There is considerable global interest in using recycled organic materials because of perceived benefits to soil health and environment. However, information on the effects of organic waste products and their optimal application rates on the quality of heavy clay soils such as Vertisols is sparse. An incubation experiment was therefore conducted using five organic amendments at various rates to identify their optimal application rates, which could improve the quality of the Vertisol. Cotton gin trash, cattle manure, biosolids (dry weight basis 7.5-120 t/ha), chicken manure (dry weight basis 2.25-36 t/ha) and a liquefied vermicast (60-960 L/ha, volumetric basis) changed the soil chemical, physical and microbiological properties compared with a control where no amendments were applied, viz. higher light fraction of organic matter, nutrient content (N and P) and soil microbial activity. Higher application of chicken manure resulted in an increase in dry-sieved mean weight diameter. Increasing rates of cattle manure increased exchangeable Na concentration and ESP. Although vermicast itself did not contribute a significant amount of N into the soil, when applied at higher rates (60-960 L/ha), its application resulted in increased concentration of NO₃-N in soil by amounts ranging from 43 to 429%. Optimal application rates for cattle manure and cotton gin trash were 30 t/ha, whereas for biosolids and chicken manure, the optimum rate was 60-18 t/ha, respectively.

Soil organic carbon (SOC) is the part of carbon (C) in the soil that is derived from living organisms and plays an important role in the C cycle (Paustian et al. 1997). Soil is a major reservoir of soil C, at 3.3 times the size of the atmospheric pool of 760 pentagrams (Pg) and 4.5 times the size of the biotic pool of 560 Pg (Lal 2004). Soils act as a reservoir of SOC and the level of storage within an ecosystem is mainly dependent on the soil type, climate, land use history, and current management practices. The quantity of SOC stored in a particular soil is dependent on the quantity and quality of organic matter returned to the soil matrix, the soil's ability to retain SOC (a function of texture and cation exchange capacity), and abiotic influences of both temperature and precipitation (Grace et al. 2005). SOC is essential for maintaining fertility, water retention, and plant production in terrestrial ecosystems with different land uses (Grace et al. 2006). Soil organic matter (SOM) maintains soil structure and productivity in agroecosystems (Lal 2010). Maintaining high levels of SOM is beneficial for all agriculture and crucial in improving soil quality. SOM has been widely used as an effective indicator of the functional response of soils to land use intensification (Dalal et al. 2003).

In Australia, the surface and subsurface soils of the majority of cotton ('Gossypium hirsutum' L.)-growing regions are sodic. Application of organic amendments can be an option to stabilize the structure of sodic Vertisols due to their potential positive effect on soil physical properties. An incubation experiment was conducted for 4 wk in a temperature-controlled (30°C) growth chamber to study the effect of organic amendments on the properties of two Vertisols with different sodicity levels. The exchangeable Na percentages (ESPs) in these Vertisol soils collected from the Australian Cotton Research Institute (ACRI) near Narrabri, New South Wales, and a commercial cotton farm near Dalby, Queensland, were modified such that three different sodicity levels resulted, i.e., nonsodic (ESP<6), moderately sodic (ESP 6-15), and strongly sodic (ESP>15). The organic amendments used were cotton gin trash (60 Mg ha⁻¹), cattle manure (60 Mg ha⁻¹), and composted chicken manure (18 Mg ha⁻¹), as well as an unamended control. The organic amendments improved the physical properties of both Vertisols by decreasing clay dispersion. In the Dalby soil, cotton gin trash produced the largest decrease (29%) in the dispersion index over the control at the moderate sodicity level, whereas in the strongly sodic soil, the lowest dispersion index resulted from the application of chicken manure. Nutrient availability (N, P, and K) was also increased significantly at higher sodicity levels for both the ACRI and Dalby soils by using organic amendments. These results indicate that using organic amendments can be beneficial for the amelioration of sodic vertisols and also to sustain soil quality.

Vietnam is one of the largest rice-exporting countries, and therefore a large amount of rice husk and rice straw is produced annually. To manage residues after harvesting, Vietnamese farmers commonly burn rice residues in the field which emits large quantities of gaseous and particulate pollution to the atmosphere and has a negative impact on the climate and the health of the population. In the last decade, using biomass to make biochar for application to cropland has received growing attention as a possible strategy to mitigate climate change by sequestering carbon from the atmosphere and suppressing soil greenhouse gas (GHG) emissions. Biochar can be produced from various biomass sources including crop residues, and at various scales from large industrial facilities, village scale and even at the household level using small-scale pyrolysis technologies.

This thesis investigated the climate change, human health (particulate matter and human toxicity) and economic impacts of biochar production from rice residues for addition to paddy soils compared with the conventional practice of open burning of residues. Life Cycle Assessment (LCA), a methodology that aims to assess impacts of products, processes, or services from “cradle to grave”, was employed to evaluate the environmental and health effects of alternative uses of biomass in rice growing systems in northern Vietnam. For this purpose, different studies related to crop residue management were defined and, for each study, the biochar system and a comparison reference system were modelled. The biochar produced in all studies was assumed to be returned to paddy rice fields from where the biomass was harvested. In study one, the carbon footprints (CF) of two rice production systems were calculated: one scenario in which rice residues are burned and another scenario where these are converted into biochar and incorporated into soil. The functional unit (FU) was the production of 1 kg of milled rice. It was assumed that households used pyrolytic cookstoves and drum ovens to produce biochar. Based on a literature review, I assumed that the agronomic effects of biochar compounds with increasing biochar application until reaching maximum benefits 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 husk and straw available. Biochar addition reduced the CF of spring rice and summer rice by 49% and 38% respectively, compared with rice produced with conventional residue disposal, after eight years of biochar addition.

Study two assessed the CF of two different biochar production systems: one scenario in which rice straw-derived biochar in raw form was applied to the paddy fields, and a second scenario using enriched biochar (biochar made from rice straw enriched with lime, clay, ash and manure). In this study, the management of 1 Mg of dry rice straw was chosen as the FU. Applying enriched biochar showed an increase in GHG emissions abatement by 126% and 309% in spring and summer seasons, compared with using rice straw-derived biochar. This was mostly due to greater reduction of soil CH₄ emissions by enriched biochar, because of the larger area treated with enriched biochar at a lower application rate.

Study three involved a comparative analysis of the climate change and health impacts of various biochar-compost (COMBI) systems 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 the conversion of rice husk into biochar were considered. In this study, the FU was the management of 1 Mg of dry rice husk. All biochar systems substantially improved environmental and health impacts of rice husk management compared with the open burning of rice husks. The differences between the three COMBI systems in the climate change and particulate matter impacts were not significant, due partially to large uncertainties. The lowest human toxicity effect 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 energy. This result highlights the significance of pyrolysis gas recycling for sustainable biochar production.

At the household or village level, economic benefits are a key factor that drives producers to adopt a new agricultural technology. The scenarios modelled in study one were used to assess the costs and benefits, and non-renewable energy use of rice production. After eight years of biochar application, the net present value of rice was enhanced by 12% and the energy intensity decreased by 27%, compared with rice production with conventional residue management. The existence of a carbon market that recognises the reduction of soil GHG emissions and carbon sequestration due to the land application of biochar could considerably raise the profitability of rice production.

These results indicate that ceasing the open burning of residue in the field and using residues to produce biochar for addition to soils can provide important benefits in climate change mitigation, human health and economic returns in rice cropping systems in Vietnam. This conclusion relied on several uncertain assumptions, including the effect of biochar on CH₄ emissions from soil. The assumed suppression of soil CH₄ emissions is a major contributor to the reduced climate effects for the biochar systems, and sensitivity analysis showed that the CF of biochar systems was highly sensitive to any variation in this factor. Therefore the soil impacts of biochar need to be confirmed by further research to enable more accurate quantification of the climate effects of biochar use in rice production.

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.

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.

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.

Application of organic waste products as amendments has been proposed as a management option whereby soil quality of Vertisols could be improved. An incubation experiment was, therefore, conducted for 4 weeks under controlled temperature conditions (30°C) to identify those potential organic amendments that might improve the quality of a Vertisol. Twelve organic amendments were investigated: cotton gin trash from three sources, cattle manure from two sources, green waste compost, chicken manure from three sources including a commercial product, biosolids and two commercial liquefied vermicomposts. Except for the biosolids, no other organic amendments had any effect on soil microbial biomass and respiration. Compared with NO₃₋N levels in the control, there was a 50% decrease in soil amended with 10 t ha⁻¹ green waste compost (65 μg g⁻¹). The three different types of chicken manures increased the NO₃₋N concentration from 75% (228 μg g⁻¹) to 226% (424 μg g⁻¹) over the control. Approximate recovery of P added by the amendment as resin-extractable soil P was 53% for cattle manure and 39% for chicken manure. Application of cattle manure resulted in a 22% increase in soil-exchangeable K over levels found in control. Organic amendments application also resulted in a significant increase in exchangeable Na concentration. Some of the organic wastes, viz. cotton gin trash (10 t ha⁻¹), cattle manure (10 t ha⁻¹), biosolids (10 t ha⁻¹) and composted chicken manure (3 t ha⁻¹) have value as a source of nutrients to soil and hence showed potential to improve Vertisol properties.

Vegetable production systems rely on frequent tillage to prepare beds and manage weeds, thereby accelerating losses of soil organic carbon (SOC). They are also characterised by scant crop residue input. Residue incorporation and organic fertiliser application could counteract SOC loss due to tillage. We tested this hypothesis in a Chromosol and a Vertosol in northern NSW, Australia, where the effects of incorporating sweet corn ('Zea mays' L. var. 'rugosa') residue in soil in a corn-cabbage ('Brassica oleracea' L.) rotation under either organic or conventional system on soil C fractions were studied during two rotation cycles (2 years). A laboratory experiment was conducted to isolate the effect of tillage on the soil organic matter (SOM) fractions, because both the residue-incorporated and without-residue treatments for organic systems received tillage for weed control in the field, whereas conventional systems did not. Residue incorporation increased particulate OC (POC) by 32% in the field experiment and 48% in the laboratory experiment, whereas dissolved OC was increased only in the organic system. Concentrations of mineral-associated OC (MOC) and total OC (TOC) were increased by residue incorporation in both field and laboratory experiments. Simulated tillage had a limited effect on POC, MOC and TOC, suggesting that cultivation for weed control may have only a minor effect on short-term SOM mineralisation rates. In both experiments, MOC accounted for ≤83% in the Vertosol and ≤73% in the Chromosol. Due to frequent tillage in vegetable production systems, physicochemical stabilisation of C predominates over protection through aggregation.