The overall aim of the project was to evaluate selected irrigated cropping systems and management practices in terms of carbon sequestration, water storage and WUE, soil quality and profitability. The management practices investigated were tillage systems, rotation crops and stubble management.

The more specific aims were:

• To determine the relationship between management practices and soil carbon sequestration, N accumulation, water storage and WUE

• To evaluate the efficacy of sowing corn in rotation with cotton on soil organic carbon sequestration in the tillage/rotation long-term experiment (LTE) at ACRI

• To evaluate the efficacy of sowing corn in rotation with cotton on soil organic carbon storage in on-farm sites

• To investigate whether systems characteristics such as water use efficiency and C sequestration could be related to qualitative indicators which could be more easily measured by cotton growers, such as rotation frequency and tillage intensity, with a view to using these indicators as surrogate indices of C sequestration, and nitrogen and water use efficiency

Key findings were:

• Legumes, although contributing large amounts of carbon to the soil were unable to retain it because their low C/N ratio facilitated rapid microbial decomposition

• Carbon inputs of C4 crops such as sorghum and corn were much larger than those of C3 crops such as wheat; a major proportion of that carbon came from their root systems Long-term SOC sequestration rates were generally negative or neutral; in most sites there was an initial rapid decrease in SOC sequestration followed by a stabilisation or an increase. This change was associated with implementation of soil conservation measures (e.g. replacement of conventional management practices with soil conserving practices such as no-tillage, or replacing saline irrigation water with good quality water).

• Estimates of carbon inputs, based on above-ground and root dry matter, together with measured sequestration rates indicated that large losses of carbon were occurring, probably due to a combination of accelerated erosion, runoff and microbial decomposition

• Factors that influenced SOC storage varied widely between sites but included dry matter inputs, average maximum temperature, soil aeration, water and N fertiliser inputs. Except for temperature, the other variables can be manipulated by cotton growers

• The temperature optima (for C sequestration) were highest in the central highlands of Queensland (30.1ºC), lower in the Namoi valley (27-28 ºC), and lowest in the Macquarie valley of NSW (25.5 ºC)

• Farming practises that could reduce emissions include eliminating inversion tillage, minimising use of groundwater, sowing winter crops in rotation with cotton, and reducing/optimising mineral N fertiliser rates.

• Substituting a legume and thus, fixed N for mineral N fertiliser although reducing nitrous oxide emissions at time of fertiliser application also required additional inputs in terms of farm operations and irrigation, thus negating the reductions achieved by lowering N fertiliser rates.

• Cotton yields and gross margin/ML were generally higher when wheat was included in the rotation with highest values occurring on permanent beds.

• Including vetch in the rotation did not result in sufficient improvements in cotton yield to compensate for the increase in production costs.

• Corn rotations reduced the incidence of black root rot infestation in a following cotton crop

• In years of plentiful water (or when crop area is the limiting factor) reducing water application rates on a continuous cotton crop was a false economy.

• In a sodic soil, a high frequency of the tillage practices intended to aerate the soil may have caused yield decreases, presumably due to exposure of more sodic soils.

• As with SOC, the factors that influenced cotton lint yields varied across sites. Variable such as frequency of minimum tillage, N rates, water and average maximum and minimum temperatures played significant roles in determining NUE (nitrogen use efficiency) of cotton but varied across sites. No one variable could strongly account for the variations in NUE across all sites.

• Variable such as depth and frequency of tillage, water inputs, N and SOC played significant roles in determining WUE (water use efficiency) of cotton; the relative importance of individual variables differed among sites for yield, WUE and NUE

• Between 20011 and 2014, two PhD students, two honours students and three work-experience students were hosted by the project.

• Project outputs were: 12 journal articles (9 published, 2 under revision, 1 under review), 24 conference and workshop papers, and 8 extension articles (printed and web). A total of 27 public presentations were given by project staff and collaborators

Key outcomes included:

• Identifying soil and crop management practices, and climatic variables that had direct impacts on soil carbon stocks, yield, water and nitrogen use efficiency in irrigated cotton soils.

• Identifying practices that could reduce carbon footprint of cotton farming systems with life cycle analysis.

• Improvement and refinement of a whole-farm model of profitability for cotton farming systems that can be used as an analytical research tool.