Threats to soil quality in Denmark - A review of existing knowledge in the context of the EU Soil Thematic Strategy

The EU Commission is preparing a proposal for a Soil Framework Directive with the purpose of protecting the soil resources in Europe. The proposal identifies six major threats to the sustained quality of soils in Europe. This report addresses the threats that are considered most important under the prevailing soil and climatic conditions in Denmark: compaction, soil organic matter decline, and erosion by water and tillage. For each of these threats, the relevance and damage to soil functions as well as the geographic distribution in Denmark are outlined. We suggest a procedure for identifying areas at risk. This exercise involves an explicit identification of: i) the disturbing agent (climate / management) exerting the pressures on soil, and ii) the vulnerability of the soil to those stresses. Risk reduction targets, measures required to reach these targets, and the knowledge gaps and research needs to effectively cope with each threat are discussed. Our evaluation of the threats is based on soil resilience to the imposed stresses. Subsoil compaction is considered a severe threat to Danish soils due to frequent traffic with heavy machinery in modern agriculture and forestry. The soil content of organic matter is critically low for a range of Danish soils, which should be counteracted by appropriate management options. Soil erosion by tillage, and to a lesser degree by water, adversely affects soil quality on much of the farmland because degradation rates are much higher than generation of soil

Soil Quality Management - Concepts and Terms

The industrialization of agriculture and the concurrent increase in societal concerns on environmental protection and food quality have put focus on agricultural management and its impact on soil quality. Soil quality involves the ability of the soil to maintain an appropriate productivity, while simultaneously reducing the effect on the environment and contributing to human health. This development has changed society’s expectations to science and there is an urgent need to improve the communication among researchers from different scientific disciplines. The interaction of scientists with decision-makers is a topic of utmost relevance for future developments in agriculture. Reflexive objectivity denotes the exercise of raising one’s consciousness of the cognitive context, i.e. societal priorities and the values and goals of the researcher. The term sustainability comprehends the priorities in the cognitive context and thus constitutes a valuable tool for expressing the basis of scientific work. Soil quality evaluations should include awareness of the stability of any given quality attribute to disturbance and stress. This implies addressing resistance and resilience of the soil functions and/or the physical form in question. Most existing literature on soil quality focuses on assessment of soil quality rather than the management tools available to influence soil quality. Identification of management thresholds rather than soil quality indicator thresholds is suggested as an important means of implementing the soil quality concept. The major challenges facing modern agriculture include proper nutrient cycling, maintained functions and diversity of soil, protection of an appropriate physical form, and avoidance of chemical contamination. It is suggested that these challenges and problems as related to the soil quality concept are discussed in the framework expounded above

Searching the critical soil organic carbon threshold for satisfactory tilth conditions – test of the Dexter clay:carbon hypothesis

The concern for deteriorating soil structure at low soil organic matter (SOM) contents calls for better knowledge of SOM interaction with soil minerals as well as guidelines for soil conservation. We measured clay dispersibility in a field with a textural gradient. Our results support the concept of differentiating soil content of clay in a complexed and non-complexed part although our data did not point out an exact clay/OC ratio threshold. Our results also indicated that labile fractions of SOM may play an important role in soil physical behavior. We revisited literature data and found evidence that soil content of fines (<2 or <20 μm) is a major determinant of soil specific surface area (SA). We noted that soil SA coverage with SOM changed dramatically at a specific ratio of either clay (<2 μm) or clay+silt (<20 μm) with soil OC. This is an indirect support of the recently suggested quantification of the soil mineral ‘saturation’ hypothesis. More studies are needed on the causal relationships. We conclude that clay/OC~10 or (clay+silt20μm)/OC~20 are corresponding indices reflecting shift in soil physical behavior

Carbohydrates in hot water extracts of soil aggregates as affected by long-term management

Microbial carbohydrates are immediate by-products of microbial metabolism and play an important role in the formation and stabilization of soil structure. The effect of long-term management on soil carbohydrate content and monosaccharide composition was investigated in five. Danish sandy loams under organic and conventional management with animal manure and mineral fertilizers. Hot-water (80°C)extraction was used to measure the distribution and composition of carbohydrates in aggregate size. Carbohydrates released to hot water were determined after hydrolysis as reducing sugars equivalent to glucose. The monosaccharide composition in hot-water extracts was analyzed as the corresponding alditol acetates. Sites with a history of long-term continuos management practices were used. Three treatments from the >100 year Askov long-term field experiment were used to show results of contrasting fertilization on soil carbohydrate content. These were all grown to a four-course crop rotation. Total carbohydrate content was signifcantly infuenced by long-term management practices, with a signifcantly higher carbohydrate content in soils fertilized with either mineral fertilizers or animal manure (1200 to 800 mg C kg-1 DM aggregate)than in an unfertilized soil (600 to 500 mg C kg-1 DM aggregate). These results were as true for micro-aggregates (,0.25 mm)as for the 0.5–1. 0mmand 4.0–8.0 mm fractions. The organically managed soil (>40years) was sited at a commercial farm with forage crop rotations, organic manure and nouse of crop protection chemicals. These results showed signifcantly higher levels of carbohydrate both in micro-aggregate and macro-aggregates (1200 to 900 mg C kg-1 DM aggregate) than an adjacent conventionally managed soil with annual cash crop, mineral fertilizers and use of cropprotection chemicals (960 to 760 mg C kg-1 DM aggregate). Carbohydrate Ccontent generally increased as aggregate size decreased in both soils. Monosaccharide distribution was generally similar among three aggregate size classes studied. In all soils the content of monosaccharide was highest in micro-aggregates and lowest in macro-aggregates. Mannose and galactose were normally the most common monosaccharides in the hot-water extracts of aggregate fractions, indicating a predominantly microbial origin

Soil porosity as a habitat for microorganisms

The soil pore system has to be characterized quantitatively in order to describe the soil as a habitat for microorganisms. Soil pore morphology as determined by soil structure may be as important as the size distribution for the transport of gases and nutrients. This study adresses the physical properties of differently textured soils in undisturbed and remoulded state and their effect on microbial activity. Bulk soil was retrieved from 0-20 cm depth at six locations along a textural gradient in an arable field in Denmark. The samples ranged in clay content from 11 to 45%. The soils were crushed in air dry state, mixed, re-moistened, and exposed to a 9 month period of structure regeneration. Following application of 14C-labelled organic residues, the soils (labelled NA1 - NA6) were exposed to a further 8 month period of structure regeneration. Intact soil cores were then sampled and drained to four different matric potentials, analyzed for physical characteristics and subjected to a 15 weeks period of incubation with collection of evolved carbon dioxide. Cores of undisturbed but recently tilled topsoil from each sampling location in the field (labelled RE1 - RE6) were included as reference samples for the physical characteristics. The previously disturbed (NA) soils regained some of their secondary structure during storage under field-like conditions, but they were still structurally different from undisturbed (RE) soils of similar texture. The habitable and the protective pore space defined as the volume of pores ranging in size from 0.8-30 µm and 0.8-3 µm, respectively, appeared to be nearly constant across soils with different clay content. NA soils had slightly smaller volume of protective pore space compared with RE soils. Although partly aggregated, disturbed clayey soils showed less continuous / more tortuous pore systems than the texturally corresponding and structurally intact soils. Generally, the disturbed NA soils had a pore system with relatively small pores 'enmeshed' in the soil matrix, whereas the undisturbed RE soils exhibited larger pores. Measurements of oxygen diffusion confirmed that the relative water content - often quantified as the percentage of water filled pore space (WFPS) - may be a reasonable parameter in models simulating the activity of aerobic micro-organisms in remoulded soils. However, differences in the structure of undisturbed soils were shown to invalidate the use of WFPS as a universal parameter in simulation models applied to field conditions. The results indicated that turnover of soil OM was mainly controlled by the soil volume occupied by water. Soil texture affected turnover indirectly through its effect on the soil water regime. The turnover of the native OM was regulated by the total volume of water, whereas water in micropores and cavities <0.2 µm in diameter was not involved in the decomposition process of recently added OM. The studies encourage further investigations of the activity of soil microorganisms as related to the soil physical framework and the physical processes of importance for the function of the microbial community

Soil nutrient levels define herbage yield but not root biomass in a multispecies grass-legume ley

The response of above- and below-ground biomass to soil nutrient availability is crucial for estimating belowground carbon input and predicting changes in soil carbon storage. However, the response is far from clear at plant community level, especially for grassland systems. Using a long-term field experiment initiated 123 years ago with varying soil nutrient levels (deficient, sub-optimal, optimal and over-optimal) established by use of two nutrient sources (animal manure or mineral fertiliser), we examined the effects of soil nutrient level and source on herbage yield and composition, root biomass and root-to-shoot (R/S) ratio of an unfertilised multispecies grass-legume ley. Increased nutrient levels enhanced herbage yield, but did not affect root biomass. The R/S ratio decreased from deficient to sub-optimal level, but remained constant from optimal to over-optimal level. Nutrient source did not influence herbage yield, root biomass or R/S ratio, but the legume proportion increased in soils previously receiving mineral fertiliser. The R/S ratio decreased with herbage yield, but did not vary with herbage composition. We conclude that soil nutrient level and herbage yield rather than nutrient source and herbage composition determine biomass allocation between aboveground and belowground in temperate grassland leys

Epigenetic variation during the adult lifespan: cross-sectional and longitudinal data on monozygotic twin pairs

The accumulation of epigenetic changes was proposed to contribute to the age-related increase in the risk of most common diseases. In this study on 230 monozygotic twin pairs (MZ pairs), aged 18-89years, we investigated the occurrence of epigenetic changes over the adult lifespan. Using mass spectrometry, we investigated variation in global (LINE1) DNA methylation and in DNA methylation at INS, KCNQ1OT1, IGF2, GNASAS, ABCA1, LEP, and CRH, candidate loci for common diseases. Except for KCNQ1OT1, interindividual variation in locus-specific DNA methylation was larger in old individuals than in young individuals, ranging from 1.2-fold larger at ABCA1 (P=0.010) to 1.6-fold larger at INS (P=3.7×1

Manuring and stable nitrogen isotope ratios in cereals and pulses: towards a new archaeobotanical approach to the inference of land use and dietary practices

This paper explores the impact of animal manure application on the δ15N values of a broad range of crops (cereals and pulses), under a range of manuring levels/regimes and at a series of locations extending from northwest Europe to the eastern Mediterranean. We included both agricultural field experiments and areas where ‘traditional’ farming is practised. Our aim is to ground-truth interpretation of δ15N values in archaeobotanical crop remains as evidence of past growing conditions and husbandry practices. The results confirm the potentially radical impact of manuring on δ15N values in cereals, depending on manuring level, but indicate only a slight effect on pulses, which can fix atmospheric nitrogen. The expected geographical trend towards greater δ15N with increasing climatic aridity is not apparent, probably because the growing conditions for crops are ‘buffered’ through crop management. Each of these observations has fundamental implications for archaeobotanical interpretation of δ15N values as evidence of land use practices and (together with analysis of bone collagen/tooth enamel in potential consumers) palaeodiet