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

Authors: Franzluebbers, Alan

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/15/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Grazing lands can be a valuable conservation land use that benefit both farmer and society due to erosion control, storage of soil carbon and nitrogen, and providing open space for wildlife habitat. The stability of soil organic matter during accumulation with pasture age and management conditions relative to other conservation land uses has not received much attention. An ARS scientist in Raleigh North Carolina collected soil from 31 different farms in Virigina under grazing land, no-till cropland, and woodland. Management-induced changes in labile and stable soil organic matter fractions were greater under grazing lands and woodlands than under croplands. A large pool of stabilized soil organic carbon and nitrogen was present under all land uses but the more labile fractions accumulated and were important precursors to further changes in stable soil organic matter. These results will be important for agricultural advisors, farmers, extension specialists, and scientists in the region to promote more efficient, carbon-storing practices for agriculture to simultaneously meet the production and environmental demands for a sustainable future.

Technical 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 0-10-, 10-30-, and 30-60-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 (0-30-cm depth) of particulate organic C (Mg C/ha) (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. Feeding hay increased root-zone enrichment of soil-test biological activity (49 kg CO2-C/3 d/[Mg hay]). 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.