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

Hanegraaf, M.C.; Hoffland, E.; Kuikman, P.J.; Brussaard, L.

Résumé

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