Nitrogen dynamics of cover crops with sorghum for increased sustainability

View the project final report

• 2013 annual report
• 2014 annual report

Commodities

• Agronomic: sorghum (milo), sugarbeets

Practices

• Crop Production: application rate management, catch crops, continuous cropping, cover crops, crop rotation, fallow, nutrient cycling, organic fertilizers, tissue analysis
• Education and Training: mentoring
• Energy: bioenergy and biofuels
• Production Systems: agroecosystems, holistic management
• Soil Management: green manures, nutrient mineralization, organic matter, soil chemistry, soil quality/health
• Sustainable Communities: local and regional food systems

A comparison of multiple winter rotational crops with summer sorghum under moderate (100 Kg N ha^-1) and low (100 Kg N ha^-1) nitrogen fertilization determined that some rotation crops like clover and sugar beet tops provided a fertilizer N credit of approximately 30 kg N ha^-1 to sorghum grown in North Central Florida. Our results also showed that growers need to sow subsequent crops quickly (ideally within 2 weeks) after incorporation of rotation crop residues, as much of the nitrogen from the residues was released within 4-6 weeks after incorporation. Rye reduced nitrate-nitrogen availability in the soil after incorporation.

Introduction

High levels of chemical nitrogen fertilization are a staple of modern agricultural practices in the Unites States, with 11.6 million metric tons of nitrogen applied to agricultural lands in 2011 (USDA ERS, 2014). These inputs, combined with other factors, have significantly increased agricultural productivity over the last 50 years, but also have negative effects on agroecosystems, including decreasing soil fertility and increasing erosion and leaching losses (Matson et al., 1997; Tilman et al., 2002). Chemical nitrogen fertilizer application has also been indicted as a major driver of increasing N₂O concentrations in the atmosphere, an important greenhouse gas and cause of concern with regard to climate change (Park et al., 2012). Recently, growing concern over the impacts of chemical fertilizers on agroecosystems and the broader environment has spurred research on means to reduce nitrogen inputs and mitigate the detrimental effects of fertilization, while maintaining yields (Grant et al., 2002; Smith et al., 1997). Based on prior research, Grant et al. (2002) concluded that continuous rotated cropping systems can provide multiple benefits to subsequent crops and the agroecosystem, particularly in no-till systems, relative to fallow systems, provided proper management strategies are followed.

Rotational cropping and residue return have been advocated to decrease the need for external nitrogen inputs, but crop selection and return practices must be properly managed. Vigil and Kissel (1991) determined that return of crop residues with a C:N ratio greater than 40, which typically corresponds to tissue N concentrations of or lower than 10 g N kg^-1, will generally result in net soil nitrogen immobilization, and not a return of N to a subsequent crop. Residues with higher tissue N concentrations have the potential to return significant quantities of nitrogen to the soil and a subsequent crop. Yamoah et al. (1998) showed that grain sorghum grown following soybean had higher average yields over an 18 year period relative to continuous sorghum (5130-7120 kg ha^-1 versus 4050-6260 kg ha^-1 relatively) in Nebraska, and that soybean could contribute up to 83 kg N ha^-1 yr^-1 to sorghum depending on climactic factors. However, Havlin et al. (1990) demonstrated that continuous sorghum cropping provided the greatest increase in soil nitrogen when compared with sorghum-soybean or continuous soybean systems, and that the effects of fertilizer application on soil organic nitrogen were minimal. These results indicate that the residue fertilizer nitrogen credit to a subsequent crop is strongly affected by environmental conditions as well as crop selection (Cameron et al., 2013). Lacking from these studies though is a quantified understanding of the temporal dynamics of nitrogen in the rooting zone of the subsequent crop, as these studies integrated the cumulative effects of years.

Prior research on grain sorghum following a crimson clover crop has shown the potential for legumes to offset fertilizer requirements. However, even when clover contained 202 to 216 kg N ha^-1, only an estimated 128 kg N ha^-1 of fertilizer N was replaced by the clover supplied N, or an N recovery rate of 59-63% (Hargrove, 1986). McVay et al. (1989) reported similar effects, with crimson clover preceding grain sorghum replacing 21-80 kg fertilizer N ha^-1 based on yield relative to an unfertilized sorghum system with cool season fallow. In contrast, Singh et al. (2012) reported nitrogen uptake rates of 133 to 139 kg N ha^-1 by two sweet sorghum cultivars grown in the southeastern U.S. for two years at two sites when fertilized with chemical N at 135 kg ha^-1, a recovery rate of 99-103%, which is considerably higher than found with the legume residue studies. These differences in nitrogen uptake are attributable to multiple factors, including cover crop decomposition rate and microbial lockup of nitrogen, but must be considered when selecting rotational crops and calculating nitrogen offsets. In order to accurately determine how much nitrogen may be supplied to a subsequent crop, soil nitrogen mineralization and availability should be investigated for various rotations and environmental conditions.

Nitrogen mineralization and availability in the soil can be measured in numerous ways (Bai et al., 2012; DiStefano and Gholz, 1986). Soil incubation in buried plastic bags or columns is a common measure of soil mineralization, but may produce an unnaturally constrained environment for nitrogen mineralization, including excessive moisture, even in the sandy soils of North Florida (DiStefano and Gholz, 1986; He et al., 2000). Ion exchange resins have the ability to accumulate cations or anions from water moving through the soil and have been widely used and reviewed as a method for in situ comparisons of nitrogen dynamics as they mimic the effects of plant roots and do not allow ion concentrations to build in the surrounding soil (Ziadi et al., 2006). Additionally, the use of ion exchange methods has been shown to be sensitive to smaller perturbations in soil ion availability than other methods while maintaining a more natural, and potentially less disturbed, environment after insertion.

1. Ascertain yields and tissue nitrogen concentrations of sorghum grown under high and low fertility in rotation with winter cover crops.

2. Monitor soil nitrogen pools and organic matter content, and quantify monthly availability of nitrogen in the rooting zone of sorghum and winter cover crops, and correlate with crop yields and tissue nitrogen concentrations.