EVALUATION OF SOIL ORGANIC CARBON UNDER WOODS, COOL-SEASON, AND WARM-SEASONGRASSES IN THE NORTHEASTERN U.S.

Authors: Corre, Marife; Schnabel, Ronald; SHAFFER, J. - PENN STATE UNIVERSITY

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: An important consideration when selecting riparian vegetation is the quantity and quality of soil organic carbon (SOC) they provide to microbes for removing nonpoint-source pollutants. The commonly used riparian vegetation in the northeastern U.S. are forest and cool-season grasses. However, there is an increasing interest of using native warm-season grasses in riparian areas, especially to replace cool-season grasses. We undertook this research to evaluate and compare the total and microbially- available organic carbon within 1-m depth of soils under woods, cool- and warm-season grasses in the northeastern U.S. The conventional wisdom is that the most effective riparian zones for removal of pollutants such as nitrate will be those containing a sufficient amount of microbially- available organic carbon. We found that neither woods nor cool- and warm- season grasses consistently supported the highest levels of total and microbially-available organic carbon. The amount of microbially-available organic carbon was uniform with soil depth, suggesting that energy source for microbial activity from surface to 1-m depth was similar, irrespective of the vegetation type. The conversion of cool-season grass to warm-season grass caused a loss of SOC during the early years of warm-season grass establishment. This loss generally occurred at the surface depth. It took 16 to 18 years after planting for total SOC content under warm-season grass to approach that under the paired cool-season grass. This time lag in SOC accumulation following a change in vegetation is an important consideration for restoring riparian areas. A similar time lag is anticipated in reforesting riparian ecosystems.

Technical Abstract: One goal of creating riparian buffers is to provide an environment that favors denitrification in order to reduce the concentration and load of nitrogen in throughflow to streams. When selecting riparian buffer vegetation, an important consideration is the quantity and quality of soil organic carbon (SOC) they provide. Our objectives were to evaluate and compare the accumulation and bioavailability of SOC within 1-m depth of soils under woods, cool-season and warm-season grasses in the northeastern U.S., and to evaluate the replacement of SOC from cool-season grass (C3) with SOC from warm-season grass (C4). Six locations in the northeastern U.S. were chosen to have mature stands of C4 grass, C3 grass, and woods. The total SOC and dissolved and bioavailable organic C were measured to a depth of 1 m. We found that across all locations neither woods nor C3 and C4 grasses consistently provided the highest levels of total, dissolved, and bioavailable organic C. While total SOC decreased markedly with depth the measures of SOC more closely associated with microbial activity (dissolved and bioavailable organic C) were more uniform with depth. This suggests that the energy source needed for denitrifiers to remove nitrate from surface to 1-m depth was similar irrespective of the vegetation type. An average of 41% and 50% of dissolved organic C was biodegradable above 60 cm and below 95 cm, respectively. With the use of d13C method, we found that conversion of C3 grass to C4 grass caused soil organic C to be lost during the early years of C4 grass establishment. This loss generally occurred at 0-5 cm depth. It took 16 to 18 years after planting for the total SOC content under C4 grass to approach that under the paired C3 grass. From the total SOC under 16-yr and 18-yr C4 grass, the contribution