Stratification ratio of soil organic matter pools as an indicator of carbon sequestration in a tillage chronosequence on a Brazilian Oxisol

Long-term no-tillage (NT) leads to profile stratification of soil organic matter (SOM) pools, and the soil organic carbon (SOC) stratification ratio (SR) is an indicator of soil quality. The objective of this report is to assess the feasibility of using SOC-SR as an index for estimating SOC sequestration in NT soils. The effect of a plow tillage (PT) and NT chronosequence on the SR of SOM pools was assessed in an Oxisol in Southern Brazil (50°23′W and 24°36′S). The chronosequence consisted of six sites: (i) native field (NF); (ii) PT of the native field (PNF-1) involving conversion of natural vegetation to cropland; (iii) NT for 10 years (NT-10); (iv) NT for 20 years (NT-20); (v) NT for 22 years (NT-22); (vi) conventional tillage for 22 years (CT-22). Soil samples were collected from four depths (0–5 cm; 5–10 cm; 10–20 cm; 20–40 cm layer) and soil parameters comprised by SOM pools [i.e., C, N, S, particulate organic C (POC), particulate N (PN), stable C (SC) and stable N (SN), microbial biomass C (MBC) and microbial biomass N (MBN), basal respiration (BR), dissolved organic C (DOC), total polysaccharides (TP) and labile polysaccharides (LP)] were measured. In undisturbed NF soil, the SR of all parameters increased with increase in soil depth. In contrast, the SR decreased in PT, and the SOM was uniformly distributed in the soil profile. All NT treatments restored the SR, and were characterized with higher values of all measured parameters compared to NF. The SR for SOC ranged from 1.12 to 1.51 for CT-22 compared with 1.64–2.61 SR for NT surface and sub-soil layers, respectively. The SR for POC and PN were higher than those for stable C and N. However, SR for the biological pools (e.g., MBC, MBN and BR) were the highest and strongly correlated with the rate of SOC sequestration. An increase in SR of SOC was also positively correlated with the rate and amount of SOC sequestered. Regression analyses indicated a strong correlation between SR of SOC and all parameters monitored in this study. The data showed that the SR of SOC is an efficient indicator of C sequestration in long-term NT management.