An Evaluation of Contrasting Cover Crop Root Systems On Nitrate Leaching During a Winter Rainy Season.

Groundwater nitrate pollution poses a serious challenge in regions with intensively managed agricultural systems. Therefore, it is important to develop crop management practices that limit nitrate transport past the root zone.  In California, leaching can be significant during winter months, when the soil typically is left fallow and precipitation is highest.  In some systems, winter cover crops have been shown to reduce leaching by incorporating nitrate into biomass and/or altering water movement through the soil.

In an ongoing field study, we aim to quantify nitrate leaching during the rainy season and assess the effectiveness of two commonly grown cover crop species, triticale (x Triticosecale) and bell bean (Vicia faba).  The two cover crops generate contrasting root systems: triticale has a fibrous root system while bell bean produces a taproot system.  Furthermore, bell bean nodules fix N late in the season.  The two cover crops were grown separately and fallow fields served as a control. Nitrate concentrations were measured using both ceramic cup suction lysimeters and ceramic plate lysimeters. Nitrate leaching was calculated as the accumulation of nitrate collected by the plate lysimeters during the growth of the cover crop.  Roots were obtained from washed soil cores and root length density was assessed using WinRhizo imaging software.

During the 2012-2013 winter, triticale roots grew significantly deeper than bell bean.  Also, bell bean roots were more concentrated near the surface than triticale roots (95% of triticale roots were above 150 cm, compared to 95 cm for bell bean).  Triticale reduced nitrate leaching by approximately 68%, while bell bean increased nitrate leaching.  Furthermore, triticale depleted soil nitrate in the root zone (77%) to a greater extent than bell bean (34%). These results will be used in a model to obtain best management practices for the use of cover crops to minimize nitrate leaching.

Support from CDFA Grant #SCB11013