Evaluation of the soil aggregation induced by the plant roots in an Oxisol by turbidimetry and water percolation

Plant roots modify aggregation of rhizosphere soil. Oxisols, in turn, have very small, granular, stable and resistant aggregates. So, it is necessary a sensitive method to detect differences between rhizosphere soil and non-rhizosphere soil or bulk soil. The objective of this work was to evaluate aggregation of rhizosphere soil and bulk soil under six cropping systems by turbidimetry and water percolation methods. By the turbidimetry method, readily-dispersible clay (RDC) and mechanically-dispersible clay (MDC) were obtained, and by the water percolation method, the water flow (WF) was obtained as a measure of the stability of aggregates. The cropping systems or treatments were the following: Control, Grains, Livestock, Phytomass, iCL and Diversified. Rhizosphere soil presented better aggregation than bulk soil, and the soil aggregation was different among the cropping systems, being the iCL the most recommended. The results obtained for RDC, MDC and WF were correlated with soil organic carbon (SOC). The correlations between RDC and WF with SOC were practically nil. Regarding the correlations between MDC and SOC, the rhizosphere soil in the Phytomass treatment and the bulk soil in the Diversified treatment presented positive correlation between these variables (r = 0.273 and r = 0.224, respectively). In the other treatments, both rhizosphere soil and bulk soil with more SOC resulted in less MDC (r varied from −0.133 to – 0.720). The results showed that the influence of plant roots on soil stability can be positive or negative and denoted MDC as a good indicator of the aggregate stability for reveal differences between rhizosphere soil and bulk soil in an Oxisol.