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Dynamics of Soil State Variables and Related Processes Across a Land Use Gradient in Spatial and Temporal Transition
Wendroth, O. O., M. Coyne, R. McCulley, A. Karathanasis, J. Grove
Department of Plant and Soil Sciences
Land use system and management intensity cause huge impacts on soil water, carbon and nitrogen dynamics in the vadose zone, soil structure and related processes in the surface soil, e.g. dynamics of soil carbon, gas exchange, water infiltration, and soil aggregates. Understanding processes during transitions of land use in time and space is of crucial importance with respect to long-term land use planning and its impact on environmental quality.
The purpose of this project is to monitor relevant soil ecological and vegetative processes in two established land use systems over one year, followed by a study of the same processes during transition from pasture to agricultural cropland and cropland to pasture and to model the measured variables using spatial and temporal statistical methods. The experimental facility established in this project will be maintained as a long-term project beyond the three years of this study. In this fundamental research project, an innovative experimental and diagnostic approach to increasing the understanding of spatial-temporal soil physical and bio-geochemical processes, their interrelationships and association with plant biomass is developed.
To study processes before and during land use transition, on top of inherent field-scale soil variability with this group of soil and plant experts, provides a unique opportunity to improve experimental designs, including more contemporary spatial and temporal elements, while enhancing basic knowledge on soil ecological processes. In this work, functional characterization of soil processes will occur at both field and lab scales. Moreover, the data collected in the proposed project becomes a strong dataset for the testing of other approaches to stochastic and deterministic modeling. Local and temporal representation of widely used soil variables, and transport and transfer coefficients, will be quantified based on spatially and temporally intensive measurements which manifest the novel character of the proposed approach. After the three-year project period is expired, the field experiment will be maintained, as the processes investigated will certainly change over longer terms.
Results of the proposed study will contribute to an improved understanding of soil processes in agricultural ecosystems, and their relationships to each other. Results will be used to improve understanding of land use-, soil structure- and vegetation-related soil water, gas, carbon and nitrogen dynamics and changes in important rate constants and kinetics. From the results of this study a novel database will be compiled, one of the first datasets that would allow physically-deterministic and stochastic modeling of processes observed in a landscape, and how the continuities of these processes behave in spatial transition zones between different land use systems and during the early period of land use transitions.
2010 Project Description
In addition to the basic site specific soil survey that was undertaken in the previous report period, further experimental installations were performed in this year that support the study of spatio-temporal patterns of soil water, carbon, and nitrogen dynamics under two differently managed land use systems, i.e., pasture and cropland.
Collars for measuring flux of some greenhouse gases were installed, so that a complete set of 60 sampling points is now established. Moreover, soil water content access tubes were installed so that the spatial installations were completed as well. To recall, the spatial design consists of a basic sampling interval of 5 m, and of four nests in which six locations are separated by 1 m distance. This nested sampling structure was chosen in order to be able to identify a spatial range of representativity even for variables that are hardly spatially structured even over distances of 5 m.
Main outputs: At a biweekly schedule, greenhouse gas fluxes at the land surface have been measured by one of our graduate students together with soil profile water content distribution using a Diviner capacitance probe. Currently, data sets are being analyzed.
In general, there is a large impact of soil temperature and soil water content on CO2 respiration rate. However, this effect can only be observed over a large time period and for measurements at many locations. On a particular day, the impact of soil water content and surface soil temperature on gas flux is concealed by the effect of soil spatial variability. Therefore, we currently work on a detrending algorithm that will allow to process raw measurements for spatial analysis so that we can assume, the detrended data would reflect what we would have found, if all the measurements of one day could have been taken at the same time and under identical temperature conditions.
Another new fundamental outcome is that temporal stability of carbon-dioxide measurements was detected. This spatial behavior has been shown so far only for soil water status, and this study is one of the first if not the first time that temporal stability behavior was shown for gas flux measurements as well. Hence, over a limited time period, one or several locations in the field can be identified to represent the mean of the distribution. Other locations manifest the upper and lower limit of 1 standard deviation.
Moreover, the spatial distribution of soil carbon gas flux proceeds in a structured fashion with different ranges among land use systems, and spatial variance behavior changing with different moisture content and temperature levels. A detailed analysis of this space-time-variance behavior will be presented at the coming SSSA annual conference.
Progress was made on soil gas diffusivity measurements at each of the 60 main investigations points for which all other state and rate variables were measured as well. For this purpose, a simple quasi-stationary measurement method was established in the UK soil physics laboratory. At different soil water pressure heads, different volume fractions of the total pore space are air-filled.
Establishing a specific soil water pressure head before measuring the apparent gas diffusion coefficient, controls the size (equivalent diameter) of the smallest air-filled pore. Knowing the volume of specific pore size classes and the associated relative apparent gas diffusion coefficient allows to derive the continuity or tortuosity of specific pore size classes. Currently measurements are being analyzed for this quantification of pore geometry, i.e., an important soil structural factor. Moreover, a procedure is tested to modify the measurement system of oxygen concentration time series which has so far accomplished with gas chromatography and hand sampling of gas using a syringe. An electronic system using oxygen sensors that can be automatically logged is tested for measurement noise, accuracy and increased efficiency of the method.
Soil hydraulic conductivity at soil water status close to saturation was investigated across the same domain and at the same locations for which gas flux and other measurements were obtained. Tension infiltrometers were used for this purpose and the hydraulic conductivity was measured at soil water pressure heads of -1, -5 and -10 cm. These pressure heads imply that all pores with an equivalent diameter smaller than 3 mm, 600 μm, and 300 μm, respectively, participated in the transport of water. Spatial structure was found for the distribution of K-values measured at -10 and -5 cm. Huge point-point-fluctuations that can be expected as a consequence of local variability in soil structure-affected macro pores, their abundance and their geometry caused that only moderate to weak structure of K at -1cm was found.
Currently, other soil structural properties (aggregate size distribution and stability) are investigated to identify spatial relationships between structure-relevant state variables. Results of this analysis will be presented as well at a national conference. The innovative aspect of spatial measurements of soil state and rate variables in this study is the fact that most of these measurements were undertaken at the plot scale for quantitative comparisons between different land use systems so far. In hardly any studies, the range of spatial and temporal representativity was quantified, nor were relationships derived in a spatial context that cause a variability pattern across the landscape.
There obviously exists lack of knowledge on the spatial correlation and cross-correlation behavior of greenhouse gas fluxes. Gas diffusivity as a function of the air-filled porosity has never before been investigated in a spatial context. Spatial investigations on soil hydraulic properties and water fluxes do exist, however, gas flux behavior over a spatial behavior has not been investigated before together with other relevant soil properties that reflect soil structure.
We have so far been very dedicated to data collection and establishing experimental lab and field procedures. For this coming experimental period, solute transport behavior, especially nitrate transport will be intensively investigated at our experimental site.
Ole Wendroth, R.L. McCulley, M.S. Coyne, A. Karathanasis, and J.H. Grove. 2010. Dynamics of Soil State Variables and Related Processes Across a Land Use Gradient in Spatial and Temporal Transition. Poster, 15th Annual KY EPSCoR Conference, Science's Grand Challenges, May 24, 2010, Lexington, KY.