Agronomic and environmental factors influencing the marginal increase in nitrate leaching by adding extra mineral nitrogen fertilizer

Abstract

Nitrate leaching from agricultural soils is a considerable environmental concern related to nitrogen (N) application, and in some countries N fertilizer application is regulated by legislation to reach environmental goals. Information about the change in nitrate leaching with changes in mineral N fertilizer rates is important under such conditions. The increase in nitrate leaching due to extra mineral N application near the economic optimal N application for crop production, called marginal nitrate leaching, depends on a number of factors like soil type, crop types, climatic conditions and N fertilization rates. In this study, we collected published experimental data from 44 site-years from Denmark and 31 site-years from other European countries (Germany, Sweden, UK) with measured nitrate leaching at increasing mineral N fertilizer rates. We focused on obtaining marginal nitrate leaching around optimal N rates based on available information from the different field experiments. The measured nitrate leaching varied from 3 to 92 kg N ha-1 in the Danish dataset. For the European data set, it varied from 1 to 124 kg N ha-1. The median and mean of the estimated marginal nitrate leaching at the optimal N rates were 17.0% and 20.5%, respectively, in the Danish dataset and 9.0% and 14.9%, respectively, in the European dataset. At the optimal N rate, a positive relationship was found between yearly nitrate leaching and marginal nitrate leaching. Both nitrate leaching and marginal nitrate leaching at the optimal N rate were positively correlated with precipitation during the hydrological year and winter periods, but not to any other environmental factors tested. There was no significant difference in either marginal nitrate leaching or nitrate leaching between growing spring cereals versus winter cereals. No significant effect of winter vegetation cover on marginal nitrate leaching was detected. Furthermore, the marginal nitrate leaching at the optimal N rate in the first year of the experiments with increasing N rates was significantly lower than the accumulated effect of two or more years with low or high N rates. Therefore, the long-term effects of N rates should be accounted for when estimating marginal nitrate leaching. The marginal nitrate leaching is highly variable between years and only a part of this variation was explained in this study. The positive correlation between marginal nitrate leaching and nitrate leaching at optimum N rates implies that cropping system and management factors reducing nitrate leaching also reduce the marginal nitrate leaching.