Soil pH moderates the resistance and resilience of C and N cycling to transient and persistent stress

The resilience of microbial functions like carbon (C) and nitrogen (N) cycling to stress is likely heavily dependent on pH. Past research, however, has been limited to laboratory manipulations or a pH gradient resulting from differences in soil mineralogy. In this study, soils were collected from a >50-year field trial where plots have been maintained at pH 4.9, 6 and 7.1. We selected copper (Cu) and heat to represent persistent and transient stresses, respectively. Changes in C mineralization, ammonia oxidation, denitrification, and gene (16S rRNA, nirK, nirS and amoA) abundance were immediately measured after heat- (40 °C for 16 h) and Cu- (500 μg Cu soil g⁻¹ or 1 mg Cu soil g⁻¹) induced stresses, during subsequent recovery over 56 days, and compared to an unstressed control. Higher soil pH significantly increased C mineralization (by 217 %), ammonia oxidation (by 617 %), and the gene abundances of 16S rRNA (by 77 %), nirK (by 976 %) and nirS (by 997 %). Soil pH had a significant (P < 0.001) selection effect on the phylotypes of bacterial communities and ammonium oxidizing bacteria (AOB). Ammonia oxidation was significantly (P < 0.05) more resistant and resilient to both Cu stresses in the pH 7.1 soil. C mineralization in the soil at pH 7.1 was significantly (P < 0.05) more resilient to low Cu than the soil at pH 4.9. Correspondingly, significantly (P < 0.001) distinct bacterial communities were present in these soils, indicating that bacterial composition triggered by the adaptation and tolerance to stress is a central factor governing functional resilience. Denitrification in the pH 7.1 soil was significantly (P < 0.05) more resilient to low and high Cu, compared to the soil at pH 4.9. Similarly, the abundances of nirS and nirK genes were greater in the higher pH soil. Although soil pH directly affects Cu but not heat stress, our results indicated that neutral soils harboured greater resilience of C and N cycling to both Cu (persistent) and heat (transient) stresses.