Assessing organic carbon sequestration in soil aggregates for building high quality carbon stocks in improved grazing lands

Abstract

Soil aggregation is critical for increasing soil organic carbon (SOC) sequestration and ecosystem services in improved grazing lands (IGLs). Evaluating soil aggregation under various IGL practices can better inform SOC accumulation potential. Assessing SOC within soil aggregates and examining quality indicators can provide additional insights for building stable SOC stocks. The objective of this study was to analyze soil aggregation and aggregate-SOC content and its quality parameters to evaluate the long-term (>50 years) impacts of different grazing and fertilization practices on SOC-sequestration in comparison to a minimally managed native pine forestry site (FS). Experimental treatments in IGLs included two grazing pressures of high (HGP) and low (LGP), and two nitrogen (N) fertilization of either only organic-N (oN) through a legume rotation or only inorganic fertilized (iN). Soil cores collected from a 0–60 cm soil profiles were separated by depth, which were further separated into aggregate fractions through dry sieving. Individual aggregate fractions were analyzed for SOC content, CN ratio (CNR) and fungal to bacterial ratio (FBR) to evaluate the quantity and quality of SOC. Results indicated contrasting trends between soil aggregation and SOC in response to grazing and N management. Sand-free Mean Weight Diameter (sfMWD, mm 50 g^–1) was higher in HGiN (1.03 mm), HGoN (1.00 mm), and in FS (0.95 mm) compared to LGoN (0.76 mm) and LGiN (0.65 mm). Whereas aggregate-SOC (g kg^–1) and stocks (Mg ha^–1) were significantly higher in LGiN (7.64 and 40.3, respectively) and LGoN (5.06 and 39.3, respectively), than HGoN (3.77 and 20.8, respectively) and HGiN (5.63 and 26.7, respectively), but comparable to FS (4.61 and 44.2, respectively). Higher SOC in LGP was largely due to higher independent-MOC (mineral occluded carbon in silt+clay). Higher soil aggregation under HGP did not increase SOC, confirming the potential loss due to overgrazing effects. Comparison of the summed-index based on several SOC quality indicators, including CNR and FBR, ranked FS highest, followed by LGoN and LGiN. Although LGiN accumulated the highest SOC stocks, a combined quantitative and qualitative assessment of SOC sequestration revealed that low intensity grazing with reduced N input and integration of legumes was a superior IGLs practice. These findings highlight the importance of incorporating aggregate-SOC quality parameters to evaluate SOC-sequestration potential.