Nitrogen mineralization: a review and meta‐analysis of the predictive value of soil tests

Accurate estimation of mineralizable nitrogen (N) from soil organic matter is essential to improve fertilizer management in agricultural systems. Mineralizable N refers to the amount of N in soil that is released during a certain period (ranging from 1 week to the length of a growing season). It has been estimated from increases in inorganic N during incubation or from N uptake by plants grown in a greenhouse or field. Many chemical soil tests measuring extractable organic N (EON) fractions have been proposed to predict mineralizable N. We evaluated the predictive value of these soil tests, using 2068 observations from 218 papers. Meta‐analysis was used to find the best soil test, to analyse differences between field and laboratory experiments, and to determine whether their predictive value is affected by extraction intensity (% of total soil N that is extracted). The concentration of EON was positively related to mineralizable N, explaining on average 47% of the variation. It did not, however, explain more of the variation than total N. Best predictions (57% < R 2 < 74%) were obtained when EON was extracted with hot CaCl 2 , acid KMnO 4 , acid K 2 Cr 2 O 7 , hot water or hot KCl. Extraction intensity was not related to the strength of the above‐mentioned relationship. Predictions of mineralizable N were significantly worse when mineralization was measured in the field compared with measurements under controlled conditions. We found no evidence of a causal and direct relationship between EON and mineralizable N. Accuracy of soil testing may improve when the current ‘single soil test approach’ changes to a more complex approach, which includes different soil tests, soil properties and environmental conditions.