The importance of inorganic carbon in soil carbon databases and stock estimates: a case study from England

Many national and regional databases of soil properties and associated estimates of soil carbon stock consider organic, but not inorganic carbon (IC). Any future change in soil carbon stock resulting from the formation of pedogenic carbonates will be difficult to set in context because historical measurements or estimates of IC concentration and stock may not be available. In their article describing a database of soil carbon for the United Kingdom published in this journal, Bradley et al. [Soil Use and Management (2005) vol. 21, 363–369] only consider data for organic carbon (OC), despite the occurrence of IC‐bearing calcareous soils across a substantial part of southern England. Robust techniques are required for establishing IC concentrations and stocks based on available data. We present linear regression models (R² between 0.8 and 0.88) to estimate IC in topsoil based on total Ca and Al concentrations for soils over two groups of primary, carbonate‐bearing parent materials across parts of southern and eastern England. By applying the regression models to geochemical survey data across the entire area (18 165 km²), we estimate IC concentrations on a regular 500‐m grid by ordinary kriging. Using bulk density data from across the region, we estimate the total IC stock of soil (0–30 cm depth) in this area to be 186 MtC. This represents 15.5 and 5.5% of the estimated total soil carbon stock (OC plus IC) across England and the UK, respectively, based on the data presented by Bradley et al. [Soil Use and Management (2005) vol. 21, 363–369]. Soil geochemical data could be useful for estimating primary IC stocks in other parts of the world.     

利用地统计学研究土壤有机质空间分布：评估意大利中部地区土地退化的重要指标

Soil organic matter (SOM) content is one of the main factors to be considered in the evaluation of soil health and fertility. As timing, human and monetary resources often limit the amount of available data, geostatistical techniques provide a valid scientific approach to cope with spatial variability, to interpolate existing data and to predict values at unsampled locations for accurate SOM status survey. Using geostatistical and geographic information system (GIS) approaches, the spatial variability of some physical and chemical soil parameters was investigated under Mediterranean climatic condition in the Abruzzo region of central Italy, where soil erosion processes accelerated by human induced factors are the main causes of soil degradation associated with low SOM content. Experimental semivariograms were established to determine the spatial dependence of the soil variables under investigation. The results of 250 soil sampling point data were interpolated by means of ordinary kriging coupled with a GIS to produce contour maps distribution of soil texture, SOM content related to texture, and C/N ratio. The resulting spatial interpolation of the dataset highlighted a low content of SOM in relation with soil texture in most of the surveyed area (87%) and an optimal C/N ratio for only half of the investigated surface area. Spatial location of degraded area and the assessment of its magnitude can provide decision makers with an accurate support to design appropriate soil conservation strategies and then facilitate a regional planning of agri-environmental measures in the framework of the European Common Agricultural Policy.     

Effects of heavy metal contamination and remediation on soil microbial communities in the vicinity of a zinc smelter as indicated by analysis of microbial community phospholipid fatty acid profiles

Heavy metal contamination in an area immediately surrounding a zinc smelter has resulted in destruction of over 485 hectares of forest. The elevated levels of heavy metals in these soils have had significant impacts on the population size and overall activity of the soil microbial communities. Remediation of these soils has resulted in increases in indicators of biological activity and viable population size, which suggest recovery of the microbial populations. Questions remain as to how the metal contamination and subsequent remediation at this site have impacted the population structure of the soil microbial communities. In the current study, microbial communities from this site were analyzed by the phospholipid fatty acid (PLFA) procedure. Principal component analysis of the PLFA profiles indicated that there were differences in the profiles for soils with different levels of metal contamination, and that soils with higher levels of metal contamination showed decreases in indicator PLFAs for mycorrhizal fungi, Gram-positive bacteria, fungi, and actinomycetes. PLFA profiles for remediated sites indicated that remediated soils showed increases in indicator PLFAs for fungi, actinomycetes, and Gram-positive bacteria, compared to unremediated metal contaminated soils. These data suggest a change in the population structure of the soil microbial communities resulting from metal contamination and a recovery of several microbial populations resulting from remediation.     

Fluoro‐mobilization of metals in a Slovak forest soil affected by the emissions of an aluminum smelter

Depositions originating from a central Slovak Al smelter may increase metal solubility in adjacent soils because they contain F (mainly HF). The reason for fluoro‐mobilization of metals may be the formation of soluble fluoro‐metal complexes or the mobilization of organic matter and subsequent formation of organo‐metal complexes. The objectives of our work were (1) to assess the extent of metal mobilization by fluoride in a Slovak Lithic Eutrochrept affected by the emissions of an Al smelter and (2) to model the dissolved metal species with the help of a chemical equilibrium model (MINEQL+). The O (Moder), A, and B horizons were equilibrated with solutions at F concentrations of 0, 0.9, 2.7, and 9.0 mmol l^—1. In the extracts, the concentrations of Al, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Ni, Pb, Zn, dissolved organic carbon (DOC), free and complexed F, and the pH and electrical conductivity (EC) were determined. The heavy metal concentrations in the O horizon (Cd: 0.99, Cr: 18.0, Cu: 44, Ni: 26, Pb: 110, and Zn: 84 mg kg^—1) were 2.5 to 9 times larger than those in the A and B horizons. The concentrations of H₂O‐soluble F decreased from the O (261 mg kg^—1) to the A (103 mg kg^—1) and B horizon (92 mg kg^—1). In batch experiments increasing addition of F increased the equilibrium concentrations of Al, Cr, Cu, Fe, Ni, Pb, and DOC in all samples, of Cd in the A, and of K in the B horizon. At the same time the concentrations of complexed F and pH increased whereas EC decreased. Chemical equilibrium modelling indicated that the mobilizing effect of F resulted from the formation of fluoro‐Al complexes and organo‐complexes of all other metals.     

Chemical forms of cadmium in a calcareous soil treated with different levels of phosphorus and cadmium and planted to spinach

The objective of the investigation was to evaluate the effect of applied phosphorus (P) and cadmium (Cd) on Cd chemical forms determined by sequential extraction and the relationship between these forms and plant responses, i.e., dry weights, concentration, and total uptake of Cd and P in a greenhouse experiment. Treatments consisted of five levels of Cd (5, 10, 20, 40 and 80 mg kg^?1 soil as cadmium sulfate) and four levels of P (0, 15, 30 and 60 mg kg^?1 soil as monocalcium phosphate), which were added to the soil and left to equilibrate for 1 month under greenhouse conditions. Spinach seeds (Spinacea oleracea L., cv Viroflay) were sown and then grown for 8 weeks. The chemical composition in the aerial part of the pant and soil was determined. Application of Cd decreased plant dry matter and increased Cd concentration in the plant, whereas at each level of applied Cd, P increased plant dry matter and decreased plant Cd concentration. All chemical forms of Cd in soil, as determined by a fractionation method, were increased following Cd application, the highest being the carbonatic form. Phosphorous application decreased exchangeable and carbonatic forms of Cd, whereas it increased other forms.     

硅胶管气样原位采集技术研究土壤N_2O浓度及通量变化

箱法被广泛用于监测土壤N_2O排放通量,但在原位采集高浓度土壤N_2O、全天候监测N_2O通量变化、动态研究土壤剖面N_2O的行为等方面存在弊端.本研究通过室内模拟硅胶管对N_2O的通透性,探索硅胶管用于原位采集土壤气样的理论可行性.田间试验设施用铵态氮肥(NH_4~+)、施用硝态氮肥(NO_3~-)及施用硝态氮肥加葡萄糖(NO_3~-+C)等3个处理,同时安置硅胶管和采样箱,验证硅胶管法在原位采集高浓度土壤N_2O气样、监测土壤N_2O浓度以及排放通量的实际效果,并与箱法进行比较.结果表明,硅胶管内外的N_2O气体经2.9 h达到95%的平衡,完伞能满足大田采样要求;用硅胶管法原位采集高浓度土壤N_2O气样的效果显著优于箱法采样.其浓度变化表现出明显的时间规律,浓度梯度法计算的N_2O排放通量与箱法测定结果呈显著正相关,但数值偏低;偏低的程度取决于采样位置和土壤中N_2O产生位置的匹配程度.建议采用埋于土壤表层的硅胶管计算地面N_2O排放通量,或在不同土层埋人硅胶管研究土壤剖面N_2O行为的时空变异.     

Cation binding in a soil with low exchange capacity: Implication for the structural rigidity of soil organic matter

Two previous studies suggested that part of the cation sorption sites in soil organic matter with low exchange capacity have to be considered as “lonely”, i.e., too far from each other to allow direct cross‐linking by bivalent cations. The objective of this contribution was to understand the mechanisms controlling structural rigidity and physicochemical aging of the SOM (soil organic matter) and the role of water molecule bridges (WaMB) therein. For this, we evaluated the matrix rigidity of an organic surface layer of a Haplic Podzol on a quantitative basis, by assessing WaMB transition temperature (T*) directly after treatment with bivalent cations (Mg²⁺, Ca²⁺, or Ba²⁺) and after eight weeks of aging. Cation loading as well as cation type influenced matrix rigidity. Ba²⁺ induced the most rigid matrix and Mg²⁺ the weakest, which is in line with their binding strength in terms of Langmuir coefficient. The matrix rigidity increased with the cross‐linking activity, which is the product of loading and Langmuir constant of the respective cation. The aging process, however, was slowed down by the initial matrix rigidity, and the rigidity of the aged matrix decreased with increasing Langmuir constant. The degree of aging increased with increasing hydration enthalpy of the cation and decreased with increasing cation loading. Thus, directly after cation treatment, direct cross‐links by multivalent cations were most relevant, but WaMB increasingly gained influence on the matrix rigidity during aging. The untreated sample revealed a considerable number of WaMB, resulting in a fairly rigid and strongly cross‐linked matrix which, however, flexibly reacts on external influences like change in cation concentration or relative humidity. With these findings, the ideas on the relevance of indirect CaB‐WaMB associations between distant sorption sites for the rigidity and flexibility of the OM matrix as proposed in previous studies were confirmed on a mechanistic basis in this study.     

Hand-feel soil texture observations to evaluate the accuracy of digital soil maps for local prediction of soil particle size distribution: A case study in Central France

Digital maps of soil properties are now widely available. End-users now can access several digital soil mapping (DSM) products of soil properties, produced using different models, calibration/training data, and covariates at various spatial scales from global to local. Therefore, there is an urgent need to provide easy-to-understand tools to communicate map uncertainty and help end-users assess the reliability of DSM products for use at local scales. In this study, we used a large amount of hand-feel soil texture (HFST) data to assess the performance of various published DSM products on the prediction of soil particle size distribution in Central France. We tested four DSM products for soil texture prediction developed at various scales (global, continental, national, and regional) by comparing their predictions with approximately 3 200 HFST observations realized on a 1:50 000 soil survey conducted after release of these DSM products. We used both visual comparisons and quantitative indicators to match the DSM predictions and HFST observations. The comparison between the low-cost HFST observations and DSM predictions clearly showed the applicability of various DSM products, with the prediction accuracy increasing from global to regional predictions. This simple evaluation can determine which products can be used at the local scale and if more accurate DSM products are required.     

Changes in the succession and diversity of protozoan and microbial populations in soil spiked with a range of copper concentrations

We studied microbial and protozoan activity, diversity and abundance as affected by Cu²⁺ amendments ranging from 0 to 1000 μg g⁻¹ over a 70-day period. At the end of the experiment the microbial population size, as indicated by substrate-induced respiration, had normalized for all Cu²⁺ concentrations, but 1000 μg g⁻¹. Protozoan abundance was negatively affected by Cu²⁺, although, only in the first few weeks. A more detailed analysis of the individual components that make up the microbial and micro-faunal populations (phospholipid fatty acid (PLFA) profile and protozoan morphotypes), however, yielded a somewhat more complex picture. For the three highest Cu²⁺ amendments (160, 400 and 1000 μg g⁻¹), there still was a significant reduction in number of differentiable protozoan morphotypes. The bacterial PLFA pattern suggested a shift from Gram-negative towards Gram-positive bacteria for the high amendments, a process where protozoan grazing most likely played a significant role. The ratio of the trans/cis isomers of the 16:1ω7 fatty acid indicated that Cu²⁺, even at low and medium concentrations, induced physiological changes in the microbial population. The relatively slight changes in total microbial and micro-faunal abundance and activity, also at the highest Cu²⁺ concentrations, probably reflected the ability of the community to compensate for loss of taxa by functional substitution.     

Influence of soil phosphorus status,texture, pH and metal content on the efficacy of amendments to pig slurry in reducing phosphorus losses

Cost‐effective strategies for using chemically amended organic fertilizers need to be developed to minimize nutrient losses in surface and groundwater. Coupling specific soil physical and chemical characteristics with amendment type could increase their effectiveness. This study investigated how water‐extractable phosphorus (P) was affected by chemical amendments added to pig slurry and how this effect varied with soil properties. A 3‐month incubation study was conducted on 18 different mineral soils, stored at 10 °C and 75% humidity and treated with unamended and amended slurry which was incorporated at a rate equivalent to 19 kg total P (TP )/ha. The amendments examined were commercial‐grade liquid alum, applied at a rate of 0.88:1 [Al:TP ], and commercial‐grade liquid poly‐aluminium chloride (PAC ), applied at a rate of 0.72:1 [Al:TP ]. These amendments were previously identified by the authors as being effective in reducing incidental losses of P. The efficacy of the amendments varied with the soil test P, the degree of P saturation (DPS ) and the Mehlich aluminium, iron and calcium, but not soil texture. Chemical amendments were most effective in soils with DPS over approximately 20%. Due to their high cost, the incorporation of amendments into existing management practices can only be justified as part of a holistic management plan where soils have high DPS .     

Changes in the carbon concentrations and other soil properties of some Scottish agricultural soils: Evidence from a resampling campaign

The change in soil carbon (C) concentration, soil pH and major nutrients for approximately 1,000 topsoil sampled from on-farm experimental sites over a thirty-year period from 1950 to 1980 in north-east Scotland are summarized. This period coincided with increased agricultural intensification, which included regular liming and fertilizer additions. During 2017, 37 of these sites were resampled and reanlaysed. While pH and extractable phosphorus (P) and potassium (K) increased over this period, there was no detectable change in the percentage loss on ignition. Composite soil samples were taken by auger from a depth of 0–15 cm and compared with the corresponding archived samples collected at the initiation of each experiment. Analysis of these resampled soils indicated no significant change in soil carbon (C), although soil pH, extractable magnesium (Mg) and K and Nitrogen (N) concentrations were significantly greater (p < .001) but extractable soil P concentration was significantly less (p = .015) compared with the original samples. Even though measuring C concentration alone is a poor indicator of overall changes in soil C stocks, it does provide a relative quick “early warning” of C losses that would justify a more comprehensive measure of stocks.     

Ammonium accumulation in soil: the long‐term effects of tillage,rotation and N rate in a Mediterranean Vertisol

Plant nutrition requires organic nitrogen to be mineralized before roots can absorb it. A 13‐year field study was conducted on typical rain‐fed Mediterranean Vertisol to determine the effects of tillage system, crop rotation and N fertilizer rate on the long‐term NH₄⁺–N content in the soil profile (0–90 cm). The experiment was designed as a randomized complete block with a split–split plot arrangement and three replications. The main plots tested the effects from the tillage system (no‐tillage and conventional tillage); the subplots tested crop rotation with 2‐year rotations (wheat–wheat, wheat–fallow, wheat–chickpea, wheat–faba bean and wheat–sunflower) and the sub‐subplots examined the N fertilizer rate (0, 50, 100 and 150 kg N/ha). Soil NH₄⁺–N content was greatest in the rainiest years and greater under the no‐tillage (NT) system than the conventional tillage (CT) system (57 and 48 kg/ha, respectively). The deepest soil (30–60 and 60–90 cm) contained a greater NH₄⁺–N content (21.0 and 21.4 kg/ha, respectively) than the shallowest soil (19.5 kg/ha in 0–30 cm). This observation may be related to Vertisol characteristics, especially crack formation that allows greater mineralization in the deepest layers by displacing organic matter.     

Microbial activity in the rhizosphere soil of six herbaceous species cultivated in a greenhouse is correlated with shoot biomass and root C concentrations

The stimulation of rhizosphere microorganisms by exudates released from roots is important for nutrient cycling and differs between plant species. The reasons for this between-species variability are poorly understood. We studied correlations between shoot biomass, soluble and non-soluble root C concentrations and rhizosphere bacterial abundance (CFU: colony forming units) and an index of microbial activity (in vitro utilization of [U-¹⁴C]glucose by soil microorganisms). We studied Briza media and Rumex acetosella (nutrient-poor habitats), Epilobium hirsutum, Eupatorium cannabinum, Rumex obtusifolius and Urtica dioica (nutrient rich habitats) cultivated in a greenhouse for 5 weeks in a forest soil. We found significant differences among species for the bacterial abundance and microbial activity in the rhizosphere. These differences poorly reflected the nutrient richness of the common habitats for these species, possibly because the soil conditions were not optimal. Nevertheless, microbial activity was positively correlated with root soluble C concentration and shoot biomass and negatively correlated with the concentration of non-soluble C in roots. These preliminary results suggest that the carbon economy could be an important control of the between-species variability of microbial activity in the rhizosphere.     

Determinants of the degradation of soil structure in vineyards with a view to conversion to organic farming

In perennial crops such as grapevine, there is a considerable risk of soil degradation caused by mechanization. Organic farming may increase traffic and result in more intensive soil structure degradation, especially on wet soil. Soil structure was observed in 69 soil profiles from 12 vineyards (Languedoc‐Roussillon, Bordeaux, Provence, Burgundy, Rhône Valley) to study the relationships between permanent soil characteristics (texture, stoniness, waterlogging), cultivation practices (technical operations, traffic, farm equipment) and soil structure. Compaction zones were identified in soil profiles viewed perpendicularly to the grapevine rows. The percentage of compacted area and a score for compaction intensity were assessed in the top 0.5 m of the soil profile in three soil compartments defined by the distance from the grapevine row: (1) 0 to 0.2 m (2) 0.2 m to 0.5 m and (3) 0.5 m to the middle of the inter‐row. Most soil profiles exhibited considerable compaction: 75% of the soil profiles were compacted in compartments 2 or 3. Intensity correlated with depth of compaction in compartment 2 – the deeper the compaction, the greater the compaction intensity. Four types of soil profiles could be identified, depending on the location of compacted zones and of their intensity of compaction. The factors that determined the types of soil profiles were as follows: soil vulnerability to compaction, use of moderating practices (practices that limit the risk of compaction) and traffic geometry. A statistical analysis resulted in a decision tree that provides a useful basis to choose cultivation practices that limit damage to or improve soil structure.     

Contribution of nitrification and denitrification to the emission of N2O in a freeze‐thaw event in an agricultural soil

The amounts of N₂O released in freeze‐thaw events depend on site and freezing conditions and contribute considerably to the annual N₂O emissions. However, quantitative information on the N transformation rates in freeze‐thaw events is scarce. Our objectives were (1) to quantify gross nitrification in a Luvisol during a freeze‐thaw event, (2) to analyze the dynamics of the emissions of N₂O and N₂, (3) to quantify the contribution of nitrification and denitrification to the emission of N₂O, and (4) to determine whether the length of freezing and of thawing affects the C availability for the denitrification. ¹⁵NO was added to undisturbed soil columns, and the columns were subjected to 7 d of freezing and 5 d of thawing. N₂O emissions were determined in 3 h intervals, and the concentrations of ¹⁵N₂O and ¹⁵N₂ were determined at different times during thawing. During the 12 d experiment, 5.67 mg NO ‐N (kg soil)^–1 was produced, and 2.67 mg NO ‐N (kg soil)^–1 was lost. By assuming as a first approximation that production and loss occurred exclusively during thawing, the average nitrate‐production rate, denitrification rate, and immobilization rate were 1.13, 0.05, and 0.48 mg NO ‐N (kg soil)^–1 d^–1, respectively. Immediately after the beginning of the thawing, denitrification contributed by 83% to the N₂O production. The ratios of ¹⁵N₂ to ¹⁵N₂O during thawing were narrow and ranged from 1.5 to 0.6. For objective (4), homogenized soil samples were incubated under anaerobic conditions after different periods of freezing and thawing. The different periods did not affect the amounts of N₂ and N₂O produced in the incubation experiments. Further, addition of labile substrates gave either increases in the amounts of N₂O and N₂ produced or no changes which suggested that changes in nutrient availability due to freezing and thawing are only small.