Ecological agriculture intensification through crop-pasture rotations does improve aggregation of Southeastern-Pampas Mollisols

Abstract

Soil physical health is strongly dependent on aggregate stability (AS) given it determines most soil physical properties. Aggregate stability is associated with total (SOC) and particulate (POC) soil organic carbon and all of them are very sensitive to management practices. Crop-pasture rotations and tillage systems could be combined pursuing ecological agriculture intensification (EAI) to manage soil manipulation and carbon input to improve or maintain AS and SOC and POC. We hypothesized that 1) EAI through rotating pasture periods with cropping improves AS through macroaggregate (MA) formation and stabilization regardless of the tillage system, and 2) reduced disturbance due to no-tillage (NT) during cropping periods allow reducing pasture period frequency without affecting MA formation and stabilization. The aim of this work was to evaluate the effect of different crop-pasture rotations and tillage systems (NT and conventional tillage (CT)) on soil AS and MA proportion and stability, and SOC and POC content of a mollisol of the Southeastern Argentinean Pampas (37° 45' 09’’ S, 58° 18' 48’’ W). Soil samples were taken at 0–5 and 5–20 cm in a long-term experiment (since 1976) to determine aggregate size distribution and stability and SOC and POC content. The sequence intensification index on a monthly basis (SIIm) was calculated and carbon input through crops and pastures was estimated. The increase of SIIm due to crop-pasture rotations led to higher AS and MA stability in the arable layer and especially in the surface 5 cm. This was due to the greater aboveground and root carbon input and lower frequency of tillage associated with pastures respect to cash crops. Cropping systems under NT failed to sequester organic carbon as compared to CT except for the uppermost soil layer, but they successfully improved AS and MA stability. Anyway, regardless of the tillage system, two years of pasture after a cropping period were not enough to take AS up to the level shown by permanent pasture, except under NT at 0–5 cm. The results suggest that despite carbon sequestration failure, cropping periods under NT could be extended (<SIIm) without compromising soil physical health. Contrarily, under CT soil health decreases sharply with the decrease of SIIm. Therefore, the only way to grant better soil functioning under CT is through increasing SIIm with more frequent or longer pasture periods and/or introducing other crops between cash crops. Evidence collected in this loam high-organic-matter soil was not enough to reject the hypotheses.