Tracking fertiliser and soil nitrogen in irrigated cotton: uptake, losses and the soil N stock

Abstract

This paper explores the importance of the N loss pathways relative to the immobilisation and soil mineral N supply during a cotton season. Despite using an agronomic practice of splitting urea application to reduce losses and an optimal rate (232kg urea-N ha–1) for the experiment, the average fertiliser recovery was 32%, which indicates that soil N mineralisation is a key source of N for irrigated cotton production systems. A large amount of the fertiliser (62kgNha–1) was immobilised in the soil at the end of the season and during the season the soil supplied 159kgNha–1 to the plant via mineralisation. During the season, large N losses occurred from the field via the atmospheric, deep drainage and surface run-off pathways (143kgNha–1). The losses occurred directly after fertilisation, predominantly at the start of the season when the majority of the urea fertiliser was applied (180kg urea-N ha–1). This indicates that the form, placement and timing of the fertiliser did not synchronise with soil and crop N dynamics and irrigation practice. Over the course of the measurement season, based on the N inputs, losses and storage budget, a 42kgNha–1 soil deficit was observed. Further longer term work is required to quantify the magnitude and significance of the soil N stock across different systems.

The current status of nitrogen fertiliser use efficiency and future research directions for the Australian cotton industry

“…The risk of N run-off and leaching loss is greater early in the irrigation season when the crop is small (Macdonald et al 2017;Macdonald et al 2016c). Another timing option increasingly practiced by cotton growers is to split N applications between pre-plant and in-crop applications in conjunction with early season irrigation events.…”

“…Historically, most of the cotton grown in Australia has been on cracking clay soils (Constable and Rochester 1992;Hulugalle and Scott 2008), and as such the bulk of Australian cotton nitrogen researches have taken place on these soils (Grace et al 2016;Macdonald et al 2016c;Scheer et al 2016). Key findings like Rochester's (2014) optimal NFUE window have been determined on these cracking clay soils.…”

“…Over the last 30 years NFUE in the Australian cotton industry has been assessed using 15 N and apparent N uptake studies, revealing that only 40%-60% of the N applied prior to sowing is taken up by the plant (Constable and Rochester 1988;Humphreys et al 1990;Macdonald et al 2016c;Rochester 2011aRochester , 2012. A common misconception is that the bulk of the fertiliser N not taken up by the plant is immobilised in the soil and will be released in subsequent years.…”

“…This is not the case, with most studies showing that only 10%-30% of the applied N remains in the soil after the cropping season, stored in macro and micro biota or in the soil's organic or inorganic pools (Constable and Rochester 1988;Humphreys et al 1990;Macdonald et al 6c;Rochester 2011aRochester , 2012. Overall, the soil supplies 30%-50% of the N required by cotton plants during a season (Macdonald et al 2016c;Rochester 1994), with fertile soils resulting in improved NFUE (Humphreys et al 1990). This means that management of the soil N budget is critical to achieving higher NFUE (Rochester and Bange 2016).…”

Canopy temperature of high‐nitrogen water‐stressed cotton

“…The main reason for the high N application is as insurance against N‐deficit‐related yield loss (Roth, 2013). This overapplication of N unnecessarily increases the cost of production, N leaching (Macdonald, Ringrose‐Voase, et al., 2016), runoff losses (Macdonald, Chang, Nadelko, Tuomi, & Glover, 2017; Silburn & Hunter, 2009), and the potential for greenhouse gas emissions (Macdonald, Chang, & Warneke, 2016; Macdonald, Nadelko, Chang, Glover, & Warneke, 2016), as well as reducing NUE (Rochester, 2011). In contrast with high N application, water use in the Australian cotton production system is constrained by its scarcity and increasing competition from other sources (Richards, Bange, & Johnston, 2008).…”