The legacy effect of synthetic N fertiliser

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 ¹⁵N 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.

Keywords: 15N; cereal production; fertiliser requirement; long‐term experiment; nitrogen recovery; nitrogen use efficiency; soil N retention; soil N supply; synthetic fertiliser N.

FIGURE 1

Locations of long‐term experiments in North America (a) and Europe (b) included in this study

FIGURE 2

Examples of short‐term treatments within a long‐term trial which allow for the calculation of ∆RE. The two shaded large fields have continuously received either synthetic N fertiliser or no N fertiliser. Often such treatments are part of a larger setup with multiple N rate treatments. At the right, examples of a ¹⁵N treatment and of a new control subplot are shown superimposed on the original (long‐term fertilised) treatment. The ¹⁵N subplot receives the same N rate as the historic N rate, but now the fertiliser is ¹⁵N labelled. The numbers indicate the amount of applied synthetic N fertiliser (these are examples only). Between brackets is indicated what is measured on these plots, with notation corresponding to Equations ((1), (2), (3))

FIGURE 3

Hypothetical development of total N uptake and fertiliser N recovery, with continuous synthetic N application over time. The short‐term recovery fraction (dark grey) increases over time in this graph, possibly by long‐term improvements in, for example, cultivars or management. It could also decrease, for example, by changing biotic or abiotic stresses. The light grey area indicates the additional recovery (∆RE) of synthetic fertiliser N via increased soil N supply. The total N recovery in a certain year is called long‐term recovery (RE^LT), indicated by the solid black arrow. Uptake from the native soil N stock (that existed prior to the start of LTE and dwindles over time) is not shown here

FIGURE 4

First season recovery and long‐term recovery for winter wheat, spring barley and maize (N = 66). The diagonal solid black line indicates RE^LT = RE^1st; the difference between that black line and the red diagonal dashed line indicates the estimated average ∆RE. Point‐size indicates the weight based on sample size. Note that data from the LTE Bad Lauchstädt is excluded from this graph because separate values of long and first season recovery could not be calculated and ∆RE was assessed differently (see Table S1; see Equation S2)

FIGURE 5

Violin plots of the distribution of ∆RE, separated for (a) crop types, (b) crop residue retention (no N = 59, yes N = 8), and (c,d) method; within each violin plot, a boxplot indicates the median, lower (1st) and upper (3rd) quantile of the data. Plot (c) illustrates the distribution of all data points (¹⁵N N = 47, subplot N = 20). Plot (d) contains only data from an experiment, which allowed for both ¹⁵N and the Subplot method (N = 16). If upper case letters near the median are identical then there is no significant difference