The legacy effect of synthetic N fertiliser

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.