Li, Jinbo2023-11-022023-11-022023https://hdl.handle.net/10182/16584Nitrate (NO3-) leaching from agriculture is a growing environmental concern, and various strategies have been proposed to mitigate these losses. While many strategies aim to lower soil NO3- concentrations, they may not always be effective because NO3- leaching is also influenced by frequency and intensity of drainage events, which are affected by hydrological processes. No tillage (NT) has been proposed as an effective management practice to reduce NO3- leaching by influencing drainage. However, limited research has been conducted to identify how, and under what conditions, NO3- leaching can be reduced by regulating drainage with NT management practices. The aims of this study were to determine: (1) the relative importance of hydrological (i.e., drainage) and biochemical risk factors (i.e., NO3- concentration) and associated mechanisms in determining NO3- leaching losses at the global scale, (2) how and under what conditions NO3- leaching reduction can be achieved through regulating drainage with NT practices, and (3) the potential mechanisms explaining the effect of NT on drainage and NO3- leaching. It was hypothesized that (1) hydrological factors had a greater impact on global NO3- leaching variability than biochemical factors, (2) NO3- leaching was more sensitive to drainage in areas with higher nitrogen application rates and lower risks of fast flow, and that reducing NO3- leaching losses could be achieved by regulating drainage under these conditions, (3) the impact of NT on drainage and NO3- leaching was associated with tillage type (inversion vs. non-inversion tillage), soil properties, climate factors, and management practices; and (4) the greater drainage and leaching of NO3- associated with NT would be primarily attributed to higher soil water content rather than preferential flow. These hypotheses were tested through a combination of two meta-analyses of globally published data and field experimentation in New Zealand. The principal aim of the first meta-analysis was to assess the extent to which drainage contributed to NO3- leaching and the degree of sensitivity of NO3- leaching to drainage, aiming to identify the specific conditions under which regulating drainage could be more effective in reducing NO3- leaching. The second meta-analysis aimed to identify the specific conditions under which a reduction in NO3- leaching from NT practices may be feasible. The results of the first meta-analysis indicated that NO3- leaching variability was more closely linked to drainage than NO3- concentration, and that NO3- leaching was more sensitive to drainage in scenarios where fast flow drainage was less probable, such as NT and non-inversion tillage, and high N fertilizer rates were used. The results of the second meta-analysis revealed that NO3- leaching under NT was typically 7% higher than under inversion tillage but was comparable to non-inversion tillage. Greater NO3- leaching under NT was primarily attributed to drainage, and long-term NT cropping systems on high-SOC (soil organic carbon) soils were found to offer the most significant potential for mitigating NO3- leaching. Finally, field experiments were conducted in Canterbury to investigate the impact of NT on soil hydraulic properties and preferential solute transport, with the aim of elucidating the mechanism behind the increased risk of NO3- leaching under NT relative to inversion tillage using New Zealand as a case study. The findings indicated that NT resulted in greater average soil water content than inversion tillage, but there was no evidence to suggest that NT increased the risk of preferential flow. This suggests that greater drainage and NO3- leaching under NT relative to inversion tillage may be due to increased soil water content rather than preferential flow. Collectively, this study emphasizes the important role of drainage management in reducing global NO3- leaching risks, particularly in situations where fast flow drainage is less common (e.g., conservation tillage) and high N fertilizer rates are utilized. This study also highlights that adopting NT on average had greater NO3- leaching losses than inversion tillage and the greater NO3- leaching loss under NT is mainly through changes to soil hydrological properties that modulate intensity and frequency of drainage events. However, these effects are production system specific due to non-linear interactions between environmental and management conditions. NT increases the risks of NO3- leaching on low SOC soils and where NT adoption is short-term. In contrast, NO3- leaching is reduced by NT where it is practiced in the longer-term and on soils with high SOC content.enhttps://researcharchive.lincoln.ac.nz/pages/rightsNO3- leachingdrainagemeta-analysissensitivitypreferential flowno tillagesoil organic carbonsoil hydraulic propertiesnitrate leachingevaporationThesisANZSRC::410605 Soil physicsANZSRC::370702 Ecohydrology