Browsing by Author "Uhlgren, Oona"

Phosphorus is one of the three main nutrients for plants and sufficient phosphorus uptake is a prerequisite for crop production. Phosphorus exists in organic and inorganic forms in soil, and the stability and biological availability of different forms of phosphorus vary. Fertilizer phosphorus is efficiently adsorbed by aluminium and iron oxides in mineral soils, and due to positive phosphorus balances in the last decades, cultivated soils contain residual phosphorus. In addition to topsoil, phosphorus can accumulate in soil layers below the plough layer. Knowing the size of the phosphorus reserves in soil profile and the proportions of different forms of phosphorus could provide a more reliable assessment of the possibilities for utilizing and considering phosphorus reserves for example in fertilization. The aim of this study was to investigate the distribution and stocks of different forms of phosphorus in depth profiles of conventionally and organically managed clay soil and in unmanaged natural grassland. In a field experiment that has lasted 24 years before the sampling, conventional plots were fertilized with inorganic fertilizers, and organically managed plots with organic fertilizers. Total phosphorus, organic phosphorus, and water-extractable phosphate phosphorus were determined in 10 cm layers to an average depth of 70 cm. The stocks of soil total carbon and nitrogen were also determined. Changes in the carbon-to-phosphorus (C:P) and carbon-to-nitrogen (C:N) ratios of soil can affect the mineralization of soil organic matter and thus the release of nutrients for use by organisms. In addition, oxalate-extractable aluminium and iron, representing the concentrations of aluminium and iron oxides, were determined from the samples. Stocks were calculated using the equivalent soil mass (ESM) method, which quantifies stocks in terms of soil mass layers rather than depth layers. The differences in the concentrations and total stocks of the investigated forms of phosphorus between management systems were found to be relatively small. The slight differences in total stocks may indicate that phosphorus fertilization in the cultivated plots has corresponded well with the amount of phosphorus taken up by the crops. However, both conventional and organic plots also showed indications that phosphorus had accumulated in organic form below the plough layer. The use of organic fertilizers on the organically managed plots appeared to maintain the organic carbon content at deeper soil depths, as the carbon concentration of the organically managed plots and the unmanaged grassland with the highest carbon total stock did not differ statistically significantly at depths of 20–30 cm and below 40 cm. In all management systems, approximately one-third of the total phosphorus stock was in organic form. Based on the carbon-to-organic-phosphorus ratios (C:Porg), the soil organic matter in the cultivated plots contained more phosphorus than the unmanaged grassland, which may indicate that more phosphorus net mineralization occurs in the cultivated plots than in the unmanaged grassland.