Trends in soil organic matter contents in Dutch grasslands and maize fields on sandy soils

There is considerable concern in Europe that soil organic matter (SOM) contents are declining, which would threaten both agriculture and the environment. We performed a trend analysis of SOM contents in sandy soils, using historic data from routine agricultural soil analyses. Data were selected from grass, grass-maize rotation and maize fields in four adjacent provinces that had been sampled four to five times during the period 1984¿2004. Absolute (at least 1%) and relative changes (SOMt=20/SOMt=0) were calculated and regressed against initial SOM contents. Mean SOM content showed a north-south gradient per cropping system. We found no single uniform trend in SOM contents for any of the three systems. Over the 20-year period, SOM declined in c. 25% of all grasslands, amounting to 185 000 of the 635 000 hectares of land under grass and forage crops in the four provinces, and increased in a total of 267 000 hectares. Carbon accumulation in grassland sandy soils was calculated at 39 g C m¿2 year¿1 (top 5 cm). For the grasslands, initial SOM contents were linearly and negatively related to absolute changes in SOM; the relation with the relative change was best explained by using log-transformed values of SOM. We conclude that in grassland soils in the Netherlands, conservation of SOM requires identification of high-risk fields rather than high-risk areas. For continuous maize on sandy soils, the entire area may be denoted as high-risk, because all fields could reach the critical limit of 3.4% SOM in the near future