Atmospheric ammonia concentration modulates soil enzyme and microbial activity in an oak forest affecting soil microbial biomass

The present work was carried out to assess the effect of atmospheric ammonia (NH3) on soil physicochemical properties, soil enzymatic activities (β-glucosidase -β-GLU-, nitrate reductase -NR-, urease -UR-, protease -PRO-, acid phosphatase -PHO-, dehydrogenase -DHA-), soil microbial biomass and soil respiration. The study was conducted along a NH3 gradient in a Q. pubescens Milld. forest in the vicinity of two livestock farms. Because of NHy (NHy: NH3 and NH4+) deposition, N saturation was detected up to 330 m from the farms. This excess of N led to a decrease in soil C:N and an increase in soil nitrification processes, which resulted in an accumulation of the heavy N isotope (15N) in the soil. N saturation was also reflected in the activity of NR enzyme, which was inhibited. On the other hand, while UR enzyme was inhibited close to the farms possibly due to the high amount of N-NH4+ resulting from the hydrolysis of NH3, PRO activity was stimulated by the presence of organic nitrogen compounds and the need of soil organisms to meet the C demand. In addition, the activity of PHO and β-GLU enzymes was regulated by the relative amount of C and P that organisms need. Regarding biological variables, enhanced NH3 reduced soil microbial biomass and biomass respiratory efficiency. Finally, soil enzyme activities and soil microbial biomass have proved to be good biological indicators of soil quality.