Changes In Soil Amino Composition and Microbial N Acquisition Strategies In Response to Woody Plant Invasion of Grasslands.

Changes in land cover have the potential to alter nutrient cycling through changes in carbon input chemistry and microbial community 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 soil 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 rates and chemistry from the grassland to the woodland transition was reflected in soil nitrogen forms.  In addition, we measured the activity of two microbial N-acquiring soil-bound enzymes, arylamidase and β-N-acetylglucosaminidase (NAG), to determine whether microorganisms responded to this ecosystem shift by altering their N acquisition strategies.  In the conversion from grassland to woodland, the total amino compound content exhibited significant decreases in C and N-normalized concentrations, as 14% of organic C and 62% of N held in amino compounds in grassland soils decreased to 11% of organic C and 47% of N in woodland soils.  These decreases indicate that a large proportion of accruing N and C in woodland soils is held in non-extractable forms.  The amino pool in woodland soils also appeared to be more chemically complex, structurally protected, or less easily degraded, as the proportions of cell-wall associated amino acids (hydroxyproline, serine) linearly increased while amino sugars decreased with the extent of woody encroachment.  Soil organic C-normalized activities of arylamidase and NAG were significantly higher in woodland soils relative to grassland soils and outpaced increases in soil N and C, suggesting increases in complex, less accessible N resulted in a greater allocation of limited microbial resources to N acquisition strategies.  These data suggest that in this globally relevant land-cover change, less accessible N pools may be selectively accruing, reducing N availability and potentially decreasing the rate of C cycling.  This work furthers our understanding of the feedbacks between plant chemistry, soil chemistry, and microbial activity in response to land cover changes.