Despite the lack of correlation between aggregate stability and AMF, rapid growth of G. mosseae hyphae suggested by Jansa et al. (2008) might be expected to result in alterations in soil structure relative to hyphae of other slower growing species. Work by Giovannetti et al. (2001, 2004) demonstrated that AMF hyphae frequently anastamose and could result in a large network of interconnecting hyphae. Within the current experiment, colonisation by G. mosseae resulted in a higher proportion of small soil pores to large ones and reduced distances between neighbouring pores. A soil with a greater proportion of small pores (∼0.4 mm2) will retain water more effectively than a soil with larger pores; furthermore, small pores act as nutrient-rich habitats for soil microflora (Nunan et al., 2001). In contrast, G. intraradices led to soil with a greater number of large pores and all combinations of G. intraradices resulted in greater distances between neighbouring pores. Bearden (2001) concluded, in a study of soil–water characteristics, that AMF hyphae resulted in soils gaining groups of small pores which corroborates the current findings, although it is interesting that different Glomus species affect the size and distance between pores differently. Bearden (2001) used a combination of six Glomus species (including G. mosseae) in her investigation of vertisols. In contrast, Milleret et al. (2009) found no evidence of G. intraradices forming small diameter structural pores and the larger size of the pores relative to the AMF led them to conclude that pore development was an indirect effect of the mycorrhiza. Since the extraradical hyphae had a smaller diameter than the pores, the authors suggested that hyphal penetration of pores induced root exudation and increased microbial activity. The data in the current investigation show that plant roots, possibly by influencing the associated microbial biomass and not colonisation by AMF per se, positively affected aggregate stability. Previous studies (e.g. Bedini et al., 2009) showed that aggregate stability is affected by the plant-fungal system rather than by plant root metabolism. Hallett et al. (2009) performed an elegant experiment in which mycorrhizal-deficient and ‘wild type’ tomato plants were grown in an agricultural field soil amended with G. mosseae and G. intraradices. These authors demonstrated that aggregate stability and porosity were increased in planted soils irrespective of the plant type (and mycorrhizal status) and they conclude that roots drive the stabilisation of soil structure. Their experiment lasted for 84 days and during this time the soil experienced wet–dry cycles which may have encouraged microbial populations to produce polysaccharides which in turn enhanced aggregate production. However in the current study, individual AMF species and combinations of co-occurring AMF species caused short-term effects on pore dynamics which may result in long-term impacts on porosity and aggregate stability. Whilst it has been shown before that different AMF species may affect aggregate stability (Schreiner et al., 1997), the focus here is on the effects of the mycelium in addition to the mycorrhiza. No correlations were observed for AMF hyphal abundance and soil physical properties, demonstrating that the differences observed may relate to growth patterns and mycelium architecture rather than density, or perhaps to AMF-specific altered C-exudation since AM fungi are known to produce extracellular soil proteins such as glomalin (Wright and Upadhyaya, 1996). The data presented here demonstrate that different AMF species within the same genus may have contrasting effects on soil pore characteristics which could alter the micro-physical habitat for other soil organisms.
small pores
∼0.4 mm2
mm2
experiment
lasted
84 days
days