Potential for nitrate-nitrogen leaching in a silvopastoral system compared with open pasture and loblolly pine plantation

Potential for nitrate-nitrogen leaching in a silvopastoral system compared with open pasture and loblolly pine plantation

Summary

At 0.3 m below soil surface, soil solution nitrate-nitrogen was less in the silvopastures than the open pasture after first warm-season forage fertilization in April, and less than in the conventionally thinned pine plantation after late season fertilization in July. For the first three months, at 1.2 m below soil surface, soil solution nitrate-nitrogen concentrations were less in the silvopastures and the open pasture than in the thinned pine plantation. These results partially corroborate our hypothesis of reduced nitrate-nitrogen leaching in silvopastures compared with other investigated production systems. Fertilization of the silvopastures for forage production generally improved tree nutritional status.

Objectives/Performance Targets

• Determine differences in soil nitrate-nitrogen leaching among silvopasture, conventionally thinned loblolly pine plantation and open pasture. Determine effects of silvopasture fertilization for forage production on tree nutrition.

Accomplishments/Milestones

The 2005 lysimeter sampling revealed peaks in soil solution nitrate-nitrogen concentrations measured at 1 ft (0.3 m) below the soil surface in all four production systems following both N fertilization events. The early season (April 21) fertilization treatment (100 lb N/acre) significantly increased nitrate-nitrogen concentrations in open pasture at 1ft (0.3 m) below the soil surface for three consecutive sampling dates between May 2 and June 27. The highest recorded nitrate-nitrogen concentration was 294 mg/l in the same treatments on May 28, or 37 days after the first fertilization event. The late season (July 1) fertilization treatment (50 lb N/acre) resulted in lesser nitrate-nitrogen concentration peaks measured at 1 ft (0.3 m) below soil surface in all four production systems. The nitrate-nitrogen concentrations measured at 1 ft (0.3 m) below the soil surface in the 5th row thinned pine plantation were significantly higher than those for the other production systems between September 27 and December 13, 2005 sampling dates. Although nitrate-nitrogen concentrations at 4 ft (1.2 m) below the soil surface were raised after the fertilization events in the open pasture and the 5th row thinned pine plantation, they were not significantly different from those in the silvopastoral treatments. For the 4 ft (1.2 m) sampling depth, only the concentrations measured between January 8 and April 3 in the 5th row thinned pine plantation were significantly higher than those in the other production systems.

From 2004 to 2005, fertilization treatments applied for forage production in the silvopastures increased tree needle concentrations of N, P, Ca, and Mg. In the same time period, there was an increase of foliar N and Mg concentration in trees grown in the similarly fertilized 5th row thinned pine plantation, while concentrations of P and Ca remained the same between the sampling periods. From 2004 to 2005, foliar K concentrations decreased in all three pine containing production systems. Despite that, at the beginning of 2005, foliar nutrient concentrations were higher than the minimum recommended for loblolly pine for all five measured macronutrients. The three pine containing production systems did not differ significantly from each other with respect to tree nutritional status.

Impacts and Contributions/Outcomes

The soil solution nitrate-nitrogen concentrations measurements at 1 ft (0.3 m) below the soil surface revealed that silvopastures had lower potential for nitrate-nitrogen leaching than the open pasture after early season N fertilization, and lower potential for nitrate-nitrogen leaching than the 5th row thinned pine plantation after the late season N fertilization. Measurements at 4 ft (1.2 m) below the soil surface showed that the 5th row thinned pine plantation had higher potential for nitrate-nitrogen leaching than all other production systems in the first three months of the year. Together these results indicate that in 2005, there was less potential for nitrate-nitrogen leaching in both silvopastoral systems compared with either the open pasture or the conventionally thinned pine plantation. This corroborates our original hypothesis that nitrate-nitrogen leaching may be less in silvopastures compared with fertilized open pastures or fertilized pine plantations.

Our results show that fertilization of silvopastures for forage production generally increased tree nutritional status. Between 2004 and 2005 sampling dates, foliage N, P, Ca and Mg increased while needle K concentrations decreased in both silvopastoral production systems. Therefore, in silvopastures, both forage and timber production may benefit from forage fertilization. In all three investigated pine production systems foliage nutrient concentrations were above minimum recommended for loblolly pine.

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