Soil respiration, fine root production, and microbial biomass in cottonwood and loblolly pine plantations along a nitrogen fertilization gradient

It is well known that carbon storage capacity of forests will change in response to management practices such as fertilization. However, the influence of fertilization on belowground processes such as soil respiration, fine root production, and microbial biomass is still unclear. We measured soil respiration, fine root biomass production, and microbial biomass along a fertilization gradient (0, 56, 112, and 224 kg N ha−1 per year) in 7-year-old cottonwood and loblolly pine plantations, established on a well-drained, Redbay sandy loam (a fine-loamy, siliceous, thermic Rhodic Paleudlt), in northwest Florida. Soil respiration was measured monthly from June 2001 to May 2002 using the soda-lime technique. Fine root biomass production was quantified using the ingrowth core method during the same period. In addition, microbial biomass, soil temperature, moisture, soil pH, and organic matter were also measured along the same gradient for both species. Annual soil respiration rate was significantly greater (781 g C m−2 per year) in cottonwood than that 692 g C m−2 per year) in loblolly pine. Nitrogen fertilization had a significant negative effect on soil respiration in cottonwood, but no effect was observed in loblolly pine stands. Mean daily soil respiration rates exhibited significant exponential relationships with soil temperature both in cottonwood (R2=0.81) and loblolly pine (R2=0.51). Annual soil respiration rates in cottonwood stands were positively correlated with fine root production (r=0.64) and soil microbial biomass C (r=0.87) and negatively correlated with soil pH (r=−0.81). Annual soil respiration in loblolly pine stands was correlated positively with fine root production (r=0.54) and with organic matter content (r=0.74). Annual fine root production was significantly greater in cottonwood (221 g m−2 per year) than that in loblolly pine (144 g m−2 per year). Fertilization did not affect fine root production in both species. Microbial biomass, however, was significantly reduced by nitrogen fertilization in both species. We also observed an optimum range of soil pH (6.0±0.4), where highest microbial activity could be expected. Multiple regression analysis indicated that microbial biomass, soil organic matter, and soil pH were the major factors affecting soil respiration in cottonwood, while fine root production and soil organic matter were the major factors affecting soil respiration in loblolly pine. These results suggest that belowground responses to fertilization can vary widely between conifers and hardwoods.