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The effects of nitrogen fertilizer and tillage systems on soil organic carbon (SOC) storage have been tested in many field experiments worldwide. The published results of this research are here compiled for evaluation of the impact of management practices on carbon sequestration. Paired data from 137 sites with varying nitrogen rates and 161 sites with contrasting tillage systems were included. Nitrogen fertilizer increased SOC but only when crop residues were returned to the soil; a multiple regression model accounted for just over half the variance (R 2 ¼ 0.56, P ¼ 0.001). The model included as independent variables: cumulative nitrogen fertilizer rate; rainfall; temperature; soil texture; and a cropping intensity index, calculated as a combination of the number of crops per year and percentage of corn in the rotation. Carbon sequestration increased as more nitrogen was applied to the system, and as rainfall or cropping intensity increased. At sites with higher mean temperatures and also in fine textured soils, carbon sequestration decreased. When the carbon costs of production, transportation and application of fertilizer are subtracted from the carbon sequestration predicted by the model, it appears that nitrogen fertilizer-use in tropical regions results in no additional carbon sequestration, whereas in temperate climates, it appears to promote net carbon sequestration. No differences in SOC were found between reduced till (chisel, disc, and sweep till) and no-till, whereas conventional tillage (mouldboard plough, disc plough) was associated with less SOC. The accumulation of SOC under conservation tillage (reduced and no till) was an S-shape time dependent process, which reached a steady state after 25 -30 years, but this relationship only accounted for 26% of the variance. Averaging out SOC differences in all the experiments under conservation tillage, there was an increase of 2.1 t C ha 21 over ploughing. However, when only those cases that had apparently reached equilibrium were included (all no till vs. conventional tillage comparisons from temperate regions), mean SOC increased by approximately 12 t C ha 21 . This estimate is larger than others previously reported. Carbon sequestration under conservation tillage was not significantly related to climate, soil texture or rotation.

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Agriculture is facing a major challenge nowadays: to increase crop production for food and energy while preserving ecosystem functioning and soil quality. Argentine Pampas is one of the main world producers of crops and one of the main adopters of conservation agriculture. Changes in soil chemical and physical properties of Pampas soils due to different tillage systems have been deeply studied. Still, not much evidence has been reported on the effects of agricultural practices on Pampas soil microbiomes. The aim of our study was to investigate the effects of agricultural land use on community structure, composition and metabolic profiles on soil microbiomes of Argentine Pampas. We also compared the effects associated to conventional practices with the effects of no-tillage systems. Our results confirmed the impact on microbiome structure and composition due to agricultural practices. The phyla Verrucomicrobia, Plactomycetes, Actinobacteria, and Chloroflexi were more abundant in non cultivated soils while Gemmatimonadetes, Nitrospirae and WS3 were more abundant in cultivated soils. Effects on metabolic metagenomic profiles were also observed. The relative abundance of genes assigned to transcription, protein modification, nucleotide transport and metabolism, wall and membrane biogenesis and intracellular trafficking and secretion were higher in cultivated fertilized soils than in non cultivated soils. We also observed significant differences in microbiome structure and taxonomic composition between soils under conventional and no- tillage systems. Overall, our results suggest that agronomical land use and the type of tillage system have induced microbiomes to shift their life-history strategies. Microbiomes of cultivated fertilized soils (i.e. higher nutrient amendment) presented tendencies to copiotrophy while microbiomes of non cultivated homogenous soils appeared to have a more oligotrophic life-style. Additionally, we propose that conventional tillage systems may promote copiotrophy more than no-tillage systems by decreasing soil organic matter stability and therefore increasing nutrient availability.

The labile component of soil organic matter (SOM) plays an important role in short‐term nutrient turnover. Our objectives were (i) to establish the relationships between carbon in soil density fractions with carbon mineralization and the microbial biomass under contrasting conditions, (ii) to compare the goodness of fit of different mathematical models to describe carbon mineralization, and (iii) to evaluate the relationships of the SOM pools and the mineralization parameters estimated by the best kinetic model. Twenty‐eight soil samples were collected from a fine, illitic, thermic Typic Argiudoll localized in Argentina. These samples differed in the soil management (pasture and agriculture), tillage systems (chisel tillage, plough tillage, and no‐tillage), crop rotation, or depths. Microbial biomass was highly correlated with total carbon and carbon in the SOM light density fraction (density < 1.59 g mL−1) but less strongly correlated to medium (density 1.59–2.0 g mL−1) and heavy (density > 2.0 g mL−1) soil fractions. Carbon in the soil light fraction was strongly related to the carbon mineralized at 10 and 160 d of incubation. The exponential and hyperbolic models showed a good description of the mineralization data (r2 > 0.982). The application of models which considered two organic matter pools could not describe the mineralization of some samples. The hyperbolic model estimated higher potentially carbon mineralizable pools (C0) and semidecomposition time periods than the exponential one. The C0 estimated by the exponential model were similar to the carbon content in the soil light fraction. This soil organic component seemed to be the driving variable of microbial activity and a good predictor of soil potential carbon mineralization.

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