Abstract
Phosphorus (P) loss from soils to water can be a major pressure on freshwater quality and dairy farming, with higher animal stocking rates, may lead to potentially greater nutrient source pressures. In many countries with intensive agriculture, regulation of P management aims to minimise these losses. This study examined the P transfer continuum, from source to impact, in a dairy-dominated, highly stocked, grassland catchment with free-draining soils over three years. The aim was to measure the effects of P source management and regulation on P transfer across the nutrient transfer continuum and subsequent water quality and agro-economic impacts. Reduced P source pressure was indicated by: (a) lower average farm-gate P balances (2.4kgha−1yr−1), higher P use efficiencies (89%) and lower inorganic fertilizer P use (5.2kgha−1yr−1) relative to previous studies; (b) almost no recorded P application during the winter closed period, when applications were prohibited, to avoid incidental transfers; and (c) decreased proportions of soils with excessive P concentrations (32–24%). Concurrently, production and profitability remained comparable with the top 10% of dairy farmers nationally with milk outputs of 14,585lha−1, and gross margins of € 3130ha−1. Whilst there was some indication of a response in P delivery in surface water with declines in quick flow and interflow pathway P concentrations during the winter closed period for P application, delayed baseflows in the wetter third year resulted in elevated P concentrations for long durations and there were no clear trends of improving stream biological quality. This suggests a variable response to policy measures between P source pressure and delivery/impact where the strength of any observable trend is greater closer to the source end of the nutrient transfer continuum and a time lag occurs at the other end. Policy monitoring and assessment efforts will need to be cognisant of this.
Highlights
The driver of food needs and the pressure (DPSIR framework; EEA, 1999) of intensive agriculture may impact water quality, via nutrient loss from land to water, causing eutrophication (Sharpley and Rekolainen, 1997)
Accounting for the estimated area of index 1 and 2 soils on these farms and their associated potential P build-up requirements to achieve optimal soil P concentrations, an “Optimal P Balance” was estimated. This Optimal P Balance indicates how the actual annual farm P imports compare to the annual imports that would be required to build the levels of soil P fertility on the farm to the agronomic optimal level, with negative values indicating that more P imports would be required
P balances and P use efficiency (PUE) in these grazed grass-based systems compare favorably with continental European dairy farms. This suggests that the Nitrates Action Programmes (NAPs) may be having the intended positive effects at the nutrient source end of the transfer continuum
Summary
The driver of food needs and the pressure (DPSIR framework; EEA, 1999) of intensive agriculture may impact water quality, via nutrient loss from land to water, causing eutrophication (Sharpley and Rekolainen, 1997). Reviews of the NAP follow a four-year cycle and are contingent, amongst other things, on water quality status and the agricultural contribution to the pressure. Dairy farming is a key sector of Irish agriculture and operates a relatively low cost, efficient and profitable system based on maximizing grazed grass in the cow’s diet (Ryan et al, 2011).
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