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

Background: 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). Methods: 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. Results: 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. Conclusion: Clearer understanding of long-term N dynamics can improve sustainability of heavily fertilized perennial grasses on dairy farms. [ABSTRACT FROM AUTHOR]