Soil organic carbon contents of agricultural land in the Netherlands between 1984 and 2004

There is some debate about the likelihood that soil organic carbon (SOC) contents of agricultural land decreases because of global warming and governmental restrictions on animal manure application rates in some countries. Here, we report on changes in the mean SOC contents of the top soils (0-5 cm) of grassland and the top soil (0-25 cm) of arable land in the Netherlands during the period 1984-2004, using a data base with similar to 2 million results of SOC determinations from farmers' fields. The analyses were made for all agricultural land on mineral soils and for agricultural land in 9 regions with distinct differences in mean soil textures and SOC contents (marine and riverine clay, peaty clays, reclaimed peat soils, and Aeolian sand and loess), and land uses (arable land and permanent grassland). Except for the regions with peaty clay and reclaimed peat soils, samples with SOC>125 g/kg were designated as peat and peaty soils and excluded from the analyses. Mean SOC content of soils under arable land in 2003 ranged from 13 to 22 g/kg for sand, loess and clay soils to 59 g/kg for reclaimed peat soils. Mean SOC content of soils under permanent grassland in 2003 ranged from 22 to 56 g/kg for sand and clay soils. The difference in mean SOC contents between grassland and arable land is in part related to the difference in sampling depth. Mean SOC contents of all mineral soils under grasslands and arable land tended to increase annually by 0.10 and 0.08 g/kg, respectively. We observed large differences in mean trends between regions. Regions with relatively low SOC contents tended to accrue C by up to 0.37 g/kg/year, while regions with relatively high SOC contents (e.g., peaty clays) tended to lose C by up to 0.98 g/kg/year. In conclusion, mean SOC contents of the top part of mineral soils of agricultural land in most regions in the Netherlands tended to increase slightly during the period 1984-2004. This result contrasts with reports from e.g., United Kingdom and Belgium that suggest decreasing C stocks in arable land possibly due to changes in land use and climate