Using the farmgate phosphorus balance to meet river and lake water quality targets

Phosphorus (P) remaining in the farming system after accounting for all P inputs (chemical fertilisers, imported slurry/manure, concentrate feed) and offtakes (agricultural products, exported slurry/manure) can be summarised as the farmgate P balance (FPB). If FPB is in surplus, P can be immediately vulnerable to runoff and also to become part of a legacy soil P store that can have longer term diffuse pollution consequences. The FPB scaled to Northern Ireland was investigated against national water quality datasets from one hundred and one rivers over 18-years. National FPBs ranged from 8.7 – 15.1 kg P/ha/yr over that period. Ninety-three of the river sites were used for Water Framework Directive (WFD) reporting of baseline soluble reactive P (SRP) concentrations. Total P (TP) data from eight major rivers were combined with river discharge for large catchment area (4,836 km²) estimates of TP load to the 304 km² Lough Neagh lake basin where lake TP is a WFD reporting requirement. River TP loads were normalised to annual flow-weighted mean concentrations (FWMC). Based on conceptual models of ‘catchment memory’ and rivers as ‘jerky conveyor belts’ of material movement, the study found a linear 1-year lag between annual FPB and mean annual baseline SRP concentration in the ninety-three river sites, and a 5-year lag between FPB and TP FWMC in the eight major rivers. The differences were explained by soluble and particulate P partitioning and fate between source and receptors. The linear model suggested that river SRP would need a stronger FPB to aim for the good/high SRP boundary (upper quartile 0.037 mg/L) to meet a river FWMC TP concentration (0.109 mg/L) that would approach the lake’s WFD TP moderate/good boundary (0.044 mg/L) in the absence of internal lake P loading. The analysis suggested that the FPB would need to be 5.5 kg/ha/yr, or half the current balance, to meet these targets in the absence of other non-FPB sources. Addressing these non-FPB sources of P pollution in the Lough Neagh catchment and Northern Ireland more generally (38% considered to be from urban and rural sewered populations), would either speed up and attain higher water quality targets, or relieve the burden to agriculture if a slightly higher FPB target was used.