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The Palace Leas meadow hay plots have received different annual manure and fertilizer applications since 1897 and represent the second oldest permanent grassland experiment in the world. Although the soil properties have diverged as a result of the experimental treatments since 1897, the soil organic carbon (SOC) contents of the plots have remained largely unchanged between the 1980s and 2006. In this study we have determined the effects of contrasting manure and fertilizer treatments on the total N contents of the soil, the recalcitrance of soil organic matter, the contributions made from different depths to the overall C mineralization, and the distribution of soil microbial biomass in the soil profiles. Relative to the control plot, which has received no manure or fertilizer additions since 1897, the two plots that have received manure or P plus K contain significantly more N, which in the case of the P plus K plot is consistent with abundant Trifolium spp. in the sward. The plot that has received farmyard manure (FYM) contained 11% more N than the control. The two plots that received large inputs of (NH)SO and which have consequently become highly acidic (around pH 4) contain 15-27% less N than the control despite the N addition and the accumulation of SOC in these plots. The plot that has only received P has also apparently lost total N (20%) relative to the control. The rates of C mineralization of the microbial biomass were closely related to total SOC contents but have been positively influenced by the increased N in the P plus K plot. Values for soil N, C mineralization rate and microbial C are reported because such data from sites that have been under known long-term known land-use are extremely rare and because there is increasing realization of the potential importance of soil data, especially biological characteristics, for monitoring environmental changes. [ABSTRACT FROM AUTHOR]

A recent report of widespread declines in soil organic C (SOC) in the UK over the 10–25 years until the early 2000s has focussed attention on the importance of resampling previously characterized sites to assess long-term trends in SOC contents and the importance of soils as a potentially volatile and globally significant reservoir of terrestrial C. We have used two sets of long-term experimental plots which have been under constant and known management for over a century and for which historical data exist that allow comparison over recent decades to determine what, if any, changes in SOC content have occurred. The plots used are the Palace Leas (PL) Meadow Hay Plots in north-east England (UK) established in 1897, and from the Park Grass (PG) Continuous Hay experiment established in 1856 at Rothamsted in south-east England. Collectively, these plots represent the only grassland sites in the UK under long-term management where changes in SOC over several decades can be assessed, and are probably unique in the world. The plots have received different manure and fertilizer treatment and have been under known management for at least 100 years. In 1982, total SOC contents were determined for the 0–27 cm layer of six of the PL plots using measurements of SOC concentrations, bulk density and soil depth. In 2006, the same six PL plots were resampled and SOC contents determined again. Four of the plots showed no net change in SOC content, but two plots showed net loss of SOC of 15% and 17% (amounting to decreases of 18 and 15 t C ha−1) since 1982. However, these differences in total SOC content were in a similar range to the variations in bulk density (6–31%) with changing soil water content. In 1959, the soil masses and SOC concentrations to 23 cm depth were measured on six PG plots with fertilizer and manure treatments corresponding closely with those measured on PL. In 2002, the SOC concentrations on the same plots were measured again. On three of the PG plots, SOC concentrations had declined by 2–10%, but in the other three it had increased by 4–8% between 1959 and 2002. If it is assumed that the soil bulk density had not changed over this period, the losses of SOC from the top soils ranged range from 10 to 3 t C ha−1, while the gains ranged from 4 to 7 t C ha−1. When the differences with time in SOC contents for the six PL and the six PG plots were examined using paired t-tests, that is, regarding the plots as two sets of six replicate permanent grasslands, there were no significant differences between 1982 and 2006 for the PL plots or between 1959 and 2002 for the PG plots. Thus, these independent observations on similar plots at PL and PG indicate there has been no consistent decrease in SOC stocks in surface soils under old, permanent grassland in England in recent decades, even though meteorological records for both sites indicate significant warming of the soil and air between 1980 and 2000. Because the potential influences of changes in management or land use have been definitively excluded, and measured rather than derived bulk densities have been used to convert from SOC concentrations to SOC amounts, our observations question whether for permanent grassland in England, losses in SOC in recent decades reported elsewhere can be attributed to widespread environmental change. [ABSTRACT FROM AUTHOR]