Recent C Inputs in Different Land-Use Systems in North Florida.

Land use is a key factor affecting C input in agricultural systems. Assessing recent C inputs provide useful information guiding long-term soil C balance. Physical fractionation of soil organic matter (SOM) by particle density, into light and heavy fractions, has been an important alternative to SOM chemical fractionation. Light SOM fraction represents recent C inputs and it is linked with important soil processes such as mineralization of SOM. Soil microorganism respiration is an additional response that characterize labile soil C. Carbon origin from C₃ and C₄ plants can also be determined using stable isotopes (δ¹³C) due to different C discrimination between these two photosynthetic groups. This research assessed light fraction SOM and soil microbial respiration in different land use systems in North Florida including sod-based rotation, bahiagrass pasture, native vegetation, and conventional tillage. Light fraction was determined in five depths (0 to 0.15, >0.15 to 0.25, 0.30-0.56, 0.61-0.87, and 0.91-1.22 m). Soil samples to determine microbial respiration were collected in the same systems at 0-15 cm soil layer. In both cases, samples were replicated three times. Light fraction of SOM was greater for the bahiagrass pasture, followed by native vegetation, sod-based rotation systems, and conventional tillage. Light fraction SOM was greater at the shallowest layer (0 to 0.15 cm), reducing its amount and variability in deeper soil layers. Soil microbial respiration did not differ among land use systems averaging 0.414 mg C-CO₂ kg^-1 soil h^-1. The δ¹³C value obtained from the CO₂ evolved, however, was less depleted (P = 0.0085) in the bahiagrass pastures (-13.36‰) and sod-based rotation system (-15.10‰) compared to conventional tillage (-15.73‰), and native vegetation (-16.16‰), demonstrating the importance of the C₄ bahiagrass increasing the pool of soil labile soil C.