Concerns about declining soil organic carbon (SOC) and increased greenhouse gas emissions due to practices such as intensive tillage and bare fallows have encouraged the adoption of practices such as no-tillage, crop rotations and residue retention. However, whilst no-till farming is suited for broadacre crops, it has not been widely adapted for most vegetable production systems. Vegetable production systems, especially organic ones, routinely use tillage to prepare beds and manage weeds. These tillage operations break soil structure and aggregates, which is known to accelerate losses of SOC stocks. Despite requiring multiple tillage operations, the vegetable systems are also characterised by little or no crop residue input, potentially further reducing SOC stocks. The effect of sweet corn ('Zea mays L. var. rugosa') residue management (RM; i.e. incorporation or removal) in a corn-cabbage ('Brassica oleracea' L.) rotation on SOC parameters in two soil management systems (SMS; i.e. organic and conventional) was examined because crop residue incorporation and application of organic fertilisers could be ways to counteract loss of SOC due to tillage in vegetable systems. The principal aim of this thesis was to examine the effect of RM in the two SMS on soil total carbon (TOC) concentrations and stock, soil carbon fractions and microbial biomass carbon (MBC) through a field experiment of a corn/cabbage rotation over two years. A laboratory experiment was performed to separate the confounding factors of SMS in the field experiment, i.e. herbicide and mineral fertilisers in the conventional SMS, and cultivation and organic fertilisers in the organic SMS. To supplement the field experiment, another laboratory experiment focused on how two potentially opposing determinants of TOC, residue incorporation and simulated tillage (sieving), influence the emission of CO₂-C. Although, the research objectives of this thesis are focused on SOC, agronomic and fertility parameters, the essential components of a crop production system, were also considered.

This study investigated the carbon footprint of two different biochar production systems for application to paddy fields. The impacts of using rice straw-derived biochar in raw form (System A) were compared with those arising from using rice straw biochar enriched with lime, clay, ash and manure (System B). The GHG abatement of the management of one Mg of rice straw in Systems A and B was estimated at 0.27 and 0.61 Mg CO₂-eq, respectively, in spring season, and 0.30 and 1.22 Mg CO₂-eq in summer. The difference is mainly due to greater reduction of soil CH₄ emissions by enriched biochar.

Concerns in Australia about agriculture and the environment have triggered calls for sustainable agricultural practices, and organic farming is a widely promoted option for addressing this need. The Australian organic sector has tripled since the 1990s, but has not attracted strong support for industry development funding. This paper discusses how organic farming may or may not have had a 'fair go' in Australia, especially in terms of government support. Support for organic agriculture has been inconsistent over time, partly due to the hands-off approach of governments, but also due to the lack of awareness among decision makers and agricultural professionals of the potential of organic systems. For some sectors, funding is lower than levies paid to government by organic producers. Industry disunity hinders the ability or desire of government to assist, further thwarting the chance of a 'fair go'. Despite strong commercial growth, supply remains stagnant in some sectors and demand is being met through imports. The current regulatory system, centred around certification standards, still causes some confusion among producers and consumers. The road may still be rockier for organic agriculture in Australia than for other parts of the developed world in achieving its full potential.

Rice, maize and wheat account for 96% of the total food grain production in Bhutan signifying their importance for food security and the socioeconomic value of the Bhutanese agriculture. However, various biotic and abiotic factors impede optimum production of these cereals with weeds as one the main biotic constraints in attaining the full realisation of potential yields. In Bhutan, weeds have the potential to cause rice yield loss up to 50%. Similarly, 50% of the labour in maize production is for hand weeding. Key challenges for weeds management are labour availability, small farm sizes and potential risks of developing herbicide resistance from unsuitable usage patterns. Currently, rice is the only crop which receives herbicide, with long-term usage of a single chemical, butachlor. Manual weeding continues to be the main form of weed management in rice and maize, though the effectiveness of herbicide in maize has been demonstrated. Wheat is currently un-weeded, and no chemicals are used. Developing alternative herbicides to butachlor with different modes of action, and providing training on proper usage are likely to become more important. Considering the existing wide-spread use of low-external-input farming systems in Bhutan, non-chemical tactics will continue to have a key role in weed management. Some of these methods include competitive cultivars, optimum planting configurations, intercropping and strategic agronomic management. Incorporating these into an integrated package that includes herbicides may have long-term benefits for farmers.

Over the recent past, increasing concerns have emerged in Australia regarding agricultural production methods that have been causing environmental damage, enhanced by extreme climatic conditions such as droughts and highly variable rainfall. This has triggered an increasing awareness of and calls for more environmentally sustainable agricultural practices. The area under organic management has tripled worldwide since the late 1990s, with Australia currently being the country with the largest area under organic management, most of which is extensive farming. This paper will discuss whether organic farming has had a 'fair go' in Australia. On the one hand, increasing demand for domestic produce is an incentive for more local production. On the other hand, for most sectors, organic farms in Australia remain smaller than large scale, export-oriented conventional enterprises, and for some organic commodities, supply remains stagnant. Conventional farms have historically received a higher level of support in the light of Australia's export potential of agricultural produce, an important contributor to the country's economic prosperity. However, support for organics, has been inconsistent, with government support being relatively low, particularly for research and development. The current regulatory system, primarily the standards, still causes some confusion among producers and consumers. Furthermore, there are still problems in providing a consistent supply of quality products, partly related to logistics and supply chain management. We conclude that for organic agriculture in Australia, the road may still be 'rockier' than for other parts of the world in achieving the full potential in production and distribution.

Annual horticultural systems rely on frequent and intensive tillage to prepare beds, manage weeds and control insects. But this practice reduces soil organic carbon (SOC) through accelerated CO₂ emission. Crop residue incorporation could counteract this loss. We investigated whether vegetable systems could be made more resilient by including a high-residue grain crop such as sweet corn ('Zea mays' L. var. 'rugosa'), in the rotation through the use of conventional (no residue, no soil sieving) and organic (residue incorporated and soil sieved) soil management scenarios. We evaluated short-term emission of CO₂-C and soil C content in incubated Chromosol and Vertosol soils (Australian Classification) with and without sieving (simulated tillage) or the incorporation of ground sweet corn residue. Residue treatment emitted 2.3 times more CO₂-C compared to the no-residue treatment, and furthermore, sieved soil emitted 1.5 times more CO₂-C than the unsieved across the two soil types. The residue incorporation had a greater effect on CO₂-C flux than simulated tillage, suggesting that C availability and form can be more important than physical disturbance in cropping soils. The organic scenario (with residue and sieved) emitted more CO₂-C, but had 13% more SOC compared with the conventional scenario (without residue and unsieved), indicating that organic systems may retain more SOC than a conventional system. The SOC lost by soil disturbance was more than offset by the incorporation of residue in the laboratory conditions. Therefore, the possible SOC loss by tillage for weed control under organic management may be offset by organic residue input.

One of the alternatives to conventional (Conv) farming system is organic (Org) farming to prevent or to mitigate negative environmental impacts of intensive agriculture. Organic farming systems are claimed to be more resilient to weather extremes and can outperform conventional systems in weather extremes such as floods and droughts due higher levels of soil organic carbon (SOC).Whether organic or conventional, crop residue management (RM) plays an important role in maintaining SOC in horticulture, especially where annual crop rotations rely on frequent tillage. Stubble retention, incorporation and burning are the main three stubbles management practices in Australia. The effects of tillage and RM are often complex and difficult to separate. Theoretically, the effects of the two practices on SOC dynamics differ and can be opposing: no-tillage reduces the rate of organic carbon breakdown and potentially can reduce soil carbon losses, while stubble retention/burning directly affects the rate of organic input. The argument that organic farming is better than conventional farming for SOC storage have been challenged by critics due to its high reliance on cultivation (tillage) for weed control although its fertility management requires the addition of high levels of organic materials.

Crop residue management (RM) plays an important role in maintaining soil organic carbon (SOC) in horticulture, especially where annual crop rotations rely on frequent tillage. A trial investigating the short-term effects of sweet corn ('Zea mays' L. var. 'rugosa') residue incorporation on crop yields in a corn-cabbage ('Brassica oleracea' L.) rotation using organic (Org) and conventional (Conv) soil management systems (SMS) was established on 14 December 2009 in two contrasting soil types (Vertosol and Chromosol). The effect of mulched corn residue incorporation on weed biomass production was also studied. Corn was grown under the two SMS and residue was retained (+RES) or removed (-RES) after harvest on 23 April 2010. Cabbage was then grown from 4 May to 14 October 2010, under the same SMS in a three-way factorial design (SMS x RM x soil type). In both systems, equal quantities of macro-nutrients were supplied. Crop yields and weed biomass and apparent electrical conductivity (ECa) of soil were measured. There was no significant difference in total corn biomass for SMS or soil type. However, cabbage yield was significantly greater at the Chromosol site. The SMS x RM x soil type interaction was significant for weed biomass in cabbage, with Org having less weed biomass at the Vertosol site, especially in -RES. The +RES treatment had reduced weed biomass by 20 and 64% in conventional and organic SMS, respectively, in comparison to -RES in Chromosol. Soil ECa was significantly different for soil type only. The reduction of weed biomass in +RES treatment could be attributed to the mulching effect of the incorporated corn residue, the differences in weed seed bank and drainage between two sites. In conclusion, crop yields and soil ECa were not influenced by SMS or RM in short-term, but incorporation of residue in soil reduced weed biomass.

This study presents a comparative analysis of the environmental impacts of different biochar-compost (COMBI) systems in North Vietnam relative to the conventional practice of open burning of rice husks. Three COMBI systems, using different pyrolysis technologies (pyrolytic cook-stove, brick kiln and the BigChar 2200 unit) for conversion of rice husk into biochar were modelled. Biochar was assumed to be composted with manure and straw, and the biochar-compost produced from each system was assumed to be applied to paddy rice fields. Life Cycle Assessment (LCA) showed that the three COMBI systems significantly improved environmental and health impacts of rice husk management in spring and summer compared with open burning, in terms of climate change, particulate matter (PM) and human toxicity (HT) impacts. The differences between the three COMBI systems in the climate change and PM impacts were not significant, possibly due to the large uncertainties. In all systems, the suppression of soil CH4 emissions is the major contributor to the reduced climate effect for the COMBI systems, comprising 56% in spring and 40% in summer. The greatest reduction in the HT impact was offered by the BigChar 2200 system, where biochar is produced in a large-scale plant in which pyrolysis gases are used to generate heat rather than released into the atmosphere.

This study investigated economic returns and energy use of alternative rice production systems in North Vietnam with various residue management options. The traditional practice of open burning of rice residues (System A) was compared with the alternative of converting residues to biochar, which was returned to the paddy fields (System B). It was assumed that households used improved cook-stoves and drum ovens to produce biochar, and that the agronomic impacts of biochar compound with increasing biochar applications until reaching maximum benefit at 18 Mg ha⁻1 . This amount of biochar would take eight years to be produced in pyrolytic cook-stoves and drum ovens using the rice residues produced onsite. The net present value (NPV) of producing rice in the two systems was calculated based on their expected streams of costs and benefits. Biochar addition enhanced the NPV of rice by 12% and reduced the non-renewable energy intensity by 27%, relative to System A, after eight years of application. The difference in NPV values between production systems significantly increased to 23% and 71% by crediting GHG emissions abatement in low and high carbon price scenarios, respectively. These findings demonstrate the potential economic benefits of converting rice residues to biochar for soil application.