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 River. 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 to13 m 2 ha −1