Nitrogen Mineralization In Cover Crop Above- and below-Ground Biomass Added to Soils.

Cover crops are grown to protect soil against wind and water erosion and/or to increase nitrogen (N) input into soil following incorporation of the biomass into soil. However, contributions of the above- and below-ground biomass to the total N mineralized pool are not well understood. A study was initiated to examine N mineralization in three soils amended with above or below-ground biomass of crimson clover, black oat, Sudan grass, or hairy vetch. The objectives of the study were to determine N mineralization rates in three Alabama soils and to establish relationships between soil properties and N mineralization kinetic parameters. Surface soils (0-15 cm) were selected to represent a range of chemical properties. The soil sample (< 2-mm) was mixed with silica sand and treated with above or below ground cover crop biomass to give a concentration of 300 mg N kg-1 soil. The mixture was packed into a leaching tube incubated at 25�1�C and leached every two weeks with 5 mM CaCl2 for 20 weeks. The inorganic N contents of the leachate were analyzed by steam distillation. A non-linear regression approach for N mineralization was used to estimate the readily mineralizable organic N pools (No) and the first-order rate constant (k). Cumulative ammonium (NH4+)-N and nirate (NO3-)-N were plotted against incubation time (weeks). Results showed that incubation of the soils without cover crop biomass increased NH4+-N and NO3- -N released only until four weeks but then stabilized at < 10 mg-N kg-1 soil. Addition of the cover crop biomass increased the amount of NH4+ and NO3- released significantly by (P < 0.05) as compared with the non-amended soils. On average, the cumulative NH4+-N released in the soils were 60, 30, and 20 mg kg-1 soil for Sucarnoochee, Maytag, and Colbert soils, respectively.