Soil organic Matter, Alfalfa Root and Shoot Decomposition, and Modifications of Mineral Nitrogen Fluxes

Rasse, D. and A.J.M. Smucker

Presented at the All Scientist Meeting (1996-07-16 to 1996-07-17 )

Dynamics of mineral nitrogen in the soil profile have been monitored in the soil profile of alfalfa and bare soil plots since December 1993. Four treatments are considered:Soil solutions were sampled by suction lysimeters in the Ap, Bt and upper C horizons (depths 15, 35, and 60 cm) and analyzed for NO₃ and NH₄. Volumetric soil water contents of the same horizons were measured by time domain reflectometry (TDR) at every sampling date. Destructive sampling for extractable NO₃ and NH₄ and total nitrogen was conducted in 1996 to depths of 150 cm.Maximum N-NO₃ accumulations of 28 and 45 µg g^-1 were measured in treatments A and AT during August. Nitrate concentrations in the upper C horizons were consistently low (<8 ppm) throughout the year for treatment A. Maximum N-NO₃ concentrations, of 108 and 104 µg g^-1 were observed in the Ap horizons for treatment BT in October-November and December for B. Accumulation of NO₃ was greater for BT than B throughout the growing season.In the Fall, N-NO₃ concentrations in the upper 60 cm of the soil profile declined much faster in treatment BT than in B. This difference can be explained by a differential drainage flux, as soil volumetric water contents were consistently higher for BT than B. The presence of living root systems induced much higher recharge rates after precipitation compared to the bare soil treatments. Soil extractions conducted in May 1996 showed that treatments B and BT had accumulated 8 times more N-NO₃ in the upper 150 cm of the soil profile than did A and AT.Maximum quantities of N-NO₃ were found in the C horizons. This suggests that much of the N-NO₃ which has not been absorbed or immobilized is leached to depths greater than 1 m. At the first harvest, alfalfa shoots were broadcasted on the BT and AT plots. The plant regrowth appeared to be impeded by the residues in the AT plots. The following harvests were conducted with alfalfa shoots applied by hand in between the rows. By the fourth harvest, AT yields were significantly higher than A yields. This indicates that the decomposition products of alfalfa shoots are not directly or indirectly (soil pathogens, …) harmful to a well established alfalfa stand.