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Straw incorporation, crop rotation and organic fertilizer applications have been proposed to counter the negative effects of straw burning, inorganic N fertilizer application and intensive agriculture practices for wheat production in the state of Sonora (México). A laboratory study was done to investigate how these alternative agriculture practices applied for 9 years affected carbon dioxide (CO 2 ), nitrous oxide (N 2 O) and nitrogen (N 2 ) emissions and inorganic N in soil. Emission of CO 2 decreased 1.2 times in soil burned compared to soil where residue was incorporated, but emissions of N 2 increased 4.1 times and the increase in inorganic N or the N mineralization rate 1.4 times. Including Sesbania spp. in the crop rotation reduced N mineralization rate 1.2 times when residue was burned, but increased it 1.5 times when it was incorporated compared to clean fallow or the cultivation of maize. Fertilizing soil with urea increased the N mineralization rate 4.9 times and the N 2 O emission 2 times while chicken manure increased it 11.4 times and 3.7 times, respectively, compared to the unamended soil when residue was burned. It was found that burning of crop residue increased N mineralization and N 2 emissions, but decreased microbial activity. Addition of chicken manure increased emissions of N 2 O and CO 2 and concentrations of NO 3 − .

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The effects of cultivation in hillside (> 30% slope) on soil C and N stocks after forest clearance and subsequent cultivation in new re-growth forest sites not always induce rapid soil organic matter (SOM) loss in subtropical areas. In the present study we evaluated the sensitivity to SOM changes of material floatable in water (Fw), coarse sand (> 250 µm), fine sand (50-250 µm), silt (2-50 µm) and clay (0-2 µm) fractions by the impact of cultivation of Mexican Ferrasols. This research also determined the relative degree of C and N saturation in the silt and clay size particles. The soil physical fractions were obtained after ultrasonic vibration and sedimentation. Soil organic matter stock (0-20 cm depth) in cultivated sites ranged from 54 to 146 Mg C ha-1 and from 3.9 to 7.1 Mg N ha-1 and in the forest soil 46-94 Mg C ha-1 and 3.7-5.1 Mg N ha-1. The relative distribution of C and N was lower in the coarse sand and highest in the silt fraction; the latter paralleled the changes of whole soil C and N. Coarse sand was the most sensitive fraction responding to cultivation, thus representing a diagnostic pool to assess the SOM shifts under cultivation. Half of our studied soil showed a low degree of C and N saturation in the silt and clay fraction indicating potential for accumulation of C and N, while the other half were well above the saturation limit.