Short- and long-term effects of animal manures and mineral fertilizer on carbon stocks in subtropical soil under no-tillage

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Establishing the long-term effects of different types of animal manures on soil organic carbon (SOC) stocks has both agronomic and environmental implications. However, only a few studies have analyzed the effects of the types of animal manure on SOC storage in subsurface soil layers. Furthermore, the effect of animal manure on labile and stable soil organic matter (SOM) fractions has been scarcely reported for subtropical no-till soils. We evaluated the effect of animal manure application in the short and long term (4 and 12 years) on SOC storage at different depths throughout a 0–70 cm soil profile and on the physical fractions of SOM in the 0–20 cm layer of a sandy Alfisol cropped under no-tillage. The study involved five treatments: pig deep-litter (PDL), cattle slurry (CS), pig slurry (PS), mineral fertilizer (MF) and a control without fertilization (CL). The SOC stock in the soil profile (0–70 cm) was influenced by manure application in only the long term, which was due to high SOC accumulation in the surface (0–20 cm) in the short-and long-term. The PDL and CS applications resulted in 16% (8.6 Mg ha−1) and 10% (5.2 Mg ha−1) increases in the SOC stock, respectively, compared with those in the CL, but only PDL resulted in greater SOC stocks than MF. These results are related to the greater C addition and higher SOC retention coefficient with the PDL application (17%) than with the CS (15%) and PS (7%) applications. Most of the manure-induced changes were observed in the silt plus clay-sized heavy fraction (SCHF), with PDL and CS being more efficient at increasing SOC storage as organic-mineral complexes than PS or MF in the 0–20 cm soil layer. However, in the subsurface layer (20–70 cm), manure caused a decrease in SOC compared with that in the CL, resulting in negative SOC storage rates in this layer, which partially or completely neutralized the SOC storage rates observed in the surface layer (0–20 cm). In the long term, this condition resulted in SOC storage rates in the 0–70 cm soil profile of 0.48 Mg ha−1 year−1 for PDL, 0.21 Mg ha−1 year−1 for CS, −0.03 Mg ha−1 year−1 for PS and 0.01 Mg ha−1 year−1 for MF. These results indicate that PDL and CS are suitable alternatives to MF for increasing the SOC stock and that the SOC retention is greater with solid manure than with liquid manure (PDL vs CS and PS) and with CS than with PS. Moreover, the results reveal the importance of monitoring SOC changes in the long term and within deep soil layers to better quantify the effects of successive applications of animal manure on SOC storage potential in subtropical sandy soils under no-tillage.