Abstract
Soil C-CO2 emissions are sensitive indicators of management system impacts on soil organic matter (SOM). The main soil C-CO2 sources at the soil-plant interface are the decomposition of crop residues, SOM turnover, and respiration of roots and soil biota. The objectives of this study were to evaluate the impacts of tillage and cropping systems on long-term soil C-CO2 emissions and their relationship with carbon (C) mineralization of crop residues. A long-term experiment was conducted in a Red Oxisol in Cruz Alta, RS, Brazil, with subtropical climate Cfa (Köppen classification), mean annual precipitation of 1,774 mm and mean annual temperature of 19.2 ºC. Treatments consisted of two tillage systems: (a) conventional tillage (CT) and (b) no tillage (NT) in combination with three cropping systems: (a) R0- monoculture system (soybean/wheat), (b) R1- winter crop rotation (soybean/wheat/soybean/black oat), and (c) R2- intensive crop rotation (soybean/ black oat/soybean/black oat + common vetch/maize/oilseed radish/wheat). The soil C-CO2 efflux was measured every 14 days for two years (48 measurements), by trapping the CO2 in an alkaline solution. The soil gravimetric moisture in the 0-0.05 m layer was determined concomitantly with the C-CO2 efflux measurements. The crop residue C mineralization was evaluated with the mesh-bag method, with sampling 14, 28, 56, 84, 112, and 140 days after the beginning of the evaluation period for C measurements. Four C conservation indexes were used to assess the relation between C-CO2 efflux and soil C stock and its compartments. The crop residue C mineralization fit an exponential model in time. For black oat, wheat and maize residues, C mineralization was higher in CT than NT, while for soybean it was similar. Soil moisture was higher in NT than CT, mainly in the second year of evaluation. There was no difference in tillage systems for annual average C-CO2 emissions, but in some individual evaluations, differences between tillage systems were noticed for C-CO2 evolution. Soil C-CO2 effluxes followed a bi-modal pattern, with peaks in October/November and February/March. The highest emission was recorded in the summer and the lowest in the winter. The C-CO2 effluxes were weakly correlated to air temperature and not correlated to soil moisture. Based on the soil C conservation indexes investigated, NT associated to intensive crop rotation was more C conserving than CT with monoculture.
Highlights
The greenhouse effect is a natural phenomenon responsible for the partial retention of the incident solar radiation on the earth surface, maintaining the global mean temperature close to 15 °C (IPCC, 2001)
In Brazil, these latter are the major sources of greenhouse gas (GG) because of the high deforestation rates associated with frequent biomass burning and high soil organic matter (SOM) decomposition rates resulting of the soil mobilization under conventional tillage (Cerri & Cerri, 2007)
This study aimed to evaluate the effect of tillage systems on crop residue carbon mineralization of main cash and cover crops regularly used in Southern Brazil
Summary
The greenhouse effect is a natural phenomenon responsible for the partial retention of the incident solar radiation on the earth surface, maintaining the global mean temperature close to 15 °C (IPCC, 2001). The increase of greenhouse gas (GG) concentrations in the last century has amplified this natural effect and, raised the global mean temperature. The increase of GG concentrations has been associated to industry and transportation sectors due to fossil fuel consumption (IPCC, 2007). Changes in land use and agricultural activities have been considered important sources of GGs. In Brazil, these latter are the major sources of GGs because of the high deforestation rates associated with frequent biomass burning and high SOM decomposition rates resulting of the soil mobilization under conventional tillage (Cerri & Cerri, 2007). Possible consequences of climate changes for Brazil include yield loss in the major grain and cash crops (Siqueira et al, 2001; Streck, 2005)
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