4. Conclusions
The results suggest that soil property distributions on the field scale vary more randomly than the specific intra-field structures. Controversially, higher degrees of structured spatial dependences were found on the catenary scale, and to a slightly lesser degree on the watershed scale. The existence of the relationships between physical and hydrological soil properties on the one hand and topography on the other enables the application of co-regionalization techniques in order to optimize soil sampling/mapping on the watershed scale by using readily available secondary information as terrain variables calculated from digital elevation models. Furthermore, the application of remote sensing data tends to be suitable to indicate different states of water supply and soil hydrological characteristics such as the hydraulic conductivity of the plough pan.
Results obtained from field and laboratory scale investigations revealed the dynamic dual porosity nature of the paddy soils with a macropore network penetrating the plough pan. The plough pan controls vertical water losses towards the subsoil, seems to be less permeable for the older and more developed paddy soils as compared to younger ones. Functioning of plough pans seems to increase with ‘age’ (i.e., duration of paddy management) of the paddy fields. Preferential flow paths indicate a potential for water loss and solute leaching at particular times during flooding the paddy fields. In the plough pan both dynamic (cracks and root channels) and relatively static (i.e., worm burrows) macropore systems could be identified as flow paths. In opposite to earlier, exclusively laboratory-based studies, we could demonstrate that the consolidation of the paddy's plough pan is likely to continue for decades.
Repeated puddling increases sealing and decreases mechanical top soil-swelling potential such that the plough pan grows (sedimentation) in thickness and shrinkage is reduced. However, even field sites that have been under paddy cultivation for centuries significant water and compound losses may occur through the bunds surrounding the field. Dye tracer experiments demonstrated that the bunds are distinctly more vulnerable to preferential water losses to the groundwater than the actual paddy field.
Puddling and the destruction of soil structure directly impacts microhabitats especially the development of intra-aggregate pores as described on the aggregate-scale. The response of microbial dynamics is varying with respect to soil texture and the management-stages of puddling, flooding, and drainage. The combination of micropedological investigations and specific detection of microorganisms is appropriate to achieve a complex description of microhabitats in paddy soils.
The outcome of the conducted research suggests that paddy fields are highly dynamic systems with periodical water and matter losses. Although paddies belong to agricultural systems that are strongly impacted by anthropological activities often over centuries, it is advisable to maintain equilibrium conditions with respect to water status as long as possible. If rice paddies are subjected to regular drying and wetting cycles even within the growing period, the dual porosity nature of the soil is further enhanced and significant water and matter losses may be the consequence. Since many of the rice fields are operated manually, it could even be possible to maintain ponded conditions during harvest in order to decrease the risk of crack formation. The puddling itself (and its regular repetition) may be considered as a consolidation and water conservation measure since it reduces the hydraulic conductivity of the plough pan.
4. ConclusionsThe results suggest that soil property distributions on the field scale vary more randomly than the specific intra-field structures. Controversially, higher degrees of structured spatial dependences were found on the catenary scale, and to a slightly lesser degree on the watershed scale. The existence of the relationships between physical and hydrological soil properties on the one hand and topography on the other enables the application of co-regionalization techniques in order to optimize soil sampling/mapping on the watershed scale by using readily available secondary information as terrain variables calculated from digital elevation models. Furthermore, the application of remote sensing data tends to be suitable to indicate different states of water supply and soil hydrological characteristics such as the hydraulic conductivity of the plough pan.Results obtained from field and laboratory scale investigations revealed the dynamic dual porosity nature of the paddy soils with a macropore network penetrating the plough pan. The plough pan controls vertical water losses towards the subsoil, seems to be less permeable for the older and more developed paddy soils as compared to younger ones. Functioning of plough pans seems to increase with ‘age’ (i.e., duration of paddy management) of the paddy fields. Preferential flow paths indicate a potential for water loss and solute leaching at particular times during flooding the paddy fields. In the plough pan both dynamic (cracks and root channels) and relatively static (i.e., worm burrows) macropore systems could be identified as flow paths. In opposite to earlier, exclusively laboratory-based studies, we could demonstrate that the consolidation of the paddy's plough pan is likely to continue for decades.Repeated puddling increases sealing and decreases mechanical top soil-swelling potential such that the plough pan grows (sedimentation) in thickness and shrinkage is reduced. However, even field sites that have been under paddy cultivation for centuries significant water and compound losses may occur through the bunds surrounding the field. Dye tracer experiments demonstrated that the bunds are distinctly more vulnerable to preferential water losses to the groundwater than the actual paddy field.Puddling and the destruction of soil structure directly impacts microhabitats especially the development of intra-aggregate pores as described on the aggregate-scale. The response of microbial dynamics is varying with respect to soil texture and the management-stages of puddling, flooding, and drainage. The combination of micropedological investigations and specific detection of microorganisms is appropriate to achieve a complex description of microhabitats in paddy soils.The outcome of the conducted research suggests that paddy fields are highly dynamic systems with periodical water and matter losses. Although paddies belong to agricultural systems that are strongly impacted by anthropological activities often over centuries, it is advisable to maintain equilibrium conditions with respect to water status as long as possible. If rice paddies are subjected to regular drying and wetting cycles even within the growing period, the dual porosity nature of the soil is further enhanced and significant water and matter losses may be the consequence. Since many of the rice fields are operated manually, it could even be possible to maintain ponded conditions during harvest in order to decrease the risk of crack formation. The puddling itself (and its regular repetition) may be considered as a consolidation and water conservation measure since it reduces the hydraulic conductivity of the plough pan.
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