Reconciling nitrogen fertilization effects on soil nitrification with acidification.

Nitrogen (N) fertilization supports nearly half of the global population but disrupts the N cycle. While N addition is assumed to stimulate soil nitrification by increasing substrate supply and nitrifier activity, it may also inhibit nitrification via soil acidification. Resolving this paradox is key to understanding the nitrification response to N addition (βnit). Using a global dataset, we show that βnit is best explained by final soil pH and ecosystem type. The βnit-pH relationship is positive in croplands, weak in forests, and negative in grasslands. Notably, βnit shifts from negative to positive above a pH threshold of 5.7 in croplands. Structural equation modeling reveals that final pH affects βnit in croplands by altering ammonia-oxidizing bacteria abundance and ammonium concentration in response to N addition. Our findings underscore the critical role of pH in regulating βnit and improving predictions of ecosystem N dynamics and plant productivity under future N inputs.