Agricultural managed aquifer recharge (Ag-MAR) is a concept in which farmland is flooded during the winter using excess surface water to recharge the underlying groundwater. In this study, we show how different recharge practices affect NO 3 − leaching and mineralization-denitrification processes in different soil systems. Two contrasting soil textures (sand and fine sandy loam) from the Central Valley, California, were repeatedly flooded with 15 cm of water at varying time intervals in field and soil column experiments. Nitrogen species (NO 3 -, NH 4 + , total N), total C, dissolved O 2 , and moisture content were measured throughout the experiments. Results show that when flooding occurs at longer intervals (every 1-2 wk), N mineralization increases, leading to an increase of mobile NO 3 − in the upper root zone and leaching of significant quantities of NO 3 − from both soil textures (137.3 ± 6.6% [sand] and 145.7 ± 5.8% [fine sandy loam] of initial residual soil NO 3 − ) during subsequent flooding events. Laboratory mineralization incubations show that long flooding intervals promote mineralization and production of excess NO 3 − at rates of 0.11-3.93 mg N kg -1 wk -1 (sand) and 0.08-3.41 mg N kg -1 wk -1 (fine sandy loam). Decreasing the flooding frequency to 72 h reduces potential mineralization, decreasing the amount of NO 3 − leached during flooding events (31.7 ± 3.8% [sand] and 64.7 ± 10.4% [fine sandy loam] of initial residual soil NO 3 -). The results indicate that implementing recharge as repeated events over a long (multiple-week) time horizon might increase the total amount of NO 3 − potentially available for leaching to groundwater. INTRODUCTIONDependence on groundwater for irrigation and consumptive use has resulted in the widespread depletion of groundwater aquifers across the world (Dalin et al., 2019;Wada et al.,

Contents 1. Introduction 2. Managed Aquifer Recharge in Urban Settings 2.1 Centralized Managed Aquifer Recharge Approaches 2.1.1 Conjunctive Use and In-Lieu Recharge 2.1.2 Use of Treated Wastewater in Centralized Urban Managed Aquifer Recharge 2.2 Decentralized Managed Aquifer Recharge Approaches 2.2.1 Water Quality Considerations in Decentralized Urban Managed Aquifer Recharge 3. Managed Aquifer Recharge in Agricultural Settings 3.1 Background 3.

In the past few decades, farmers and researchers have firmly established that biologically diversified farming systems improve ecosystem services both on and off the farm, producing economic benefits for farmers and ecological benefits for surrounding landscapes. However, adoption of these practices has been slow, requiring a more nuanced examination of both barriers and opportunities to improve adoption rates. While previous research has demonstrated that both individual and structural factors shape farmers' decisions about whether to adopt diversification practices, this study aims to understand the interaction of these individual and structural factors, and how they relate to farm scale. Based on 20 interviews with organic lettuce growers on the Central Coast of California, as well as 8 interviews with technical assistance providers who work with these growers, we constructed a typology to help elucidate the distinct contexts that shape growers' decisions about diversification practices. This typology, which reflects the structural influence of land rent and supply chains, divides growers into three categories: limited resource, mid-scale diversified, or wholesale. In this economic context, limited resource and wholesale growers both experience significant barriers that constrain the adoption of diversification practices, while some mid-scale diversified growers have found a “sweet spot” for managing agroecosystems that can succeed in both economic and ecological terms. The key enabling factors that allow these farmers to choose diversification, however, are not directly related to their farm size, but have more to do with secure land tenure, adequate access to capital and resources, and buyers who share their values and are willing to pay a premium. By focusing on these key enabling factors with targeted policies, we believe it is possible to encourage diversification practices on farms at a variety of scales within California's Central Coast.

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Nitrate (NO 3 − ) contamination of freshwater resources from agricultural regions is an environmental and human health concern worldwide (Rodell et al., 2018). In agriculturally intensive regions, it is imperative to understand how management practices can enhance or mitigate the effect of nitrogen loading to freshwater systems. In California, managed aquifer recharge on agricultural lands is a proposed management strategy to counterbalance unsustainable groundwater pumping practices. Agricultural managed aquifer recharge (AgMAR) is an approach in which legally and hydrologically available surface water flows are captured and used to intentionally flood croplands with the purpose of recharging underlying aquifers (Kocis & Dahlke, 2017). AgMAR represents a shift away from the normal hydrologic regime wherein high efficiency irrigation application occurs mainly during the growing season. In contrast, AgMAR involves applying large amounts of water over a short period during the winter months. This change in winter application rates has the potential to affect the redox status of the unsaturated (vadose) zone of agricultural regions with implications for nitrogen (N) fate and transport to freshwater resources.Most modeling studies targeting agricultural N contamination of groundwater are limited to the root zone; these studies assume that once NO 3 − has leached below the root zone, it behaves as a conservative tracer until it reaches the underlying groundwater (

Synthetic nitrogen (N) fertilizer formulations vary in their effects as substrates on nitrous oxide (NO) emissions. Mitigation of NO emissions can potentially be achieved through appropriate choice of N fertilizer sources combined with stabilizers. The effects of three N fertilizers and urease and nitrification inhibitors on NO emissions, crop N uptake, and yields were determined in a furrow-irrigated corn ( L.) system in Reiff loam soil in the Sacramento Valley of California for one growing season. Aqua ammonia (Aq. NH), urea ammonium nitrate (UAN), and calcium nitrate were sidedressed at the rate of 202 kg N ha. The control treatment received only starter fertilizer (20 kg N ha). Total seasonal emissions were in the order Aq. NH > UAN > calcium nitrate = control with 1.38, 0.97, 0.35, and 0.27 kg NO-N ha, respectively. A novel, positively charged form of dicyandiamide, KAS-771G77 (G77), was combined with Aq. NH and UAN to test the effectiveness of this nitrification inhibitor in reducing NO emissions. When combined with Aq. NH, G77 did not reduce the emissions, but G77 significantly lowered them in the UAN treatment. A similar reduction of NO emissions in the UAN treatment was achieved with the urease and nitrification inhibitor AgrotainPlus. Yields and N use efficiency did not differ among the fertilized treatments. Ammoniacal fertilizers had higher NO emissions than nitrate-based fertilizers, which could imply nitrification pathways as a source of NO emissions. The use of G77 or AgrotainPlus, when applied with UAN, was an effective NO mitigation practice.

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