The effects of stand age, site index and parent material on soil biochemical properties related to biomass (extractable C, microbial C and metabolic quotient) were examined in the 0-15 cm mineral soil layers of Pinus pinaster and Pinus sylvestris stands from NW Spain. Two productivity levels (low and high site index), two ages (young and old) and two parent soil materials (granite and acid schists) were considered. The data indicated that there were differences in microbial parameters in soils under different species. In general in P. pinaster forest higher values of biochemical parameters, expressed on organic C basis, were observed in the stands of high site index as compared with the low ones; in contrast, in P. sylvestris no differences among stand site index were detected. In both species different results were also observed depending on parent material and a significant effect of stand age was detected for extractable C and microbial C in P. pinaster forest developed over granite. The data seem to indicate that measured parameters may have the potential to be used as indicators of the effect of forest management on soil organic matter quality.Key words: Microbial C, extractable C, metabolic quotient, reforestation, Pinus pinaster, Pinus sylvestris. ResumenBiomasa microbiana edáfica en pinares del noroeste de España: influencia de la edad de la plantación, índice de sitio y material de partida Se examina la influencia de la edad de la plantación, índice de sitio y material de partida sobre las propiedades bioquímicas relacionadas con la biomasa microbiana (C extraíble, C microbiano y coeficiente metabólico) del horizonte superficial de suelo (0-15 cm) de plantaciones de Pinus pinaster y Pinus sylvestris del Noroeste de España. Se consideraron dos índices de sitio (baja y alta calidad), dos edades de plantación (joven y vieja) y dos materiales de partida (granito y esquistos). Los datos indicaban que hubo diferencias entre los parámetros microbianos de suelos bajo diferente vegetación. En general, para P. pinaster se observaron valores más altos de los parámetros bioquímicos, expresados en base al contenido de C orgánico, en las plantaciones de alta calidad que en las de baja calidad; por el contrario en las plantaciones de P. sylvestris no se detectaron diferencias con respecto al índice de sitio. En las plantaciones de ambas especies se observaron resultados diferentes dependiendo del material de partida y también se observó un efecto significativo de la edad de la plantación sobre el C extraíble y el C microbiano en suelos desarrollados sobre granito bajo P. Pinaster. Los resultados parecen indicar que las propiedades bioquímicas medidas pueden ser utilizadas como indicadores del efecto del manejo forestal sobre la calidad de la materia orgánica del suelo.Palabras clave: C microbiano, C extraíble, coeficiente metabólico, plantación forestal, Pinus pinaster, Pinus sylvestris.
The effects of fallow agriculture on soil organic matter (SOM) dynamics and CO 2 emissions were assessed in the tropical Andean páramo ecosystem. Possible changes during the cultivation-fallow cycle were monitored in four areas of the Quebrada Piñuelas valley (Venezuela). Uncultivated soils and plots at different stages of a complete cultivation-fallow cycle were incubated, and SOM mineralization kinetics was determined. Soils exhibited a low SOM mineralization activity, total CO 2 evolved never reaching 3% of soil carbon, pointing to a stabilized SOM. Potential soil CO 2 effluxes differed significantly according to their plot aspect: northeast (NE)-aspect soils presented higher CO 2 effluxes than southwest (SW)-aspect soils. Soil CO 2 emissions decreased after ploughing as compared to virgin páramo; low CO 2 effluxes were still observed during cropping periods, increasing progressively to reach the highest values after 4-5 y of fallow. In all cases, experimental C mineralization data was fitted to a double exponential kinetic model. High soil labile C pool variability was observed, and two different trends were identified: NE-oriented soils showed more labile C and a wider range of values than SW-facing soils. Labile C positively correlated with CO 2 effluxes and negatively with its instantaneous mineralization rate. The instantaneous mineralization rate of the recalcitrant C pool positively correlated with %C evolved as CO 2 and negatively with soil C and Al 2 O 3 contents, suggesting the importance of aluminum on SOM stability. The CO 2 effluxes from these ecosystems, as well as the proportion of soil C released to the atmosphere, seem to depend not only on the size of the labile C pool but also on the accessibility of the more stabilized SOM. Therefore, fallow agriculture produces moderate changes in SOM quality and temporarily alters the CO 2 emission capacity of these soils.
Stable isotopes natural abundance (13C, 15N) was studied in soils from P. pinaster Ait. and P. sylvestris L. plantations located in Galicia, NW Spain, to evaluate the use of isotopic techniques for stand quality estimation. Combinations of old/young forest plantations of high/low site index, growing on acidic soils over two different parent materials (granite/schists) were studied. Significantly lower 13C in soils under P. pinaster (–27.03‰) as compared with soils under P. sylvestris (–26.13‰) were found (P < 0.001). However, soil 15N of both species did not differ significantly. Significant linear regressions between soil 13C and site index for P. pinaster were found, pointing at the 13C signature of the soil as a good stand quality indicator, especially over schists. For P. sylvestris over schists, linear regression between soil 15N and site index suggests that 15N could be used as a complementary tool in quality determinations. As a general trend for both tree species, stands with high site index showed more depleted soils in 13C and 15N as compared with low site index stands.
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