The effects of rotation crops and stubble management on soil quality, carbon sequestration, deep drainage, nutrient leaching, yield and profitability of succeeding cotton in irrigated and dryland Vertosols was studied from 2005 to 2008 in seven irrigated experiments (near Ashley via Moree, Narrabri and ACRI in NSW and Goondiwindi in Queensland), and one dryland experiment in Queensland (Brigalow in the Darling Downs). Key management issues considered were tillage systems, rotation crops and stubble management, sowing cotton into standing wheat and vetch stubble, in particular.

Within the overall aim of the project there were three key objectives pertaining to cropping systems in Vertosol-based cotton farming systems:

- Determine the effects of sowing cotton into standing wheat and vetch stubble on soil and water quality and conservation. Find solutions to management problems associated with in situ mulch in furrow-irrigated Vertosols.

- Compare cotton-wheat and cotton-vetch rotations in terms of soil quality, water conservation and long-term cotton production.

- Determine efficacy of organic and inorganic amendments in stubble-mulched irrigated

and dryland Vertosols.

Measurements taken in all experiments were: soil physical and chemical properties (e.g. soil organic matter, plastic limit, soil structure, exchangeable Ca, Mg, K and Na, ESP, pH, electrical conductivity). Profile water content to 1.2 m, crop growth, , nitrate-N, cotton lint yield and fibre quality were also measured. Economic returns in irrigated sites at ACRI and Ashley were evaluated by comparing seasonal and cumulative gross margins. Spatial and temporal deep drainage (with the chloride mass balance model) and nutrient leaching were measured at ACRI, and drainage at Narrabri and Ashley. Investigations were also conducted at ACRI into soil and cotton crop management practices and machinery which could overcome problems associated with sowing cotton into standing rotation crop stubble.

Sowing cotton into standing wheat stubble facilitated drainage and leaching of salts, and

water conservation through rainfall harvesting. Leaching of nutrients such as nitrates was also higher. Due to drought during 2006 and 2007, winter rotation crop growth was poor, and consequently carbon sequestration did not differ significantly from control treatments. Under restricted water availability and on a whole-farm basis minimum-tilled cotton-wheat was more profitable than continuous cotton, whereas with unlimited water or on an individual field basis the reverse was true. In comparison with infrequent irrigation (10-14 day interval), frequent irrigation (7-10 day interval) doubled cotton lint yield and profitability (measured as gross margins), and improved fibre quality. Growers would, therefore, be better off reducing the area of cotton sown and giving it sufficient water rather than reducing irrigation frequency over a larger area. Within soil layers in the cotton root zone, drainage with frequent irrigation was greater than that with infrequent irrigation. Drainage out of the crop root zone was, however, similar under both irrigation frequencies and may be related to differing drainage pathways.

Vetch in a cotton-wheat-vetch sequence responded positively in terms of growth and N

fixation to phosphate fertiliser whereas in a cotton-vetch sequence it did not. N fixation by vetch in the former rotation was also higher due to a longer growth period (sown in Late- February vs. later May) and wetter soil profile at sowing (sown into fallow vs. sowing immediately after cotton). Wheat grain yield and quality was improved by including vetch in the rotation (i.e. cotton-wheat-vetch) relative to cotton-wheat rotations. Cotton yield was highest when a wheat crop was included in the rotation. However, in comparison with cottonwheat where stubble was incorporated, the cotton-wheat (standing stubble)-vetch sequence required less N fertiliser (due to N fixation by the vetch) and irrigation water (due to better 5 subsoil water storage and presumably, reduction of evaporation by the in situ mulch). Under restricted water availability and on a whole-farm basis, profitability was in the order of cotton-wheat-vetch > cotton-wheat > cotton-winter fallow-cotton > cotton-vetch-cotton. Adding vetch to a cotton-wheat rotation is more profitable but adding vetch to a continuous cotton rotation is less profitable. The “Mulch Manager”, a machinery attachment which is able to kill vetch while minimising herbicide application rates and trafficking was developed.The amount of C added to soil C stocks by the roots of Bollgard II-Roundup Ready Flex varieties was less than that added by non-Bollgard II varieties. Above-ground stress such as insect pressure also reduced cotton root growth and C addition to soil, whereas minimum tillage and wheat rotation crops increased them. In comparison with above-ground dry matter, however, contribution by cotton root material to soil C stocks is small.

Sowing corn in rotation with cotton increased concentrations of the light carbon fraction but not total soil carbon. A close relationship was present between the light carbon reaction and microbial activity. Microbial activity and hence, nutrient cycling may be improved by including corn as rotation crop. Including vetch in a cotton-corn rotation increased SOC and exchangeable K, and decreased exchangeable Na concentrations.

Furrow soil in continuous cotton systems sown with minimum tillage had lower pH and

higher SOC than that under conventional tillage. In comparison with non-wheel-tracked

furrows, EC1:5 and geometric mean diameter of aggregates were higher in wheel-tracked

furrows, and plastic limit lower. Differences were small between conventionally-tilled and

minimum-tilled furrows, and between wheel-tracked and non-wheel-tracked furrows. Large inter- and small intra-seasonal changes also occurred with respect to soil physical and chemical properties in furrows. Interactions between surface soil factors in furrows may not, therefore, play a major role in influencing water application efficiency and infiltration within a season. Inter-seasonal differences could, however, affect hydrological processes.

In a K-deficient dryland Vertosol with high subsoil salinity and sodicity, only application of cattle manure (16 t FW/ha) resulted in a sustained improvement in soil quality, whereas gypsum and inorganic fertilisers had no effect.

Between 2005 and 2008, training was provided for two postgraduate students, and one

honours student. During the same period, 6 journal articles, 5 conference papers and 12 cotton industry and extension publications were published by project research and technical staff. A total of 21 public presentations were given by project and associated staff.

Key outcomes included:

- identifying cotton-wheat-vetch with in-situ stubble mulching as one which can reduce

cotton’s N fertiliser and irrigation water requirements while maintaining yields;

- identifying the practice of irrigating with treated sewage effluent as potentially risky to

soil health;

- determining that increasing complexity of cropping systems (i.e. sowing rotation crops)

under conditions of restricted water availability can improve whole farm profitability;

- identifying carbon sequestering management practices such as minimum tillage, vetch

rotation crops and manure application

- identifying corn as a rotation crop which could facilitate nutrient cycling;

- Identifying manure as a soil amendment which could alleviate K deficiency.