Soil Organic Carbon and Nitrogen Content and Distribution in a Vertisol under Mixed Land-Use.
David Sotomayor-Ramírez, Univ of Puerto Rico - Agronomy and Soils Dept, PO Box 9030, Mayagüez, PR 00681, Yusmary Espinoza, Instituto Nacional de Investigaciones Agrícolas (INIA), Centro Nacional de Investigaciones Agropecuarias (CENIAP), Maracay, Venezuela, Veronica Acosta-Martinez, USDA-ARS, Cropping Systems Research Laboratory, 3810 4th St., Lubbock, TX 79415, and Luis Pérez-Alegría, Univ of Puerto Rico, Agriculture and Biosistems Engineering Dept, Mayagüez, PR 00681.
Soils can play a significant role in the amounts of Carbon (C) sequestered from the atmosphere which can mitigate increased atmospheric CO2. The amounts of C and nitrogen (N) stored in soil is the net result of inputs and outputs, which will vary due to inherent soil properties that impart protection to organic matter, soil management, and land-use. There is scant information that describes the functioning of Vertisols in the tropics with regards to C and N storage capability and distribution, mineralization, and biological indicators of ecosystem health as affected by land-use. A Vertisol (Fine, smectitic, isohyperthermic Typic Haplusterts) in the southwestern part of Puerto Rico under cultivation, forest and pasture was sampled using intact cores to four depths (0-15, 15-30, 30-60, 60-100 cm). The forest area consisted of separate replicated plots of pure stands of Eucaliptus robusta or Leucaena leucocephala. The improved pasture areas had mixed species of Urochloa spp., Digitaria eriantha, Cynodon nlemfuensis and Pennisetum purpureum which were cropped for haylage. The agricultural land areas had a history of intensive horticultural and agronomic crop production that were maintained by moldboard and chisel tillage alternated with fallow rotations. All sites were under the specified land uses for at least 20 years and the agriculture and pasture sites received fertilizer N. At 0-15 cm depth, the Total Organic C (TOC) and Total Organic N (TON) concentrations were in the order of: Leucaena ³ Eucalyptus ³ Pasture > Agriculture. While TOC concentrations decreased with depth, there was an accumulation of TON at the deeper depth intervals, and was more prevalent for sites that received N fertilizer (Pasture and Agriculture). The TOC contents to 1 m depth were 22.8, 18.6, 17.2, and 13.0 kg C/m2 for Leucaena, Eucalyptus, Pasture, and Agriculture, respectively. The TON contents to 1 m depth were highest for Pasture with values of 3.07 kg N/m2 and were similar among the other land uses with values of 2.60, 2.50, and 2.46 kg N/m2 for Leucaena, Eucalyptus, and Agriculture, respectively. The Microbial Biomass Carbon (MBC) and Nitrogen (MBN) were in the order of Pasture > Leucaena = Eucalyptus > Agriculture at 0-15 cm. Land use effects on MBC and MBN were not as apparent at 15-30 cm which were significantly lower than at 0-15 cm. The proportions of MBC and MBN comprising TOC and TON were nearly 1.0% in Pasture and less than 0.50% in the Leucaena, Eucalyptus, and Agriculture. In general, ß-glucosidase, ß-glucosaminidase, and total arylsulfatase activities decreased with depth. The ß-glucosidase activity at 0-15 cm depth was in the order of: Leucaena = Eucalyptus > Pasture > Agriculture, while ß-glucosaminidase activity was in the order of: Eucalyptus > Leucaena = Pasture > Agriculture. The results suggest that more cellulose and similar biochemical substrates are being mineralized in soils under Leucaena and Eucalyptus, while more chitin-like products are being degraded in soils under Eucalyptus. Total arylsulfatase activity was similar in Pasture and forest sites, which were significantly higher than Agriculture. The results obtained are in agreement with previous studies in other regions reporting that cropping Vertisols will reduce C contents, nutrient cycling potential, and organic matter turnover in comparison with undisturbed counterparts under forested N-fixing tress, non N-fixing trees, and Pasture. In the region studied, proliferation of Leucaena as volunteer species is probably a better option for recuperating resting or abandoned cropland as it will result in greater C accumulation than in soils under pasture. Soils under Pasture had greater proportion of their C and N allocated to labile forms such as the soil microbial biomass, and the greater TON content points to either increased N preservation or reduced mineralization, than in the other land uses.