Challenges and Opportunities for Sustainable Nitrogen Management in Dairy Systems

Abstract

Nitrogen (N) is central to agricultural productivity, yet its mismanagement drives water and air pollution across the world. Ireland’s grass-based dairy systems are among the most N-intensive in the European Union (EU), with high inorganic and organic fertilizer sustaining productivity but creating persistent surpluses that threaten groundwater and surface water quality. Despite major policy efforts, Ireland continues to struggle to meet EU Water Framework Directive (WFD) chemical targets for good water status. However, Ireland is still seeking the renewal of its Nitrates Derogation, which allows exceptionally high stocking rates up to 220 kg N ha⁻¹ yr⁻¹. This tension between economic needs and environmental compliance defines one of the country’s greatest agri-environmental challenges. The EU is moving toward tighter nutrient limits and nature restoration objectives, making it essential to understand whether sustainable dairy production can coexist with future regulatory expectations. One of the main obstacles to achieving water-quality goals is the temporal disconnect between management interventions and measurable improvements, which can erode stakeholder confidence and obscure the true impact of mitigation policies. In Ireland, the EU WFD program of measures (PoMs) is carried out under the Nitrates Directive, which include nutrient management, land management and farmyard management strategies to protect water quality. These lags are increasingly attributed to legacy N i.e., reactive N accumulated in soils and subsoils from past surpluses that continue to leach long after inputs decline. While groundwater legacy effects are recognized (i.e., the time it takes water to travel through the soil termed hydrologic time lag), few studies worldwide have directly quantified soil legacy N (i.e., biogeochemical time lags), and none had done so in Ireland prior to this research. Understanding the scale, distribution, and persistence of these soil pools is critical for designing realistic mitigation timelines and adaptive policies. The overarching aim of my research was therefore to assess N dynamics and environmental outcomes in Irish dairy systems by evaluating mitigation scenarios and quantifying legacy soil N accumulation to understand how current and historical management, drainage class, and hydrogeological setting influence both near-term losses and the pace of environmental recovery. I combined process-based modelling, multi-decadal farm data, deep soil coring, and groundwater monitoring to connect farm management decisions with both short- and long-term system responses. Together, these studies form the first integrated assessment of soil N legacies in Irish dairy systems. In Chapter 2, I used the €riN-MDSM model to simulate N flows, surpluses, and losses in a well-drained dairy farm operating under derogation conditions. This model, developed to represent N cycling in Irish grass-based systems, quantifies losses of nitrate (NO₃⁻), ammonia (NH₃), nitrous oxide (N₂O), and dinitrogen (N₂) from urine, dung, slurry, dairy soiled water, and fertilizer. I simulated a range of management scenarios, including reduced inorganic N rates (200–225 kg N ha⁻¹) and organic rates (170–430 kg N ha⁻¹), substitution of calcium ammonium nitrate (CAN) with protected urea, and restrictive grazing during vulnerable winter–spring months. Results showed that integrated approaches combining restrictive grazing, protected urea, and reduced fertilizer inputs lowered NO₃⁻ leaching by up to 44 % and NH₃ volatilization by 31 %, bringing water losses close to the 30 kg N ha⁻¹ threshold for good groundwater quality. These findings demonstrated that substantial environmental gains are possible through system-level optimization, but that even under improved management, N surpluses remain high, implying persistent risks to water and air quality. This modelling work provided a critical benchmark for assessing what levels of mitigation might be achievable within the derogation framework and highlighted the need to understand how historical surpluses continue to affect recovery, setting the stage for the legacy N analyses that followed in the next phase of this research study. In Chapter 3, I conducted a 24-year investigation (2001–2024) at Moorepark Teagasc Research Farm (known as Curtins Research Farm locally) in southern Ireland, a well-drained, karstic site with low denitrification potential. I reconstructed multi-decadal N budgets from detailed farm records and collected 75 soil cores across 15 paddocks, 1m deep profiles representing a gradient of historical management intensity. Annual N surpluses frequently exceeded 200 kg N ha⁻¹ yr⁻¹, leading to cumulative soil N accumulation of 4,000–5,500 kg N ha⁻¹ in the top 50 cm. Groundwater NO₃⁻ loads declined from over 70 kg N ha⁻¹ in the early 2000s to under 30 kg N ha⁻¹ by 2024, yet concentrations have plateaued rather than continuing to fall. This persistent signal alludes that subsoil N stored from past decades continues to mineralize and leach, sustaining groundwater nitrate levels despite reduced inputs. These findings provide the first direct quantification of legacy soil N in Irish dairy systems, showing that deep soil stores act as long-term sources of reactive N, constraining the pace of water quality recovery even when surface management improves. In Chapter 4, I expanded the investigation to include the Johnstown Castle Teagasc Dairy Research Farm, a variably drained site in the southeast with finer-textured soils and higher denitrification potential. I analyzed 45 soil cores from 9 paddocks, 1 m deep profiles covering an 18-year management period and compared results to those from Curtins. Despite lower annual surpluses (~100–150 kg N ha⁻¹ yr⁻¹), Johnstown Castle soils contained 4,000–11,000 kg N ha⁻¹ in the upper 50 cm, substantially higher than the well-drained Curtins profiles. The difference reflected higher clay and silt content, which enhanced N retention through adsorption and organic-mineral associations, as well as shallower water tables and moderate denitrification that reduced nitrate transport to groundwater but trapped nitrogen in the soil profile. These results revealed a clear trade-off: well-drained systems potentially act as “fast transmitters,” showing rapid leaching but quicker recovery when management improves, whereas variably drained systems are possibly “slow retainers,” buffering groundwater in the short term but accumulating persistent legacy N stores that prolong recovery. By linking long-term management records, soil data, and investigating groundwater trends across these contrasting systems, I demonstrated that N accumulation is governed by the interaction of soil texture, soil drainage class, hydrology, denitrification potential, and historical management intensity. Across both sites, total soil N accumulation exceeded 3,000–11,000 kg N ha⁻¹, far higher than values reported for most temperate cropland systems, confirming the exceptional capacity of Irish grassland soils to store reactive N from decades of intensive management. This thesis makes several novel contributions. It provides the first empirical evidence of soil legacy N magnitudes in temperate dairy grasslands, quantifies their long-term influence on water quality and nitrate dynamics, and develops a conceptual framework concerning drainage, soil texture, and hydrology to N retention and release. It also demonstrates how soil legacy N can be reframed as both a risk and a resource—a potential nutrient reservoir that, if managed strategically, could offset fertilizer needs during the transition to lower-input systems. These findings have direct implications for Ireland’s compliance with the EU Nitrates and WFD. Current six-year reporting cycles are too short to capture recovery in legacy-affected catchments, creating the perception of policy failure. Integrating soil monitoring to 1 m depth alongside existing high-resolution catchment and groundwater networks would enable more accurate assessment of progress and support realistic, site-specific mitigation timelines. Legacy N must be explicitly incorporated into nutrient models, regulatory assessments, and PoMs to ensure that both soil and water systems are managed as coupled components of the nitrogen cycle. Ultimately, this research underscores that Ireland’s path to sustainable dairy production requires addressing both current N surpluses and historical legacies. The methods and evidence developed here — combining modelling, deep soil sampling, and long-term monitoring offer a blueprint for future national assessments and international comparisons. As EU policy evolves toward stricter nutrient limits and nature restoration goals, understanding and managing legacy N will be fundamental to aligning agricultural productivity with environmental resilience.