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The nitrate to ammonium ratios in nitrogen (N) compounds in wet\udatmospheric deposits have increased over the recent past, which is a cause\udfor some concern as the individual effects of nitrate and ammonium deposition\udon the biomass of different soil microbial communities and enzyme activities\udare still poorly defined. We established a field experiment and applied\udammonium (NH4Cl) and nitrate (NaNO3) at monthly intervals over a\udperiod of 4 years. We collected soil samples from the ammonium and nitrate\udtreatments and control plots in three different seasons, namely spring,\udsummer, and fall, to evaluate the how the biomass of different soil\udmicrobial communities and enzyme activities responded to the ammonium\ud(NH4Cl) and nitrate (NaNO3) applications. Our results showed that\udthe total contents of phospholipid fatty acids (PLFAs) decreased by 24 and\ud11 % in the ammonium and nitrate treatments, respectively. The inhibitory\udeffects of ammonium on Gram-positive bacteria (G+) and bacteria, fungi,\udactinomycetes, and arbuscular mycorrhizal fungi (AMF) PLFA contents ranged\udfrom 14 to 40 % across the three seasons. We also observed that the\udabsolute activities of C, N, and P hydrolyses and oxidases were inhibited by\udammonium and nitrate, but that nitrate had stronger inhibitory effects on the\udactivities of acid phosphatase (AP) than ammonium. The activities of\udN-acquisition specific enzymes (enzyme activities normalized by total PLFA\udcontents) were about 21 and 43 % lower in the ammonium and nitrate\udtreatments than in the control, respectively. However, the activities of\udP-acquisition specific enzymes were about 19 % higher in the ammonium\udtreatment than in the control. Using redundancy analysis (RDA), we found that\udthe measured C, N, and P hydrolysis and polyphenol oxidase (PPO) activities\udwere positively correlated with the soil pH and ammonium contents, but were\udnegatively correlated with the nitrate contents. The PLFA biomarker contents\udwere positively correlated with soil pH, soil organic carbon (SOC), and total\udN contents, but were negatively correlated with the ammonium contents. The\udsoil enzyme activities varied seasonally, and were highest in March and\udlowest in October. In contrast, the contents of the microbial PLFA biomarkers\udwere higher in October than in March and June. Ammonium may inhibit the\udcontents of PLFA biomarkers more strongly than nitrate because of\udacidification. This study has provided useful information about the effects\udof ammonium and nitrate on soil microbial communities and enzyme activities

Contrasting effects of ammonium and nitrate additions on the biomass of soil microbial communities and enzyme activities in subtropical China