Abstract
In grasslands, N mineralization and nitrification are important processes and are controlled by several factors, including the in situ microbial community composition. Nitrification involves ammonia oxidising archaea (AOA) and bacteria (AOB) and although AOA and AOB co-exist in soils, they respond differently to environmental characteristics and there is evidence of AOA/AOB niche differentiation. Here, we investigated temporal variation in N mineralization and nitrification rates, together with bacterial, archaeal and ammonia-oxidiser communities in grassland soils, on different geologies: clay, Greensand and Chalk. Across geologies, N mineralization and nitrification rates were slower in the autumn than the rest of the year. Turnover times for soil ammonium pools were <24 h, whilst several days for nitrate. In clay soils, bacterial, archaeal, AOA, and AOB communities were clearly distinct from those in Chalk and Greensand soils. Spatially and temporally, AOA were more abundant than AOB. Notably, Nitrososphaera were predominant, comprising 37.4% of archaeal communities, with the vast majority of AOA found in Chalk and Greensand soils. AOA abundance positively correlated with nitrate concentration, whereas AOB abundance correlated with ammonium and nitrite concentrations, suggesting that these N compounds may be potential drivers for AOA/AOB niche differentiation in these grassland soils.
Highlights
Grasslands cover ~40% of the Earth’s land surface, are high in organic matter and represent a large reservoir of nitrogen (N) (Cam bardella and Elliott, 1992)
Changes in ammonia oxidising archaea (AOA) abundance were found to positively correlate with changes in nitrate concentration, whereas Ammonia oxidising bacteria (AOB) abundance correlated with ammonium coef 1⁄4 2.00, z 1⁄4 4.62, P < 0.001) and nitrite con centrations (Fig. 3D)
Whilst we found that AOA were more abundant than AOB, spatially and temporally, we cannot attribute the nitrification rates measured here to either AOA or AOB
Summary
Grasslands cover ~40% of the Earth’s land surface, are high in organic matter and represent a large reservoir of nitrogen (N) (Cam bardella and Elliott, 1992). Grasslands were identified as a priority habitat under the UK Biodiversity Action Plan (BAP) as they are important habitats for biodiversity and carbon sequestration; with UK grasslands sequestering 240 200 kg of carbon per hectare per year (Natural England, 2008; Ostle et al, 2009). Mineralization of soil organic matter by microorganisms is an important process in grassland N cycling, and is crucial for regulating the available N in soils for plant growth as well as preventing a net loss of N from the environment (Yao et al, 2011; Van der Heijden et al, 2008; Zhang et al, 2012). Other factors may control soil N mineralization and nitrifi cation rates, including soil organic nitrogen (SON), soil water avail ability, total N, soil organic C (SOC), C:N ratio and microbial community composition (Dalal and Meyer, 1987; Fisk and Schmidt, 1995; Von Lutzow and Ko€gel-Knabner, 2009)
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