The aims of the project were to determine the effect of selected management practices on

carbon sequestration, soil quality and hydrology, crop agronomy and profitability in irrigated

and dryland Vertosols using a combination of field and laboratory experiments, and desktop

studies. Management practices were tillage systems, rotation crops, soil amendments,

irrigation and stubble management. Measurements included environmental variables such as

soil quality, carbon storage and sequestration, greenhouse gas emissions, deep drainage and

soil water storage, and agronomic variables such as above- and below-ground crop growth

and cotton lint yield. Economic returns in irrigated sites at ACRI were evaluated by

comparing seasonal and cumulative gross margins. Partial life cycle analyses of greenhouse

gas emissions were made using a desktop approach. The “Mulch Manager”, a machinery

attachment which was able to kill vetch while reducing herbicide application rates and

trafficking was completed and assessed.

In general, SOC stocks in the 0-60 cm depth ranged between 50 and 70 t/ha. 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. Increasing water availability and reducing tillage improved root growth. SOC sequestration rates were generally negative or neutral, except where a stressed soil (disease, sodicity, salinity) was in the process of recovering.

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. SOC storage was positively related to dry matter inputs, average maximum temperature, soil aeration and water availability but was negatively associated with N fertiliser inputs. Except for temperature, the other variables can be manipulated by cotton growers. Average maximum temperature and soil organic carbon in the 0-60 cm depth had a curvilinear relationship. The temperature optima were higher in the Namoi valley (27-28 oC) than in the Macquarie (25.5 oC). Farming practises that could reduce emissions include eliminating inversion tillage, minimising use of groundwater, sowing winter crops in rotation with cotton, reducing/optimising mineral N fertiliser rates, substituting a legume an thus, fixed N for mineral N fertiliser. Long-term cropping-related K depletion may be minimised by regular application of cattle manure. Gypsum application did not improve subsoil structure under dryland conditions, probably because of the erratic rainfall pattern.

Water losses through drainage can be reduced and soil water storage increased (i.e. water

conservation improved) by including a wheat crop in the rotation with in situ stubble retention under less frequent irrigation. Management systems that conserve all rainfall received in situ, thereby reducing irrigation water requirements can contribute greatly to the sustainability of irrigated cropping. Deep drainage in cropped plots under normal or low rainfall conditions was many times higher than that in fallow plots, and reflects the higher water inputs in the former. When rainfall was frequent and no irrigation was required, drainage was higher under fallow, with fallow length being positively correlated to drainage. A model was developed that used rainfall and potential evaporation to estimate soil evaporation from beds where stubble was either incorporated or retained in situ. A model that used EM38 measurements, soil water storage and sodicity (ESP) was able to accurately estimate chloride in non-saline soils. These values could then be used to estimate drainage using chloride mass balance models.

Cotton yields and gross margin/ML were generally higher when wheat was included in the

rotation with highest values occurring on permanent beds. Amendments such as gypsum or

manure did not improve crop yields under dryland conditions, even though soil quality was

improved. Including vetch in the rotation did not result in sufficient improvements in cotton

yield to compensate for the increase in production costs. 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.

Cotton lint yields, in general, were positively related to water and N inputs, soil aeration in

some sites and average annual daily maximum temperature in cooler or poorly-drained sites

but were lowered by higher average annual daily minimum temperature. 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. Depth and frequency of tillage, average annual maximum and minimum temperature, N and SOC directly affected WUE of cotton. Except for SOC, which had no effect, all of the above variables directly affected NUE of cotton, particularly N fertiliser rate, which was negatively related, and legumes, which were positively related. The relative importance of individual variable differed among sites for yield, WUE and NUE.

The “Mulch manager” reduced use of herbicides, decreased labour, lowered risk to operators

and had a lower carbon footprint. In comparison to spraying with an 8-row boom sprayer,

depth of compaction was more when this 4-row implement was used, although the former

resulted in more intense and shallower compaction.

Between 2008 and 2011, two postgraduate students, two honours student and a visiting fellow from Pakistan were hosted by the project. Project outputs were: 7 journal articles, 11

conference papers and 5 cotton industry and extension. A total of 16 public presentations were given by project and associated staff.

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.

• Quantifying rainfall harvested, and associated drainage and evaporation, and thus, water

saved by retaining rotation crop stubble as in situ mulch.

• Identifying practices that could reduce carbon footprint of cotton farming systems with

life cycle analysis.

• A machinery attachment for managing prostrate cover crops bed-furrow systems.

• Simplified field methods to estimate soil evaporation and deep drainage.

• A whole-farm model of profitability for cotton farming systems that can be used as an

analytical research tool