SOIL-NITROGEN AND CARBON STATUS FOLLOWING CLOVER PRODUCTION IN LOUISIANA

Field studies were conducted for four to seven years on two soils, Tan gi silt loam (Typic Fragiudalf, fine-silty, mixed, thermic) and Dexter loam (Ultic Hapludalf, fine-silty, mixed, thermic), to determine the effects of phosphorus (P) applications on growth and nitrogen (N) cont ent of white clover (Trifolium repens L.) and subterranean clover (Tri folium subterranum L.) and on ammonium (NH4+)- and nitrate (NO3-)-N, t otal N, and organic carbon (C) levels in the soils at the end of the s tudy. Phosphorus applications consistently and significantly increased forage yields and led to significantly higher N yields by the clovers . Increases in plant yields and N2-fixation, however, were not reflect ed in higher soil N and C levels. On Tangi soil, NH4+- and NO3--N leve ls were lowest where no P was applied but no statistically significant differences (P < 0.05) were found among P rates above 20 kg/ha. On th e Dexter soils, no significant differences were found at any P applica tion level. Significant differences due to higher clover yields at inc reasing P rates were not found in total N or organic C levels in eithe r soil. Greenhouse evaluations showed no differences in bermudagrass y ield, N concentration, or total N recovery despite increasing subclove r yields in the field during the previous seven years. Harvesting near ly all above-ground clover growth caused plant roots to be the major N and C contributor to the soil. It is possible that root production wa s not increased in proportion to forage production as P applications i ncreased. Perhaps increased microbial activities and some leaching los ses also minimized accumulations of N and C released by clover roots.