Dynamics of carbon and nitrogen mineralization in relation to stand type, stand age and soil texture in the boreal mixedwood

Abstract

In the boreal mixed forest, stand composition generally changes from deciduous to mixed to coniferous stands during post-disturbance succession. Our objective was to determine the influence of forest composition on the quality of soil nitrogen and carbon as determined by C and N mineralization during the course of a long-term in vitro incubation (282 days). Three stand types (Populus tremuloides, Betula papyrifera, and conifers (mixture of Abies balsamea and Picea glauca)), of two ages (50 and 124 years since fire) growing on two soil types (clay and till) were considered. Expressed on an organic C basis, our results showed a greater N mineralization for deciduous than coniferous stands, both in the mineral soil and the forest floor, a higher N mineralization in the mineral soil of older stands as compared with young ones, and in till than in clay soils. Mineralization of C was higher in the forest floor of clay soils as compared to till soils while the opposite was found in the mineral soil. It was also higher in both soil layers of older sites. The observed differences in N and C mineralization among stand types, stand ages and deposits appeared to be due to differences in the most labile fraction of soil organic matter because these differences were observed within 100 days of incubation. The ratio of C mineralized–N mineralized was greater in coniferous than deciduous soils in both soil layers, indicating a lesser quality of organic matter under coniferous stands. Despite significant differences among the above-listed factors for N and C mineralization on an organic C basis, the pool size of mineralized (or mineralizable) N and C was generally not significantly different between the different forest and soil types because of an inverse relationship between quality and quantity of soil organic matter. Correlation and multiple regression analysis indicated that clay content (negatively), C–N ratio (negatively), available P as well as exchangeable Mn were related to the mineralization of N.

Introduction

Boreal forest soils contain large amounts of organic nitrogen of which only a small portion is potentially available to the vegetation over a growing season, and the productivity of these forests is often N limited. Fertilization trials have often shown a growth response to N addition in both deciduous (Van Cleve, 1973, Weetman et al., 1978) and coniferous boreal stands (Binkley and Högberg, 1977, Weetman et al., 1978). Apart from soil climate, the rate of N mineralization is dependent upon the chemical characteristics of organic matter that affect the activity of the decomposer community. Soil environmental conditions such as temperature, pH, heavy metals, redox and texture also influence the activity of the decomposer community (DeLaune et al., 1981, Parton et al., 1987, Carreiro and Koske, 1992, Cotrufo et al., 1995).

Chemical attributes of plant litter, such as lignin, nitrogen, C–N and lignin–N ratio are believed to determine the quality of soil organic matter (SOM) for decomposers (Berg and McClaugherty, 1987, Taylor et al., 1989, Riffaldi et al., 1996). However, as noted by Ågren and Bosatta (1987), there is no universally applicable expression that provides an immediate link between litter properties and SOM decomposition rates. Litter composition can vary widely among tree species and forests of different composition and can potentially affect soil N and C dynamics (Gower and Son, 1992, Hobbie, 1992, Paré and Bergeron, 1996, Prescott and Preston, 1994). Coniferous litter has been found to reduce the availability of soil N because of its high lignin and low N content (Pastor et al., 1987). Nadelhoffer et al. (1991) showed that the quality of organic matter was more important than temperature in controlling N and C mineralization rates in tundra plant communities. In the boreal mixedwood of Quebec, current forestry practices favor the maintenance of a stable forest composition. However, without human intervention, these ecosystems will generally show a succession from deciduous to mixed, and finally to coniferous dominated forests following disturbance (Bergeron and Dubuc, 1989, Bergeron and Harvey, 1997). Our objective was to determine how forest composition can affect the dynamics of N and C mineralization and how this effect is influenced by soil type and stand age.

Several techniques can be used to assess soil N availability (Binkley and Richter, 1987); the most common include short-term incubations in the field or under controlled laboratory conditions. These techniques can be problematic in boreal forest soils because of the low and often negative rates of net N mineralization (Bonan, 1990, Paré and Van Cleve, 1993, Brais et al., 1995, Paré and Bergeron, 1996). Ruess et al. (1996) estimated that in situ incubation grossly underestimates net N mineralization in boreal forest soils. Several authors have proposed that SOM should be conceptually separated into several fractions with different turnover times (Bonde and Rosswall, 1987, Parton et al., 1987, Nicolardot et al., 1994). Commonly used terms for these pools are active, slow, and passive. The active pool, which is the most labile with regard to mineralization, can be assessed through long-term incubations. This technique allows us to determine potentially mineralizable N (N₀) and C (C₀) through following the accumulation of mineral N and soil respiration (Stanford and Smith, 1972).