Search

Deep‐ploughing far beyond the common depth of 30 cm was used more than 50 years ago in Northern Germany with the aim to break root‐restricting layers and thereby improve access to subsoil water and nutrient resources. We hypothesized that effects of this earlier intervention on soil properties and yields prevailed after 50 years. Hence, we sampled two sandy soils and one silty soil (Cambisols and a Luvisol) of which half of the field had been deep‐ploughed 50 years ago (soils then re‐classified as Treposols). The adjacent other half was not deep‐ploughed and thus served as the control. At all the three sites, both deep‐ploughed and control parts were then conventionally managed over the last 50 years. We assessed yields during the dry year 2019 and additionally in 2020, and rooting intensity at the year of sampling (2019), as well as changes in soil structure, carbon and nutrient stocks in that year. We found that deep‐ploughing improved yields in the dry spell of 2019 at the sandy sites, which was supported by a more general pattern of higher NDVI indices in deep‐ploughed parts for the period from 2016 to 2021 across varying weather conditions. Subsoil stocks of soil organic carbon and total plant‐available phosphorus were enhanced by 21%–199% in the different sites. Root biomass in the subsoil was reduced due to deep‐ploughing at the silty site and was increased or unaffected at the sandy sites. Overall, the effects of deep‐ploughing were site‐specific, with reduced bulk density in the buried topsoil stripes in the subsoil of the sandy sites, but with elevated subsoil density in the silty site. Hence, even 50 years after deep‐ploughing, changes in soil properties are still detectable, although effect size differed among sites. [ABSTRACT FROM AUTHOR]

Background and aims: Phosphorus (P) is an essential element for crop growth. However, while links of P turnover in soils to carbon (C) and nitrogen (N) availability have been described, it remains to be clarified how combinations of fertilizer C and N additions affect stocks and cycling of distinct P fractions at different soil depths. The objectives of our study were (1) to assess how soil total P stocks are affected by organic amendments and N fertilization, (2) to evaluate how different soil P fractions respond to N fertilization, and (3) to verify whether N fertilization increases soil biological P cycling. Methods: We collected soil samples from a long‐term field experiment established in 1984 in Rauischholzhausen, Germany. The soil is a Haplic Luvisol and received either no organic fertilizer (NOF), farmyard manure (FYM) or a combination of organic and mineral N fertilizer (OMF). Each treatment additionally received three levels of mineral N: 0 kg ha−1 y−1 (N0), 100 kg ha−1 y−1 (N100), and 200 kg ha−1 y−1 (N200). The organic fertilizers were applied by a manure spreader and the N fertilizer (calcium ammonium nitrate) was applied in spring as top dressing by a plot fertilizer machine. We estimated stocks of P in fractions isolated by sequential P fractionation, and assessed the oxygen isotopic composition of 1 M HCl‐extractable phosphate (δ18OP). Results: We found that increased organic matter (OM) addition and mineral N inputs caused significant decreases in the stocks of resin‐ and NaHCO3‐extractable P in the topsoil (0–30 cm). Mineral N fertilization alone resulted in significant increases in stocks of resin‐, NaHCO3‐, and NaOH‐extractable P in the upper subsoil (30–50 cm). These changes occurred for both inorganic and organic P. The subsoil δ18OP values were closer to expected equilibrium values in soil fertilized with mineral N, indicative of more intensive biological P cycling than in the treatments without mineral N inputs. Conclusions: These findings suggest that long‐term OM and mineral N fertilization promotes topsoil P losses from labile fractions by crop uptake with an enrichment of these P forms in the subsoil, and an overall increase in biological P cycling in both top‐ and subsoil horizons upon N fertilization. [ABSTRACT FROM AUTHOR]

Aims: Agricultural soils in Germany store 2.54 Pg of organic carbon (C). However, information about how and when this C entered the soils is limited. This study illustrates how depth profiles of organic matter can shed light on different entry paths of organic C. Methods: Machine learning was used to explain total organic C (TOC), C:N, particulate organic C (POC), δ13C and δ15N values down to 100 cm depth based on pedology, geology, climate and management-related variables from the German Agricultural Soil Inventory. We estimated TOC turnover rates based on the relationship between the proportion of maize (only C4 plant) in crop rotations and soil δ13C values. Results: In the upper 30 cm of cropland, fresh photosynthates added on average 0.2 to 0.8 Mg C ha− 1 year− 1. Organic fertiliser was another source of topsoil C, especially in grassland. Sandy sites in north-west Germany contained historic C from past heathland and peatland. One third of German agricultural land was found to be on colluvial and alluvial deposits, in which allochthonous C from upstream and upslope areas evidently increased the TOC content of subsoils. In and below hardpans, TOC content and C:N and POC:TOC ratios were low, indicating restricted root-derived C input. Conclusions: Our data indicate that ongoing management in German agricultural soils mainly affects topsoil C, while C storage in subsoils reveals significant legacies from allochthonous, buried or translocated C inputs. Specific attention should be focused on the sustainable loosening of hardpans that could result in a slow, but significant increase in subsoil C stocks. [ABSTRACT FROM AUTHOR]

Liming is widely used to alleviate soil acidity in western and central Europe, but its role in the cycling of magnesium (Mg) in arable soil–plant systems is still ambiguous. Here, we systematically analysed Mg concentrations and the natural Mg stable isotope compositions (δ26Mg) of two Mg pools in soil profiles and a major crop (winter rye) in a long‐term German agricultural experimental field that has been managed with and without liming since 1923. The results showed that the δ26Mg signatures of the bulk soil Mg pool in the studied Albic Luvisol displayed limited variation with depth and between the liming treatments. In contrast, the exchangeable soil Mg pool exhibited an increase in δ26Mg values with depth down to 50 cm, which was more pronounced in the limed plots. We attributed this enrichment of light Mg isotopes in upper layers to the Mg addition from "Dolokorn 90" lime, as well as to the removal of heavy Mg isotopes by plant uptake. The subsequent use of a simple isotope‐mixing model suggested that only 25% of the remaining Mg in the soil exchangeable pool stemmed from the last liming practice. The other parts of the exchangeable soil Mg pool had either interacted with the bulk soil matrix or had been utilized by the plants. Almost 100 years of liming enhanced Mg uptake by the vegetation, probably via elevated Mg contents in the grain, and reflected by the stronger depletion of heavy Mg isotopes in the soil exchangeable Mg pool relative to non‐limed plots. Whole winter rye plants were enriched in heavy Mg isotopes but they displayed similar Mg isotope compositions among plant organs in all plots, indicating identical Mg uptake and translocation strategies in the different trials. Tracing the stable isotope compositions of soil and plant Mg thus opens novel opportunities to evaluate soil management impacts on the cycling and fate of this essential nutrient in agricultural systems. Highlights: Mg concentrations and stocks in an Albic Luvisol were more heterogeneous in subsoils than in topsoilsThe variation of Mg isotope compositions of bulk soil was limited with depth and between liming treatmentsLiming induced a pronounced negative shift of δ26Mg values in soil exchangeable Mg pools down to a 50‐cm soil depthUptake of Mg by plants in limed plots was enhanced relative to that in non‐limed plots [ABSTRACT FROM AUTHOR]

Sustainable arable cropping relies on repeated liming. Yet, the associated increase in soil pH can reduce the availability of iron (Fe) to plants. We hypothesized that repeated liming, but not pedogenic processes such as lessivage (i.e., translocation of clay particles), alters the Fe cycle in Luvisol soil, thereby affecting Fe isotope composition in soils and crops. Hence, we analysed Fe concentrations and isotope compositions in soil profiles and winter rye from the long‐term agricultural experimental site in Berlin‐Dahlem, Germany, where a controlled liming trial with three field replicates per treatment has been conducted on Albic Luvisols since 1923. Heterogeneity in subsoil was observed at this site for Fe concentration but not for Fe isotope composition. Lessivage had not affected Fe isotope composition in the soil profiles. The results also showed that almost 100 years of liming lowered the concentration of the HCl‐extractable Fe that was potentially available for plant uptake in the surface soil (0–15 cm) from 1.03 (standard error (SE) 0.03) to 0.94 (SE 0.01) g kg−1. This HCl‐extractable Fe pool contained isotopically lighter Fe (δ56Fe = −0.05 to −0.29‰) than the bulk soil (δ56Fe = −0.08 to 0.08‰). However, its Fe isotope composition was not altered by the long‐term lime application. Liming resulted in relatively lower Fe concentrations in the roots of winter rye. In addition, liming led to a heavier Fe isotope composition of the whole plants compared with those grown in the non‐limed plots (δ56FeWholePlant_ + Lime = −0.12‰, SE 0.03 vs. δ56FeWholePlant_‐Lime = −0.21‰, SE 0.01). This suggests that the elevated soil pH (increased by one unit due to liming) promoted the Fe uptake strategy through complexation of Fe(III) from the rhizosphere, which favoured heavier Fe isotopes. Overall, the present study showed that liming and a related increase in pH did not affect the Fe isotope compositions of the soil, but may influence the Fe isotope composition of plants grown in the soil if they alter their Fe uptake strategy upon the change of Fe availability. Highlights: Fe concentrations and stocks, but not Fe isotope compositions, were more heterogeneous in subsoil than in topsoil.Translocation of clay minerals did not result in Fe isotope fractionation in the soil profile of a Luvisol.Liming decreased Fe availability in topsoil, but did not affect its δ56Fe values.Uptake of heavier Fe isotopes by graminaceous crops was more pronounced at elevated pH. [ABSTRACT FROM AUTHOR]

Historically, large areas of forest in Europe were managed as coppice woodland to produce wood-based fuel for the smelting industry. We hypothesized that this practice produced a legacy effect on current forest ecosystem properties. Specifically, we hypothesized that the historical form of coppicing may have produced a legacy of elevated stocks of soil organic carbon ( SOC), nutrients and black carbon ( BC) in soil as fire was routinely used in coppiced woodland to clear land. We further hypothesized that these changes in soil properties would result in increased biodiversity. To test these hypotheses, we sampled the surface soil (0-5, 5-10 and 10-20 cm) from a chronosequence of forest sites found in the Siegerland (Germany) that had been coppiced and burned 1, 2, 3.5, 6, 8, 11 and 17 years before present. Mature beech and spruce forests (i.e., >60 years) were also sampled as reference sites: to provide a hint of what might occur in the absence of human intervention. We measured stocks of SOC, BC, NO3-N, P, K, Mg, as well as cation exchange and water-holding capacity, and we mapped plant composition to calculate species richness and evenness. The results showed that coppicing in combination with burning soil and litter improved soil nutrient availability, enhanced biodiversity and increased SOC stocks. The SOC stocks and biodiversity were increased by a factor of three relative to those in the mature beech and spruce forests. This study shows that traditional coppicing practice may facilitate net C accrual rates of 20 t ha−1 yr−1 and maintain high biodiversity, indicating that aspects of traditional practice could be applied in current forest management to foster biodiversity and to mitigate climate change. [ABSTRACT FROM AUTHOR]

Black carbon (BC) is the product of incomplete burning processes and a significant component of the passive soil organic carbon (SOC) pool. The role of BC in the global carbon cycle is still unclear. This study aimed to quantify and characterize BC in major grassland ecosystems of the world. Twenty-eight representative soil profiles (mainly Mollisols) were sampled in the Russian Steppe, the U.S. Great Plains, the Argentinian Pampa, the Manchurian Plains in China, and the Chemozem region in central Germany. Black carbon contents were estimated using benzene polycarboxylic acids (BPCA) as a molecular marker, and indications about the origin of the BC were derived from bulk and compound-specific δ13C analyses and radiocarbon dating of bulk soil organic matter (SOM). Our findings suggest that between 5% and 30% of SOC stocks consist of BC. Maximum BC contributions to SOC frequently were found at deeper parts of the A horizon with 14C ages younger than 7000 years BP; that is, incorporation of C as charred particles accompanied ecosystem development since the mid-Holocene. Most of this BC formed from local vegetation, as indicated by a 13C isotope signature similar to that of bulk SOM. At some sites, also nonlocal sources contributed to soil BC, e.g., fossil fuel BC inputs at the German sites. Black carbon stocks were highest in Chernozems and lowest in Kastanozems. The Russian Steppe and Chinese Manchurian sites stored about 3-4 times more BC (around 3 kg m-2) than did the other sites because of thicker A horizons that were rich in BC. On a global scale, we estimate that steppe ecosystems contain between 4 and 17 Pg BC. [ABSTRACT FROM AUTHOR]

Tracing back the N use efficiency of long-term fertilizer trials is important for future management recommendations. Here we tested the changes in natural N-isotope composition as an indicator for N- management within a long-term fertilization lysimeter experiment in a low mountain range pasture ecosystem at Rengen (Eifel Mountains), Germany. Cattle slurry (δ15N = 8.9 ± 0.5‰) and mineral fertilizers (calcium ammonium nitrate; δ15N = −1.0 ± 0.2‰) were applied at a rate between 0 and 480 kg N ha−1 yr−1 throughout 20 years from 1985 onwards. In 2006, samples were taken from different grass species, coarse and fine particulate soil organic matter, bulk soil and leachates. Total soil N content hardly changed during fertilization experiment. As also N leaching has been small within the stagnant water regime, most N was lost through the gaseous phase beside plant uptake and cutting. Unlike N uptake by plants, the process of N volatilization resulted in strong discrimination against the 15N isotope. As a consequence, the δ15N values of top soil samples increased from 1.8 ± 0.4‰ to 6.0 ± 0.4‰ and that of the plants from −1.2 ± 1.3‰ to 4.8 ± 1.2‰ with increasing N fertilizer rate. Samples receiving organic fertilizer were most enriched in δ15N. The results suggest that parts of the fertilizer N signal was preserved in soils and even discovered in soil organic matter pools with slow N turnover. However, a 15N/14N isotope fractionation of up to 1.5‰ added to the δ15N values recovered in soils and plants, rendering the increase in δ15N value a powerful indicator to long-term inefficient N usage and past N management in the terrestrial environment. [ABSTRACT FROM AUTHOR]

Abstract: There is evidence of black carbon (BC) contributing to stable humus in the soil environment but its quantity and fate are poorly examined. We used benzene polycarboxylic acids (BPCAs) as markers to assess the contents and distribution of BC in soils and soil fractions of the long-term field experiments in Halle (Haplic Phaeozem), Bad Lauchstädt (Haplic Chernozem), Rotthalmünster (Haplic Luvisol) and an additional site near Bad Lauchstädt (Haplic Phaeozem), Germany. Black carbon comprised 11.9–13.2% of organic C in the top soils (0–10 cm) of the black soils located around Halle and Bad Lauchstädt while it explained only 2.7% of organic carbon in the Haplic Luvisol of Rotthalmünster. The BPCA pattern at Bad Lauchstädt and Halle suggested that two thirds of BC were of fossil origin. In general, BC contents (in g kg−1 soil) decreased with increasing soil depth. The C-normalized BC concentrations, however, increased and reached 35±7% of organic carbon at a depth of 87–114 cm. With increasing depth BC was increasingly localized in the coarse silt and sand fractions and the heavy mineral fraction. This indicated that BC was connected to the minerals of this size and preserved there. Inorganic fertilization for >30 years did not affect BC contents. [Copyright &y& Elsevier]

Lake sediments can provide useful archives to reconstruct past vegetation changes or fire history. To comprehend how vegetation and fire history have correlated during the last 130,000 years, we used two lake sediment records with known patterns of pollen and botanical macro remains and supplemented this data by analyses of lignin-derived phenols as markers for local vegetation inputs and by benzene polycarboxylic acids (BPCAs) as markers for total fire residue inputs (black carbon, BC). The two sediment archives originated from two maar lakes in the Eifel, which is part of the low mountain ranges in central Germany. A lignin-derived phenol index showed woody angiosperms and gymnosperms as fire fuel in the periods with the highest BC amounts. We recorded 3 g BC per kg sediment for phases covered by forest, while BC contents during colder and drier climates were much smaller (≤0.2 g BC per kg sediment), confirming the hypothesis that fires mostly occurred during humid periods in which forest were established. Both records pointed to forested phases with strong fire activity from 48,000–60,000 years before the year 2000 (b2k). Furthermore, the record reaching back to 130,000 years showed a strong fire activity at about 28,000–30,000 yr b2k, and was free of vegetation macro remains and fire markers from 60,000–118,000 yr b2k. All paleobotanical and chemical biomarker results documented the next oldest forested and fire interval, from 118,000 to 130,000 yr b2k, which includes the Eemian of the last interglacial. Thus, nearly all forested phases were accompanied by fire peaks. Overall, biomarker analyses confirmed that analyses of pollen and botanical macro remains. Fire events were highest during warm and humid forested phases. • Paleovegetation and -fire reconstruction via lignin and black carbon. • Black carbon marker peaked at periods when forests were most abundant. • The comparison of two different maar cores indicate local vegetation patches. [ABSTRACT FROM AUTHOR]

Gamma spectrometric field measurements may provide high resolution information on topsoil texture. Yet, calibrations for the estimation of texture data usually have to be done site-specifically. The lack of site-independent calibrations thus limits the easy and universal use of proximal gamma-ray sensing in soil mapping and precision agriculture. Our objective was to develop a study site-independent prediction model for topsoil texture from gamma-ray spectra. We surveyed ten study sites across Germany with 417 reference samples (291 for calibration, 126 for test set-validation), providing soils from a broad range of parent materials and with widely varying soil texture. First, study site-specific models were calibrated by a linear regression approach. These models provided reliable estimations of sand, silt, and clay for most of the study sites. Second, study site-independent models were calibrated via i) linear regression and ii) support vector machines (SVM), the latter being mathematical methods of data pattern recognition. Based on the non-linear relationship between gamma spectrum and soil texture, which varied widely between the different parent materials the linear models are not appropriate for satisfactory soil texture prediction (averaged R 2 of 0.73 for sand, 0.61 for silt, and 0.18 for clay and averaged absolute prediction errors of 9 to 5%, respectively). In contrast, the SVM calibrated prediction models revealed reliable performance also for site-independent calibrations. With the non-linear SVM approach we were able to include all sites in one single prediction model for each texture fraction although the different mineralogical composition of their parent materials led to complex and partly opposing relationships between gamma features and soil texture. Site-independent predictions via SVM were often even better than site-specific linear regression models. The site-independent SVM calibrated predictions yielded an averaged R 2 of 0.96 (sand), 0.93 (silt), and 0.78 (clay), and corresponding averaged absolute prediction errors of 2 to 4%, respectively. To summarize, (i) non-linear prediction models are a feasible approach for capable site-independent texture estimations across a wide range of soils and (ii) gamma spectrometry-based texture predictions are a valuable input for applications that require highly resolved texture information at low costs and efforts. [ABSTRACT FROM AUTHOR]

Agricultural management can influence soil organic carbon (SOC) stocks and thus may contribute to carbon sequestration and climate change mitigation. The soil depth to which agricultural management practices affect SOC is uncertain. Soil depth may have an important bearing on soil carbon dynamics, so it is important to consider depth effects to capture fully changes in SOC stocks. This applies in particular to the evaluation of carbon farming measures, which are becoming increasingly important due to climate change. We sampled and analysed the upper metre of mineral cropland soils from ten long-term experiments (LTEs) in Germany to quantify depth-specific effects on SOC stocks of common agricultural management practices: mineral nitrogen (N) fertilisation, a combination of N, phosphorus (P) and potassium (K) fertilisation, irrigation, a crop rotation with preceding crops (pre-crops), straw incorporation, application of farmyard manure (FYM), liming, and reduced tillage. In addition, the effects of soil compaction on SOC stocks were examined as a negative side effect of agricultural management. Results showed that 19 ± 3 % of total management effects on SOC stocks were found in the upper subsoil (30–50 cm) and 3 ± 4 % in the lower subsoil (50–100 cm), including all agricultural management practices with significant topsoil SOC effects, while 79 ± 7 % of management effects were in the topsoil (0–30 cm). Nitrogen and NPK fertilisation were the treatments that had the greatest effect on subsoil organic carbon (OC) stocks, followed by irrigation, FYM application and straw incorporation. Sampling down to a depth of 50 cm resulted in significantly higher SOC effects than when considering topsoil only. A crop rotation with pre-crops, liming, reduced tillage and soil compaction did not significantly affect SOC stocks at any depth increment. Since approximately 20 % of the impact of agricultural management on SOC stocks occurs in the subsoil, we recommend soil monitoring programs and carbon farming schemes extend their standard soil sampling down to 50 cm depth to capture fully agricultural management effects on SOC. • Ten 32- to 112-year-old long-term experiments were sampled down to 100 cm depth. • Sampling only topsoil ignores 20 % of agricultural management effects on SOC stocks. • Mineral fertilisation had the greatest effect on subsoil organic carbon stocks. • None of the agricultural measures had significant effects deeper than 50 cm. • Standard soil sampling should therefore be extended to 50 cm depth. [ABSTRACT FROM AUTHOR]

Forest clear-cuts may have severe effects on the soil structure and related nutrient cycling, though with yet unknown consequences for nutrient pools such as phosphorus (P) within microaggregates. We sampled the bulk mineral topsoil prior to clear cut as well as 1 and 2 years thereafter from the experimental forest site Wüstebach, Germany, and we assessed the degree of oxygen isotope exchange in HCl-extractable soil phosphate of two microaggregate size fractions (< 20 μm, 20–250 μm) after incubating soil with 18 O-labeled water. We found that after the clear-cut, microaggregate phosphates exchanged significantly more oxygen with the incubation water than before clear cut. One and two years after clear cut, the respective δ 18 O values of soil phosphates (δ 18 O P,HCl ) were elevated by 16 and 38% (< 20 μm) and by 43 and 53% (20–250 μm) than before the clear-cut, respectively, indicating that additional microaggregate P had been made available to biological P cycling. The degree of oxygen exchange after the clear-cut was significantly greater in larger soil microaggregates than in the smaller sized ones, reflecting that also at microaggregate level size controlled the increase in the bioavailability of P with changes in land management. [ABSTRACT FROM AUTHOR]

A sustainable use of soil resources is urgently required to cope with the increasing demand for agricultural products during climate change. To inspire farmers on new soil cultivation methods like subsoil management requires not only yield measurements but also nutrient use efficiency measurements for which analytical tools are still missing. Stable isotopes of the macronutrient magnesium (Mg) are a potential novel subsoil management evaluation tool in agronomy and soil/plant sciences because its isotope ratios shift considerably during Mg uptake by crops. The feasibility of Mg stable isotopes was first demonstrated conceptually by simulating subsoil management on soils with low, middle, and high inventories of bioavailable Mg and crop plants typically cultivated in Germany. This simulation showed that the magnitude of Mg isotope shifts among crops and the exchangeable fraction of Mg in soil is resolvable from the long-term external precision of Mg isotope analyses only if three conditions are met. First, the crop uptake-related Mg isotope fractionation factor should be at the upper end of hitherto published fractionation factors. Second, a high Mg uptake flux of crop plants (e.g., sugar beets) is matched by a low Mg supply from the exchangeable fraction in soil (e.g., sandy soils). Third, subsoil management causes a considerable deepening of the rooting system (e.g., flipping the topsoil root cluster below 30 cm depth). If these conditions are met, Mg stable isotopes can be used in a qualitative manner to identify the main Mg uptake depth, and in a quantitative manner by calculating the Mg use efficiency, defined here as the ratio of Mg uptake versus Mg supply, solely from Mg isotope ratios. This concept was tested for Alfisols on field trials by conducting deep loosening with and without the incorporation of compost. Magnesium isotope shifts in crops and the exchangeable fraction of Mg in soil were mostly unresolvable from the long-term external precision of Mg isotope analyses, which positively tested the Mg isotope concept for well nurtured soils. However, systematic Mg isotope shifts among bulk crops cultivated on and beside a melioration strip were found and attributed to the uplift of isotopically distinct compost-derived Mg on the melioration strip and root restricting layers beside the melioration strip. Given that the Mg isotope composition of the exchangeable fraction barely varies with depth, field-based crop uptake-related 'apparent' Mg isotope fractionation factors of winter wheat and spring barley could be determined, which differed from one another (Δ26Mg wheat-rem. exch. = 0.63 ± 0.05‰, Δ26Mg barley-rem. exch. = 0.37 ± 0.12‰). Nonetheless, the quantitative approach of Mg isotopes was violated when calcareous fertilizer was applied to the field as differences in the isotope-derived Mg use efficiency could be attributed to the uneven distribution of lime-derived Mg with depth. Using Mg stable isotopes as a new geochemical routine for agronomy and soil/plant sciences requires future work focussing on isotope fractionation factors related to crop uptake and intra-plant translocation of Mg – which may depend on species, environmental conditions, and nutrient status – to allow minimally invasive sampling of the soil-plant system and to reduce sample sets. • A conceptual Mg isotope model was established and positively tested in the field. • Mg stable isotopes are a powerful novel tool for subsoil management evaluation. • Mg use efficiency of crops can solely be quantified from Mg stable isotopes. • Lime application limits Mg use efficiency determination solely from Mg isotopes. • A criteria catalogue ensures a successful application of Mg isotopes in agronomy. [ABSTRACT FROM AUTHOR]

Fires accompanied human development throughout the Holocene, leaving behind black carbon (BC) as residues from incomplete biomass burning. Here we used molecular fire markers, benzene polycarboxylic acids (BPCAs), to reconstruct fire history in two Eifel maar lakes, Germany. We hypothesized to find indications for (i) changes in BC related to ecosystem changes, (ii) an increase in BC influx at the onset of agriculture until modern times, and (iii) a change in BC quality due to technical progress in combustion, e.g., at the beginning of agriculture and at the onset of the Bronze Age. To calculate absolute BC influx into the maar lakes, we multiplied BC contents with sedimentation rates. The BC influx rates were elevated during tundra-like vegetation in the Late Pleistocene (up to 7.7 g BC m −2 a −1 ), followed by relatively constant 2.5 g BC m −2 a −1 from the Bølling interstadial (>13.7 kilo years before present, ka BP) until the early Atlantic when forest began to develop. Thereafter, BC influx increased with the onset of land use of Neolithic cultures in the region from 7.5 ka BP to rates of 7–9 g BC m −2 a −1 . Noteworthy, also the quality of BC changed: higher ratios of five-to six-times carboxylated benzenes (B5CA/B6CA) pointed at colder, arable fires approximately 1000 years after first Neolithic activity from 6 to 4 ka BP (B5CA/B6CA increased from 1.0 to 2.0). From 4 ka BP (Bronze Age) to modern times increasing burning temperatures as indicated by dropping B5CA/B6CA ratios (from 2.0 to 1.0) were related to metallurgy and industrialization. Between 2.5 and 1 ka BP maximum BC influx rates were reached with ca 15 g BC m −2 a −1 . With increasing combustion efficiency and a reduction of wild fires during the last centuries, total BC influx decreased, suggesting that fossil fuel combustion contributed less to total BC input into the lake sediments than former vegetation fires. [ABSTRACT FROM AUTHOR]

The use of charcoal as a soil amendment is currently of great interest to sequester carbon and improve soil fertility, however, studies of sites where charcoal amendments to the soil have been made many years ago are lacking at the moment. In this study we investigated historical charcoal production sites in Germany that have not been in use for >60 years, and evaluated the effects of the former charcoal inputs on soil and vegetation parameters relative to those of adjacent, unamended areas. Surface soil samples (0-5, 5-20 cm) were taken from five sites located on extremely acidic (Siegerland, pH 3.8-4.1) and base rich soils (Eifel, pH 4.8-5.3) in species poor (Luzulo-Fagetum) and species rich (Hordelymo-Fagetum) beech forests, respectively. We determined stocks of black carbon (BC) and natural soil organic carbon (SOC=total C minus BC) as well as of soil nutrient stocks (NO3-N, P, K, Mg), cation exchange capacity and water holding capacity, and we mapped plant composition to calculate richness and evenness. The results showed that historical charcoal production sites were enriched with BC and also exhibited increased stocks of natural SOC and total N possibly due to enhanced stabilization of natural SOC by the charcoal. The availability of nitrate-nitrogen, phosphate and potassium was increased when the charcoal was added to the base rich soils and less so when charcoal was added to the extremely acidic soils. Plant biodiversity was not different between the sites of historical charcoal addition and the reference sites. We conclude that charcoal additions may increase soil carbon storage capacity over prolonged periods of time without negatively affecting plant ecological interactions over the long term. [ABSTRACT FROM AUTHOR]

Highlights: [•] Black carbon (BC) was preserved in up to 130,000yr old loess–palaeosol. [•] No vertical translocation of BC to loess and palaeo-subsoils found. [•] Palaeosol BC provided comprehensive regional information on past fire regimes. [•] Maximum BC amount occurred at times of intensive pedogenesis. [Copyright &y& Elsevier]

Abstract: In absence of commercial fertilizers prehistoric agriculture relied heavily on nutrient supply from soil. This study was designed to reconstruct the phosphorus (P) status of prehistoric arable soils in order to highlight the possible importance of soil P limitation in Neolithic (after the Early Neolithic, i.e. around 4400 until 2200BC) and Bronze Age (2200 until 700BC) agriculture. For this purpose, we sampled buried archaeological topsoil relicts that have been preserved in prehistoric pit fillings outside ancient settlements in the loess areas in Central and Western Germany. Adjacent subsoils and recent topsoils served as references. Samples were characterized using sequential P extraction and selected samples were characterized using 31P NMR spectroscopy. Prehistoric topsoil relicts in the investigated regions showed larger contents of organic matter and extractable P (sum of all sequentially extracted inorganic and organic P fractions) than the adjacent subsoils. Sampling the soils at higher depth resolution did not provide any evidence of P-leaching. The P content in the topsoil relicts amounted to approximately 86% of the P content of the modern, fertilized topsoils. Most of the extractable P (>80%) comprised inorganic P. Organic P fractions were dominated by monoester-P. Intriguingly, plant available P contents (sum of resin-Pi, NaHCO3-Pi and NaOH-Pi) in the prehistoric topsoils and the recent topsoils were similar (23% and 20% of extractable P, respectively), suggesting that even plant available P was maintained in the prehistoric topsoil relicts. Also the contents of Ca-associated P were large, but reflected differences in pedogenesis among the sampling sites (Chernozem, Phaeozem and Luvisol). Hence, we have to conclude that there was no significant P limitation in the prehistoric farmland of the two loess areas, either because the soils had been fertilized or because the inherent P stocks were sufficient to sustain ancient farm yields. In any case our data do not support the hypothesis that prehistoric P limitations may have reduced crop yields. [Copyright &y& Elsevier]

• Apple replant disease is reflected in a huge in-field variability of tree growth. • Plants showing most stunted shoot growth revealed lowest δ 15N values in the rhizosphere. • Apple replant disease severity is correlated to a reduced micronutrient supply of Mn and Co for infected plants up to 100 cm shoot length. Orchards affected by apple replant disease may show a distinct in-field variability of the severity of apple replant disease (ARD) symptoms. As abiotic soil properties can affect pathogen survival but show considerable variability at the scale of a few meters, we hypothesized that the variability of abiotic soil properties may be used to explain parts of the in-field variability of ARD. We sampled soil and rhizosphere of 32 apple trees with different degree of ARD symptoms after two years of growth at the horticultural research station Heidgraben, Germany. The sandy soils correspond to Entic Podzols. Soil analyses comprised soil aggregate-size fractionations, the assessment of the stocks of nitrogen and essential macro- and micronutrients, as well as a tracing of stable δ 15N natural abundance both in rhizosphere and non-rhizosphere soil. The results showed that there was indeed a huge in-field variability of tree growth, following a Gaussian distribution, but classified here in shoot heights of < 40, 40–80, 80–120, and >120 cm, respectively. An effect of soil aggregation on ARD or vice versa was not detected. Elevated tree growth, however, went along with a consumption of soil macronutrients (R2: = 0.3 – 0.8 for the n = 4 ARD classes; significant only for K on individual plant basis). Intriguingly, opposite correlations were observed for micronutrients, where reduced tree growth occurred at subsites short in micronutrient supply (R2 = 0.9 for Mn and Co, significant for Mn and Co for ARD infected plants up to 100 cm shoot length on individual plant basis). Moreover, the plants showing most stunted shoot growth revealed lowest δ 15N values in the rhizosphere, as typical for reduced N cycling. Our data therefore confirm that at least parts of the in-field variability of ARD severity is correlated to reduced micronutrient supply and N cycling, possibly via effects on habitat properties of pathogenic soil microbiota. [ABSTRACT FROM AUTHOR]

The mean residence time of organic matter in soil ranges from days to millenia. Although chemical and physical processes are known to control the storage of organic matter in soil, the role of spatial arrangements in soil remains largely unknown due to the lack of in-situ techniques operating on the micro-scale. We asked whether deterministic patterns in element arrangement occur in large and small soil microaggregates (250-53 and 53-20 µm), and how these contribute to the storage of organic matter. To do so, we studied surface soils with increasing clay contents (sandy to loamy Luvisols, Germany) and subjected 60 individual aggregates to elemental mapping by electron probe micro analysis (EPMA), which recorded C, N, P, Al, Fe, Ca, K, Cl, and Si contents at micrometer scale resolution. We further developed the first laser-ablation isotope ratio mass spectrometry technique in soil science detecting d13C composition on the micro-scale (LA-IRMS) and employed this to trace micro-gradients in undisturbed soil and soil microaggregate samples. We found a pronounced heterogeneity in aggregate structure and composition, which was not reproducible for different microaggregates from the same soil fraction, and which was largely independent from clay content in soil. However, neighborhood analyses revealed close spatial correlations between organic matter debris (C:N app. 100:10) and microbial organic matter (C:N app. 10:1), indicating a spatial relationship between source and consumer. There was no systematic relationship between soil minerals and organic matter, suggesting that well-established macroscale correlations between pedogenic oxides and clay with soil organic matter storage do not apply to soil microaggregates. From first applications of LA-IRMS to soil we found also a high d13C variability of up to 9 per mill along spatial gradients of less than 300 µm, suggesting the appearance of very small hotspots of isotope enrichment and organic matter turnover by metabolic processes. [ABSTRACT FROM AUTHOR]