Evaluation of soil organic carbon under forests, cool-season and warm-season grasses in the northeastern US

There is insufficient information on whether or not soil organic carbon (SOC) under forest and grass vegetation differs, and such information is needed by conservation programs targeted for C sequestration. When these contrasting types of vegetation are used for restoration of degraded riparian areas, evaluation of water-extractable and bioavailable dissolved organic carbon (WEOC and BDOC, respectively) is also important for assessing their potential in supplying available SOC for microbial degradation of nonpoint-source pollutants (e.g. nitrate removal by denitrification). Our objective was to compare the total SOC, WEOC and BDOC under forests, cool-season (C(3)) and warm-season (C(4)) grasses in the northeastern U S. Six locations were selected which had mature stands of forests, C(3) and C(4) grasses. The total SOC, WEOC and BDOC were measured to a depth of 1 m. Analysis based on pooled data from all locations showed no difference in total SOC under forest (averages between 17-48 Mg C ha(-1) at 0-5 cm depth), C(3) (19-35 mg C ha(-1)) and C(4) grasses (13-39 mg C ha(-1)). However, analysis conducted at each location indicated that total SOC was, in part, influenced by vegetation age. When vegetation age is the same, temperature was also implicated to influence changes in SOC. Neither forests nor C(3) and C(4) grasses consistently supported the highest amounts of WEOC, BDOC and the proportion of BDOC to WEOC (%BDOC) across locations. The %BDOC ranged from 2 to 84% and averages were 47% under forest, 49% under C(3) grass, 39% under C(4) grass, 41% above 60 cm depth, 47% below 60 cm depth. The uniform %BDOC with depth suggested similar amounts of available C resource for denitrifiers under these vegetation types. Conversion of C(3) grass to C(4) grass resulted to a loss of SOC during the early years of C(4) grass establishment. It took 16 to 18 y after planting for the total SOC under C(4) grass to approach that under the original C(3) grass. Under 16-y and 18-y C(4) grasses, the contribution of C(4)-derived SOC ranged from 53% to 72% of the total SOC under the original C(3) grass. The slow accumulation of C(4)-derived SOC is an important consideration for its use in restoring riparian and conservation areas in the northeastern US.