P-legacy effect of soluble fertilizer added with limestone and phosphate rock on grassland soil in subtropical climate region

Abstract

P-fertilization can be applied to cropped soil through two main ways, namely: by using soluble phosphates to increase P pools, rapid release and uptake by plants or by using less soluble rock phosphate powder to trigger synchronization between P solubilization and its uptake by plants. The aims of the current study were to evaluate whether there is no dissolution of P-bearing minerals in the long-term when phosphate rock is used, as well as to analyze its response associated with P-legacy and plant dry-matter production in comparison to that of fertilization based on soluble phosphate added with limestone. A 20-year-old field experiment is in progress at Pampa grassland, Southern Brazil. Two rates of phosphate rock and triple superphosphate plus limestone (118 and 250 kg ha−1 of P), as well as a P-free control, were tested. The P-legacy effect of fertilizers were analyzed by phosphorus lability and apatite minerals recalcitrance. Additionally, a historic data set of dry matter production was considered to access phosphorus use efficiency. The use of soluble P source and limestone (triple superphosphate at rate of 250 kg ha−1 of P add with limestone at rate of 3.2 Mg ha−1) resulted in greater nutrient use efficiency (42 kg kg−1 - five times greater than that of phosphate rock) and in higher dry matter yield (22 %). In addition, the use of phosphate rock (at rate of 250 kg ha-1 of P) has led to higher total P and moderate-lability P levels in the soil due to low apatite dissolution (even under favorable soil thermodynamics conditions). P-apatite minerals were found in sand and silt soil fractions subjected to phosphate rock treatment, which presented up to 6-year P dissolution and promoted low P bioavailability. Agronomic use of soluble phosphate added with limestone is the most suitable technique used to increase grassland yield and P-use efficiency.