Biomass, carbon, nitrogen and soil respiration dynamics within riparian buffers and adjacent crop fields

Natural and re-established riparian buffers reduce nonpoint-source pollutants derived from upland agricultural lands and enhance terrestrial and aquatic habitat. This study was conducted in multi-species riparian buffers, cool-season grass buffers and adjacent crop fields to determine biomass, carbon, nitrogen and soil respiration dynamics. The multispecies buffers were composed of poplar (Populus x euroamericana' Eugenei ) and switchgrass (Panicum virgatum L.). Crop fields were under annual corn-soybean rotation. Aboveground biomass was determined by clipping grasses in 25 x 25 cm quadrats. The dynamics of fine (0--2 mm) and small roots (2--5 mm) were assessed by sequentially collecting 5.4 cm diameter, 35 cm deep cores for the first year and 125 cm deep cores for the second year from April through November. Coarse roots were described by excavating 1 x 1 x 2 in pits and collecting all roots in 20 cm depth increments. Root distributions within the soil profile were determined by counting roots that intersected the walls of the excavated pits. Soil respiration was measured monthly from July 1996 to July 1998 using the soda-lime technique. Over the sampling period, live fine-root biomass and root C and N in the riparian buffers were significantly higher than in the crop fields. Poplar had the greatest aboveground live biomass and N and C, while switchgrass had highest aboveground dead biomass, C and N. Roots of trees, cool-season grasses, and switchgrass extended to more than 1.5 m in depth, with switchgrass roots being more widely distributed in deeper horizons. Root density was significantly greater under switchgrass and cool-season grasses than under corn or soybean. Soil respiration was significantly greater in both buffer systems than in the cropped fields. Annual soil respiration rates correlated strongly with soil organic carbon (R = 0.75, P < 0.001) and fine root (<2 min) biomass (R = 0.85, P < 0.001). Abundant fine roots, deep rooting depths, and high soil respiration rates in the multispecies riparian buffer zones suggest that these buffer systems added more organic matter to the soil profile, and therefore provided better conditions for nutrient sequestration within the riparian buffers.