Vegetation community impacts on soil carbon, nitrogen, and trace gas fluxes

Changes in C and N cycling were pronounced with mesquite (Prosopis spp.) N inputs into a previously N limited grassland or riparian area. Expansion of mesquite into semi-arid grasslands (Sporobolus spp.) increased soil C content and decreased C isotope values compared with soils of non-mesquite grasslands. Cool season litter (2.9 % N) collection (October 2001 to March 2002) recovered 66 g mesquite C and 4.5 g mesquite N m-2. Measurement of surface litter present in the different riparian plant communities ranged from 750 g litter m-2 (mesquite-sacaton), 598 g litter m-2 (mesquite) to160 g litter m-2 (annuals and forbs). An estimated 3 to 8 years of high quality plant litter remained on the soil surface under mesquite due to yearly inputs. Soil cores removed from the mesquite understory and incubated at constant moisture potentials determined CO2–C was linearly respired for up to 80 d indicating the litter remaining on the mesquite soil was highly labile, but the disconnect between high litter fall and low moisture conditions present in the ecosystem allowed the mesquite litter to accumulate. Changes in the chemistry of the mesquite soil, especially the N content, resulted in greater fluxes of the greenhouse gas nitrous oxide compared to grass or sacaton soils, but no differences were found in carbon dioxide evolved from the three sites. Of interest was the finding that the semi-arid soils were a strong sink for atmospheric methane during the majority of the year