Changes in soil carbon, nitrogen, phosphorus along two temperate slope-floodplain-creek catenas under different land use - A case study in Southeast Germany

The impacts of land use on organic carbon (OC) and nitrogen (N) in different ecosystems have been widely studied. However, less attention has been paid to its influence on the change of total phosphorus (P_t) and its two fractions, inorganic P (P_i) and organic P (P_o), along slopes. In addition, since N and P are the main factors causing the eutrophication of aquatic ecosystems, it is essential to link the terrestrial and aquatic ecosystems.

Two slope-floodplain-creek catenas were chosen in the low mountain region of the eastern Bavarian Forest. The slopes of the two catenas have been extensively managed for grassland and cropland use, respectively. The floodplain was under grassland use. They were fertilized with sludge, occasionally with farmyard manure, under the same regime. Additional carbonic magnesium lime was applied annually at the cropland. Soil profiles at the top, middle and bottom slope, as well as the floodplain area, were sampled according to their horizons and replicated with three soil cores in each catena. Liquid-nitrogen freeze-cores of the aquatic sediments were sampled to extend each catena into the below-slope creek.

We found the grassland slope had higher OC and N content in the topsoil (0-10 cm) and higher P_t and P_o content in both the topsoil (0-40 cm) and subsoil (40-60 cm). The P content in the topsoil (0-40cm) of the grassland slope was dominated by P_o, while the cropland slope was P_i -dominated. The OC and N stocks on the grassland slope were significantly higher than at the corresponding position on the cropland slope. Except for the lowest P stock found on the top slope of grassland, the P_t stock in topsoil (0-20 cm) was not land-use affected while the P_o stock was significantly higher on the grassland slope. An accumulation of OC, N and P_o in topsoil (0-20 cm) from top to bottom of the slope was observed. In the floodplain, upslope cropland use decreased the OC, N, P_t content and stocks, but reversely for P_o. The upslope land use effect on OC, N and P content in the creek sediment was limited.

In conclusion, this study provides a field-based observation on the change of OC, N and P (P_t and P_o) under different land use upslope from terrestrial to aquatic systems through two slope-floodplain-creek catenas.