D.W. Hopkins1,2,3, I.S. Waite4 and A.G. O’Donnell4
1 Scottish Crop Research Institute
2 School of Biological and Environmental Sciences, University of Stirling
3 Present address: School of Life Sciences, Heriot-Watt University
4 Faculty of Natural and Agricultural Sciences, University of Western Australia

Concerns about the volatility of soil organic C and the potential contribution it could make to atmospheric CO2, particularly under warmer climate conditions, have focussed attention on sites with long-term (decades or centuries) stable management and reliable base-line data which can be used to make accurate assessments of any changes. The Palace Leas Meadow Hay Plots in north-east England, UK, established in 1897, is the second oldest grassland experiment in the world and they have received known and constant management including fertilizer and manure applications ever since. Systematic, replicated measurements of the soil organic C and total N contents and the bulk densities were made in the 1980s for six contrasting plots at 3 cm depth increments down the profile to enable accurate estimates of soil organic C and N stocks to be made on an areal basis. We repeated these measurements in 2006 to enable a thorough examination of any changes in the amounts and distribution of soil organic C free from confounding influences of land-use change and uncertainties associated with the use of derived (rather than measured) bulk densities.  There have been no consistent differences in the soil organic C stocks during the 20+ year sampling interval despite a detectable increase in soil temperatures over the same period. Measurements of soil organic C mineralization during laboratory incubation for 200 days and determination of soil microbial biomass indicated that the distribution of microbial activity differed between the plots, but these factors did not contribute to changes in the overall soil organic C stocks of the soils. The absence of consistent changes in soil organic C may suggest that any changes in soil organic C mineralization as a result of elevated soil temperature have been insufficiently large to be detected; or that feedback processes, such as increased primary production or partitioning of plant biomass below ground, have off-set any increases in soil organic C mineralization; or that the estimation of small changes in soil organic C stocks has been confounded by the spatial heterogeneity of the soils. The data also highlight the importance of bench-mark sites under stable management and with reliable base-line data for long-term monitoring of soil organic C, especially in an environmental change context.