Root-zone enrichment of soil-test biological activity and particulate organic carbon and nitrogen under conventional and conservation land management

Abstract

Biologically active and intermediately labile fractions of soil organic matter are important features of conservation agricultural management systems. Depth distribution of soil-test biological activity (STBA) and particulate organic carbon (C) and nitrogen (N) were characterized under cropland, grassland, and woodland across 25 research stations in North Carolina. Most (87%) of the 310 fields sampled were on Ultisols (udults or aquults). Variation in STBA in the surface 30-cm of soil was controlled primarily by land use and to a lesser extent by soil texture and physiographic region. Baseline concentrations of STBA and particulate organic C and N at 30-cm depth were relatively narrowly constrained compared with contents in the 0- to 10-cm depth. Root-zone enrichment (i.e., total minus baseline contents) of STBA (mean ± standard error; kg CO₂–C ha⁻¹ 3 d⁻¹) followed the order: conventional-till cropland (194 ± 17) < no-till cropland (274 ± 26) < woodland (421 ± 17) < grassland (537 ± 15) across locations. Proportion of root-zone enrichment of soil organic C as particulate organic C was 43 ± 2%. Root-zone enrichment of particulate organic C (Mg C ha⁻¹) followed the order: conventional-till cropland (3.8 ± 0.6) < no-till cropland (8.7 ± 0.8) < grassland (14.1 ± 0.5) < woodland (16.3 ± 0.6) across locations. The C:N ratio of root-zone-enriched particulate organic matter was greater under woodland than under other land uses likely because of N input limitations. Conservation land uses increased root-zone enrichment of these active and intermediately labile fractions independent of soil types and regions in the state.