Effects of tillage pan on soil water distribution in alfalfa-corn crop rotation systems using a dye tracer and geostatistical methods

The present study was designed to assess the potential effects of tillage pan on the distribution and the movement of soil water. Two different tillage regimes of an alfalfa-corn crop rotation system were carefully selected. Plot 1 contained alfalfa without tillage for the past 10 years. Plot 2 contained alfalfa with no tillage for the first 5 years and corn with conventional tillage for the succeeding 5 years. A dye tracer was introduced to these plots and the different types of water flow were visualized using classified dye-stained patterns. In addition, the kriging maps of gravimetric soil water content were used to indicate potential distribution characteristics. Semivariance analysis was performed to determine spatial variability on a centimeter scale. The role of macropore flow was weaker in the alfalfa plot than in the corn plot because of the difference in flow path and distribution. A belt of high soil moisture content appeared in the vertical soil profile of the alfalfa plot, indicating that the macropore flow showed a deeper stained depth. Several scattered locations in the corn plot contained high amounts of water, suggesting that macropore flow also occurred. The soil water content in the corn plot demonstrated infiltration patterns with higher degrees of fragmentation than that in the alfalfa plot. Moreover, a lateral flow with a wider stained width was triggered by the compacted tillage pan in the corn plot. Thus, the role of lateral flow was more dominant in the corn plot than in the alfalfa plot. The spatial correlation distance of the soil water content of infiltration patterns was 23.08cm in the alfalfa plot compared with 10.52cm in the corn plot. This trend indicated a high spatial variation of moisture in water infiltration patterns of the alfalfa field. In addition, strong spatial autocorrelation was observed in soil water infiltration patterns in both plots (nugget percentages of <25%). Our results highlighted a significant change of soil water behaviors in crop rotation from alfalfa to corn. In particular, downward water flow was restricted by a compacted tillage pan; thus, water was stored in the loose soil structure of the corn plot.