Context-dependent tree species effects on soil nitrogen transformations and related microbial functional genes

• Published in: Biogeochemistry Vol. 140; no. 2; pp. 145 - 160
• Main Authors: Ribbons, Relena R., Kepfer-Rojas, Sebastian, Kosawang, Chatchai, Hansen, Ole K., Ambus, Per, McDonald, Morag, Grayston, Sue J., Prescott, Cindy E., Vesterdal, Lars
• Format: Journal Article
• Language: English
• Published: Cham Springer Science + Business Media 01-09-2018; Springer International Publishing; Springer Nature B.V
• Subjects: Abundance; Acidic soils; Acidification; Ammonia; Ammonia-oxidizing bacteria; Ammonification; Archaea; Bacteria; Beech; Biogeosciences; Community composition; Cycles; Denitrification; Dilution; Earth and Environmental Science; Earth Sciences; Ecosystems; Environmental Chemistry; Fungi; Genes; Immobilization; Land use; Life Sciences; Microbial activity; Microorganisms; Mineral nutrients; Monooxygenase; NirK protein; Nitrate reductase; Nitrification; Nitrogen; Nitrogen cycle; Nitrogen isotopes; Nutrient cycles; ORIGINAL PAPERS; Oxidation; Pine trees; Plant species; Soil; Soil lime; Soil microorganisms; Soils; Species; Tree species; N transformation rates; Soil microbial communities; qPCR; N isotope pool-dilution
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1007/s10533-018-0480-8

Although it is generally accepted that tree species can influence nutrient cycling processes in soils, effects are not consistently found, nor are the mechanisms behind tree species effects well understood. Our objectives were to gain insights into the mechanism(s) underlying the effects of tree species on soil nitrogen cycling processes, and to determine the consistency of tree species effects across sites. We compared N cycling in soils beneath six tree species (ash, sycamore maple, lime, beech, pedunculate oak, Norway spruce) in common garden experiments planted 42 years earlier at three sites in Denmark with distinct land-use histories (forest and agriculture). We measured: (1) net and gross rates of N transformations using the ¹⁵N isotope pool-dilution method, (2) soil microbial community composition through qPCR of fungal ITS, bacterial and archaeal 16S, and (3) abundance of functional genes associated with N cycling processes—for nitrification the archaeal and bacterial ammonia-monooxygenase genes (amoA AOA and amoA AOB, respectively) and for denitrification, the nitrate reductase genes nirK and nirS. Carbon concentrations were higher in soils under spruce than under broadleaves, so N transformation rates were standardized per g soil C. Soil NH₄⁺ parameters (gross ammonification, gross NH₄⁺ consumption, net ammonification (net immobilization in this case), and NH₄⁺ concentrations, per g C) were all lowest in soils under spruce. Soils under spruce also had the lowest gene abundance of bacteria, bacterial:-fungal ratio, denitrifying microorganisms, ammoniaoxidizing archaea and ammonia-oxidizing bacteria. Differences in N-cycling processes and organisms among the five broadleaf species were smaller. The ‘spruce effect’ on soil microbes and N transformations appeared to be driven by its acidifying effect on soil and tighter N cycling, which occurred at the previously forested sites but not at the previously agricultural site. We conclude that existing characteristics of soils, including those resulting from previous land use, mediate the effects of tree species on the soil microbial communities and activities that determine rates of N-cycling processes.