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

Summary 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 S outh A frican H ighveld. 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 ( P i ) to the total P of bulk soil increased from 37 to 63%, with a corresponding decrease in organic P ( P o ). After approximately 60 years of cultivation, a steady‐state equilibrium was approached in all P fractions, which was characterized overall by smaller P o and larger P i contents. These temporal dynamics were controlled mainly by P pools in the sand fraction and by the 0.1 m NaOH pool from the H edley fractionation. Increases in P i may reflect inorganic fertilizer applications, whereas losses in P o 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 P i and P o 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 N a OH pool from H edley fractionation