ARS | Publication request: Changes to soil organic N dynamics with leguminous woody plant encroachment into grasslands

Submitted to: Biogeochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 11, 2012
Publication Date: August 5, 2012
Citation: Creamer, C.A., Filley, T.R., Olk, D.C., Stott, D.E., Dooling, V., Boutton, T.W. 2012. Changes to soil organic N dynamics with leguminous woody plant encroachment into grasslands. Biogeochemistry. Available: http://www.springerlink.com/content/587464236rk51k7u/. DOI:10.1007/s10533-012-9757-5.

Interpretive Summary: An accumulation of nitrogen in soil usually results in a more productive soil. This cannot always be assumed correct however, because we do not clearly understand how nitrogen accumulates in soil. For example, grasslands cover a large proportion of the earth’s land surface, and nitrogen has often accumulated in their soils when woody plants become established. For grassland soils in southern Texas, we confirmed that nitrogen accumulated in soil under recently established woody plants. Our results suggest, though, that this additional nitrogen is less available to soil microorganisms and plants than was the original soil content of nitrogen. Therefore, the nitrogen and its associated carbon that accumulated in soil under newly established woody plants would have relatively greater stability in the soil. This knowledge will benefit rangeland managers and soil scientists who study nitrogen and carbon accumulation.

Technical Abstract: The encroachment of nitrogen-fixing trees and shrubs into grasslands and savannas occurs worldwide. In the Rio Grande Plains region of southern Texas, previous studies have shown that woody encroachment by leguminous Prosopis glandulosa (mesquite) trees increases soil and microbial biomass nitrogen (N) and accelerates N mineralization and nitrification. We examined responses of the dominant organic N sources (amino acids and amino sugars) and two soil-bound protein-N acquiring enzymes (arylamidase and ß-N-acetylglucosaminidase) along a grassland-to-woodland successional chronosequence to determine how N availability and cycling are altered following woody encroachment. The proportion of total N held within amino compounds was significantly lower in the woodlands (47%) relative to the original grassland soils (62%), suggesting that a large proportion of N accrual in wooded areas was in forms other than extractable amino-N. This increase in non-hydrolysable N was accompanied by increases in cell wall associated amino acids (e.g. hydroxyproline, serine) and losses of microbial amino sugars, indicating the woodland amino pool was comprised of more chemically complex or structurally protected N compounds. Carbon-normalized activities of both N-acquiring enzymes were significantly higher in woodland soils, consistent with changes in the biochemical composition of organic N in these soils. This indicated that microbial communities in wooded areas were allocating appreciable resources towards N acquisition despite soil N accrual on those wooded portions of the landscape, likely in response to a more complex and less accessible N pool. An accumulation of more refractory N under woodlands would limit microbial N accessibility and slow the cycling of soil organic carbon.