Soil organic matter dynamics and land use change at a grassland/forest ecotone

In the grassland/forest ecotone of North America, many areas are experiencing afforestation and subsequent shifts in ecosystem carbon (C) stocks. Ecosystem scientists commonly employ a suite of techniques to examine how such land use changes can impact soil organic matter (SOM) forms and dynamics. This study employs four such techniques to compare SOM in grassland (Bromus inermis) and recently forested (similar to 35 year, Ulmus spp. and Quercus spp.) sites with similar soil types and long-term histories in Kansas, USA. The work examines C and nitrogen (N) parameters in labile and recalcitrant SOM fractions isolated via size and density fractionation, acid hydrolysis, and long-term incubations. Size fractionation highlighted differences between grassland and forested areas. N concentration of forested soils' 63-212 mu m fraction was higher than corresponding grassland soils' values (3.0 +/- 0.3 vs. 2.3 +/- 03 mg g(fraction)(-1), P < 0.05), and N concentration of grassland soils' 212-2000 mu m fraction was higher than forested soils (3.0 +/- 0.4 vs. 2.3 +/- 0.2 mg g(fraction)(-1), P < 0.05). Similar trends were observed for these same fractions for C concentration; forested soils exhibited 1.3 times the C concentration in the 63-212 mu m fraction compared to this fraction in grassland soils. Fractions separated via density separation and acid hydrolysis exhibited no differences in [C], [N], delta N-15, or delta C-13 when compared across land use types. Plant litterfall from forested sites possessed significantly greater N concentrations than that from grassland sites (12.41 +/- 0.10 vs. 11.62 +/- 0.19 mg g(litter)(-1)). Long-term incubations revealed no differences in C or N dynamics between grassland and forested soils. delta C-13 and delta N-15 values of the smallest size and the heavier density fractions, likely representing older and more recalcitrant SOM, were enriched compared to younger and more labile SOM fractions; delta N-15 of forested soils' 212-2000 mu m fraction were higher than corresponding grassland soils (1.7 +/- 0.3 parts per thousand vs. 0.5 +/- 0.4 parts per thousand). delta C-13 values of acid hydrolysis fractions likely reflect preferential losses of C-13-depleted compounds during hydrolysis. Though C and N data from size fractions were most effective at exhibiting differences between grassland and forested soils, no technique conclusively indicates consistent changes in SOM dynamics with forest growth on these soils. The study also highlights some of the challenges associated with describing SOM parameters, particularly delta C-13, in SOM fractions isolated by acid hydrolysis. (c) 2006 Elsevier Ltd. All rights reserved.