Excessive N fertilizer use leads to enhanced nitrous oxide (N2O) emissions from cotton (Gossypium hirsutum L.) production systems. The objective of the study was to quantify nitrous oxide emissions from the ridges within a furrow‐irrigated field during the growth of a cotton crop that had been fertilized with urea at 0, 120, 200, or 320 kg N ha−1. No measurements were taken from the furrows; we assumed similar N2O emissions from the furrows in this system. The N2O emissions increased exponentially with N fertilizer rate. Over the cotton‐growing season, N2O emissions totalled 0.51, 0.95, 0.78, and 10.62 kg N2O‐N ha−1, for the four respective N fertilizer rates. The cotton phase of the cotton–faba bean (Vicia faba L.)–fallow rotation was the main contributor to the total N2O emission. Over this 2‐yr rotation, emissions totalled 1.23, 1.65, 1.44, and 11.48 kg N2O‐N ha−1. However, <0.35% of the N fertilizer applied was emitted as N2O for the complete rotation where the economic optimal N fertilizer rate for the cotton crop was not exceeded. More than 3.5% of the N fertilizer was emitted as N2O where 320 kg N ha−1 was applied, which was estimated to represent about 11 kg N ha−1. These data indicate that supra‐optimal N fertilizer applications increase the net emissions of N2O from the ridges in high‐yielding furrow‐irrigated cropping systems. The N2O emissions could be decreased further by reducing or eliminating the time in fallow.

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.

Deep drainage under irrigated cotton is not only a waste of a scare resource but also has the potential to cause groundwater levels to rise and cause salinity. Drainage is difficult and expensive to measure directly, so most estimates have relied on modelling or chloride mass-balance calculations. However, direct, accurate measurements of drainage are required to understand drainage processes in cracking clay soils and to provide some certainty about other estimates. A variable-tension lysimeter was installed at 2.1 m depth in a Grey Vertosol under a furrow-irrigated, cotton–wheat rotation. The collection trays were installed without disturbing the overlying soil. A vacuum was applied to the trays and was continuously adjusted to match the matric potential in the surrounding soil at the same depth, thus maintaining the same hydraulic gradient as in the surrounding soil. The lysimeter was used to measure drainage and other components of the water balance from 2006 to 2011, including three cotton crops, one wheat crop and a long fallow. During this period, two types of drainage were observed. Matrix drainage occurred after an extended period during which rainfall exceeded evapotranspiration. This caused a wetting front to move through the soil over a period of months until it reached the lysimeter and was measured as drainage. Matrix drainage extended over a period of 1 month but at a low rate of ~0.5 mm/day. During the cotton season, the earlier irrigations generally caused a sharp peak in drainage of up to 3.5 mm day–1 ~25 h after irrigation. However, the water content and soil-water potential at 2.1 m were largely unaffected, and in some cases, the hydraulic gradient was upwards while drainage was occurring. This suggests this drainage is caused by irrigation flowing rapidly through the profile bypassing the soil matrix. Later in the season, when soil-water deficits developed in the subsoil at 0.5–1.0 m between irrigations, the peaks in drainage rate became much smaller. Bypass drainage appears to account for most of the drainage during the measurement period. Apart from lowering the water use efficiency, it is also more unpredictable and difficult to manage. In addition, bypass drainage is less efficient at removing salt from the soil profile, so that a higher leaching fraction may be required to prevent excessive salt accumulation.