The need for a large-scale program aimed at developing sustainable forest management practices has never been more pressing. A method to achieve reliable and systematic assessments of soil quality might be to collect reference biological values. There are two basic ways to evaluate indicators for sustainability: (i) compare parameters or indicators with reference values and (ii) evaluate evolution of the indicator itself through time. In the Basque Country, the area once covered with native species has been reduced by the spread of an exotic and rapid growth species, radiata pine. This exotic species now accounts for 60 percent of the forest area. These exotic plantations are harvested every 30-40 years and heavy machinery has been used for site preparation since the early 1980's. Forest management practices can greatly alter plant litter input to soils and may also lead to degradation of soil properties and to erosion. Moreover, modifying the soil environment could alter the composition and function of soil microbial communities as well. One objective of this study was to investigate whether soil microbial community composition and function differed among native and exotic forest species in established undisturbed/mature forests, and to gather information on undisturbed/mature forest communities as possible reference values. A second objective was to determine how soil microbial community composition and function varied among three exotic forest plantations of different ages and different management practices. Three different forest types were selected: two stands with native species, one with Quercus robur and another one with Fagus sylvatica and three stands with one exotic species, Pinus radiata (old, young, and newly planted). Oak and beech forests were old-growth forests with no intensive management for over 120 years. The old pine stand was a > 40 year even-aged plantation established with no mechanical site preparation. The young pine forest was a 15 year-old even-aged second-growth unmechanized plantation. The newly planted pine forest was established in 2002 using mechanical site preparation with ripper plowing and blading.

Soils were sampled in January 2005 and within each forest type three topsoil samples were collected randomly at 0-5 and 5-15 cm depths. The following analyses were run on each sample:

-Chemistry was analyzed using standard methods,

-Mineralogy was analyzed using X-ray diffraction,

-Physical analyses; Soil texture was analyzed using laser diffractometry and gravimetric soil moisture was determined,

-Biological analyses; Soil-basal and glucose-induced respiration was analyzed by MicroResp^TM, extracellular enzyme activities were analyzed using methylumbelliferone (MUB)-linked substrates, PLFAs (phospholipid fatty acids) were analyzed using gas chromatography, and particulate organic matter (POM) as an indicator of labile organic matter was also determined.

There were no significant differences in soil mineralogy or texture among different stands; soils contain mainly quartz and mixed layered illite-vermiculite with small amounts of kaolinite. Soil texture is clay loam. However, pH of soil from the oak forest was significantly higher, reflecting the heterogeneous presence of parent material rich in decimetric calcite. As expected, microbial community function estimates using extracellular enzyme activities and soil respiration showed significant reductions in deeper soil layers, probably due to reductions in labile organic matter. In the newly planted pine plantation, measurements of soil organic carbon, nitrogen, phosphorous, soil basal respiration and extracellular enzyme activities such as xylosidase, cellobiosidase, phosphatase, and leucinepeptidase in the 0-5 cm layer were very similar to those levels found in deeper layers of other stands. These parameters, associated with organic matter, could indicate loss of the upper soil horizon as a result of blading during mechanical site preparation. Particulate organic matter in the 0-5 cm layer differentiated native and exotic species; soils at the exotic species stands had less labile organic material than that corresponding to native species. Soil microbial community structure based on the Shannon diversity index of PFLAs did not differ among species and silvicultural history, however total PLFAs as an indicator for microbial biomass in the new plantation (161 nmol PLFA g^-1 soil) were below the levels of undisturbed/mature soils (210, 221 and 239 nmol PLFA g^-1 soil in oak, beech and old pine, respectively). However, total PLFAs were highest in the young pine forest (304 nmol PLFA g^-1 of soil). Discriminant analysis based on extracellular enzyme activities and PLFAs clearly separated different forest types. The discrimination along the axis 1 was due to leucinepeptidase activity and fungal PLFA 18:2ω6c with high positive weights and PLFA 18:1ω9c was given a negative weight. The two first discriminant functions explained 98 percent of the total variance and 86.7 percent of cross-validated grouped cases were classified correctly. In contrast, discriminant analysis based on standard chemical and physical analyses showed 53.3 and 46.7 percent respectively of correctly classified cross-validated grouped cases. The results suggest that compositionally and functionally distinct soil microbial communities exist in forests with different dominant species and silvicultural history.