The data was collected as part of a study (“Waterbuddies”) of grassland agroecosystems of the northwest German (NWG) coastal region. The aim of the study was to identify the sources and discharge paths of nutrients like phosphorus that play an important role as contaminants for surface waters and the North Sea. The NWG coastal region is characterized by a dense network of drainage ditches that directly connect agricultural areas with surface waters. The Jade catchment area was selected to represent the different landscape areas of the NWG coastal region.
Therefore, the study differentiates between the landscape areas of the geest with mostly sandy soils (Geest), the peatlands with bogs (Hochmoor) and fens (Niedermoor) as well as the marsh (Marsch) with a predominance of clay-rich soils. In addition to these landscape areas, two transition sites were identified. Both are oragnomineral in character but are either increasingly characterized by clay in the transition to the marsh (ÜMoMa), or increasingly characterized by sand in the transition to the geest (ÜGeMo). Besides drainage ditches, grassland areas are often drained via surface drainage and drainage pipes, which mark a second discharge path for nutrients. Surface drainage mainly is used for clay soils, whereas drainage is used in sandy and organic-rich soils. Surface drainage in the marsh region is permanently installed, leading ditch run-off via end pipes into higher order waterbodies. The Geest differs in this respect, here, temporary surface drainage is installed during the runoff period without an end pipe. This way, stagnating water on the field plots are drained as needed. Within this study area Phosphate-P contents of soils and phosphate-P concentration of discharge and ditch waters were determined to get a further insight into how different factors (soil type, surface vs. subsurface drainage) are influencing the P-dynamics of the ditches as surface waters of the intensively drained coastal region of Germany.
Sampling method:
Soil and water samples were taken between January 2019 and April 2022 at 49 study sites representative across the different landscapes. With the exception of the organomineral soils of the transition locations, soil samples were taken at selected locations in October 2019, 2020 and 2021 and in January 2020. The soil samples were taken with a Pürckhauer auger for the depths of 0 – 30 cm and 30 - 60 cm for determination of plant available P. In addition, soil samples of 0 – 10 cm were taken in spring 2019 to determine the organic matter content of the soil (Corg). Water samples were taken monthly in 2019 and at selected times from 2020 to 2022. A measuring cup on a telescopic rod was rinsed twice with ditch water or discharge water before water was collected for analysis and stored in 250 ml HDPE bottles. The samples were cooled for transport to the laboratory.
Analytical procedure:
The plant-available P was extracted using the DL (double lactate) method of VDLUFA (1991). In deviation from VDLUFA instructions, the method was modified as following: for peat soils, 10 g of soil were mixed with 250 ml of DL solution and shaken in an overhead shaker for 2 hours (Reax 20, Heidolph). Eluates were filtered through a cellulose filter with a pore size of 2 - 3 µm (ROTILABO round filter type 15A), the extracts were stored in 50 ml PE bottles and frozen at -20 °C until measurement. For measuring phosphate of the extracts, the cited method was modified. The ammonium heptamolybdate solution was prepared by using 10 g to 1 L by adding 140 ml of 97% sulfuric acid. The ascorbic acid solution was prepared at 0.5 g per 100 ml. The standard series with a maximum of 8 mg L-1 phosphate was prepared from a stock solution with 1000 mg L-1 phosphate. The phosphate content was measured in a spectrophotometer (UV-mini 1240 Shimadzu) by measuring 5 ml of standard or sample together with 3 ml of ammonium heptamolybdate solution and 2 ml of ascorbic acid. The solution was measured after 5 min in boiling water (99 °C) and a subsequent cooling time of 5 min at 820 nm. To determine soil organic matter content, the loss on ignition was determined in a muffle furnace (Nabertherm LVT 15/11) at 430 °C until constant weight was reached.
