Nitrogen dynamics under contrasting management systems
Woodruff, Lisa Kimberly
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The abundance of poultry litter (PL) in states such as Georgia makes it an ideal choice as a source of N and other nutrients to grow organic produce that have high demand. However, research is needed to exploit such benefits in a way that minimizes the undesirable impacts. We evaluated the use of PL in an organic N management system to determine if it could be used to provide comparable yield to conventional fertilizers without significant accumulation of potential pollutants. Sweet corn was grown in plots receiving four treatments: Control (no N), ammonium sulfate at 112 (AS1) or 224 (AS2) kg N ha-1 and PL (in combination with a cover crop) at 112 kg available N ha-1. Average 3-year yield was greater in the AS and PL treatments than Control, with no significant difference among AS and PL. Post-season NO3--N in the 15-30 cm for the 3-year average was the greatest for AS2 (16.0 kg N ha-1), followed by AS1 (7.8 kg N ha-1), with no significant difference between PL and Control. Thus downward movement of N in PL was limited, possibly due to the use of cover crop. We also calibrated and validated a computer simulation submodel, CERES-N, which was used to predict plant-available N from the cover crops. Crimson clover was incorporated in the soil and samples were collected over 120 d for three years. Root mean squared error (18.9 to 63.1 kg ha-1), FLOFIT (p > 0.05), and 95% confidence intervals indicated an adequate fit of modeled and measured values. The use of such models to predict N availability from cover crops that are used in combination with PL avoids excess application of PL and hence N loss and accumulation of pollutants. Finally, we examined impacts of the two N management systems on abundance and function of ammonia-oxidizing bacteria and archaea (AOB and AOA), which mediate the rate-limiting first step in production of nitrate that is highly mobile in soils. AOA estimates of abundance based on amoA gene copy numbers were higher than (or no different from) AOB abundance for all treatments, with the highest AOA to AOB ratios in control and organically managed systems. However, the abundance of AOB showed stronger correlation with nitrification potential than AOA, indicating their functional dominance in the AS treatments. The differential response of AOB and AOA suggests the need for targeted approaches to maximize N-use efficiency in the two systems. The study addressed interrelated soil, plant and microbial factors that are important in achieving N efficiency in an organic system that uses PL as an amendment.