Spatial and Temporal Variability of Water-Soluble Organic Carbon in a Cropped Field

Abstract

Little is known about the spatial and temporal variability of water-soluble organic carbon (WSC) within agricultural fields. The purpose of this research was to characterize the distribution, the mean, and the variance of WSC that occurs within a cropped field as a function of space and time. Freezing of soil samples showed no significant change in water-soluble organic carbon compared with that in fresh samples taken from a depth of 0.2 m. Thus, water-soluble organic carbon was extracted from frozen soil samples taken from a 200-point grid established on a 1.2-ha field, a 60-point grid within a 0.4-ha field, and a 55-point transect of a field amended with 45 Mg/ha of manure. The initial sampling was in the fall after the harvest of a sorghum crop. The concentrations ranged from 23 to 274 mg/kg within the 1.2 ha field. Over 90% of the concentrations were grouped around the mean of 39.8 mg/kg. The higher values caused the distribution to be greatly skewed, such that a ln-normal distribution characterized the data better than a normal distribution. To be within 10% of the population mean at the 95% confidence level, 26 samples would be required if ln-normal distribution was assumed compared with 88 samples under normal distribution. Soil-water content was found to be well described by a normal distribution for the 200-point grid. The means, variances, and frequency distributions of WSC changed dynamically throughout a 1-year period during which wheat and sorghum were grown. Mean WSC concentrations were highest in the spring and fall and lowest in the summer. The fall samplings had large variances and were best described by ln-normal frequency distributions, whereas other samplings during the year could be adequately described by normal distributions. The WSC concentrations showed only slight spatial dependency for samples taken 1.37 m apart and thus can be assumed to be randomly distributed. The addition of manure to the field resulted in higher WSC concentrations than in the unamended field, but concentrations were nearly equal after a year. Changes in soluble organic carbon in the field are closely related to the degree and duration of drying of the soil before extraction. These changes occur primarily in the upper 0.05 m of soil. Irrigation causes a drastic decrease in the soluble organic carbon in the surface soil, most likely because of leaching of soluble carbon.