Fit-for-purpose phosphorus management: do riparian buffers qualify in catchments with sandy soils?

Abstract

Hillslope runoff and leaching studies, catchment-scale water quality measurements and P retention and release characteristics of stream bank and catchment soils were used to better understand reasons behind the reported ineffectiveness of riparian buffers for phosphorus (P) management in catchments with sandy soils from south-west Western Australia (WA). Catchment-scale water quality measurements of 60 % particulate P (PP) suggest that riparian buffers should improve water quality; however, runoff and leaching studies show 20 times more water and 2 to 3 orders of magnitude more P are transported through leaching than runoff processes. The ratio of filterable reactive P (FRP) to total P (TP) in surface runoff from the plots was 60 %, and when combined with leachate, 96 to 99 % of P lost from hillslopes was FRP, in contrast with 40 % measured as FRP at the large catchment scale. Measurements of the P retention and release characteristics of catchment soils (<2 mm) compared with stream bank soil (<2 mm) and the <75-μm fraction of stream bank soils suggest that catchment soils contain more P, are more P saturated and are significantly more likely to deliver FRP and TP in excess of water quality targets than stream bank soils. Stream bank soils are much more likely to retain P than contribute P to streams, and the in-stream mixing of FRP from the landscape with particulates from stream banks or stream beds is a potential mechanism to explain the change in P form from hillslopes (96 to 99 % FRP) to large catchments (40 % FRP). When considered in the context of previous work reporting that riparian buffers were ineffective for P management in this environment, these studies reinforce the notion that (1) riparian buffers are unlikely to provide fit-for-purpose P management in catchments with sandy soils, (2) most P delivered to streams in sandy soil catchments is FRP and travels via subsurface and leaching pathways and (3) large catchment-scale water quality measurements are not good indicators of hillslope P mobilisation and transport processes.

Phosphorus status and saturation in soils that drain into the Peel Inlet and Harvey Estuary of Western Australia

“…The accumulation of P, measured as soil test P and often termed legacy P, is a well-documented threat to water quality and requires effort to draw down soil P reserves to manage the threat (Rowe et al 2016; Withers et al 2019). This challenge posed by legacy P stores and soluble P (Summers et al 2014;Weaver and Summers 2014) also exists in the Peel-Harvey coastal catchment and is a problem that is increasing with time (Figs 1g,h,8). When coupled with other soil constraints (Fig.…”

“…Sandy soils with limited P retention capacity located near the Peel-Harvey estuarine system have little capacity to build up P or to stop P from entering into nearby waterbodies and are a high priority for management. In this environment, the traditional practice of riparian revegetation is ineffective at retaining P, which is transported in a soluble form (McKergow et al 2003;2006a;2006b;Summers et al 2014;Weaver and Summers 2014); therefore, minimising P application or increasing soil P retention are important strategies. Although voluntary soil testing has been used to encourage landholders to apply only the nutrients required, this appears to have had limited, if any, effect on reducing legacy P stores to date, as evidenced by gradual increases in the P 90 fertility index (Fig.…”

Does Riparian Filtration Reduce Nutrient Movement in Sandy Agricultural Catchments?

“…80% in 2011) that in-drain sediment supply due to management can alter the form of P measured in the drain. In these sandy catchments it is likely that soluble forms of P are supplied from adjacent agricultural soils, which is then transformed to particulate P dependent on sediment supply (Weaver & Summers, 2014).…”

Pronounced surface stratification of soil phosphorus, potassium and sulfur under pastures upstream of a eutrophic wetland and estuarine system

“…In the Peel-Harvey catchment, leaching of P into shallow water tables is often the primary path of P movement into waterways and open drains located at low points in the landscape (Weaver and Summers 2014). However, in areas with very limited slope and/or subsurface impediments to groundwater flow, the movement of P through surface water pathways (infiltration excess or saturation excess overland flow and, perhaps, return flow of infiltrated water) may dominate, particularly once the soil profile is saturated (Ruprecht and George 1993).…”