Particulate organic carbon and nitrogen and soil-test biological activity under grazed pastures and conservation land uses

Abstract

Soil organic matter is assumed to accumulate with conservation management. On-farm surveys using paired-farm and chronosequence approaches could validate assumptions, as well as provide a diversity of soils and management conditions to assess organic matter fractions. The objective of this study was to assess how land use and pasture management affected the gradient of organic matter fractions from active to resistant. Particulate and non-particulate (also called mineral-associated) organic C and N fractions were isolated and soil-test biological activity was measured from 304 profiles at 0- to 10-, 10- to 30-, and 30- to 60-cm depths on 31 private farms in Virginia. Root-zone enrichment contents of these soil properties were calculated following subtraction of a baseline from total stocks of each soil profile. Land use was a key factor affecting root-zone enrichment (0- to 30-cm depth) of particulate organic C (Mg C ha⁻¹) (p ≤ 0.05): conventional-till cropland (6.3) = no-till cropland (9.2) < grassland (14.2) < woodland (16.7). Root-zone enrichments of soil C and N fractions increased with pasture age to a plateau achieved by 25 years and were optimized at cattle stocking rate of 1.0 ± 0.2 Mg live weight ha⁻¹. Feeding hay increased root-zone enrichment of soil-test biological activity (49 kg CO₂-C 3 day⁻¹ [Mg hay]⁻¹). Although non-particulate organic C and N dominated, accumulation with time and steady-state concentration of particulate fractions were vital steps toward greater soil organic matter under conservation management. Root-zone enrichment calculations from on-farm sampling were helpful in distinguishing true effects of management from pedogenesis on soil organic matter formation.