Labile phosphorus in peat and other organic soils: baseline data and sampling protocols for paludiculture

Plant-available phosphorus (P) is the quantity of labile P that can be utilised by soil biota. This P pool is a major driver of plant growth, affecting the mineralisation of soil organic matter and the risk of P leaching. A common agronomic soil test (calcium-lactate extraction, P_CAL) is regularly used to assess the status of plant-available nutrients in cultivated soils, while extraction with a bicarbonate-buffered dithionite solution (P_DT) is suggested as a proxy for redox-sensitive P that might be released upon rewetting. However, systematic P studies on peat and other organic soils are scarce. The few studies that are available mostly describe P stocks over large depth increments or focus on leaching risks only. Organic soils are characterised by a high heterogeneity of the accumulated substrates and by a complex differentiation of the soil horizons. In addition, peatland management practices vary considerably from region to region, strongly influencing the level of water and nutrient management. Therefore, assessment of P status requires a specific soil sampling approach that reflects the genuine characteristic of organic soils and consider specific differences in peatland management. This is particularly relevant for paludiculture sites, where information is needed on both the beneficial and potentially harmful aspects of labile P (i.e. plant nutrition and risk of eutrophication). However, there are no agreed sampling and analysis methods especially for wet organic soils.

Here, we analyse data on the labile P pool (P_CAL and P_DT) from about 100 sites comprising more than 500 horizons of the German Peatland Monitoring Programme, covering a wide range of organic soils and land use types. In addition, data from a fen paludiculture project are used to elucidate spatial and temporal variability. These results will allow us to derive a baseline data set of the labile P pools in different organic soils depending on land use type, land use intensity and water management. Furthermore, appropriate sampling schemes will be derived specifically for paludiculture sites. Thus, the results can be used to contextualise specific (future) paludiculture site conditions with respect to biomass production and P leaching risks.