Cover crop varieties, tillage dates and irrigation on-demand: their impact on nitrogen and soil water dynamics in Austria’s semi-arid Marchfeld region

Cover crops (CC) have shown promise in reducing nitrogen (N) leaching, but in semi-arid regions, they might compete with subsequent field culture for water availability. In Austria's Marchfeld region, which is intensively used for agriculture, nitrate levels in groundwater exceed threshold values due to N surplus and limited dilution of seepage water caused by low annual precipitation. Early sown CC with sufficient emergence and N uptake might reduce such groundwater contamination. Further, later tillage dates keep the N stored in the organic pool and reduce mineralisation during autumn and winter. However, CC induced changes in soil water availability could impact the follow-up crop.
This study investigates the impact of cover crop (CC) varieties with different i) seed compositions, ii) tillage dates and iii) on-demand irrigation on N leaching and soil water availability for subsequent field culture.

The randomized block trial included a) frosting CC – autumn conversion, b) frosting CC – spring conversion, c) a mixture of winter hardy and frosting CC – spring conversion and d) fallow plots. On-demand irrigation was performed at plots with same varieties to enhance CC emergence and simulate conditions of both wet and dry years. Within each plot soil moisture sensors and suction cups were installed. Some plots were equipped with matrix potential sensors. Monthly soil samples were analysed for plant available N and plant samples were taken twice. Evaporation was evaluated using four mini-lysimeters, one for each CC composition. STOTRASIM, which is a soil water and mass transport model was used to model the amount of seepage water of each plot and was calibrated on matrix potential measurements.

In general, the results show that all tested varieties of CC significantly reduce plant available nitrogen during winter compared to fallow. Despite the relatively low levels of leached N, in semi-arid regions even minor amounts pose a risk of groundwater contamination. Soil water content analysis revealed no significant differences between the CC varieties. The yield of the subsequent crop remained unaffected by the different CC.
While CC reduced N leaching and did not compete with the subsequent field culture, integrating practical considerations like phyto-sanitation, seedbed preparation, tillage methods, crop rotation and succeeding crop selection into CC practices is crucial to prevent adverse effects on subsequent field culture.