Kalkningens effekter på fosformineralisering : organisk fosfor i det permanenta fältförsöket i Röbäcksdalen (R3-1002)

Abstract: The use of phosphorus within today´s intensive farming, represents a future chal-lenge that grows as the world's stores of easily accessible phosphate minerals are being depleted. At the same time, large amounts of organic matter is annually re-turned to the world´s arable land, organic matter whose mineralization contributes to the plant nutrition supply of the subsequent crop. Several studies have evaluated and reported effects of liming on the solubility of inorganic phosphorus in soil. Current pH and liming recommendations are based on the findings in these stud-ies. In what way liming affects the mineralization of organic phosphorus is, by contrast, less well known. The purpose of this study was to determine what effects the three different levels of liming (unlimed, and liming to 70 % and 100 % base saturation, respectively), combined with three PK-levels and two different harvest-residue treatments, had had on the mineralization of organic phosphorus under long-term field conditions. Soil samples from the ‘permanent liming’ field trial at Röbäcksdalen close to Umeå in northern Sweden, were analyzed in terms of or-ganic phosphorus content (Porg), using Saunders and Williams ignition method, and soil organic carbon (C) and nitrogen (N), by dry combustion in a C:N-analyzer. An analysis of variance was performed to test the effects of liming on Porg, organic C and N contents. Liming affected the Porg content, which was higher in limed than in unlimed plots. Yet, there were no effects of liming on C and N contents. This resulted in lower C:Porg ratios in the limed plots. The P flows and pools in the soil of Röbäcksdalen were estimated based on management information and unpublished results from the field trial. These showed that the limed plots received more PK-fertilizer than the unlimed ones. Mass balance considerations indicates that this was the likely cause why the pool of Porg was larger in the limed treatments. However, soil car-bon content was similar cross the lime treatments, even though more harvest resi-dues had been returned in the limed plots because of a higher yield there. There-fore, it cannot be ruled out that the mineralization rate has been higher in limed plots. Furthermore, the analysis of variance showed a strong influence of the block factor, with effects on soil organic carbon, nitrogen and the C:Porg ratios. The re-sults do not confirm that liming would increase the P mineralization.