ABSTRACT.
Land use impacts on physical-based soil organic matter fractions on three hillside Ferrasols in Mexico

Francisco Matus1*, Claudia Hidalgo2, Carlos Monreal3, Isabel Estrada2, Mariela Fuentes2, and Jorge Etchevers2
 

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.

Keywords: Soil C and N, sand-size, SOM saturation, Ultrasonic dispersion.
1Universidad de la Frontera, Departamento de Ciencias Químicas, P.O. Box 54-D, Temuco, Chile. Current Address: Agriculture and Agri-Food Canada, Room 4046, 960 Carling Avenue, Ottawa, Ontario, K1A 0C6, Canada. *Corresponding author (Francisco.Matus@agr.gc.ca).
2Colegio de Posgraduados, Campus Montecillo, Carretera México-Texcoco km 36.5, Montecillo, Texcoco 56230, Estado de México, México.
3Agriculture and Agri-Food Canada, Room 4046, 960 Carling Avenue, Ottawa, Ontario, K1A 0C6, Canada.