Interaction of decreased crop growth and retarded mineralisation of 15N 13C labelled green manure under decreased precipitation patterns

Changes in climate will bring along changes in precipitation patterns, and as such, it will determine the availability of water in agricultural systems. We aimed to investigate the impact of climate-induced altered precipitation regimes on crop performance and soil processes such as organic matter mineralisation and nutrient release. The experiment took place at the lysimeter station located in Hirschstetten, Vienna, Austria (48° 15' 22" N, 16° 289 3" E, 160 m a.s.l.) where a future precipitation scenario was compared with current precipitation patterns on two different soil types – a sandy calcaric Phaeozem and a calcic Chernozem, both being representative for the Marchfeld region in Lower Austria. The future precipitation regime was calculated from four regionalised scenarios from Euro-Cordex out of the ÖKS 15 ensemble following the GHG emission scenarios RCP 4.5 and RCP 8.5.

Stable isotope analysis has become a useful tool for sensitively tracing biogeochemical processes in soils. In this study, plant residues of white mustard (Sinapis alba), isotopically labelled with carbon ¹³C and nitrogen ¹⁵N in a controlled laboratory environment were applied as organic fertiliser (green manure) on the lysimeter soils in April 2018. Soil, plant, gas and groundwater samples were collected from the lysimeters throughout the growing season of 2018 and 2019 and analysed using cavity ring-down spectrometry (CRDS) for ¹⁵N-N₂O in the field and by isotope ratio mass spectrometry.

Crop results showed an increase in the shoot ¹³C signatures, indicative of drought stress, which resulted in diminished plant production by -20 to -50% under the decreased precipitation. Isotope analysis showed lower decomposition and mineralisation rates of labelled green manure only during the first few days under the future precipitation treatment, followed by an increase in ¹⁵N enrichment of soil solution NO₃^- during summer, emphasising the importance of plant biomass production on root NO₃^- uptake from the soil. N₂O emissions were higher after the application of synthetic fertiliser during the first year, highlighting the importance of available NO₃^- in agricultural systems for nitrification and denitrification processes. However, lower N₂O emissions were observed during the second year, indicating possible N stress. Overall we found that N losses through NO₃^- leaching and N₂O emissions were most sensitive to reduced precipitation when NO₃^- is available, which can cause aggravating environmental problems in the future.

The stable isotope labelling technique proved to be successful for tracing and identifying drought stress effects on plant and soil processes in agricultural systems, allowing for a better understanding of soil-plant processes under changing climate conditions.