Soil organic matter in physical fractions after intensification of irrigated rice-pasture rotation systems

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Crop-pasture systems improve soil quality, but their intensification through the increase of the frequency of annual crops may reduce it. We evaluated the impacts of six no-till rice rotations systems on soil quality after five years in a field scale long term experiment established on a site with a 30 years old stabilized rice-pasture rotation. Rotations included: continuous rice (ContRc); rice-soybean (Rc-Sy); rice-soybean-rice-sorghum (Rc-Sy-Sg); rice-soybean-pasture (Rc-Sy-Past); and rice-pasture, with short (Rc-SPast) and long-term pastures (Rc-LPast). Cover crops were included in winter between cash crops. All rotation phases coexisted and were replicated three times in space. Soil quality indicators included: soil organic carbon and total nitrogen contents in bulk soil (TSOC and TN, respectively) and in particulate (>53 μm, POM-C and POM-N) and mineral associated soil organic matter fractions (<53 μm, MAOM-C and MAOM-N). Soil cores were collected at 0−5 cm and 5−15 cm soils depths (results presented at 0−5 and 0−15 cm depths). Additionally, soil samples were taken up to 60 cm soil depth every 15 cm for TSOC and TN. After five years, no differences were observed in TSOC (29.3 Mg C ha−1) or TN (3.16 Mg N ha−1) between rotations in the first 0−15 cm as well as for each layer and in the aggregated 0−60 cm of soil. Neither POM-C nor POM-N contents were different between treatments that had perennial pastures in the rotation. However, Rc-LPast had 18 and 19 % greater POM-C and POM-N respectively than the average of Rc-Sy and Rc-Sy-Sg, (6.06 Mg C ha−1 and 0.48 Mg N ha−1, 0−15 cm depth). Meanwhile, the POM-C represented 23.6 % of TSOC in Rc-LPast, but in rotations that replaced pastures (Rc-Sy and Rc-Sy-Sg) represented only 20 %. For soils in temperate zones, under a stable rice-pasture rotation, there are intensification alternatives which preserved TSOC in the midterm. However, the reduction in the particulate fractions observed in the rice rotations that substituted perennial pastures with other crops, suggests that TSOC may be more vulnerable to losses in the long term.