Nitrogen additions inhibit nitrification in acidic soils in a subtropical pine plantation: effects of soil pH and compositional shifts in microbial groups
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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.
KeywordsAcidification Atmospheric nitrogen deposition Microbial functional group Nitrification Soil nitrogen transformation
Special thanks are due to the Qianyanzhou Experimental Station of Red Soil and Hilly Land, Chinese Academy of Sciences, Jiangxi Province, China, for permission to work in their permanent slash pine plantation experiment plots. Thanks to all the staff and students of the Qianyanzhou Experimental Station for their assistance in the fieldwork. The authors acknowledge the contributions of the anonymous reviewers. This research is financially supported by the Grants from the National Key Research and Development Plan (No. 2016YFD06000202), and the National Natural Science Foundation of China (Nos. 31570443, 31130009).
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Conflict of interest
All authors declare that they have no conflict of interest.
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