Thermal properties in Luvisols under conventional and conservation tillage treatment
Soil temperature is one of the most important factors governing the exchange of energy and mass between the soil and the atmosphere as well as evaporation and aeration. Biological processes, like uptake of water and nutrients by roots, decomposition of organic matter by microbes, germination, seedling emergence and plant growth strongly depend on soil temperature and its thermal properties. These processes as well as the soil thermal regime depend also on soil texture, structure, and further physical and chemical properties. An increase in use of large machines in agriculture increases the total weight and the axle loads applied to the soil and, consequently, the risk of soil compaction leading to its degradation is greater as well. The result of increasing soil compaction through the increase in bulk density improves its thermal properties. On the other hand, however, soil compaction causes a disruption of the soil pore connectivities and flow processes of soil solutions and gas diffusion is affected as well. In recent years discussion about global climate changes has increased, contributing at the same time to an increasing interest in the influence of temperature on different factors not only in the area of biological science but also in agricultural and geological sciences and industry as well. In order to investigate the thermal properties of soils, the aim of this research was (i) to assess if there are changes in soil hydraulic and thermal properties caused by different tillage systems (conventional and conservation system) and soil compaction, (ii) to compare thermal properties of undisturbed and disturbed soil samples prepared by different bulk densities and determined by two different methods (damping depth and statistical-physical model) and (iii) to determine the effect of bulk density on soil shrinkage and pore functions ( kf, ku, kl) relating them to soil thermal properties. To determine the thermal behaviour of these soils, undisturbed and disturbed soil samples were taken at two depths of a Stagnic Luvisol derived from loess: 0-30cm and 30-60cm before and directly after wheeling. With the disturbed samples, soil volumes repacked by 2(3) bulk densities were prepared with the Load Frame device. Additionally, temperature data measured at two depths on the field were used. These data were measured by the Sugar Beet Institute in Göttingen. Changes in soil thermal properties appearing as a result of different soil management as well as caused by soil compaction (uncompacted and compacted plots) were measured for both tillage treatments and plots. To calculate thermal properties of the soil, the development of volumetric water content (with TDR needles) and temperature (with pT 100 thermistors) during the simulation of the daily fluctuation of temperature were registered in laboratory and then the thermal conductivity, volumetric heat capacity and heat diffusivity were calculated following the damping depth method and the statistical-physical model. Since soil thermal properties depend on the water-air relationship, hydraulic properties such as pore volume and size distribution, soil shrinkage, air permeability and saturated hydraulic conductivity were measured in soil samples prepared from homogenized material and different bulk densities. Estimated results show that different tillage systems as well as compaction influenced soil thermal as well as hydraulic properties. Conventional tillage treatment of annually disturbed soil surface decreases the amount of organic matter and aggregate stability and alters the heat flux through changes in soil roughness, which changes the area of the soil surface in contact with the atmosphere and in turn decreases heat conductivity. Conservational tillage treatment with more stable and better developed soil structure at a depth of 0-30cm which represents ploughing depth and decides differences between soil management presents higher water content as the main factor deciding soil thermal properties. According to the magnitude of volumetric water content conservational treatment presents greater values of thermal conductivity and volumetric heat capacity. Thermal diffusivity, however, is lower than under the conventional tillage treatment. From the latter we can conclude that under conservation tillage treatment the soil can store more heat, but at the same time and as a result of the lower thermal diffusivity, the atmospheric variations do not affect the soil thermal regime strongly. The rate of soil warming is worse compared with the conventional treatment, however, with increasing bulk density, thermal properties increase as well. As a result of tillage practises bulk density increases leading to a decrease in the pore size distribution which in turn leads to a decrease in the shrinkage behaviour of the soil. This means that the water and air flow in the soil is retarded decreasing the heat flow in soils especially if air pockets, acting as isolators, are formed. These changes in thermal properties influence biological processes in soils affecting plant growth, microbiological activity and decomposition of organic matter.