Aggregate stability changes after organic amendment and mycorrhizal inoculation in the afforestation of a semiarid site with Pinus halepensis

The recovery of soil structural stability is a precondition for successful afforestation programmes in semiarid environments. A multifactorial field experiment was carried out in a semiarid rangeland in south-eastern Spain to evaluate the influence of a fresh organic residue addition (first factor), mycorrhizal inoculation with Pisolithus arhizus (second factor), and the rhizosphere of Pinus halepensis (third factor) on soil aggregate stability. A total of 6 years after planting, the addition of residue was seen to increase the levels of stable aggregates to a greater extent than the mycorrhizal inoculation. Both reafforestation methods increased C-fractions and enzyme activities measured (dehydrogenase and phosphatase). The rhizosphere also affected aggregate stability, particularly when P. halepensis was inoculated with P arhizus. Aggregate stability in the rhizosphere of P. halepensis was strongly correlated (P < 0.01) with the C-biomass and soluble C-fractions (WSC and WSCH) as well as with dehydrogenase (r = 0.901, P < 0.05) and phosphatase (r = 0.903, P < 0.05) activities. It was concluded that the combination of residue amendment and inoculation of P halepensis with P. arhizus significantly improves soil aggregate stability, this beneficial effect appearing to be mainly due to a reactivation of microbiological activity. (C) 2002 Elsevier Science B.V. All rights reserved.