Concentration and vertical distribution of total soil phosphorus in relation to time of abandonment of arable fields

Abandonment of agricultural soils is a common practice in Western Europe to increase the area of nature and to counteract agricultural overproduction. However, it has been suggested that abrupt changes in management of land, such as abandonment of heavily fertilized agricultural fields, could trigger leaching of phosphorus into deeper soil layers and groundwater. In a previous study we observed that total phosphorus (P) in the upper 10 cm of ex-arable soils in the Netherlands was negatively related to the time of abandonment. In a subsequent study in the region reported here, we measured total P concentrations at different soil depths in four ex-agricultural fields that differed in time since abandonment to examine if the decrease in total P with increasing time of abandonment could be due to leaching of P into deeper soil layers. At each site total P concentration decreased with increasing depth, and for each soil profile depth, total P also decreased with increasing years since abandonment. We calculated, based on estimated P fertilizer gifts over the last decades and the regression coefficient of the relation between total P in a core of 95 cm and time of abandonment, the amount of net total P that should have accumulated in the oldest ex-arable field to reach the P level of the most recently abandoned field. The continuation of accumulation of P for a longer period of time in recently abandoned fields appeared to be the most reasonable explanation for the decrease of P with years of abandonment. Therefore, abandonment of agricultural land does not seem to trigger a 'chemical time bomb' to explode as no large amounts of P seem to leach into deeper soil layers.