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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]

Nitrogen (N) cycling in rangeland soils could potentially be controlled by water supply, stocking rates, or a range of other variables, such as ecosystem N stocks. To gauge the relative importance and elucidate possible interactions among these factors, we measured many abiotic variables to identify first‐order controls of δ15N for Patagonia's rangeland soils under contrasting historical grazing intensities. The results showed that δ15N values declined as water availability increased. The effects of precipitation and stocking rate on soil δ15N values were additive, and the effect of precipitation far outweighed the effects of grazing pressure. The soil N stock was a weak predictive variable for modeling variation in δ15N of the soil. Earlier assumptions about an inflection point for N cycling and δ15N values related to aridity were not confirmed. We conclude that variation in water availability drives variation in δ15N values irrespective of grazing intensity. We also conclude that meaningful interpretation of δ15N in soil will require a better mechanistic understanding of the interactions between water and N in the vadose zone than we currently possess. Core Ideas: Interpretation of δ15N values in soils remains a significant and unresolved challenge. Precipitation is the main driver of variation in δ15N values in Patagonian soils. Vadose zone research is needed to better understand the interactive effects of water and N on δ15N in soil. [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]

Stagnant water conditions may release phosphorus (P) in soil solution that was formerly bound to Fe oxides. To understand which P species are potentially involved, we obtained water extracts from the surface soils of a gradient from Cambisol, Stagnic Cambisol to Stagnosol from temperate grassland, Germany. These were filtered to < 450 nm, and divided into three procedurally-defined fractions: small-sized colloids (20-450 nm), nano-sized colloids (1-20 nm), and "dissolved P" (< 1 nm), using asymmetric flow field flow fractionation (AF4), as well as filtration for solution 31P-NMR spectroscopy. The total P of soil water extracts increased in the order Cambisol< Stagnic Cambisol< Stagnosol due to increasing contributions from the dissolved P fraction. Associations of C-Fe/Al-PO43-/pyrophosphate were absent in nano-sized (1-20 nm) colloids from the Cambisol but not in the Stagnosol. The 31P-NMR results indicated that this was accompanied by elevated portions of organic P in the order Cambisol > Stagnic Cambisol > Stagnosol. Across all soil types, elevated proportions of inositol hexakisphosphate species (e.g. myo-, scyllo-, and D-chiro-IHP) were associated with soil mineral particles (i.e. bulk soil and small-sized soil colloids) whereas other orthophosphate monoesters and phosphonates were found in the 'dissolved' P fraction. We conclude that stagnic properties affect P speciation and availability by potentially releasing dissolved inorganic and ester-bound P forms as well as nano-sized organic matter-Fe/Al-P colloids. [ABSTRACT FROM AUTHOR]

Neolithic and Bronze Age topsoil relicts revealed enhanced extractable phosphorus (P) and plant available inorganic P fractions, thus raising the question whether there was targeted soil amelioration in prehistoric times. This study aimed (i) at assessing the overall nutrient status and the soil organic matter content of these arable topsoil relicts, and (ii) at tracing ancient soil fertilizing practices by respective stable isotope and biomarker analyses. Prehistoric arable topsoils were preserved in archaeological pit fillings, whereas adjacent subsoils served as controls. One Early Weichselian humic zone represented the soil status before the introduction of agriculture. Recent topsoils served as an additional reference. The applied multi-proxy approach comprised total P and micronutrient contents, stable N isotope ratios, amino acid, steroid, and black carbon analyses as well as soil color measurements. Total contents of P and selected micronutrients (I, Cu, Mn, Mo, Se, Zn) of the arable soil relicts were above the limits for which nutrient deficiencies could be assumed. All pit fillings exhibited elevated δ15N values close to those of recent topsoils (δ15N>6 to 7‰), giving first hints for prehistoric organic N-input. Ancient legume cultivation as a potential source for N input could not be verified by means of amino acid analysis. In contrast, bile acids as markers for faecal input exhibited larger concentrations in the pit fillings compared with the reference and control soils indicating faeces (i.e. manure) input to Neolithic arable topsoils. Also black carbon contents were elevated, amounting up to 38% of soil organic carbon, therewith explaining the dark soil color in the pit fillings and pointing to inputs of burned biomass. The combination of different geochemical analyses revealed a sufficient nutrient status of prehistoric arable soils, as well as signs of amelioration (inputs of organic material like charcoal and faeces-containing manure). [ABSTRACT FROM AUTHOR]

Highlights: [•] Soil texture and soil organic carbon content do not affect 15N dynamic. [•] 15N release depends on fertilizer (chicken manure, mushroom compost, mineral N). [•] Mustard intercropping is a tool to enhance N efficiency. [Copyright &y& Elsevier]

• Organic matter accumulation depends on pedogenic soil differentiation. • Pedogenic processes lead to soil-group-specific organic matter accrual in subsoil. • Reference Soil Groups have unique process combinations of organic matter accrual. Soil organic matter (SOM) accumulation is different in certain soil groups with differences in parent material, degree of weathering and mineral composition. These differences are modulated by climatic factors, but also by pedogenesis, in particular by the formation of reactive mineral surfaces, by soil aggregation, as well as by translocation processes such as eluviation and illuviation and different types of turbation. However, there is still a lack of conceptualization of how such processes and thus important Reference Soil Groups influence the composition and properties of OM. Here we summarize the basic processes of OM storage as they differ from soil group to soil group, in order to present a first overview of the processes of OM formation in the different terrestrial soils of the world. We distinguish between soils of different climatic zones, i.e. Cryosols in permafrost regions, soils of limited development (Cambisols), Podzols, Phaeozems, Chernozems, Kastanozems, and Luvisols in temperate climate zones, as well as Acrisols, Ferralsols, Plinthosols and Nitisols in the subtropics and tropics. We also include soils derived from a specific parent material (Andosols, Vertisols), as well as Anthrosols (paddy soils, Terra Preta, plaggen soils) as examples for human-made SOM accumulations. The compilation of the literature shows that research on OM is clearly focused on specific Reference Soil Groups in temperate climate zones and some man-made soils, while other soils such as Nitisols and Acrisols are clearly underrepresented. The contribution of the different soil groups to global organic carbon (OC) stocks varies, with large amounts of OC found for the first metre in Cryosols, Cambisols, and Podzols, due to the large land area they cover, followed by Acrisols and Ferralsols. In part, these differences can be attributed to differences in the formation of SOM, which we ascribe to three main mechanisms. We emphasize that in all major Reference Soil Groups, both the mechanism of sorptive conservation as well as the protection within the aggregates contribute to the storage of OM. However, the reactant partners and aggregate forming agents and therewith the intensity of these stabilisation processes vary among the Reference Soil Groups. As a result, there are differences in the SOM composition in the topsoil. Within the entire soil profile, however, pedogenic processes lead as third mechanism to soil-group-specific accrual of SOM in the subsoil, e.g. by means of illuviation, by cryo-/bio-, and peloturbation, as well as by management. We conclude that the specific pedogenic environment must be considered in the assessment of global SOM storage potentials and thus also in future global C models. [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]

• Nanocolloids dominated soil colloids in Acrisols. • Fine and medium-sized colloids dominated soil colloids in Terra Preta soils (TP). • Elevated pH and Ca2+ conc. correlated with SOM involvement in colloid formation in TP. • Enriched water-extractable and colloidal P in TP. Amazonian Dark Earths (or terra preta de índico) are known as highly fertile soils that can maintain elevated crop yields for centuries. While this fertility was frequently ascribed to the presence of black carbon, the availability and colloidal binding of major nutrients received limited attention. We examined the size distribution and the elemental compositions of water-dispersible colloids (WDC) in both forested and cultivated Terra Preta topsoils (0–10 cm, Anthrosols), as well as in their adjacent non-Terra Preta controls (Acrisols) via asymmetric flow field-flow fractionation (FFF). Liquid-state 31P-nuclear magnetic resonance (NMR) spectra, black carbon content, and scanning electron microscope (SEM) images were also obtained. We found that WDC in Terra Preta soils contained a significant proportion of organo-mineral associations in the size range 30–300 nm, whereas, in contrast, water-dispersible nanoparticles with a diameter < 30 nm were dominant in the adjacent Acrisols. The shifts to larger WDC sizes in the Terra Preta soils went along with elevated pH values, as well as with elevated contents of Si, Al, Fe, Ca and organic matter-containing particles. Also P concentrations were enriched in both the water-extractable phase (WEP) and WDC extracts of Terra Preta soils relative to the adjacent Acrisols. We assume that the higher pH values and Ca ion concentrations promoted the involvement of soil organic matter (SOM) into the formation of larger-sized colloids consisting of kaolinite-like clay minerals, iron oxides and Ca ions in the Terra Preta soils. The elevated content of Ca in Terra Preta soil colloids may also contribute to the retention of P, likely via bridging of anionic P like orthophosphate to SOM. Preventing soil acidification is thus not only to be recommended for Acrisols, but also for maintaining colloidal structures and related fertility in Terra Preta soils. [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]

Nitrogen (N) deposition to soils is globally rising, but its effect on soil organic carbon (SOC) turnover is still uncertain. Moreover, common theories of stoichiometric decomposition and microbial N mining predict opposing effects of N supply on SOC turnover. We hypothesized that the effect of N deposition on SOC turnover depends on initial soil nutrient conditions. Thus, we sampled tropical forests and rubber gardens with pronounced gradients of nutrient availability from the topsoil to the deep subsoil (up to 400 cm) and measured substrate-induced respiration (SIR) for 30 days in four treatments (C, CN, CP, CNP additions). A natural 13 C abundance approach was conducted to quantify priming effects (PE) of the added C on SOC mineralization. For this purpose we assessed the 13 CO 2 isotope composition after adding a C4 sugar to the C3 soil; to correct for isotopic fractionation a treatment with C3 sugar additions served as control. We found that nutrient additions to topsoil did neither alter cumulative CO 2 release within 30 days (SIR acc ) nor PE (PE = 1.6, i.e., sugar additions raised the release of SOC-derived CO 2 by a factor of 1.6). In the upper subsoil (30–100 cm), however, both CN and CP additions increased SIR acc (by 239% and 92%, respectively) and the PE (PE = 5.2 and 3.3, respectively) relative to the treatments that received C only (PE = 1.7), while CNP additions revealed the largest increase of SIR acc (267%) and PE (PE = 6.0). In the deep subsoil (>130 cm depth), only the CNP addition consistently increased SIR acc (by 871%) and PE (PE = 5.2) relative to only C additions (PE = 2.0). We conclude that microbial activity was not limited by nutrients in the topsoil but was co-limited by both N and P in the subsoil. The results imply that microbes mine nutrients from previously unavailable pools under the conditions that 1) deficiency actually exists, 2) co-limitation is alleviated, and 3) nutrient reserves are present. Yet, as opposed to microbial nutrient mining theories, we showed that the subsoil PE is highest when nutrient supply matches microbial demand. As a result also N deposition might exert variable effects on SOC turnover in tropical soils: it might have no effect in nutrient-rich topsoils and in co-limited subsoils without P reserves but might increase SOC turnover in co-limited subsoils with potentially acquirable P reserves. [ABSTRACT FROM AUTHOR]

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]

Peatlands comprise major global stocks of soil organic carbon (SOC). Many degraded peatlands are currently being restored, but little is known to which degree former disturbances leave a 'legacy' in such restored peatlands, and subsequently how this impacts their response to global change. Our aims were to investigate if after 20 years of restoration (i) carbon stability may still be affected by the former land use and if (ii) restored bogs are less susceptible to nutrient input but (iii) more sensitive to temperature rise. We sampled the top- and subsoil of a formerly drained, a previously drained and afforested part and an unmanaged control site of a Scottish bog. We incubated peat from each part for determination of potential basal respiration, nutrient limitation and temperature sensitivity (Q 10) of aerobic peat degradation. Lowest respiration rates were identified at the afforested site while nutrient addition had no significant effect on topsoil organic matter decomposition at all sites. Q 10 values were significantly higher in the topsoil (2.6 ± 0.3 to 2.8 ± 0.2) than in the subsoil. For the subsoil, the drained site (2.0 ± 0.0) showed significantly lower Q 10 values than the afforested one (2.6 ± 0.6), while the control site had a Q 10 of 2.1 ± 0.0, indicating contrasting temperature sensitivities of potential SOC losses following specific forms of disturbance. Overall, our data indicate that afforestation left a legacy on potential subsoil SOC losses with global warming. Such effects must be considered when integrating restored bogs into global data bases on peatlands' responses to global change. Image 1 • The rate of aerobic peat decomposition depends on former land use. • Formerly afforested peat shows reduced respiration and high temperature sensitivity. • Former drainage of peat does not affect its temperature sensitivity. • 20 years of restoration have levelled effects of former drainage on respiration rates. [ABSTRACT FROM AUTHOR]

• Assessment of woody encroachment in different tenure based rangeland management systems. • Communal areas exhibited higher bush encroachment after management change. • Woody encroachment went along with an improvement of soil properties, especially SOM. • Management change was an important driver for bush encroachment. Woody encroachment is increasingly threatening savanna ecosystems, but it remains unclear how this is driven by different land tenures and management systems. In South Africa, communal land is mainly managed under continuous grazing, while commercial land is under rotational grazing. We hypothesize that woody encroachment has increased since the end of the Apartheid era in 1994, when rotational grazing systems in communal land switched to continuous grazing. To test this hypothesis, we sampled six subsites in each of three replicates of the two tenure systems in the savannah biome, South Africa, and monitored the degree of woody encroachment and effects on soil using remotely-sensed normalized difference vegetation index (NDVI), dendro-ecology, and grid-based soil analyses. The results confirmed that there has been a positive greening trend over the past 25 years in communal areas due to an increase in woody cover, particularly for tree in the 50–150 cm height class. These trees corresponded to Senegalia tree ages between 10 and 25 years. Greater woody cover in communal areas was accompanied by height-dependent elevated nutrient and organic matter concentrations that increased in the topsoils by a factor of up to 1.5, relative to the freehold systems. Isotopic analyses identified debris of C 3 bushes and trees as main carbon input into the soil of communal areas, where the δ13C value of −20.1‰ was significantly lower than it was at private farms (−19.2‰). Isotopic values also indicated that this carbon input took place beyond the edges of the bush canopy. We conclude that the tenure-based management system in the communal areas has promoted woody encroachment, with the associated changes in soil properties homogenizing the savanna system. [ABSTRACT FROM AUTHOR]