Water repellence and soil aggregate dynamics in a loamy grassland soil as affected by texture S. D E G RYZE a , L. J ASSOGNE a,b ,H.B OSSUYT a , J. S IX c &R.M ERCKX a a Laboratory for Soil and Water Management, K.U. Leuven, Kasteelpark Arenberg 20, 3001 Heverlee, Belgium, b Department of Soil and Land Systems, University of Adelaide, Australia, and c Department of Agronomy and Range Sciences, University of California, Davis, CA 95616, USA Summary It has been hypothesized that aggregate stability is partly caused by subcritical (i.e. mild) water repellence. We conducted both a field study at a pasture site and an incubation experiment in the laboratory to characterize the relationship between aggregate dynamics and water repellence. In the field study, we investigated 53 samples from a natural texture gradient for water repellence and aggrega- tion. In the 3-week incubation experiment, we added wheat straw to crushed soil material from five of the 53 soils representative of the texture gradient (a sandy loam, two loams, a silt loam and a clay loam), and followed changes in aggregate formation and water repellence. Although there was a dramatic increase (P < 0.0001) in aggregation in all the soils during incubation (the mean weight diameter increased from about 300 mm to at least 900 mm), we observed a concomitant increase in water repellence only in the clay loam soil (P ¼ 0.0003). We found no significant correlations between water repellence and aggregation (n ¼ 53, P > 0.05) in the field. Whereas correlation between aggregate formation and texture was weak in the incubation experiment, we found a significant correlation between aggregation and textural parameters in the field. The amount of large macro-aggregates (> 2000 mm) was positively correlated with clay content (n ¼ 53, r ¼ 0.53, P < 0.001), and negatively with sand content (n ¼ 53, r ¼0.46, P < 0.001). These results indicate that (i) microbially induced water repellence might only become apparent when enough easily decomposable substrate is available, (ii) soil texture affects aggregate stabilization rather than aggregate formation, and (iii) aggregate formation is not necessarily associated with an increase in water repellence. Introduction Soil structural stability is an important aspect of soil quality; it determines root penetration, organic matter stabilization, soil erodibility, susceptibility to compaction, etc. Despite the extensive use of soil stability measurements in recent soil research, few studies have tried to explain the underlying mechanisms involved (Six et al., 2004). Most authors agree that aggregate formation is to a large extent induced by phy- sical enmeshment by plant roots or fungal hyphae, and plant or microbial exudation. It has been noted that the effect of these exudates on soil structure is dual (Tisdall & Oades, 1982). First, easily decomposable, hydrophilic substances (e.g. carbohydrates) are formed rapidly during plant residue decomposition and increase soil stability by enhancing the inter-particle binding (Piccolo & Mbagwu, 1999). Second, the water-repellent metabolites such as aliphatic fractions and humic acids can form coatings on soil pore walls (Capriel et al., 1990; Capriel, 1997; Chenu et al., 2000). These coatings enhance water repellency of the soil pore system and can therefore reduce the detrimental effect of slaking on soil structure (Piccolo & Mbagwu, 1999). Caron (1996) demon- strated that slaking due to air pressure buildup in pores was a function of wetting rate, which is directly influenced by water repellence. In contrast to the significant amount of research that has been carried out on the effect of carbohydrates on soil structure, the impact of water-repellent components on this has been much less investigated. Nevertheless, recent studies have clearly demonstrated that subcritical (mild) water repellence is a much more widespread phenomenon in soils than hitherto assumed (Hallett & Young, 1999; Chenu et al., 2000). Besides the importance of water repellence in soils for struc- tural stability, it is also an important soil physical property in its own right. Water repellence in soils increases the occurrence of preferential flow, possibly causing fast leaching of xeno- biotics or nutrients to groundwater (White et al., 2000). Additionally, water repellence possibly restricts substrates to larger pores where fewer microorganisms reside (Strong et al., Correspondence: S. De Gryze. E-mail: sdegryze@advalvas.be Received 6 August 2004; revised version accepted 21 March 2005 European Journal of Soil Science, April 2006, 57, 235–246 doi: 10.1111/j.1365-2389.2005.00733.x # 2006 British Society of Soil Science 235