Changes in soil phosphorus pools during prolonged arable cropping in semiarid grasslands

Abstract

It is assumed that land-use change has less effect on soil phosphorus (P) than on soil carbon (C) and nitrogen (N). Little is known, however, about the changes in soil P pools during prolonged arable cropping in semiarid grasslands. In this research, we measured changes in phosphorus pools of soil that had been cropped for 0–98 years in the South African Highveld. Hedley sequential extraction was carried out on bulk soil samples and size fractions to quantify inorganic and organic P pools along this temporal gradient. Total P content did not change with prolonged arable cropping, indicating that P removed by crops was compensated for by fertilizer application. The contributions of inorganic P (Pi) to the total P of bulk soil increased from 37 to 63%, with a corresponding decrease in organic P (Po). After approximately 60 years of cultivation, a steady-state equilibrium was approached in all P fractions, which was characterized overall by smaller Po and larger Pi contents. These temporal dynamics were controlled mainly by P pools in the sand fraction and by the 0.1 m NaOH pool from the Hedley fractionation. Increases in Pi may reflect inorganic fertilizer applications, whereas losses in Po were attributed to crop harvest, erosion and mineralization of organic matter. Arable cropping affected both labile and stable P pools after land conversion, indicating that stable P pools were not mere sinks, but also slowly available sources of plant-available P. Highlights: Phosphorus pools in semiarid grassland soil respond to changes in management over decades. Contributions of inorganic P to total P of bulk soil increased, with corresponding decreases in total organic P. Modelling of temporal changes in total Pi and Po indicated a steady-state equilibrium after ∼60 years. The long-term response of P pools was controlled mainly by dynamics of the sand fraction and the 0.1 m NaOH pool from Hedley fractionation.