Soil nitrogen and carbon after twenty years of riparian forest development

Soil C and N were measured from a chronosequence of adjacent riparian forest sites located along the floodplain terrace of the San Antonio Fiver. The communities were 25, 39, 45, 47, 49, and 53 yr past long-term farmland use. Four of these sites (25, 45, 49, and 53 yr old) were also sampled 20 yr ago. Soil C was significantly greater after 20 yr of community development; however, the increase was dependent on the community age. Soil C increased by 38 g kg(-1) in the 49-yr-old community during the 20-yr period, but only 11 to 15 g kg(-1) in the 25, 45-, and 53-yr-old communities. Soil N was significantly greater (1.7-2.5 g kg(-1)) after 20 yr of development for communities 25, 45, and 49 yr old. In the oldest community (53 yr old), no significant increase was seen in soil N after 20 yr. No significant spatial differences [underneath the canopy of Acacia farnesiana (L.) Willd. vs. intercanopy grassland] were found in soil C and N in the 25-yr-old community. The late successional dominant Celtis laevigata Willd., total woody plant density, and total woody plant basal area could be explained by soil C or N. Changes in soil C and N through time parallel other studies, increasing through the early- and mid-successional stages and decreasing from the mid- to late-successional stages. Spatial differences in soil C and N that existed in the 15-yr-old community disappeared in the 25-yr-old community as A. farnesiana basal area increased from 0.7 to 13 m(2) ha(-1).