773.5 kg P2O5 ha-1 yr-1) and low P use efficiency (PUE, 14.7%). Such long-term excessive fertilizer P input significantly increased the TP, Olsen-P, and P fraction concentrations and significantly reduced the proportions of Org-P and reduction-P (Red-P) in both the surface (0–20 cm) and subsurface (20–40 cm) soils but increased the proportions of easily soluble P (Sol-P), aluminum-P (Al–P) and iron-P (Fe–P) rather than calcium-P (Ca–P). Furthermore, the P fractions exhibited a corresponding increasing trend and a significant linear (or two-stage linear) relationship with the soil P surplus. There is a serious risk of P loss when the P surplus in the surface soil exceeds 4128 kg P ha-1. Al–P had the highest correlation with Olsen-P (R = 0.984, p < 0.01), followed by Sol-P, Fe-P, Ca-P, Org-P and Red-P (R = 0.973, 0.908, 0.8783, 0.820 and 0.697, respectively, p < 0.01). However, only Sol-P and Al–P had a major direct impact on Olsen-P. In general, long-term excessive P application exerted a remarkable and differentiated impact on the soil P fractions. An increase in the Sol-P and Al-P fractions could lead to enhanced P bioavailability and environmental risk in acidic red soils. Therefore, P management in the PO production system needs to control the P fertilizer input and monitor the soil P fractions, which merits further investigation.