Effectiveness of Voluntary Nutrient Management Measures to Reduce Nitrate Leaching on Dairy Farms Using Soil N Surplus as an Indicator

A pilot study with 18 dairy farms in recharge areas of five vulnerable drinking water abstractions in the Dutch province of Overijssel aimed to reduce nitrate leaching risks to the upper meter of groundwater through improved farm management. The pilot employed a voluntary, mutual gain approach, promoting measures that enhanced both nutrient efficiency and groundwater quality. Over the research period (2011–2017), nitrogen surpluses on the soil balance declined significantly from 153 to 96 kg N per ha per year, achieving the target of 100 kg N per ha per year. Despite this decline, average nitrate concentrations in the upper meter of groundwater fluctuated annually, showing no significant reduction in grassland but a noticeable decrease in maize. Economic evaluation showed that relative fodder profitability (RFP) increased over time, suggesting positive financial effects of implemented measures, as acknowledged by participating farmers. However, the adoption of measures perceived as complex or less financially rewarding remained limited, highlighting the challenges of relying solely on voluntary implementation. The absence of farm-specific feedback on nitrate leaching emerged as a critical limitation, emphasizing the need for additional monitoring tools, such as residual soil nitrogen assessments, to provide actionable insights at the farm or field level. These findings underscore the potential for further reducing nitrate leaching through enhanced feedback systems, precise execution of measures, and collaborative efforts integrating farmer expertise and scientific knowledge.