Vertical Distribution of Carbon and Nitrogen in Pastures Fertilized with Broiler Litter or Mineral Fertilizer with Two Drainage Classes

Abstract

Nitrogen cycling in pasture soils differing in drainage characteristics and fertilization legacy needs more research to determine efficient nutrient management strategies. This study compared differences in nitrate (NO3−), ammonium (NH4+), inorganic N (IN = NO3− + NH4+), potentially mineralizable nitrogen (PMN), loss-on-ignition carbon (C), and soil pH in 10, 0.7 ha pastures in Eatonton, Georgia, historically fertilized with the same amount of N as either broiler litter (BL; >15 years, 6 pastures) or mineral fertilizer (Min; 4 pastures). We sampled to 90 cm (0–5, 5–10, 10–20, 20–40, 40–60, and 60–90 cm) on a 20 m grid. An analysis of variance indicated that below 5 cm BL pastures had significantly greater amounts of NO3−, IN, PMN, and soil pH compared to Min pastures. Comparisons of drainage classes (well drained~WD, moderately well drained~MWD, and somewhat-poorly drained~SPD) for each BL and Min were analyzed using linear regression for C:IN, C:PMN, pH: NO3−, and pH: NH4+ with all depths combined. In MWD soils, BL had 0.1 and 0.2 mg N kg−1 greater PMN and IN, respectively, for each unit increase in C. In WD soils NO3− decreased in BL by 7.4 and in Min by 12.1 mg N kg−1, while in MWD soils, this level decreased in BL by 7.8 and in Min by 4.5 mg N kg−1 for each pH unit. Five years after N fertilization stopped, BL soils have retained more inorganic N but are losing more NO3− at a greater rate in the MWD soils when all depths are considered. These losses are a combination of plant uptake, emissions, runoff and leaching. While more research is needed, these results strongly suggest the need to design N fertilization practices with drainage class and fertilization legacy in mind to improve N-use efficiency.