Forecasting the decline of excess soil phosphorus in agricultural catchments

Abstract

Measures within the European Union Nitrates Directive National Action Programmes in many member states aim to reduce soil phosphorus (P) levels from excessive to agronomically optimum. This is to reduce the risk of diffuse P losses to water and ultimately help achieve the Water Framework Directive targets of good water quality status. In this study, a ‘Soil P Decline’ model was used to evaluate this expectation for soils in four intensive agricultural catchments. Realistic annual P‐balance scenario deficits (−30, −15, −7 kg P/ha) were used to estimate the average time required for soils to decline to optimum levels in two predominantly arable and two grassland catchments with excessive soil test P (STP). Depending on the STP concentration and total P reserves, for the smallest field P deficit scenario (−7 kg P/ha), the model predictions were for an average of between 5 and 20 yr for agronomically optimum levels to be reached. Under the largest P deficit scenario (−30 kg P/ha), it was forecast to take between 2 and 10 yr. These predictions highlight the likely time lag that exists between implementation of soil P mitigation regulations and the desired outcome of few or no fields with excessive soil P. Expectations for water quality improvement through diffuse P source mitigation must also factor in additional time for P decline model uncertainty, land management variability and time for P sources to transfer to and within river networks.

A Global Perspective on Integrated Strategies to Manage Soil Phosphorus Status for Eutrophication Control without Limiting Land Productivity

“…While current research suggests that P inputs can be safely omitted in the short term at many sites (typically up to 10 yr), there is significant site and seasonal variation in crop reactions to lack of freshly applied P, which is poorly understood (Rowe et al, 2016). The APLE model outputs presented here suggest that P loss reductions achievable from STP drawdown will also be slow (30–40+ years); such long‐term and variable timespans for STP drawdown have been reported elsewhere (Sharpley et al, 2013; Wall et al, 2013). Management options for more aggressive drawdown of STP seem limited because agricultural crops are not P accumulators.…”

“…Although clear guidance exists on managing soil P status to build up and maintain soil P fertility, management guidelines on STP drawdown are notably lacking. Tensions understandably exist over how soil P status can be reduced without sacrificing crop productivity (Buckley and Carney, 2013; Wall et al, 2013). While current research suggests that P inputs can be safely omitted in the short term at many sites (typically up to 10 yr), there is significant site and seasonal variation in crop reactions to lack of freshly applied P, which is poorly understood (Rowe et al, 2016).…”

Determining the longer term decline in plant‐available soil phosphorus from short‐term measured values

“…() and Wall et al . (). Johnston & Poulton () showed that the decline in Olsen P from c .66 mg/kg to c .11 mg/kg was curvilinear over the period 1903 to 1974 on the Exhaustion Land experiment at Rothamsted when the annual P offtake was small.…”

Integrating soil testing phosphorus into environmentally based manure management in peri-urban regions: A case study in the Beijing area

“…For fields with excessive Olsen-P values where manure applications have been stopped, it will be important to understand how quickly crop P offtake removes soil P to below the environmental threshold value (<60 mg kg À1 ) (Wall et al, 2013).…”