Direct Exports of Phosphorus from Fertilizers Applied to Grazed Pastures

Abstract

Since its discovery in 1669, phosphorus (P) in the form of fertilizer has become an essential input for many agroecosystems. By introducing a concentrated P source, fertilizers increase short‐term P export potential soon after their application and longer‐term export potential by increasing soil fertility (legacy P). The 4R concept was developed to help mitigate P exports from the fertilizers that sustain agricultural productivity. This review investigates the factors affecting P exports soon after the application of mineral fertilizers to pasture‐based grazing systems and studies quantifying its potential impact in different systems, with an emphasis on Australasia. Initially, P fertilizers and reactions that might affect their short‐term P export potential are reviewed, along with P transport pathways, the forms of P exported from grazing systems, factors affecting P mobilization into water, and studies demonstrating the possible short‐term effects of fertilizer application on P exports. Using that foundation, we review studies quantifying the short‐term impact of fertilizer application in different regions; they show that under poor management, recently applied fertilizer can contribute a considerable proportion (30–80%) of total farm P exports in drainage, but when fertilizer is well‐managed, that figure is expected to be <10%. We then use three model systems of varying hydrology that are common to Australasia to demonstrate the principles for selecting fertilizers that are likely to minimize P exports soon after their application. Core Ideas Fertilizers increase P exports directly or by increasing P cycling. We review P pathways and processes relevant to grazing systems. Direct fertilizer P exports can be comparatively small (<10% of annual exports). Fertilizer selection to minimize P exports in three model systems is demonstrated.

A framework for sustainable management of ecosystem services and disservices in perennial grassland agroecosystems

“…For example, Owens et al (1994) reported groundwater NO 3 -N concentrations beyond USEPA potable water standards in watersheds connected to fertilized monoculture grass pastures that were grazed by beef cattle, compared to similar pastures where inter-seeded alfalfa was utilized to enhance soil fertility. These differences in nutrient concentration in drainage water also vary with the timing of nutrient applications and stocking rates (Julian et al 2017, Nash et al 2019. Julian et al (2017) reported a significant reduction in nutrient loading in catchment areas, after reducing the number of dairy cattle and sheep in grazing pastures.…”

The Ability to Reduce Soil Legacy Phosphorus at a Country Scale

“…In addition, it is important to understand that 10-80% of P losses can come from other parts of the farm such as recent fertilizer or manure (and dung) applications or unutilized forage (McDowell et al, 2007). Strategies are also available to reduce losses from these sources such as the application of sparingly soluble P fertilizers (Nash et al, 2019) or avoiding the grazing of wet areas that are well-connected to nearby streams . Each strategy varies in its cost and effectiveness and while costeffectiveness can be improved by targeting strategies to the sources of P loss, this may not be sufficient to enable WEP losses to meet the target nor do so without significantly reducing farm profitability (McDowell and Nash, 2012).…”

Phosphorus mirabilis: Illuminating the Past and Future of Phosphorus Stewardship

“…Ippolito et al (2019) show how irrigation type influences soil P dynamics, noting a difference in soil P availability between sprinkler and furrow irrigation, with greater Olsen‐extractable P in long‐term furrow irrigation. In Australasian settings, Nash et al (2019) detail the implications of varying hydrology on direct exports of P from fertilizer applications. They also describe how the use of information on local hydrology can help farmers identify principles for selecting fertilizers, to minimize risk of P loss, soon after application.…”