Long‐term leakage of nitrate from bare unmanured soil

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Abstract
The Rothamsted Drain Gauges built in 1870 comprise blocks of soil, 0.5, 1.0 and 1.5 m deep, isolated laterally by brickwork and undermined for the collection of drainage water but otherwise undisturbed. The soil has not been cropped, manured or cultivated subsequently. The annual nitrate leakages from these blocks were recorded for the 38 years from 1877/8 to 1914/5. The soil in the 0.5 and 1.5‐m gauges lost on average about 45 kg ha−1 of nitrate‐N per year during the first seven years of this period; that in the 1.0‐m gauge lost slightly less. The overall decline in leakage was masked by large annual fluctuations attributable mainly to variation in rainfall. Fitting a simple function that assumed an exponential decline and took account of rainfall fluctuations gave a rate constant for each gauge from which the half‐life could be estimated for the organic nitrogen feeding the leakage. The half‐life for the 1.5‐m gauge was 41 yr. The average nitrate leakage during the first seven years of the record differs little from estimates of the current leakage from soil carrying fully fertilized crops of winter wheat. This and the long half‐life of the leakage show that pollution of drainage water by nitrate will not be controlled by limiting the use of fertilizer in the short term.