Climate efficiency of long-term nutrient management: Cross-site greenhouse gas responses in temperate cropping systems

Long-term nutrient management is central to climate-smart agriculture, yet comparative evidence across crop rotations, soils, and fertilisation strategies remains limited. Fluxes of nitrous oxide, carbon dioxide and methane were quantified, together with crop productivity, across three German long-term experiments encompassing four cropping systems: silage maize following cover crops, potatoes under organic rotations, and Persian clover and winter rye under contrasting nutrient management strategies. Gas fluxes were measured using dynamic chamber systems, with intensified sampling after fertilisation, tillage, cutting, and freeze–thaw cycles. Across sites, greenhouse gas fluxes were largely driven by short-term responses to weather and management events. Depending on the treatment, annual nitrous oxide emissions varied by more than two orders of magnitude, ranging from 10.8 to 26.8 in maize, 2.22–2.54 in potatoes, 0.67–4.71 during the clover phase and 0.86–9.24 kg N ha⁻¹ yr⁻¹ during the winter rye phase. Cover-crop treatments significantly affected nitrous oxide emissions in maize, with lower emissions following Phacelia and a four-mixed cover crop than after Oat or Fallow, whereas organic fertilisation strategies showed no significant effects. Nutrient replacement strategies significantly influenced emissions during the clover phase, with higher emissions under farmyard manure-only and combined mineral-organic fertilisation with liming than under complete mineral fertilisation or nitrogen-omission treatments. Carbon dioxide emissions, ranging from 8 to 24 Mg C ha⁻¹ yr⁻¹ across sites, reflected soil respiration driven by crop growth and disturbance, while methane fluxes were small and soils generally acted as weak sinks. Consequently, total greenhouse gas balances (nitrous oxide and methane) were dominated by nitrous oxide and shaped by management practices specific to site and crop phase, highlighting the need to minimise excess mineral nitrogen while sustaining productivity. • Greenhouse gas fluxes were quantified across three German long-term experiments. • Nitrous oxide emissions varied strongly by site and crop (0.67–26.8 kg N ha⁻¹ yr⁻¹). • Cover-crop composition affected nitrous oxide emissions in maize at Asendorf. • Organic fertilisation had little effect on emissions in potatoes at Frankenhausen. • Nutrient omission strongly affected emissions in a 100-year experiment (Dikopshof).