Impact of integrated crop-livestock system (ICLS) on endogenous organic matter in a subtropical Oxisol

In this work, soil organic matter (SOM) content and composition was investigated in a Brazilian Red Oxisol submitted to ICLS during 15 years. The experiment was conducted in a clayey Oxisol in South Brazil in randomized blocks (n=3) with different grazing intensities according to pasture height: 10 (P10), 20 (P20) and 40 cm (P40) cm). The ICLS system consisted of black oat (Avena strigosa) and ryegrass (Lolium multiflorum) in winter and soybean (Glycine max) in summer (C3 plants). Litter and soil samples were collected within 1 m depth from treatments and from secondary bush forest (SF) soil. Previous to experiment, the whole area had been used for conventional agriculture for about 45 years. Before that (circa 60 years ago), the area was under native pasture (Aristida pallens, C4 plant). C and N contents and isotopic signature δ¹³C were determined and SOM chemical composition was investigated in HF-concentrated samples by ¹³C NMR CP/MAS spectroscopy. δ¹³C from a modal profile under native vegetation was determined as well. Grazing intensity did not affect C contents, that varied from 31.5 g kg^-1 in surface to 7.6 g kg^-1 at 1 m depth in ICLS. The greatest C contents were observed under SF down to 20 cm depth (47.8 to 20 g kg^-1), evidencing the more relevant contribution of forest vegetation on C sequestration in this area. C/N ratio under ICLS tended to increase with depth from 11 to 16. In contrast, no such trend was observed under SF, where C/N ratio was smaller than under ICLS in all analyzed layers. For SF and P40 soils, δ¹³C values were around -22 ‰ at 0-5 cm (typical for C3 plants SOM), changing abruptly to -17 ‰ at 5-10 cm layer and increasing steadily downward the profile to -14 ‰. In the modal profile, δ¹³C values were typical for C4 plants and varied from –12.4 ‰ in A horizon to -9.7 ‰ at Bw horizon. It follows that residues from past and recent agriculture use contributed relevantly to surface layer (0-5 cm) SOM, whereas below 5 cm, endogenous SOM was the main constituent. SOM chemical composition at 0-5 cm layer was dominated by O alkyl C groups (45-110 ppm) that contributed with 52 to 54 %, followed by alkyl C groups (0-45 ppm) with 18 to 26 % and aromatic C groups (110-160 ppm) with 9 to 16%. In all analyzed sites, alkyl C decreased and aromatic C proportions increased with depth, reaching 14 to 17 % and 19 to 23 %, respectively, at 80-100 cm layer. Nevertheless, at this depth, O alkyl C proportion tended to remain high (49 to 52%). Under P10, alkyl C/O alkyl C ratio at surface was lower than in P40, indicating that grazing affected SOM composition mainly at the surface layer. Our results suggest that in the studied Oxisol profile, endogenous SOM resists to be exchanged by “recent vegetation” SOM from ICLS due to stabilization via interactions with Fe-Oxides that also preserve biochemically labile C groups.