Cover Crop Root Distribution and Its Effects on Soil Nitrogen Cycling

Abstract

Nitrate N in the soil is susceptible to leaching with fall and winter precipitation and can pollute groundwater. Winter cover crops to scavenge residual NO‐3 through root extension are a possible solution. A 2‐yr field study was conducted on a Greenville fsl (fine, kaolinitic, thermic Rhodic Kandiudults) in central Georgia to determine root distribution of legume and nonlegume winter cover crops and their ability to absorb soil NO‐3 and accumulate it in aboveground biomass. Cereal rye (Secale cereale L.), hairy vetch (Vicia villosa Roth), and crimson clover (Trifolium incarnatum L.) were planted in a prepared seedbed in the fall and accumulated biomass was incorporated into the soil in the spring. Seasonal variations in root distribution were measured by minirhizotron and soil separation methods. Soil mineral N concentration and aboveground biomass yield and N uptake were determined at regular intervals during the growing season. Total minirhizotron root count (MRC; no. roots cm−2 soil profile) at the 1‐ to 50‐cm soil depth increased at the rate of 0.01 roots cm−2 d−1 in hairy vetch in the fall to 0.38 roots cm−2 d−1 in crimson clover in the spring, as temperature increased. Roots were well distributed to the 50‐cm soil depth. Compared with the other cover crops, rye had significantly greater total MRC from Dec. 1996 to Feb. 1997 and total root length density (RLD; cm root length cm−3 soil) at the 0‐ to 30‐cm depth from Nov. 1995 to Apr. 1996 and in Jan. 1997, and the subsequent NO‐3 inorganic N concentration in the soil was lower and aboveground biomass yield was greater. MRC was positively correlated with RLD in Nov. 1995, Apr. 1996, and Jan. 1997. A significant positive correlation was observed between MRC and aboveground biomass yield or N uptake (r = 0.52 to 0.68, P ≤ 0.05) and a negative correlation between MRC and soil NO‐3 concentration (r = ‐0.51 to ‐0.55, P ≤ 0.05) early in the growing season. Rye had the greatest root density and aboveground biomass, and scavenged more soil NO‐3 early in the growing season. Nonlegume cover crops, such as rye, may be more effective in reducing residual NO‐3 and potential leaching of NO‐3 from the soil early in the growing season than are legume cover crops, such as hairy vetch or crimson clover.