Tuesday, November 14, 2006 - 3:45 PM

Use of 15N and Soil-N Uptake Monitoring (SUM) System for Evaluating Potential Competition Between Citrus and Perennial Peanut (Arachis glabrata) and Bermuda Grass (Cynodon dactylon).

Jose Linares, Univ of Florida, Gainesville, FL 32603, Johannes Scholberg, Univ Florida, 304 Newell Hall, Gainesville, FL 32611-0500, Donald Graetz, Univ of Florida,106 Newell, Gainesville, FL 32611, Robert McSorley, Entomology and Nematology, Natural Area Dr, Gainesville, FL 32611, Wadell Altom, The Samuel Roberts Noble Foundation, The Noble Foundation, PO Box 2180, Ardmore, FL 73402, and Carlene Chase, Univ Florida, Horticulture Science Dept, 1245 Fifield Hall, Gainesville, FL 32611.

Cover crops (CC) can suppress weeds in conventional citrus groves and other agronomic and forest systems. However, no detailed information is available on the effects of cover crops, on either nutrients or water uptake of citrus. A Soil-N Uptake Monitoring (SUM) system was used consisting of large (32 L) columns filled with a sandy soil to determine N uptake dynamics of perennial peanut, bermuda grass, and citrus and to assess potential competition for uptake between system components.  Treatments included: citrus (CIT); perennial peanut (PP); bermuda grass (BG); CIT+BG; CIT+PP; bare soil (Control). Nitrogen (1 g/column) was applied as CaNO3 at biweekly intervals. After 14 days residual N was leached using 3 pore volumes of water combined with a partial vacuum (12 cbar). Nitrogen uptake for a specific treatment was determined by: Nupt= Ncont x Vcont – Ntrtm X Vtrt; where “N”= N concentration in leachate, “V”= volume of leachate, and “cont” refers to soil columns without plants. In order to validate the SUM system and to better assess the effect of competition on N uptake between cropping systems and plant components we also applied 15N two weeks before the last sampling using 10% enrichment applied as calcium nitrate. SUM system uptake values correlated well with 15N based measures and overall N recovery from non-planted SUM columns was 97.2 +/- 5.1%. Overall plant N uptake was greatest for Bermuda grass (BG) and intermediate for both citrus and PP, respective values were 649, 364, and 304 mg N/column during the summer season. Nitrogen uptake followed a cyclic pattern related to clipping cycle, seasonal temperatures changes and growth flushes (citrus). Citrus and BG appeared to compete for N during the summer season, while citrus and PP did not appear to significantly compete for N.