ARS | Publication request: Changes in soil amino composition and microbial N acquisition strategies in response to woody plant invasion of grasslands

Submitted to: ASA-CSSA-SSSA Annual Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: April 30, 2011
Publication Date: N/A

Technical Abstract: Changes in land cover have the potential to alter nutrient cycling through changes in carbon input chemistry, microbial community structure, and even soil structure. In the Rio Grande plains region of southern Texas, overgrazing and fire suppression have resulted in progressive encroachment of N-fixing woody plants into native grasslands. Increases in above- and belowground productivity with woody encroachment coincide with carbon and nitrogen accrual and increased rates of nitrogen cycling. We extracted amino compounds along a chronosequence of woody encroachment to determine whether altered input chemistry from the grassland to the woodland transition was reflected in the soil. In addition, we measured the activity of two N-acquiring soil-bound enzymes, arylamidase and ß-N-acetylglucosaminidase (NAG), to determine whether microorganisms responded to this ecosystem shift by altering their N acquisition strategies. When normalized to soil mass, all amino compounds and NAG activity increased with the extent of woody encroachment. However, when normalized to soil C and N, this trend disappeared and nearly every amino compound was found in higher concentrations in grassland soils due to a greater buildup of non-amino C and N in the woodland soils. Hydroxyproline was an exception; it still increased linearly with woody encroachment when normalized to C and N (R2 = 0.79), potentially as a result of the changing amino composition of woodland inputs. Arylamidase and NAG activities, when normalized to soil C, were higher in woodland soils, indicating that increases in N acquiring enzyme activity were surpassing actual increases in soil N and C. This work furthers our understanding of the feedbacks between plant chemistry, soil chemistry, and microbial activity in response to land cover changes.