Labile and total carbon and nitrogen dynamics in two grassland ecosystems

Abstract

Natural perennial grasslands are characterized by a high degree of spatial and temporal variability in total and labile soil organic matter (SOM). The objective of this research was to quantify the spatial distribution and relationships between total and labile carbon (C) and nitrogen (N) in two semi-arid grasslands, a shortgrass steppe (SGS) and a Northern mixed-grass prairie (NMP). Soil samples (0-5 and 5-10 cm depth) were collected from the two ecosystems in 2001. Analyses of water soluble organic carbon (WSOC) and water soluble nitrogen (WSN) were performed along with 21-day aerobic incubations for potentially mineralizable C and N to examine the relationships between these parameters and root biomass, total soil organic C (SOC), total N and inorganic N (iN). Results indicated that nitrogen immobilization processes dominate in NMP, which had greater microbial activity, root biomass and total SOM than SGS, but lower mass of field-moist WSOC (WSOCFM) and iN, which represented labile C and N pools. Incubation results indicated that microbes from bare-ground microsites were more C-limited than microbes from grass-occupied soils. Rhizoplane soil associated with coarse and fine roots accounted for approximately 28 % and 16 % of the total soluble C present in the 0-10 cm depth in NMP and SGS ecosystems, respectively. Concentration of WSOC in rhizoplane soil was one to two orders of magnitude higher than bulk soil. Concentrations of WSOC in grass crown-associated soil were on average 4.2 fold higher than in bulk soil. Additional data were collected in 2001, 2002, and 2003 in a NMP plant community. Continuous moderate grazing, along with two different lengths of rest from grazing treatments, were used to examine above- and below-ground ecosystem responses to a change in management. Bare microsites had lower root biomass in surface soils and higher temperatures on the soil surface and at 5 cm depth than grass-occupied microsites. Total pools of C and N were relatively unresponsive to changes in management, but did respond to year-to-year variations in precipitation. Results suggested that recovery from continuous grazing can occur rapidly, and rates of recovery are likely to be dependent on precipitation levels.