Copper is mobilized in soil by dissolved organic matter (DOM) but the role of DOM quality in this process is unclear. A one-step resin-exchange method was developed to measure the Cu-Mobilizing-Potential (CuMP) of DOM at pCu 11.3 and pH 7.0, representing background values. The CuMP of DOM was measured in soil solutions of 13 uncontaminated soils with different DOM extraction methods. The CuMP, expressed per unit dissolved organic carbon (DOC), varied 10-fold and followed the order water extracts > 0.01 M CaCl2 extracts > pore water. Soil solutions, obtained from soils that were stored air-dry for a long time or were subjected to drying-wetting cycles, had elevated DOC concentration, but the DOM had a low CuMP. Prolonged soil incubations decreased the DOC concentration and increased the CuMP, suggesting that most of the initially elevated DOM is less humified and has lower Cu affinity than DOM remaining after incubation. A significant positive correlation between the specific UV-absorption of DOM (indicating aromaticity) and CuMP was found for all DOM samples (R(2) = 0.58). It is concluded that the DOC concentration in soil is an insufficient predictor for the Cu mobilization and that DOM samples isolated from air-dried soils are distinct from those of soils kept moist.

Summary Most soil tests for available phosphorus (P) perform rather poorly in predicting crop response. This study was set up to compare different established soil tests in their capacity to predict crop response across contrasting types of soil. Soil samples from long‐term field experiments, the oldest >100 years old, were collected in five European countries. The total number of soil samples (n = 218), which differed in cropping and P treatment, and originated from 11 different soil types, were analysed with five tests: ammonium oxalate (Ox), ammonium lactate (AL), Olsen P, 0.01 m CaCl2 and the diffusive gradient in thin film (DGT). The first three tests denote available P quantity (Q), whereas the last two indicate P intensity (I) of the soil solution. All five tests were positively related to the crop yield data (n = 317). The Q‐tests generally outperformed I‐tests when evaluated with goodness of fit in Mitscherlich models, but critical P values of the I‐tests varied the least among different types of soil. No test was clearly superior to the others, except for the oxalate extraction, which was generally poor. The combination of Q‐ and I‐tests performed slightly better for predicting crop yield than any single soil P test. This Q + I analysis explains why recent successes with I‐tests (e.g. DGT) were found for soils with larger P sorption than for those in the present study. This systematic evaluation of soil tests using a unique compilation of established field trials provides critical soil P values that are valid across Europe. Highlights We compared soil P tests for predicting crop response across contrasting soil types. No test was clearly superior to the others except for the oxalate extraction, which was generally poor. This study suggests that intensity tests do not perform markedly better than quantity tests. The evaluation of soil P tests on this unique dataset provided critical soil P values across Europe.

It is well established that dissolved organic matter (DOM) mobilizes copper (Cu) in soils but it is unknown to what extent variable DOM quality affects this. During a 5 month period, 250 leachates of an uncontaminated agricultural soil were sampled at 45 cm depth using passive capillary wick samplers. The dissolved Cu and organic carbon (DOC) concentrations varied sevenfold and were weakly correlated (r ¼ 0.56). The [Cu] : [DOC] ratio varied fivefold and exhibited a significant positive correlation (r ¼ 0.77) with the specific UV-absorbance of DOM at 254 nm (SUVA), indicating that the more aromatic DOM had higher Cu affinity. The dissolved Cu concentrations were predicted by an assemblage model in WHAM6 using the composition of the solid phase above the wick samplers and that of the solution, including DOC. The predicted [Cu] : [DOC] ratio was almost constant when assuming default DOM properties with 65% of all DOM active as fulvic acid (%AFA). The %AFA was subsequently varied proportionally to the SUVA of DOM and using the SUVA of pure FA (SUVA FA ) as a fitting parameter. In that case, the variation in the predicted [Cu] : [DOC] ratio was much larger and the predicted Cu concentrations were within a factor of 1.4 of the measured values for 90% of the samples. The fitted SUVA FA was 38 l g À1 cm À1 , in excellent agreement with that of Suwannee River FA (SUVA FA ¼ 37 l g À1 cm À1 ). It is concluded that the DOM quality, e.g. the aromaticity, should be taken into account when estimating Cu mobility in soils.

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Background: When maize (Zea mays L.) is grown in the Northern hemisphere, its development is heavily arrested by chilling temperatures, especially at the juvenile phase. As some endophytes are beneficial for plants under stress conditions, we analyzed the impact of chilling temperatures on the root microbiome and examined whether microbiome-based analysis might help to identify bacterial strains that could promote growth under these temperatures. Results: We investigated how the maize root microbiome composition changed by means of 16S rRNA gene amplicon sequencing when maize was grown at chilling temperatures in comparison to ambient temperatures by repeatedly cultivating maize in field soil. We identified 12 abundant and enriched bacterial families that colonize maize roots, consisting of bacteria recruited from the soil, whereas seed-derived endophytes were lowly represented. Chilling temperatures modified the root microbiome composition only slightly, but significantly. An enrichment of several chilling-responsive families was detected, of which the Comamonadaceae and the Pseudomonadaceae were the most abundant in the root endosphere of maize grown under chilling conditions, whereas only three were strongly depleted, among which the Streptomycetaceae. Additionally, a collection of bacterial strains isolated from maize roots was established and a selection was screened for growth-promoting effects on juvenile maize grown under chilling temperatures. Two promising strains that promoted maize growth under chilling conditions were identified that belonged to the root endophytic bacterial families, from which the relative abundance remained unchanged by variations in the growth temperature. Conclusions: Our analyses indicate that chilling temperatures affect the bacterial community composition within the maize root endosphere. We further identified two bacterial strains that boost maize growth under chilling conditions. Their identity revealed that analyzing the chilling-responsive families did not help for their identification. As both strains belong to root endosphere enriched families, visualizing and comparing the bacterial diversity in these communities might still help to identify new PGPR strains. Additionally, a strain does not necessarely need to belong to a high abundant family in the root endosphere to provoke a growth-promoting effect in chilling conditions.