Just Accepted

This article has been peer reviewed and accepted for publication. It is in production and has not been edited, so may differ from the final published form.

Physical properties and organic carbon in no-tilled agricultural systems in silty Pampas soils of Argentina

Guillermo Peralta, Rodolfo Gil, María Agosti, Carina Alvarez , Miguel Ángel Taboada

Abstract

Context: Under continuous long term no till farming, many silty soils develop platy and massive compacted structures in topsoil, ascribed to low crop diversification and intense agricultural traffic. Aims: We hypothesise that agricultural scenarios of greater diversification and cropping intensity should increase carbon inputs, total and particulate organic carbon, resulting in the disappearance of these platy and massive compacted structures and soil compaction. Methods: The hypothesis was tested in 55 selected production fields (lots or macro-plots of trials with a cultivated area greater than 15 ha) and five non-cultivated sites across the Rolling Pampas of Argentina. The whole area was covered by fine, illitic, thermal, silty loams (Typic Argiudolls, US Soil Taxonomy; Typic Phaeozems, FAO Soil Map). Based on estimations of the crop intensity index (CII; proportion of days in the year with active crop growth) and recent agricultural history of crop sequences, sampled fields were grouped into five categories: soybean (Glycine max) monoculture (CII= lower than 0.45; mean CII= 0.39); low intensity cropping sequence (CII= from 0.45 to 0.60 mean CII=0.50); high intensity cropping sequence (CII= from 0.60 to 0.80 mean CII= 0.66); pastures for hay bales production (CII= 1.0); and quasi pristine situations (areas with non-implanted and non-grazed grass vegetation or with negligible stocking rate, CII= 1.0). Key Results: Total C inputs to soil varied between ~ 1400 and 7800 kg C ha-1 year-1 and were significantly and positively related to crop intensity index (P <0.0001, r = 0.83). The highest (P< 0.05) soil organic carbon levels were observed in the first 0.05 m of soil, quasi-pristine conditions (even higher than under pasture) and the lowest (P< 0.05) under soybean monoculture. In the 0.05-0.20 m soil layer, quasi-pristine conditions had significantly (P< 0.05) higher soil organic carbon levels, the other situations did not differ. Soil organic carbon and particulate organic carbon levels (0-0.05 m layer) were related to both crop intensity index and annual C input. Platy structures and Clods > 0.1 m (0-0.2 m layer) were negatively related to crop intensity index (r= -0.59 and r= -0.45; P < 0.0001) and C inputs from crops (r= -0.6 and r= -0.29; P<0.01). Nevertheless, this did not result in soil compaction alleviation, as shown by soil bulk density, maximum penetration resistance and water infiltration variations. About 92% of the samples with soil bulk density above the threshold (1.35 Mg m-3), and about 32% of the total records, presented levels of maximum penetration resistance, aeration porosity and / or water infiltration beyond the values suggested as critical. Conclusions: Although soil organic carbon in topsoil varied as hypothesized, the studied soil physical properties did not. This partially rejects our proposed hypothesis. Implications: This study underscores the intricate interplay between crop intensity, SOC enhancement, soil structure improvement, and the persistent challenge of subsoil compaction.

SR23205  Accepted 03 April 2024

© CSIRO 2024