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Many European cropped soils have high soil test P (STP) values in the top soil because of P accumulation over many years of fertilizer application. This should allow to save P fertilizer applications for some years without STP values decreasing to a level that might negatively impact crop yield. However, the way STP develops under omitted P fertilizer application is not well understood. We examined STP development under omitted P fertilizer application for timeframes between 7 and 46 years on 96 unfertilized treatments (P0 treatments) of 43 European long-term P field experiments, using five different STP methods. For comparability, values obtained by different STP methods were converted to Olsen-P concentrations. We fitted exponential decay curves to Olsen-P data of each P0 treatment defined by initial Olsen-P values (Olsen-Pi), rates of decrease (k) and asymptotes (A), reflecting minimum obtainable STP. Subsequently, we analysed whether the variables most commonly recorded in experiments, are sufficient to explain the variation in model parameters, these variables being P export, clay content, Corg and pH as well as average annual temperature and precipitation. We found that out of our predictor variables, soil clay content, precipitation and temperature were showing the most prominent effects on the parameters Olsen-Pi, A or k. However, the amount of variation explained by the considered variables was too low to potentially facilitate a prediction of STP decrease, and various P0 treatments showed no clear Olsen-P decrease or unexpectedly high asymptotes. This hints at a strong influence of the P sorption capacity of the soil with often high potential for replenishment from less available P pools. In connection with P introduction from the subsoil or possibly from surrounding plots, the extension of timeframes of omitted P fertilizer application without reaching critical STP values for crop production, might be explainable. Corresponding effects could not be analys

Phosphorus (P) fertilizer recommendations of individual countries may differ in many aspects, but often the main principle is to reach or maintain a target range of plant-available P in soil. Within this target P fertility class, the soil is expected to supply enough P to the crop, while P fertilization replaces what is exported by the harvested crop. However, the threshold values of the target P fertility classes are based on a multitude of different soil test P (STP) methods and vary by a factor of up to three, even for countries using the same STP method. This study aimed to provide a comparison of the thresholds of target P fertility classes of different European countries and critical soil test P values (Pcrit; STP below which the average relative yield falls below 95% due to P insufficiency) derived from the analysis of data from 55 long-term field experiments in eight European countries. To overcome the issue of diverging STP methods, all values were converted to Olsen-P using empirically based conversion equations from the literature. Converted threshold values varied by a factor of up to five. For the experimental data, we fitted multi-level Mitscherlich-type models to determine Pcrit values of unfertilized soils corresponding to 95% of maximum yield. We found an average Olsen-Pcrit value of 15 mg P kg−1 (adj. R² = 0.37; RMSE = 14.1% relative yield; n = 2368; 55 experiments), which lies far below several country-specific thresholds of target P fertility classes. Crop-specific analyses resulted in higher Olsen-Pcrit values for sugar beet (22 mg P kg−1), potato (19 mg P kg−1) and winter rapeseed (18 mg P kg−1). Among the texture classes (loam, sand, silt and clay), sandy soils exhibited the highest average Olsen-Pcrit value (22 mg P kg−1). We consider a reevaluation of extraordinarily high country-specific thresholds as well as an inclusion of crop type and soil texture (where not already implemented) to be a reasonable measure towards more cost-effective and

Phosphorus (P) fertilizer recommendations in most European countries are based on plant-available soil P contents and long-term field experiments. Site-specific conditions are often neglected, resulting in excessive P fertilizer applications. P fertilization experiments including relevant site and soil parameters were evaluated in order to analyze the yield response. The database comprises about 2000 datasets from 30 field experiments from Germany and Austria. Statistical evaluations using a classification and regression tree approach, and multiple linear regression analysis indicate that besides plant-available soil P content, soil texture and soil organic matter content have a large influence on the effectiveness of P fertilization. This study methodology can be a basis for modification and specification of existing P fertilization recommendations and thus contribute to mitigate environmental impacts of P fertilization.

Anhand von acht Laborversuchen wird die Wirkung von Einflussfaktoren auf die Freisetzung von Stickstoffgasennach einer Gärrestapplikation demonstriert und werden dazu Vergleiche zwischen den Gasen vorgenommen.Übereinstimmungen ergaben sich bei der NH3- und N2OFreisetzung in Bezug auf die Temperatur und die Art derDüngung. Unterschiede zeigten sich im zeitlichen Verlauf der Gasfreisetzungen und in der Reaktion auf die Applikationsmethode, die Bodenfeuchte, die Gärrest-N-Menge und den Zusatz von Stroh sowie eines Nitrifikationsinhibitors. Die N2O- und N2-Freisetzungen nahmen mit steigender Bodenfeuchte zu. Im hohen Bodenfeuchtebereich stieg besonders die N2O-Freisetzung an und lag über der Freisetzung von N2. Der Zusatz von Stroh zur Gärrestapplikation kann einen differenzierten Einfluss auf die N2O- und N2-Freisetzungen ausüben, der offensichtlich vom Bodenfeuchtegehalt abhängig ist.On the basis of eight laboratory experiments, the effect of factors is demonstrated on the release of nitrogen gases after fermentation residue application and comparisons between gases are made. Similarities where found in the behaviour of NH3 and N2O release with respect to the influence of temperature and the applied fertilizer. Differences occurred in the temporal dynamics of gas release and in response to the application method, the soil moisture content, the rate of fertilizer N application and the addition of straw and a nitrification inhibitor. The N2O and N2 release increased with increasing soil moisture. At high soil moisture, particularly N2O release increased and was partially above the release rate of N2. The addition of straw to the application of fermentation residue showed a differential influence on N2O and N2 release, which is obviously dependent on soil moisture content.

In zwei Laborversuchen wurde die Freisetzung von Ammoniak, Lachgas, Kohlendioxid und Methan aus zehn verschiedenen Böden ohne und mit Zusatz von Gärrest geprüft. Ohne Zusatz von Gärrest kam es nur zu einer Freisetzung von CO2. Mit Gärrestzusatz traten alle untersuchten Gase auf. Bei den gedüngten Böden lag eine hohe Streubreite bei den N2O-, CO2- und NH3-Freisetzungen vor. Die engste Beziehung zwischen den untersuchten Bodenparametern und den Gasfreisetzungen bestand zwischen dem Sandgehalt und der NH3-Freisetzung. Bei Gärrestzusatz erfolgte die höchste Freisetzung von CO2-Äquivalenten (Summe aller vier Gase) bei den lehmigen Sandböden.In two laboratory experiments, the release of ammonia, nitrous oxide, carbon dioxide and methane from ten different soils was tested with and without the addition of fermentation residue. Without the addition of fermentation residue, there was only a release of CO2. With application of fermentation residue all investigated gases were released. In the fertilized soils, high scatter was present in the N2O, CO2 and NH3 release. The closest relationship between the studied soil parameters and the gas release existed between sand content and NH3 release. After application of fermentation residue the highest release of CO2 equivalents (sum of all four gases) was found in the loamy sand soils.