Soil change after planting longleaf pine on agricultural lands: Revisiting a chronosequence study after 23 years

Abstract

Efforts to restore longleaf pine (Pinus palustris Mill.) have led to millions of hectares of longleaf pine being established across the southern United States. On lands planted to longleaf pine that were formerly subjected to row cropping, few studies have tracked changes in soil carbon (C) and nutrient stocks over time. We repeated measurements of mineral soil total C, total nitrogen (N), and extractable phosphorus (P) on a chronosequence of replicated (n = 3) planted longleaf pine stands of various age classes. From the surface of the mineral soil to a depth of 50 cm, C accumulation from 1999 to 2022 was 2.8 ± 10.4 (mean ± standard deviation) Mg ha⁻¹. Soil N generally tracked changes in C. This included a pattern of C and N accumulation in the shallow mineral soil horizon (0–10 cm) of planted stands, but at depth, C and N showed a decline. Meanwhile, mean extractable P declined since agricultural fertilizers were last applied to the soils of the planted stands about 24–37 years ago, but remained elevated in comparison to naturally regenerated stands. Pairwise comparisons indicated that 30-year-old plantations had similar mineral soil C stocks to those of naturally regenerated stands. The soils of planted stands were generally C sinks despite use of frequent prescribed fire and commercial thinning. Overall, our findings suggest that planting longleaf pine on marginal agricultural lands can contribute to the objective of restoring the low native fertility of these soils and sequestering belowground C in stands.