Long-term influence of manure and mineral nitrogen applications on plant and soil 15N and 13C values from the Broadbalk Wheat Experiment

The Broadbalk Wheat Experiment at Rothamsted Research in the UK provides a unique opportunity to investigate the long‐term impacts of environmental change and agronomic practices on plants and soils. We examined the influence of manure and mineral fertiliser applications on temporal trends in the stable N (15N) and C (13C) isotopes of wheat collected during 1968–1979 and 1996–2005, and of soil collected in 1966 and 2000. The soil δ15N values in 1966 and 2000 were higher in manure than the mineral N supplied soil; the latter had similar or higher δ15N values than non‐fertilised soil. The straw δ15N values significantly decreased in all N treatments during 1968 to 1979, but not for 1996–2005. The straw δ15N values decreased under the highest mineral N supply (192 kg N ha−1 year−1) by 3‰ from 1968 to 1979. Mineral N supply significantly increased to straw δ13C values in dry years, but not in wet years. Significant correlations existed between wheat straw δ13C values with cumulative rainfall (March to June). The cultivar Hereward (grown 1996–2005) was less affected by changes in environmental conditions (i.e. water stress and fertiliser regime) than Cappelle Desprez (1968–1979). We conclude that, in addition to fertiliser type and application rates, water stress and, importantly, plant variety influenced plant δ13C and δ15N values. Hence, water stress and differential variety response should be considered in plant studies using plant δ13C and δ15N trends to delineate past or recent environmental or agronomic changes.