Nitrogen additions inhibit nitrification in acidic soils in a subtropical pine plantation: effects of soil pH and compositional shifts in microbial groups

Abstract

Plantation forests play a pivotal role in carbon sequestration in terrestrial ecosystems, but enhanced nitrogen (N) deposition in these forests may affect plantation productivity by altering soil N cycling. Hence, understanding how simulated N deposition affects the rate and direction of soil N transformation is critically important in predicting responses of plantation productivity in the context of N loading. This study reports the effects of N addition rate (0, 40, and 120 kg N ha⁻¹ a⁻¹) and form (NH₄Cl vs. NaNO₃) on net N mineralization and nitrification estimated by in situ soil core incubation and on-soil microbial biomass determined by the phospholipid fatty acid (PLFA) method in a subtropical pine plantation. N additions had no influences on net N mineralization throughout the year. Net nitrification rate was significantly reduced by additions of both NH₄Cl (71.5) and NaNO₃ (47.1%) during the active growing season, with the stronger inhibitory effect at high N rates. Soil pH was markedly decreased by 0.16 units by NH₄Cl additions. N inputs significantly decreased the ratio of fungal-to-bacterial PLFAs on average by 0.28 (49.1%) in November. Under NH₄Cl additions, nitrification was positively related with fungal biomass and soil pH. Under NaNO₃ additions, nitrification was positively related with all microbial groups except for bacterial biomass. We conclude that simulated N deposition inhibited net nitrification in the acidic soils of a subtropical plantation forest in China, primarily due to accelerated soil acidification and compositional shifts in microbial functional groups. These findings may facilitate a better mechanistic understanding of soil N cycling in the context of N loading.

Keywords

Acidification / Atmospheric nitrogen deposition / Microbial functional group / Nitrification / Soil nitrogen transformation

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