Multi-level assessment of soil quality – linking reductionistic and holistic methodologies

Soil quality is often used as a qualitative, general term. However, quantification is an important feature of the scientific approach to nature. On the other hand, addressing specific soil parameters as indicators of soil quality includes a reduction of the whole soil system. Therefore, results obtained by specialized methodologies ought to be evaluated by methods integrating the soil characteristics in situ. In this presentation, results are given from an investigation of the tilth of two differently managed loamy soils. One of the soils had been managed for decades with a forage crop system (labeled FCS), which included fertilization with farmyard manure, while the other had been grown with a continuous cereal system (labeled CCS), receiving no input of organic matter. In the field, the structure of the top 30-cm soil layer was described visually (spade analysis) and by studying the fragmentation behavior (soil drop test). Further, the field measurements included determination of soil strength by a torsional shear box method. In the laboratory, shear strength was determined on bulk soil sampled in metal cylinders, and tensile strength was estimated from crushing tests of individual, differently sized aggregates. The FCS soil appeared porous, with crumbs as structural units, while the CCS soil was compact with blocks as structural units. The soil drop test yielded the highest degree of fragmentation for the FCS soil. The torsional shear box method showed the CCS soil to have the highest bulk soil strength. This was confirmed by the laboratory shear annulus method. Finally, the tensile strength measurements revealed a much higher strength of 8-16, 4-8 and 2-4 mm dry aggregates from the CCS soil as compared to the FCS soil, while 1-2 mm aggregates were strongest in the FCS soil. This indicates a higher friability for the FCS soil, which is in accordance with the soil behavior in the field tests. In conclusion, the quality of the FCS soil – as evaluated by its mechanical behavior – was found to be higher than that of the CCS soil. An important result is the good correlation between the integrating field methods and the differentiating laboratory methods. This means that the quantifying, reductionistic scientific approach is not conflicting with the ‘holistic’ descriptions in the field

Biotic and abiotic binding and bonding mechanisms in soils with long-term differences in management

During the last decades Denmark has experienced a growing interest in low-input farming systems like organic farming. These systems rely on a high soil fertility to maintain nutrient availability and plant health. Soil aggregation contributes to this fertility, because it is crucial to soil porosity, aeration and infiltration of water. This paper reports a study of two pairs of differently managed, neighboring fields. The aim was to elucidate long-term effects of the different farming systems on physical and biological variables with influence on bonding and binding mechanisms of soil aggregation. Each pair consists of an organically grown dairy farm soil, based on a forage crop rotation system, including grass (Org-FCS(G)) and a conventionally managed soil. One of the conventional farms has a forage crop rotation with annual cash crops and no grass (Conv-FCS(NG)) and one has been grown continuously with small grain cereals and rape (Conv-CCS). Our results indicate that the Org-FCS(G) soils stimulate biotic soil aggregating agents as measured by extracellular polysaccharides (EPS) and hyphal length measurements, respectively. Generally, the Conv-CCS soil, which relies exclusively on synthetic fertilisers and cereal production, offered poor conditions for the biotic binding and bonding agents. Nevertheless this soil contained a large amount of stable macro-aggregates. This is explained by the physical results, which indicated that the strong macro-aggregation was due to clay dispersion and cementation processes rather than to biotic processes

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

Short-term effects of tillage on mineralization of nitrogen and carbon in soil

Tillage is known to decrease soil organic nitrogen (N) and carbon (C) pools with negative consequences for soil quality. This decrease is thought partly to be caused by exposure of protected organic matter to microbial degradation by the disturbance of soil structure. Little is known, however, about the short-term effects of tillage on mineralization of N and C, and microbial activity. We studied the short-term effects of two types of tillage (conventional plough- and a non-inverting-tillage) on mineralization and microbial N and C pools in a sandy loam under organic plough-tillage management. The release of active and protected (inactive) N by tillage was further studied in the laboratory by use of 15N labelling of the active pool of soil N followed by simulation of tillage by sieving through a 2 mm sieve. Results showed that the two types of tillage as well as the simulation of tillage had very few effects on mineralization and microbial pools. The simulation of tillage caused, however, a small release of N from a pool which was otherwise protected against microbial degradation. The use of soil crushing for disruption of larger macroaggregates (>425 µm) and chloroform fumigation for perturbation of the microbial biomass increased the release from both active and protected N pools. The relative contribution from the protected N pool was, however, similar in the three treatments (22-27%), thus the pools subjected to mineralization were characterised by similar degree of protection. On the basis of isotopic composition the pools of N mineralised were indistinguishable. This suggests that the released N originated from the same pool, that is the soil microbial biomass. The study points to the microbial pool as the main source of labile N which may be released by tillage, and thus to its importance for sustained soil fertility in agricultural systems

Soil quality aspects of humid sandy loams as influenced by organic and conventional long-term management

The development of low-input farming systems requires knowledge of the extent to which management characteristics influence the soil as a habitat for micro-organisms and a medium for plant growth. This paper presents a study of long-term effects of organically and conventionally cultivated systems on a range of soil physical, chemical and biological characteristics in Denmark. Seven soils were included, falling into three groups (I¯III). Each group consists of an organically managed soil (Org) referenced by one or two conventionally managed soils (Conv) with the same soil type (sandy loam) and pedological history. All organically managed soils were dairy farm soils (H), which had been organically managed for 46, 47 and 40 years, respectively. They had forage crop rotations and received animal manure. The conventional soils were either dairy farm soils or soils grown to annual cash crops (P). Undisturbed core samples were used for assessing dry bulk density and water retention. Undisturbed cubic samples were used for assessing wet stability of macro-aggregates, clay dispersibility, microbial biomass C, -glucosidase activity, ergosterol, and the abundance of different groups of fungi. Penetration resistance and ease of soil fragmentation were measured in the field. The quantifying methods were supplemented by a visual inspection of soil structure of the top 30 cm soil in the field. Irrespective of agricultural system, the use of tractors and heavy machinery had caused compaction of the subsoil in the form of a dense pan below ploughing depth. In group I, the H-soils were heavily trafficked and this overshadowed the long-term effects of the diversified crop rotations and animal manure applications on the physical parameters. Generally, the results highlight the paramount influence of soil tillage and traffic in agriculture. Further, the results confirm the positive effects of organic manures and diversified crop rotations on soil quality aspects. Microbial biomass C was found to be higher in organically than in conventionally managed dairy farm soils, and appeared to correlate linearly to the volume of 0.2¯3.0 m pores (`protective' pore space). The simple model did, however, not explain a lower microbial biomass for two soils dressed only with synthetic fertilisers. The results further indicated that the contribution of the different biotic mechanisms responsible for macro-aggregation varied from soil to soil. The results revealed promising correlations between quantitative scientific laboratory methods and descriptive methods in the field

Aggregate strength and mechanical behaviour of a sandy loam soil under long-term fertilization treatments

Current concern for soil quality has stimulated research on soil biological and chemical properties. In contrast, the mechanical behaviour of soil is somewhat neglected. We have examined the effects on soil mechanical properties of more than 100years of contrasting fertilization employing three treatments from the Askov long-term experiment: UNF (unfertilized), NPK (mineral fertilized) and AM (animal manured). We have measured tensile strength of aggregates when air-dry and when adjusted to 10, 30 and 100kPa pressure potential. Four aggregate size classes were investigated (1-2, 2-4, 4-8 and 8-16mm diameter). Soil fragmentation was characterized in the field using a drop-shatter test. Bulk soil strength was determined in the field using a shear vane and a torsional shear box. Soil texture, pH, cation exchange capacity and microbial biomass were measured. The unfertilized soil has little soil organic matter and microbial biomass and is dense. Its aggregates were strong when dry and weak when wet. In contrast, the manured soil had strong aggregates when wet and rather weak ones when dry. The NPK soil generally had intermediate properties. The differences between the soils when dry seem to be related to differences in dispersible clay content, whereas the differences when wet are related to differences in the amount of organic binding and bonding material. The optimal water content for tillage as well as the tolerable range in water content was largest in the manured soil and smallest in the unfertilized soil. Our results indicate that soil mechanical properties should be measured over a range of water regimes to determine the effects of various long-term fertilization treatments