Statistical patterns of carbonates and total organic carbon on soils of Tuber rufum and T. melanosporum (black truffle) brules.(Report)

Introduction

The properties of carbonated soils limit the development of many mycorrhizal mushrooms. Truffles (Tuber Micheli ex Wiggers) are nevertheless an exception to this rule, since the calcareous and well drained soils of Mediterranean forests afford the required habitat for many Tuber species. Tuber melanosporum Vittad. (black truffle of Perigord) has greater culinary and economic value (up to 1000 [euro]/kg) than other truffle species, such as T. brumale Vittad., T. aestivum Vittad. form meridionalis Chevalier et Riousset, T. mesentericum Vittad., and T. rufum Pico ex Fries var. rufum form rufum Montecchi et Lazzari. Tuber melanosporum cultivation has spread throughout Europe, New Zealand, Australia, the United States, Israel, Chile, and to other countries in recent years.

In truffle culture, the terms 'burn' or 'brule' are used to describe spots where truffles grow and refer to the phytotoxic capacity of the Tuber mycelia and to their ability to create clearings in the vegetation where the mycelia bear fruit (Papa 1992). The simultaneous production of ascocarps in various truffle species can be observed within the same brule; these truffles are distributed within the space of the brule in a clearly defined manner. Tuber brumale, T. rufum, T. aestivum, and T. mesentericum ascocarps are frequently collected outside T. melanosporum brules, or in their innermost part. Inside the brules, a time succession of the various truffle species can also be observed; T. rufum ascocarps are the first to be collected, then T. melanosporum, and finally T. brumale (Montacchini et al. 1972; Falini and Granetti 1998; Callot 1999; Riousset et al. 2001; Ricard 2003; Granetti et al. 2005).

Garcia-Montero et al. (2006) review the soil requirements of these truffle species to determine which soil properties have the greatest effect on truffle interactions. However, the soils inhabited by Tuber rufum have not been extensively studied. Riousset et al. (2001) indicate that T. rufum lives in chalky soils in identical habitats to those occupied by T. melanosporum, T. brumale, T. aestivum, T. uncinatum Chatin, and T. mesentericum. In many areas of Spain, collectors call the ascocarps of T. rufum 'bordes' ('edges'); this common name refers to the way in which the ascocarps of this species are collected on the outer edges of the T. melanosporum brules. Tuber rufum may also enter natural and cultivated brules when the production of T. melanosporum begins to decline (Garcia-Montero 2000; Riousset et al. 2001). Recently, lotti et al. (2007) indicated that T. rufum constitutes a threat to commercial nurseries producing plants mycorrhized with truffles, and proposed techniques of molecular biology to detect their mycorrhizae.

Regarding the soil properties of Tuber melanosporum brules, Callot (1999), Ricard (2003), and Granetti et al. (2005) suggest that there is a lower quantity of total organic carbon (TOC) content inside the brules than outside, and than in the productive locations of other truffle species such as T. brumale and T. rufum. Those authors propose that T. rufum occurs outside the brules, or in the innermost part next to the symbiont plant, because in both these places there is a greater coverage of herbaceous plants and a greater accumulation of organic matter.

In any case, these authors indicate that the biological and physical-chemical conditions relating to the development of the brules should be further clarified. Moreover, most studies on truffles fail to supply any statistical analysis of the quantitative data associated with truffle ecology, such as the relationship between soils and brule development. Garcia-Montero et al. (2006, 2007a, 2007b) report that the annual ascocarp production of Tuber melanosporum increases with the size of the brule, which explains 38% of the variance in its productivity; they also indicate that a high content of active carbonate (calcium carbonate extractable with ammonium oxalate) accounts for up to 51% of the variance in T. melanosporum brule sizes. The authors describe that a high content of active carbonate also explains 43% of the variance in T. melanosporum ascocarp production, and 42% of the variance in exchangeable [Ca.sup.2+] concentration in the brules.

Active carbonate is a finely divided fraction of calcareous rock,