Root-zone enrichment of particulate and nonparticulate organic carbon and nitrogen under grassland management and woodlands across a soil texture gradient in North Carolina, USA

Abstract

Background: Surface-soil organic matter accumulation is expected under well-managed grasslands. Characterizing particulate and nonparticulate organic C and N fractions across a soil texture gradient under different conservation land management systems would help understand how grassland management can optimize soil organic matter to meet the challenges of environmental stress. Methods: A cross-sectional study of 648 soil profiles sampled to 60-cm depth was conducted under a diversity of grasslands and woodlands in North Carolina, USA. Root-zone enrichments of particulate organic C and N were calculated as the difference between total and baseline stocks, derived from nonlinear depth distributions of C and N. Results: Particulate organic C and N declined dramatically with depth, representing 35% ± 8% of total organic C at 0–10-cm depth, 14% ± 7% at 10–30-cm depth, and 10% ± 6% at 30–60-cm depth. Sand concentration had a significant negative association with both fractions. Root-zone enrichment of particulate organic C was lower under grassland than under woodland (14.2 vs. 16.7 Mg C ha−1, respectively, p < 0.001), while root-zone enrichment of particulate organic N was greater under grassland than under woodland (0.96 vs. 0.77 Mg N ha−1, respectively, p < 0.001). Root-zone enrichment of particulate organic C and N was increased by 17%–32% with poultry litter amendment and by 4%–20% in 30-year-old pastures compared with newer pastures. Conclusions: Land use and management during the past few decades had a large influence on particulate organic C and N fractions, while pedogenic processes over millennia dominated the nonparticulate organic C and N stocks.