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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.
2011 Project Description
In this report, the special focus is on space-time field on soil carbon respiration. Fluxes of carbon dioxide and other trace gases are measured with a photo-acoustic gas analyzer provided by Dr. McCulley. Each measurement at a specific point takes approximately 10 minutes of time. Hence, about 30 locations can be measured in a day. Therefore, two full days are necessary to measure the 60 locations in the two land use systems, i.e., forage and agricultural crops, in our experimental site. The crop was again winter wheat during this year.
The problem arising from measurements taken during the spring, summer and early fall seasons is that gas fluxes increase due to the influence of temperature changes during the day of measurements. In order to statistically analyze data for different locations, i.e., for their spatial covariance behavior, measurement results need to be adjusted. Otherwise, temporal dynamics of conditions strongly affecting gas flow rates would introduce spatial trends and put a misleading bias on the results.
Therefore, a procedure needed to be developed that allows the gas flux results to be corrected as if they had been taken at exactly the same time. Such a correction procedure would not only be relevant for studies in which measurements are analyzed for their spatial covariance structure, but also for ordinary randomized treatment experiments. Two approaches were developed and examined. One was based on surface soil temperature changes during the day, and an overall regression of gas flow rates versus temperature collected over the entire season.
The second approach was based on testing the gas flux data obtained over a day for any trend in time. In case of an existing trend, it was simply removed from the time series of measurements. A manuscript is being prepared on a detailed description and comparison of the two adjustment procedures. Results are briefly described below. The second major outcome of gas flux measurements in our site was the computation of space-time fields of carbon respiration fluxes which was based on a space-time kriging of adjusted carbon dioxide fluxes.
This innovative analysis of field gas flux measurements allows identification of whether the spatial pattern of gas flux rates behaves temporally stable or random. Temporal stability implies that for a number of spatial measurement campaigns, some locations can be determined that are always among the highest, others typically reflect the mean and again others represent regions of low flow rates. This concept has been introduced initially by Vachaud et al. (1985) for soil water storage measurements.
For the adjustment of diurnal changes of gas flux, the approach based on removing a trend observed over the day has been more effective than the approach based on regression of gas fluxes on soil temperature. Possible reason for this result is the fact that the effect of temperature on gas flux is not unique throughout a whole season. Moreover, adjustment of data with the de-trending procedure can be accomplished at the end of the day whereas measurements over many days with varying temperature conditions would be necessary for the regression method.
Space-time fields of soil carbon respiration fluxes were presented at the investigators' meeting in Ashville in May 2011, and at the Annual Meeting of the Tri-Societies ASA-CSSA-SSSA in San Antonio, Texas. Spatial semivariance in both land use systems behaves more stable than temporal semivariance. Both, spatial and temporal semivariances exhibit less scatter versus lag distance in the forage system than in the crop land.
In the forage system, the magnitude of experimental semivariance observed over a given distance in time measured in days is approximately twice as high as that measured over a given distance in space measured in meters. In other words, the same variation that is observed over a spatial distance of 10 m can be expected to be observed over 5 days.
Interestingly, the semivariance observed over space and time behaves different in the crop land. Here, the variance observed over short spatial lag distances is smaller than that observed in time, however, above a lag distance of approximately 50 m or 50 days, respectively, spatial semivariance is obviously larger than temporal variance. Vachaud's (1985) method of relative differences was used to compute and visualize temporal stability of spatial measurements in both systems. From early summer on, locations behaved stable with respect to their relative magnitude in the cropland.
The winter period and following spring exhibited a different pattern of relative differences, i.e., positions change their meaning with respect to mean, high and low range zones. In the grass land system, over a period of approximately two months, temporal stability of spatial ranks can be observed, however, hardly over a period longer than two months implying that different locations in a forage system can become major or minor contributors to carbon dioxide fluxes, and so far, we cannot tell what drives this obvious change in the contribution to respiration rate.
Gas diffusivity coefficients have been measured for each of the 60 locations in the experimental site in the lab, using an undisturbed core method. The apparent gas diffusion coefficient was quantified at five different soil water pressure heads in the range between -10 cm > h > -1000 cm. The result is an empirical relationship of the relative apparent gas diffusion coefficient versus air-filled porosity for 60 soil cores. Based on this relationship, pore tortuosity and its change with air-filled porosity can be quantified. For the same 60 soil cores, soil hydraulic properties (soil water retention curve and unsaturated hydraulic conductivity function) are being determined currently.
The purpose of these measurements is to use the functions in simulation models, and to quantify soil structure in the two land use systems. Furthermore, hardly any attempts exist in the literature to quantify the spatial variability structure of soil structural properties. The current measurements are expected to provide first insights in this behavior. Two graduate students (PhD level) are involved in the investigations within this project.
Wendroth, O. 2011. Spatio-temporal soil water and related processes. Invited Keynote Lecture. Brazilian Soil Physics Meeting, September 12 -16, 2011, Department of Biosystems Engineering, Luiz de Queiroz College of Agriculture, University of Sao Paulo. ESALQ - USP, Piracicaba - SP, Brazil
Kreba, S., O. Wendroth, and R. McCulley. 2011. Temporal stability of soil water storage, carbon dioxide, and nitrous oxide flux. Poster, Annual Meeting, ASA-CSSA-SSSA, Oct. 16-19, 2011, San Antonio, Texas. Kreba, S.2, O. Wendroth, and M. Coyne. 2011. Land use impact on soil structure. Poster, Annual Meeting, ASA-CSSA-SSSA, Oct. 16-19, 2011, San Antonio, Texas.
Yang, Y., and O. Wendroth. 2011. Spatial variability of wet-range soil hydraulic conductivity as affected by land use. Oral presentation, Annual Meeting, ASA-CSSA-SSSA, Oct. 16-19, 2011, San Antonio, Texas.