The legacy effect of synthetic N fertiliser
Wytse J. Vonk, Renske Hijbeek, M. J. Glendining, D. S. Powlson, Anne Bhogal, Ines Merbach, João Vasco Silva, Hanna Poffenbarger, Jagmandeep Dhillon, Klaus Sieling, H.F.M. ten Berge
Abstract
Abstract Cumulative crop recovery of synthetic fertiliser nitrogen (N) over several cropping seasons (legacy effect) generally receives limited attention. The increment in crop N uptake after the first‐season uptake from fertiliser can be expressed as a fraction (∆RE) of the annual N application rate. This study aims to quantify ∆RE using data from nine long‐term experiments (LTEs). As such, ∆RE is the difference between first season (RE 1st ) and long‐term (RE LT ) recovery of synthetic fertiliser N. In this study, RE 1st was assessed either by the 15 N isotope method or by a zero‐N subplot freshly superimposed on a long‐term fertilised LTE treatment plot. RE LT was calculated by comparing N uptake in the total aboveground crop biomass between a long‐term fertilised and long‐term control (zero‐N) treatment. Using a mixed linear effect model, the effects of climate, crop type, experiment duration, average N rate, and soil clay content on ∆RE were evaluated. Because the experimental setup required for the calculation of ∆RE is relatively rare, only nine suitable LTEs were found. Across these nine LTEs in Europe and North America, the mean ∆RE was 24.4% (±12.0%, 95% CI) of annual N application, with higher values for winter wheat than for maize. This result shows that fertiliser‐N retained in the soil and stubble may contribute substantially to crop N uptake in subsequent years. Our results suggest that an initial recovery of 43.8% (±11%, 95% CI) of N application may increase to around 66.0% (±15%, 95% CI) on average over time. Furthermore, we found that ∆RE was not clearly related to long‐term changes in topsoil total N stock. Our findings show that the—often used—first‐year recovery of synthetic fertiliser N application does not express the full effect of fertiliser application on crop nutrition. The fertiliser contribution to soil N supply should be accounted for when exploring future scenarios on N cycling, including crop N requirements and N balance schemes. Highlights Nine long‐term cereal experiments in Europe and USA were analysed for long‐term crop N recovery of synthetic N fertiliser. On average, and with application rates between 34 and 269 kg N/ha, crop N recovery increased from 43.8% in the first season to 66.0% in the long term. Delta recovery was larger for winter wheat than maize. Observed increases in crop N uptake were not explained by proportionate increases in topsoil total N stock.