Phosphate fertilizer solubility affects the P use efficiency and the sustainability of wood production by Eucalyptus stands

Eucalyptus genus exhibits high nutrient use efficiency, but the phosphorus (P) balance in the soil under Eucalyptus stands is often negative, mainly due to higher P exportation in the wood after forest harvesting when compared with fertilization inputs. We evaluated the effect of phosphate fertilizers with varying solubilities and technologies on P balance and use efficiency in Eucalyptus, focusing on organic P (Po) mineralization. A randomized complete block design, with four blocks and six treatments, was used to test the P sources both as single sources and in mixtures. The P balance was calculated by considering fertilization as input and wood harvest as output. By adding P stocks from the soil and trees, we estimated the potential number of rotations (PNR). Relative agronomic efficiency (RAE), recovered P (Rec-P), and P use efficiency (P-UE) were also estimated. Before the experiment, soil P stocks were higher when P contents obtained through fractionation were considered. Phosphorus balance was positive in treatments with rock phosphate when considering total P applied. Organic P significantly influenced PNR, with increases ranging from one to four rotations. The P-UE was higher when soluble P was used. Treatments fertilized with phosphate complexed with humic substances had higher Rec-P and RAE. Soluble phosphate fertilizers promote high wood production but fail to meet long-term P needs due to their rapid uptake and depletion, resulting in negative P balance. In contrast, rock phosphate maintained positive P balances, suggesting it can sustain soil fertility and have a long-term effect on multiple rotations (legacy P), promoting sustainability. Nevertheless, Po is essential for soil fertility and P availability in Eucalyptus stands, contributing more than routine analysis suggests. It should therefore be considered in sustainable fertilizer management to ensure the long-term productivity and economic viability of Eucalyptus plantations.

Keywords
phosphorus balance; legacy P; minimum tillage; soil P fractionation; organic phosphorus