Nitrogen dynamics after low-emission applications of dairy slurry or fertilizer on perennial grass: a long term field study employing natural abundance of δ15N

Defining the long-term effects of liquid dairy manure (LDM) applied by a recommended low emission method is important to ensure sustainable grass production and dairy operations. We used natural abundance δ15N and conventional measurements in a long term field experiment to better understand the long-term fate of N applied as LDM, mineral fertilizer (MIN) and both (ALT). We investigated the effects of long-term applications of LDM (with low-emission trailing shoe), MIN, and alternating LDM and MIN (ALT) on grass N uptake, soil N stocks, N losses and δ15N natural abundance in grass, soil and fine heavy fraction (silt + clay size heavy soil). Nominal annual rates of total N were 200 and 400 kg ha−1 for MIN and 400 and 800 kg ha−1 for LDM. MIN and LDM (at 400 kg N ha−1) had similar NUE but LDM accumulated more total soil N (13% of applied) with less losses than MIN; ALT had high yields and losses. Herbage δ15N of MIN declined to near MIN levels after 10–15 years, indicating influence of pre trial management. Herbage δ15N of LDM was consistently lower than applied LDM, implying uptake of depleted urine N. High rates of enriched LDM had little effect on soil δ15N suggesting soil N was quite stable and enriched N was lost by non discriminating pathways like leaching. The physically protected fine heavy fraction contained most soil N. Clearer understanding of long-term N dynamics can improve sustainability of heavily fertilized perennial grasses on dairy farms.