Ammonium and nitrate additions differentially affect soil microbial biomass of different communities and enzyme activities in slash pine plantation in subtropical China
The ratios of nitrate to ammonium in wet atmosphere nitrogen (N) deposition compounds were increasing recently. However, the individual effects of nitrate and ammonium deposition on soil microbial communities biomass and enzyme activities are still unclear. We conducted a four-year N addition field experiment to evaluate the responses of soil microbial communities biomass and enzyme activities to ammonium (NH<sub>4</sub>Cl) and nitrate (NaNO<sub>3</sub>) additions. Our results showed that (1)
... s showed that (1) the inhibitory effects of ammonium additions on total mass of phospholipid fatty acid (PLFA) were stronger than those of nitrate additions. Both decreased total PLFA mass about 24&thinsp;% and 11&thinsp;%, respectively. The inhibitory effects of ammonium additions on gram positive bacteria (G<sup>+</sup>) and bacteria, fungi, actinomycetes (A), and arbuscular mycorrhizal fungi (AMF) PLFA mass ranged from 14&thinsp;%&ndash;40&thinsp;%. (2) Both ammonium and nitrate additions inhibited absolute activities of C, N, and P hydrolyses and oxidases, and nitrate additions had stronger inhibition effects on the acid phosphatase (AP) than ammonium additions. Both ammonium and nitrate additions decreased N-acquisition specific enzyme activities (enzyme activities normalized by total PLFA mass) about 21&thinsp;% or 43&thinsp;%, respectively. However, ammonium additions increased P-acquisition specific enzyme activities about 19&thinsp;% comparing to control. (3) Redundancy analysis (RDA) showed that the measured C, N, and P hydrolyses and polyphenol oxidase (PPO) activities were positively correlated with soil pH and ammonium contents, but negatively with nitrate contents; the mass of PLFA biomarkers were positively correlated with soil pH, soil organic carbon (SOC), and total N contents, but negatively with ammonium contents. (4) The soil enzyme activities varied seasonally in the order of March&thinsp;>&thinsp;June&thinsp;>&thinsp;October. On the contrary, microbial PLFA mass was higher in October than in March and June. Our results concluded that inhibition of mass of PLFA biomarkers and enzyme activities might be contributed to acidification caused by ammonium addition. Soil absolute enzyme activities were inhibited indirectly by acidification and nitrification, but specific enzyme activities normalized by PLFA were directly affected by N additions. It was meaningful to separate the effects of ammonium and nitrate additions on soil microbial communities and enzyme activities.