Conservation practice effectiveness and adoption: unintended consequences and implications for sustainable phosphorus management

Phosphorus (P) runoff from agricultural land continues to receive attention due to a widespread lack of reduction in losses combined with a series of high profile P-induced harmful algal blooms. Many widely adopted conservation practices (CPs), aimed at reducing P loss, target particulate P (PP) through reductions in erosion or entrapment of P within the terrestrial landscape. However, there is increasing evidence that in time, these CPs may in fact increase dissolved P (DP) losses. We reviewed the effectiveness of current CPs promoted in the U.S., the results from long-term in-stream monitoring following implementation of conservation schemes and field studies investigating P loss from buffer zones designed to trap PP. These studies showed that different CPs are required to target different forms of P loss and the tendency for farmers to implement strategies targeting PP over DP resulted in an increase in dissolved reactive P export post-implementation of 37-250 % in three of the five catchment monitoring studies. Buffer zones, such as grass and vegetative filter strips, managed riparian zones and wetlands were found to accumulate labile forms of soil P over time and, in some studies, became significant sources of both inorganic and organic DP. Furthermore, often overlooked microbial processes appear to play a key role in P release. Consequently, to improve the effectiveness of future conservation schemes, practices need to specifically target DP losses in addition to PP and recognize that CPs trapping P within the landscape are at risk of becoming legacy P sources.