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

The nitrate to ammonium ratios in nitrogen (N) compounds in wet atmospheric deposits have increased over the recent past, which is a cause for some concern as the individual effects of nitrate and ammonium deposition on the biomass of different soil microbial communities and enzyme activities are still poorly defined. We established a field experiment and applied ammonium (NH4Cl) and nitrate (NaNO3) at monthly intervals over a period of 4 years. We collected soil samples from the ammonium and nitrate treatments and control plots in three different seasons, namely spring, summer, and fall, to evaluate the how the biomass of different soil microbial communities and enzyme activities responded to the ammonium (NH4Cl) and nitrate (NaNO3) applications. Our results showed that the total contents of phospholipid fatty acids (PLFAs) decreased by 24 and 11 % in the ammonium and nitrate treatments, respectively. The inhibitory effects of ammonium on Gram-positive bacteria (G+) and bacteria, fungi, actinomycetes, and arbuscular mycorrhizal fungi (AMF) PLFA contents ranged from 14 to 40 % across the three seasons. We also observed that the absolute activities of C, N, and P hydrolyses and oxidases were inhibited by ammonium and nitrate, but that nitrate had stronger inhibitory effects on the activities of acid phosphatase (AP) than ammonium. The activities of N-acquisition specific enzymes (enzyme activities normalized by total PLFA contents) were about 21 and 43 % lower in the ammonium and nitrate treatments than in the control, respectively. However, the activities of P-acquisition specific enzymes were about 19 % higher in the ammonium treatment than in the control. Using redundancy analysis (RDA), we found that the measured C, N, and P hydrolysis and polyphenol oxidase (PPO) activities were positively correlated with the soil pH and ammonium contents, but were negatively correlated with the nitrate contents. The PLFA biomarker contents were positively correlated with soil pH, soil organic carbon (SOC), and total N contents, but were negatively correlated with the ammonium contents. The soil enzyme activities varied seasonally, and were highest in March and lowest in October. In contrast, the contents of the microbial PLFA biomarkers were higher in October than in March and June. Ammonium may inhibit the contents of PLFA biomarkers more strongly than nitrate because of acidification. This study has provided useful information about the effects of ammonium and nitrate on soil microbial communities and enzyme activities.近年来，大气湿沉降中氮（N）化合物的硝态氮与铵态氮比值呈上升趋势，这引发了一定担忧，因为硝态氮和铵态氮沉降对不同土壤微生物群落生物量及酶活性的单独影响仍未得到明确阐释。本研究设置了野外原位实验，在4年时间内每月施加氯化铵（NH₄Cl）和硝酸钠（NaNO₃）。我们分别在春季、夏季和秋季三个季节采集铵态氮、硝态氮处理组与对照组的土壤样品，以探究不同土壤微生物群落生物量及酶活性对铵态氮和硝态氮添加的响应。研究结果显示，铵态氮和硝态氮处理组的磷脂脂肪酸（phospholipid fatty acids, PLFAs）总含量分别下降了24%和11%。铵态氮对革兰氏阳性菌（Gram-positive bacteria, G+）、细菌、真菌、放线菌以及丛枝菌根真菌（arbuscular mycorrhizal fungi, AMF）的磷脂脂肪酸含量的抑制率在三个季节中介于14%至40%之间。我们还观察到，铵态氮和硝态氮均会抑制碳（C）、氮（N）、磷（P）水解酶及氧化酶的绝对活性，但硝态氮对酸性磷酸酶（acid phosphatase, AP）活性的抑制作用强于铵态氮。以总磷脂脂肪酸含量归一化后的氮获取特异性酶活性，在铵态氮和硝态氮处理组中分别较对照组低约21%和43%。然而，铵态氮处理组的磷获取特异性酶活性较对照组高出约19%。通过冗余分析（redundancy analysis, RDA）可知，本研究测定的碳、氮、磷水解酶及多酚氧化酶（polyphenol oxidase, PPO）活性与土壤pH值和铵态氮含量呈正相关，而与硝态氮含量呈负相关。磷脂脂肪酸生物标记物含量与土壤pH值、土壤有机碳（soil organic carbon, SOC）及总氮含量呈正相关，与铵态氮含量呈负相关。土壤酶活性存在季节波动，3月活性最高，10月活性最低。与之相反，土壤微生物磷脂脂肪酸生物标记物含量在10月高于3月和6月。由于酸化作用，铵态氮对磷脂脂肪酸生物标记物含量的抑制作用可能强于硝态氮。本研究为硝态氮和铵态氮对土壤微生物群落及酶活性的影响提供了有价值的参考信息。