Leached nitrate under fertilised loamy soil originates mainly from mineralisation of soil organic N

Summary

Animal manures are suspected to be a major source of nitrate leaching due to their low nitrogen use efficiency (NUE) by crops. However, actual measurements of nitrate leaching from animal manure under field conditions are scarce. In an on-farm field trial in Switzerland over 2.5 years, we used 15N labelling to trace the fate of N from cattle slurry in the soil-plant system and to test whether more nitrate was leached from slurry than from mineral fertiliser. The experiment was conducted on two neighbouring fields with loamy soil in an agricultural area of the Swiss midlands, where nitrate levels in the groundwater are persistently high. Both fields followed the same crop rotation (silage maize – winter wheat – grass-clover), but shifted by one year. We compared three fertiliser treatments: Control (Con), 15N mineral fertiliser (Min), and 15N cattle slurry (Slu). In order to provide a comprehensive fertiliser N balance over several years, we traced the labelled fertilisers into crop biomass, soil, and leached nitrate. In the year of application, 15N recovery in crops was 45–47% for Min, but only 19–23% for Slu. Complementary to this finding, recoveries in soil were greater for Slu than for Min, despite greater NH3 emissions from Slu. Fertiliser recovery in the succeeding crops was small (< 4.6% of the originally applied fertiliser N in the first residual year and < 2.4% in the second) and similar for both fertilisers. Depth translocation of fertiliser N was marginal, with the majority of 15N in soil still in the top 0.3 m after 2.5 years. Along with higher recoveries in soil for Slu, we found significantly more slurry N than mineral fertiliser N lost through leaching. However, less than 5% of cumulated amounts of nitrate leaching over the three crops, which reached up to 205 kg nitrate-N ha-1, originated from direct leaching of the labelled fertilisers. Our findings suggest that most nitrate leaching originated from the mineralisation of soil N.