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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.
2009 Project Description
In this study, spatio-temporal patterns of soil water, carbon, and nitrogen dynamics are investigated under two differently managed land use systems, i.e., pasture and cropland. Both land use systems were established in the same field at the Agricultural Experiment Station's experimental farm Spindletop. In the reported period, a basic soil textural survey was accomplished, showing that soil textural compositions varied in the field as follows: 0-15 cm depth: silt 65-75 %, clay 16-25 % 15-30 cm depth: silt 53-72 %, clay 17-39 % 30-60 cm depth: silt 44-67 %, clay 25-46 % 60-90 cm depth: silt: 28-53 %, clay 38-58 %. This range of variation must be considered very large, given the fact that the experimental field covers an area of 0.4 ha. The experimental design is focused on the identification of spatial processes of soil rate and state variables.
Spatial statistical analysis has revealed correlation lengths for the different textural classes as follows: (In this characterization, well, moderate or weak refer to the pronunciation of spatial structure based on the nugget/sill ratio, and "long" is mentioned if there was a tendency for a long trend observed.) 0-15 cm depth: sand 50 m (long), silt 15 m (well), clay 15 m (moderate) 15-30 cm depth: sand 100 m (well, long), silt 25 m (moderate, long), clay 50 m (moderate) 30-60 cm depth: sand 100 m (well, long), silt 15 m (weak), clay 15 m (weak) 60-90 cm depth: sand 70 m (well), silt 25 m (moderate), clay 30 m (moderate, long).
Installations of gas exchange collars have been finished by now. In this report, we refer to three measurement campaigns of carbon respiration rate in the pasture land use system: First results: During the first sampling campaign (Aug. 13, 09) at 30 locations in the pasture distributed at distances of 5 m and 3 nests with 1 m intervals, the spatial variation was considerable, and only moderately structured. The spatial correlation range was 5 m and was only revealed through the nested sampling design. However, for the two following campaigns (Aug. 26, Sept. 10, 2009) the variation structure exhibited nested behavior, i.e., one moderate structure was observed over 3-5 m at the local scale, and another strongly pronounced structure over approximately 70 m. Both campaigns resulted in a spatial pattern of carbon dioxide respiration that was temporally stable, i.e., fluctuations, local high and low values followed a very similar distribution. This distribution was quantified in a common covariance structure in a well defined crossvariogram revealing that both variables are coregionalized over 70 m.
The identification of spatial correlation structure of carbon respiration measured across the landscape is relatively new and only few studies with this focus are published in the literature. One of the objectives of this study is to contribute to a better understanding of spatial respiration behavior. It is known from the literature, that carbon respiration strongly depends on soil temperature. This result is based on measurements performed at the same location.
In our spatial sampling, the relationship between soil temperature and carbon respiration was not well manifested, probably due to changing microbial conditions from one location to the next. From the transect investigations however, overall there seems to be an inhibition of respiration with temperature above 28° C, and on the contrary a positive influence of soil temperature on microbial respiration in the range below that temperature. Further specifications of these kinetic relationships will be based on site-specific measurements over daily temperature waves.
Moreover, first investigations of the dynamics of spatial behavior are encouraging. Whether co-regionalized soil water capacitance probe measurements contribute to a better understanding of the temperature-respiration relationship will be investigated in the near future.
Furthermore, the investigations will now be expanded to both the pasture and the crop land use system. These first results indicate that the choice of the specific nested sampling design is a promising approach to a better understanding of respiration dynamics across the landscape. The INNOVA gas analyzing system (Laboratory Dr. McCulley) is very helpful in this study for accomplishing a large number of field scale measurements within a relatively short time.