Monday, 10 July 2006

Lignocellulolytic Enzyme Activities and Litter-Layer Composition (NMR Spectra) of a Pine Forest Soil, Five Years after Thinning.

Sebastian Maassen1, Minh-Phuong Huynh-Le2, George D. Cody2, and Stephan J. Wirth1. (1) Leibniz-Centre for Agricultural Landscape Research (ZALF), Institute of Landscape Matter Dynamics, Eberswalder Str. 84, MŘncheberg, D-15374, Germany, (2) Carnegie Institution of Washington, Geophysical Laboratory, 5251 Broad Branch Rd., N.W, Washington, DC 20015

Over recent years, notable efforts in forest management have been made in Central Europe to restore pure coniferous to deciduous mixed forest stands. Deciduous forests have been the original forest types in the northeastern lowlands of Germany, providing increased biodiversity and improved physical and chemical soil properties. In the state of Brandenburg with a pine forest area of 800,000 ha, selected sites were already thinned and subsequently planted with beech or oak seedlings. Thinning of forests can have crucial effects on soil organic matter turn-over and nutrient budgets. As a result of canopy removal by partial thinning, microclimatic properties are altered, as well as the quantity and quality of substrate inputs into the soil. As a consequence of these changes, the composition of the soil microbial community may be altered or microbial activities may increase. It can be hypothesized, that soil organic matter decomposition and humus dynamics might be accelerated, resulting in a loss of stabilized organic matter or nutrient leaching (humus activation). Such effects are a matter of considerable concern in forest management and therefore need detailed study. A 62-year old pine stand located in northeastern Germany (Brandenburg, Ost-Prignitz; Oberforstamt Menz, Revier Beerenbusch) was studied. The soil is an acid brown earth (L, Of, Oh, Aeh, Bhv, Bv, C) with a coarse sandy texture. The unthinned treatment (1.0) and the treatment with the highest degree of canopy opening (0.4) were sampled in May 2005 along a 50 m transect with a lag distance of 5 m between sample points. Samples of the humus layer (Of and Oh, subsequently termed O) and the top humified mineral horizon (Aeh, ca. 0-5 cm, subsequently termed A) were collected separately. The samples were sieved (5 mm) and stored at 4░C until analysis. Ligno-cellulolytic enzymes, i.e., endo-1,4-▀-cellulase (EC, exo-1,4-▀-cellulase (EC, ▀-glucosidase (EC, endo-1,4-▀-xylanase (EC, exo-1,4-▀-xylanase (EC, phenol oxidase (EC, and peroxidase (EC were assayed in soil extracts at pH 5. Moreover, solid state 13C CPMAS NMR analyses were performed on a Varian-Chemagnetics Infinity 300 spectrometer. Molecular analyses were performed via Gas Chromatography-Mass Spectrometry after chemopyrolysis with tetramethyl ammonium hydroxide. The stable isotope abundances of carbon and nitrogen were determined on a Finnigan Delta-plus IRMS. Soil enzyme activities have been suggested as criteria to evaluate soil quality, or to study the impact of management practices. The enzyme activities measured in our study were linked to the decomposition of lignocellulose complexes in order to give an overview on specific microbial activity in forest litter decay. In accordance with soil chemical properties such as soil organic carbon and total nitrogen contents, no significant differences in enzyme activities were detectable between the treatments. Thus we conclude, that no evidence supports the hypothesis that thinning would rapidly stimulate lignocellulose decomposition within few years immediately after thinning, as assessed by potential enzyme activities in vitro. These findings are confirmed by the NMR, GC-MS, and IRMS analyses that all revealed no evidence of any significant differences in the molecular distributions in soil horizons comparing the thinned and unthinned treatment.

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