Abstract
Soil organic matter (SOM) plays a crucial role in soil quality and can act as an atmospheric C-CO2 sink under conservationist management systems. This study aimed to evaluate the long-term effects (19 years) of tillage (CT-conventional tillage and NT-no tillage) and crop rotations (R0-monoculture system, R1-winter crop rotation, and R2- intensive crop rotation) on total, particulate and mineral-associated organic carbon (C) stocks of an originally degraded Red Oxisol in Cruz Alta, RS, Southern Brazil. The climate is humid subtropical Cfa 2a (Köppen classification), the mean annual precipitation 1,774 mm and mean annual temperature 19.2 ºC. The plots were divided into four segments, of which each was sampled in the layers 0-0.05, 0.05-0.10, 0.10-0.20, and 0.20-0.30 m. Sampling was performed manually by opening small trenches. The SOM pools were determined by physical fractionation. Soil C stocks had a linear relationship with annual crop C inputs, regardless of the tillage systems. Thus, soil disturbance had a minor effect on SOM turnover. In the 0-0.30 m layer, soil C sequestration ranged from 0 to 0.51 Mg ha-1 yr-1, using the CT R0 treatment as base-line; crop rotation systems had more influence on soil stock C than tillage systems. The mean C sequestration rate of the cropping systems was 0.13 Mg ha-1 yr-1 higher in NT than CT. This result was associated to the higher C input by crops due to the improvement in soil quality under long-term no-tillage. The particulate C fraction was a sensitive indicator of soil management quality, while mineral-associated organic C was the main pool of atmospheric C fixed in this clayey Oxisol. The C retention in this stable SOM fraction accounts for 81 and 89 % of total C sequestration in the treatments NT R1 and NT R2, respectively, in relation to the same cropping systems under CT. The highest C management index was observed in NT R2, confirming the capacity of this soil management practice to improve the soil C stock qualitatively in relation to CT R0. The results highlighted the diversification of crop rotation with cover crops as a crucial strategy for atmospheric C-CO2 sequestration and SOM quality improvement in highly weathered subtropical Oxisols.
Highlights
Soil organic matter plays multiple roles in agricultural soils, acting as nutrient source for crops and increasing the cation exchange capacity, bioneutralization of xenobiotic compounds, soil structural stability, water retention, soil aeration, biologic activity, and biodiversity (Lal & Sanchez, 1992; Reeves, 1997)
This study aimed to evaluate soil C sequestration under tillage and cropping systems in a long-term (19 years) experiment and to assess the improvement in Soil organic matter (SOM) quality by measuring changes in particulate and mineral-associated organic C fractions in a subtropical Oxisol in Southern Brazil
Annual crop C inputs ranged from 3.70 to 6.20 Mg C ha-1 (Figure 1), which was similar to the range reported in other long-term experiments in southern Brazil (Amado et al, 2006; Bayer et al, 2009; Boddey et al, 2010)
Summary
Soil organic matter plays multiple roles in agricultural soils, acting as nutrient source for crops and increasing the cation exchange capacity, bioneutralization of xenobiotic compounds, soil structural stability, water retention, soil aeration, biologic activity, and biodiversity (Lal & Sanchez, 1992; Reeves, 1997). West & Post (2002) reported a global average C sequestration rate of 0.57±0.14 Mg ha-1 yr-1 with the conversion from tilled in untilled soils, and 0.20±0.12 Mg ha-1 yr-1 by improvement in the cropping systems. As one of the principal Brazilian strategies for greenhouse gas mitigation in COP15 Copenhague, 2009, the projected increase in NT area in Brazil of 12 Mha until 2020 could represent an offset of 8 % of the total national C-CO2 emissions. In this prediction, C sequestration in untilled Brazilian soils was estimated at 0.5 Mg ha-1 yr-1. This estimation has to be corroborated in long-term experiments in different regions of Brazil
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