Azospirillum spp. is a well known plant-growth-promoting rhizobacterium. Azospirillum-inoculated plants have shown to display enhanced lateral root and root hair development. These promoting effects have been attributed mainly to the production of hormone-like substances. Nitric oxide (NO) has recently been described to act as a signal molecule in the hormonal cascade leading to root formation. However, data on the possible role of NO in free-living diazotrophs associated to plant roots, is unavailable. In this work, NO production by Azospirillum brasilense Sp245 was detected by electron paramagnetic resonance (6.4 nmol. g-1 of bacteria) and confirmed by the NO-specific fluorescent probe 4,5-diaminofluorescein diacetate (DAF-2 DA). The observed green fluorescence was significantly diminished by the addition of the specific NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO). Azospirillum-inoculated and noninoculated tomato (Lycopersicon esculentum L.) roots were incubated with DAF-2 DA and examined by epifluorescence microscopy. Azospirillum-inoculated roots displayed higher fluorescence intensity which was located mainly at the vascular tissues and subepidermal cells of roots. The Azospirillum-mediated induction of lateral root formation (LRF) appears to be NO-dependent since it was completely blocked by treatment with cPTIO, whereas the addition of the NO donor sodium nitroprusside partially reverted the inhibitory effect of cPTIO. Overall, the results strongly support the participation of NO in the Azospirillum-promoted LRF in tomato seedlings.

Inclusion of pastures in a rotation may reverse the effects of conventional cropping and tillage on soil degradation. We hypothesized that crop‐pasture rotations could be defined with a minimum pasture component and a maximum conventional cropping component to accomplish sustainable management. Soil organic carbon (SOC), light‐fraction carbon (LFC), microbial biomass nitrogen (MBN), and aggregate stability index (ASI) were measured in the surface 0 to 0.15 m of a fine, mixed, thermic Typic Argiudoll with 2% slope under a long‐term crop rotation experiment at Balcarce, Argentina. Treatments were continuous cropping and crop‐pasture (50:50 and 75:25) rotations. All seedbeds were prepared with conventional tillage. All soil quality indicators decreased with cropping and increased with pasture. Data for each variable were fitted to an exponential model to describe their variation in time. Soil organic C decreased 4.4 g kg−1 in 6 to 7 yr under cropping and rose to the original level (37.2 g kg−1) after 3 to 4 yr under pasture. Light‐fraction C, MBN, and ASI fell 0.9 g kg−1, 39.0 mg kg−1, and 43.9, respectively (97–100% of the estimated decline), after 7 yr under cropping, while they recovered to the values at the beginning of the cropping period (1.8 g kg−1, 99.5 mg kg−1, and 76.7, respectively) in a few years under pasture. Rotations including a maximum of 7 yr of conventional cropping alternated with a minimum of 3 yr of pasture would maintain soil properties within acceptable limits and meet the goals of sustainable agriculture under conditions similar to this experiment.

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