Changes in soil carbon and nitrogen cycling in response to woody plant encroachment into grasslands

Courtney Ann Creamer, Purdue University

Abstract

Terrestrial ecosystems represent the largest pool of actively cycling carbon on the Earth. Within terrestrial ecosystems, grasslands and savannas cover 20% of the Earth's land surface and store 30% of global soil organic carbon (Field et al., 1998). Woody plant encroachment into grasslands and savannas is a globally relevant land cover change, (Archer, 1995) driven primarily by shifts in land use, which impacts the biogeochemical cycling of soil carbon and nutrients. The overarching purpose of this dissertation is to examine the biogeochemical cycling of carbon (C) and nitrogen (N) in response to woody plant encroachment of grasslands in southern Texas. Using a combination of approaches, including soil physical fractionation, long-term incubations, and plant and microbial biopolymer analyses, we attempt to elucidate the relationships between plant input chemistry, microbial community structure and function, and the biogeochemical cycling of carbon and nitrogen. A series of year-long soil incubations were conducted of the whole soil and size (>250 μm) and density (<1.0 g cm-3) separated soil fractions. The quantity and isotopic composition of CO2 respired by soil microorganisms was measured throughout the year-long incubations, allowing us to determine the source (C3 vs. C4) and amount of CO2 respired in soils possessing different soil organic carbon protection mechanisms. The results from these incubations suggested that altered carbon chemistry as a result of the grassland to woodland land cover change slowed microbial respiration in the soil fractions. However, in the whole soil the overall greater allocation of C into these more physically unprotected soil fractions allowed for greater respiration of newer, C 3-derived carbon in well-established woody stands. Amino acids, amino sugars, and carbohydrates were extracted from the soils prior to and after the incubation to determine if there were differences in C and N losses between the remnant grassland and established woody stand soils. These data indicated preferential losses of plant-derived (over microbial-derived) carbohydrate-C during the incubation, and higher carbohydrate-C losses in grasslands and recently established woody stands than older woody stands. Amino acid and amino sugar data from this incubation indicated the formation of non-hydrolysable amino-N in older woody stand soils. This data is consistent with results from amino acids and amino sugars extracted along the chronosequence of woody encroachment and N enzyme activity, which suggested that organic N may be less available for microbial degradation as woody encroachment progresses. In general, these results show that soil organic matter (SOM) accessibility is more important than chemistry in determining SOM degradation and accrual, even in these soils with limited capacity for physical protection of SOM.

Degree

Ph.D.

Advisors

Filley, Purdue University.

Subject Area

Microbiology|Biogeochemistry|Soil sciences

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