ItemImpact of water management on methane emission dynamics in Sri Lankan paddy ecosystems(MDPI, 2023-11) Lakshani, MMT; Deepagoda, Chamindu; Li, Y; Hansen, HFE; Elberling, B; Nissanka, SP; Senanayake, DMJB; Hamamoto, S; Babu, GLS; Chanakya, HN; Parameswaran, TG; Arunkumar, PG; Sander, BO; Clough, Timothy; Smits, KPaddy ecosystems constitute a dominant source of greenhouse gases, particularly of methane (CH₄), due to the continuous flooding (CF) practiced under conventional paddy cultivation. A new management method, namely alternative wetting and draining (AWD) (i.e., flooding whenever surface water levels decline to 15 cm below the soil surface), is an emerging practice developed to mitigate CH₄ emissions while providing an optimal solution for freshwater scarcity. Despite extensive paddy cultivation in Sri Lanka, no systematic research study has been conducted to investigate CH₄ emissions under different water management practices. Thus, field experiments were conducted in Sri Lanka to investigate the feedback of controlled water management on seasonal and diel variation of CH₄ emission, water consumption, and crop productivity. Adopting the same rice variety, two water management methods, continuous flooding (CF) and alternative wetting and draining (AWD), were compared with plants (W/P) and without plants (N/P) present. The emission of CH₄ was measured using the static closed chamber method. The results show a 32% reduction in cumulative CH₄ emission, on average, under AWD when compared to CF. The yield under the AWD was slightly higher than that of CF. Although it was not statistically significant (p > 0.05) there was not any reduction in yield in AWD than in CF. The total water saving under AWD ranged between 27–35% when compared to CF. Thus, the results support (without considering the effect of nitrous oxide) AWD as a promising method for mitigating CH₄ emissions while preserving freshwater and maintaining grain yield in paddy systems. Itemδ¹⁸O as a tracer of PO₄³¯ losses from agricultural landscapes(Wiley and Authorea, 2022-03-17) Wells, Naomi; Gooddy, DC; Reshid, MY; Williams, PJ; Smith, AC; Eyre, BDAccurately tracing the sources and fate of excess PO₄³¯ in waterways is necessary for sustainable catchment management. The natural abundance isotopic composition of O in PO₄³¯ (δ¹⁸OP) is a promising tracer of point source pollution, but its ability to track diffuse agricultural pollution is unclear. We tested the hypothesis that δ¹⁸OP could distinguish between agricultural PO₄³¯ sources by measuring the integrated δ¹⁸OP composition and P speciation of contrasting inorganic fertilisers (compound v rock) and soil textures (sand, loam, clay). δ¹⁸OP composition differed between the three soil textures sampled across six working livestock farms: sandy soils had lower overall δ¹⁸OP values (21 ± 1 ‰) than the loams (23 ± 1 ‰), which corresponded with a smaller, but more readily leachable, PO₄³¯ pool. Fertilisers had greater δ¹⁸OP variability (~8‰) driven by both fertiliser type and manufacturing year. Upscaling these values showed that ‘agricultural soil leaching’ δ¹⁸OP signatures could span from 18 – 25 ‰, and are influenced by both fertiliser type and the time between application and leaching. These findings emphasise the potential of δ¹⁸OP to untangle soil-fertiliser P dynamics under controlled conditions, but that its use to trace catchment-scale agricultural PO₄³¯ losses is limited by uncertainties in soil biological P cycling and its associated isotopic fractionation. ItemFlow regulates biological NO₃¯ and N₂O production in a turbid sub-tropical stream(Wiley and Authorea, 2021-02-10) Wells, Naomi; Eyre, BStreams play a critical role in attenuating the excess reactive nitrogen generated from human activities. These systems can consequently also emit significant amounts of N₂O, a potent greenhouse gas. Models and manipulative experiments now suggest that hydrology regulates the balance between nitrogen removal and N₂O production. We aimed to empirically test this hypothesis by measuring changes in the concentration and isotopic composition of NO₃¯ (δ¹⁸O, δ¹⁵N) and N₂O (δ¹⁸O, δ¹⁵N, site preference) in hyporheic sediments and surface water of a 30 m reach over eight days of falling stream discharge (2.7 to 1.8 m³ s¯¹). The stream was persistently heterotrophic (productivity/respiration: 0.005 - 0.2), while changes in conductivity, δ¹⁸O-H₂O, and ²²²Rn indicated that hyporheic mixing decreased and net groundwater inputs increased as discharge declined. The shallow groundwater had high inorganic N concentrations (2 – 10 mg 1¯¹), but increased in groundwater inputs could not fully explain the concurrent increases in NO₃¯ (1 – 3 mg N 1¯¹) and N₂O (700 to 1000% saturation) in the surface water. Biologically, rather than solely hydrologically, regulated stream nitrogen export was confirmed by changes in N₂O and NO₃¯ isotopic composition. However, isotope patterns indicated that nitrification, not denitrification, increased surface water NO₃¯ and N₂O concentrations as hyporheic exchange decreased. These findings empirically demonstrate how flow dynamics regulate biological NO₃¯ production as well as transport, with implications for predicting aquatic N₂O emissions. ItemImpacts of no tillage on nitrate leaching and associated mechanisms in arable cropping systems : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University(Lincoln University, 2023) Li, JinboNitrate (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. ItemCopper contamination affects the biogeochemical cycling of nitrogen in freshwater sediment mesocosms(MDPI, 2023-07) Tomoiye, T; Huang, J; Lehto, NiklasTrace elements can have a wide variety of effects on microbial populations and their function in the aquatic environment. However, specific impacts on chemical and biological processes are often difficult to unravel, due to the wide variety of chemical species involved and interactions between different elemental cycles. A replicated mesocosm experiment was used to test the effect of increasing copper concentrations, i.e., from 6 mg kg‾¹ to 30 and 120 mg kg‾¹, on nitrogen cycling in a freshwater sediment under laboratory conditions. Nitrous oxide emissions from the treated sediments were measured over three consecutive 24 h periods. This was followed by measurements of iron, manganese, copper and mineral nitrogen species (nitrate and ammonium) mobilisation in the sediments using the diffusive gradients in thin films (DGT) and diffusive equilibria in thin films (DET) techniques and sequential extractions. Increasing copper concentrations are shown to have resulted in significantly reduced nitrate formation near the sediment–water interface and increased nitrous oxide emissions from the sediment overall. The concomitant mobilisation and sequestration of iron with ammonium in the sediment with the highest Cu treatment strongly imply links between the biogeochemical cycles of the two elements. Modest Cu contamination was shown to affect the nitrogen cycle in the tested freshwater sediment, which suggests that even relatively small loads of the metal in fresh watercourses can exert an influence on nutrient loads and greenhouse gas emissions from these environments.