From March through June 1996, 15N-labeled fertilizer was applied to mature pecan trees [Carya illinoinensis (Wangehn.) K. Koch] in a commercial orchard to determine the fate of fertilizer-N in the tree and in the soil directly surrounding the tree. The concentrations of 15N and total N were determined within various tissue components and within the soil profile to a depth of 270 cm. By Nov. 1996, elevated levels of 15N were greatest at depths just above the water table (280 cm), suggesting a substantial loss of fertilizer-N to leaching. Recoveries of 15N from tissue and soil at the end of 1996 were 19.5% and 35.4%, respectively. Harvest removed 4.0% of the fertilizer-N applied, while 6.5% was recycled with leaf and shuck drop. In 1997, with no additional application of labeled fertilizer, the tissue components continued to exhibit 15N enrichment. By the end of the 1997 growing season, 15N levels decreased throughout the soil profile, with the most pronounced reduction at depths immediately above the water table. Estimated recoveries of 15N from pecan tissue (excluding root) and soil at the end of 1997 were 8.4% and 12.5%, respectively. In 1996 and 1997, 15N determinations indicated an accumulation of fertilizer-N in the tissues and a loss of fertilizer-N to the groundwater. Early spring growth, flowering, and embryo development used fertilizer-N applied the previous year, as well as that applied during the current year.
The recovery of late-season (September) 15N-labeled fertilizer (N at 55 kg·ha-1) was followed in mature pecan trees [Carya illinoinensis (Wangehn.) K. Koch] and soil (0-270 cm) from 1996 (application year) through 2001 (end of study). Recovery of late-season applied 15N was compared to the recovery of six 15N applications (March through June, N at 221 kg·ha-1) of a previously reported study. By Nov. 1996, both fertilizer schedules exhibited considerable 15N accumulation below the rooting zone and just above the water table (280 cm), with 43.4% and 35.3% 15N recovered from the soil sampling profile of the September and March-June schedules, respectively. 15Nitrogen recoveries from perennial storage tissues (root and wood) were 20.6% and 10.1% under the September and March-June schedules, respectively. The 15N recoveries from annual abscission tissues (leaf, shuck, and nut) were 1.4% and 10.6% under the September and March-June schedules, respectively. By the end of the 2001 growing season, 4% and 9% of the 15N remained in the soil following the September and March-June applications, respectively. Under both fertilizer schedules, >80% of the fertilizer-N was lost to the environment through natural processes and very little was removed during harvest. Nearly 6 years following application, perennial storage of 15N remained greater in the September application (4.3% of the 15N applied) than in the March-June application (2.7% of the 15N applied). Late-season application of fertilizer-N during the kernel filling stage was stored in perennial tissues for use the following year; very little was used for current year growth of annual tissues. Increased accumulation of perennial storage N by late-season application may reduce the depletion of N caused during a heavy-cropping on-year and may moderate the alternate-bearing trend in pecan by providing a greater reservoir of N the following year.
Pecan [Carya illinoinensis (Wangenh.) K. Koch] pruning wood is usually burned, a practice that creates serious environmental concerns. Chipping and soil incorporation of prunings may be an alternative disposal method if nutrient immobilization is not a problem. Our objective was to determine if incorporation of pecan wood chips into soil would affect the availability of nitrogen (N), phosphorus (P), and potassium (K). Pecan wood chips were incorporated into a silty clay soil at rates of 0, 4484, 8968, 13,452, and 17,936 kg·ha−1 in May or June 2002, 2003, and 2004. Some plots received N (ammonium sulfate) at a rate of 0, 15.2, 30.5, 45.7, and 61.0 kg·ha−1 to adjust the C:N ratio of trimmings to 30:1. Wood chip incorporation did not significantly decrease inorganic N regardless of application rate or number of applications. When ammonium sulfate was added to balance the C:N ratio, soil inorganic N increased with the rate of wood chip application, also indicating that N immobilization did not occur. Soil-available P and K were not significantly affected after one, two, or three wood chip applications. Soil-available K increased when ammonium sulfate was added to balance the C:N ratio. Soil incorporation of pecan wood chips does not appear to immobilize N, P, or K, thus providing growers with an environmentally viable means of wood disposal.
Previous research on late-season N fertilization of pecans [Carya illinoinensis (Wangehn) K. Koch] has shown significant uptake and storage of N in perennial tissues (roots, trunk, and shoots) that was used in subsequent years. The objectives of this study were to follow the fate of 15N applied at three different stages during pecan kernel fill in both the soil and tree components. In August and September 2002, 15N-labeled ammonium sulfate (9.94% 15N atom excess) was applied (56 kg N/ha) to nine pecan research trees during the early [3 days into kernel fill (DIK)], middle (25 DIK), and late (38 DIK) stages of pecan kernel fill near Las Cruces, N.M. In November 2002, about 67% of applied 15N was recovered from the soil and 13% from tree components. More 15N was recovered in nuts from the early treatment than middle or late treatments. Recoveries for May 2003 were 27% and 60% for tissues and soils, respectively. Leaf recovery increased an average of 14% in May 2003 over November 2002 leaves. More 15N was recovered from the late treatment in all tree components for May 2003 than early or middle treatments. The primary source of N for spring growth was 15N stored in perennial tissues. Fifteen months after 15N fertilizer was applied during kernel fill in 2002 about 24% remained in the soil, 28% had been used by the tree, and 48% was lost to the environment.
The pruning wood of pecan [Carya illinoinensis (Wangenh.) K. Koch] is often burned. Chipping and soil incorporation of pruning wood is becoming more popular as a result of environmental constraints on burning. The objective of our research was to determine how pecan wood incorporation into soil affects the soil chemical and physical properties. Pecan wood chips were incorporated into a silty clay soil at rates of 0, 4484, 8968, 13,452, and 17,936 kg·ha−1 in Summer 2002, 2003, and 2004. Some plots received nitrogen at a rate of 0, 15.2, 30.5, 45.7, and 61.0 kg·ha−1 to adjust the C : N ratio of trimmings to 30 : 1. Ammonium sulfate, as a nitrogen source to balance the C : N ratio of pecan wood chips, reduced soil pH. However, the wood chip amendments alone did not reduce soil pH. Soil salinity (as determined by electrical conductivity) and bulk density were unaffected by wood chip incorporation regardless of application rate or number of applications. Incorporation of pecan chips had little effect on soil moisture content, but the soil had an inherently high waterholding capacity. Pecan wood chip incorporation significantly increased soil organic matter content and aggregate stability, particularly at the higher application rates and with repeated amendment. The incorporation of pecan pruning wood into the soil appears to improve soil tilth and aggregation while providing growers with an environmentally acceptable means of disposal.