title: Long-term management changes topsoil and subsoil organic carbon and nitrogen dynamics in a temperate agricultural system.
abstract: Soil organic carbon (SOC) and nitrogen (N) contents are controlled partly by plant inputs that can be manipulated in agricultural systems. Although SOC and N pools occur mainly in the topsoil (upper 0.30 m), there are often substantial pools in the subsoil that are commonly assumed to be stable. We tested the hypothesis that contrasting long-term management systems change the dynamics of SOC and N in the topsoil and subsoil (to 0.75 m) under temperate conditions. We used an established field experiment in the UK where control grassland was changed to arable (59 years before) and bare fallow (49 years before) systems. Losses of SOC and N were 65 and 61% under arable and 78 and 74% under fallow, respectively, in the upper 0.15 m when compared with the grass land soil, whereas at 0.3-0.6-m depth losses under arable and fallow were 41 and 22% and 52 and 35%, respectively. The stable isotopes13C and15N showed the effects of different treatments. Concentrations of long-chain n-alkanes C27, C29and C31were greater in soil under grass than under arable and fallow. The dynamics of SOC and N changed in both topsoil and subsoil on a decadal time-scale because of changes in the balance between inputs and turnover in perennial and annual systems. Isotopic and geochemical analyses suggested that fresh inputs and decomposition processes occur in the subsoil. There is a need to monitor and predict long-term changes in soil properties in the whole soil profile if soil is to be managed sustainably.
  
  
  CONCLUSIONS
  Land-use change affects soil organic carbon and nitrogen, but usually the topsoil only is considered.Grassland cultivated to arable and fallow lost 13-78% SOC and N to 0.6 m depth within decades.Isotopic and biomarker analyses suggested changes in delivery and turnover of plant-derived inputs.The full soil profile must be considered to assess soil quality and sustainability.
authors.affiliation: Department of Sustainable Soils and Grassland Systems Rothamsted Research Harpenden Hertfordshire AL5 2JQ UK.
authors.forename: A S
authors.initials: AS
authors.lastname: Gregory
authors.affiliation: Department of Sustainable Soils and Grassland Systems Rothamsted Research North Wyke, Okehampton Devon EX20 2SB UK.
authors.forename: J A J
authors.initials: JA
authors.lastname: Dungait
authors.affiliation: Department of Sustainable Soils and Grassland Systems Rothamsted Research Harpenden Hertfordshire AL5 2JQ UK.
authors.forename: C W
authors.initials: CW
authors.lastname: Watts
authors.affiliation: Terrestrial Biogeochemistry Group, Institute of Bio- and Geosciences IBG-3: Agrosphere Forschungszentrum Jülich GmbH Jülich 52425 Germany.
authors.forename: R
authors.initials: R
authors.lastname: Bol
authors.affiliation: Department of Sustainable Soils and Grassland Systems Rothamsted Research North Wyke, Okehampton Devon EX20 2SB UK.
authors.forename: E R
authors.initials: ER
authors.lastname: Dixon
authors.affiliation: Department of Computational and Systems Biology Rothamsted Research Harpenden Hertfordshire AL5 2JQ UK.
authors.forename: R P
authors.initials: RP
authors.lastname: White
authors.affiliation: Department of Sustainable Soils and Grassland Systems Rothamsted Research Harpenden Hertfordshire AL5 2JQ UK.
authors.forename: A P
authors.initials: AP
authors.lastname: Whitmore