Spatio-temporal variations of soil properties in a plot scale: a case study of soil phosphorus forms and related enzymes

Spatio-temporal variability in soil properties has long been observed on uniformly managed fields. Understanding the spatial characteristics of soil properties would be helpful in recognizing their relationship and the development of site-specific management techniques. The objective of this study was to determine the spatio-temporal variation of P forms and related enzymes and their relationship with some physico-chemical properties in a plot scale. A grid soil sampling (10 x 10 m) was used to measure the spatial variation of soil properties across a 0.4-ha field. Soil samples were collected at 50 points from the upper 20 cm of luvisols in April and August 2007. The total (P-T), available (P-A), inorganic (P-I), organic (P-O) phosphorus concentration, and acid (P-AC) and alkaline (P-AL) phosphatase activity were analyzed. Additionally, total organic carbon (TOC), soil pH(KCl), and clay content were determined. Data were evaluated using classical statistical and geostatistical methods. Both enzyme activities were significantly higher in April than in August, while the P-T and P-I concentration were significantly lower. The concentrations of P-O and P-A did not significantly differ between sampling dates. The spherical or mixed (spherical/linear) models with the nugget effect were fitted to the calculated semivariograms. The P-A and TOC concentrations on both sampling dates as well as P-AC activity in April were situated in the strong variability class, the P-I concentration in April revealed a weak spatial variability and the other properties were in the moderate variability class. The clay content revealed a pure nugget effect. The range of the influence that was calculated for the properties ranged from 16.5 to 50.0 m. Kriged maps showed that temporal variability was observed in the spatial patterns of the P-I and TOC concentrations and P-AC activity. Results from this study can help us to understand and predict the contribution of internal factors (i.e., soil type) in the total variability of soil properties that can interfere with the influence of soil management practices. The spatio-temporal variability showed that the studied area was temporally unstable and the implication of these findings is that more frequent sampling, at least several times throughout the growing season, must be included in the sampling strategy in order to better understand whether P forms and related enzymes show any permanent spatial patterns in soil all of the time or whether they are more randomized.