Organic C levels of Ap horizons in North German Pleistocene sands as influenced by climate, texture, and history of land-use

We evaluated the contents of organic carbon (C-org) of Ap horizons from 11 North German study areas along a Southeast to Northwest precipitation gradient with respect to their general levels and as related to C : N ratio, soil texture (clay content), bulk soil density, climate, and historical land-use since 1780. The focus was on sandy soils, with the largest group of samples originating from 308 km(2) of the Fuhrberg catchment north of Hannover/Lower Saxony. Data from loess areas were used for comparisons. Major aims were (1) to quantify current C-org stocks, (2) to provide data on site-specific, steady-state C, g levels in old arable soils, and (3) to identify the main controls of C-org levels in the studied sands. The mean C-org content in sandy, well-drained, old Ap horizons (uplands, > 200 years under cultivation, near steady-state) increased with precipitation from < 8 g kg(-1) in the dry eastern parts of the study area (530 min year(-1), 8.3degreesC) to 25 g kg(-1) in the moist Northwest (825 mm year(-1), 8.4degreesC). The C-org levels in lowlands which have been drained for more than 40 years were approximately 3 g kg(-1) higher than those of uplands under a similar climate. The factor clay content had no predictive value because low contents were associated with high C-org levels. Large proportions of refractory organic matter in sands resulting from specific features of historical land-use and soil development (calluna heathland, heath plaggen fertilization, podzolization) appeared to be tire most probable reason for such high C-org levels. However, the high C-org levels of these old arable sites were still exceeded by those of younger arable areas formerly under continuos grassland. A chrono-sequence suggested that a period of about 100 years is necessary until a new steady-state C-org level is established after conversion of grassland into arable land. Elevated C-org levels in current Ap horizons were also found for former woodland and heathland soils. The main conclusion is that sands can contain a lot of stable organic matter, sometimes more than finer textured soils.