Soil compaction effects on utilization of nitrogen from livestock slurry applied to grassland

Abstract

In order to determine whether wheel‐induced soil damage affected the subsequent utilization of livestock slurry by grass managed for silage production, dry‐matter yield and nitrogen offtake were compared in a replicated field experiment over three years in the presence or absence of soil compaction. Typical and zero compaction were compared in each year, and nominal controlled compaction was included in the two later years. Soil structure, as characterized by porosity and hydraulic conductivity measurements, was of poorest quality in the typical compaction treatment. Averaged over 3 years, typical compaction resulted in an annual yield reduction, relative to non‐compaction, of 2·14 t has−1 (20%) and 1·66 t ha−1 (13%) when 198 and 285 kg N ha−1 respectively were provided as ammonium nitrate fertilizer plus slurry‐derived ammonium‐N. The largest and most consistent yield reductions as a result of compaction occurred at first harvest each year following slower growth on the most compacted soil of the typical treatment. In general, compaction‐induced yield penalties increased with increased soil wetness. Offtake of nitrogen from both fertilizer alone and slurry and fertilizer combined was significantly reduced by soil compaction. First‐harvest yield response to increasing N rate was larger in the zero compaction treatment than in the typical treatment. Averaged over 3 years, the total annual apparent recovery of applied nitrogen was 55% after typical compaction and 71% after zero compaction. It was likely that the larger proportion of non‐recovered N in the former was lost as a consequence of greater surface run‐off, denitrification and ammonia volatilization. Over 2 years, the yield and nitrogen offtake trends in the controlled compaction treatment were intermediate between those of the zero and typical treatments, and in general more similar to the former than to the latter.The results indicated that when livestock slurry is recycled as a source of nitrogen, prevention of soil compaction is an important component of efficient and environmentally protective grassland management.