Carbon stock and its compartments in a subtropical oxisol under long-term tillage and crop rotation systems

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 R2intensive crop rotation) on total, particulate and mineralassociated 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 (1) Part of the Ph.D. Thesis of the first author presented at the Soil Science Post-Graduation Program of the Federal University of Santa Maria, UFSM. Received for publication in February 2010 and approved in March 2011. (2) Professor, Federal Institute of Rio Grande do Sul, Ibirubá Campus, IFRS. 1111 Nelsi Fritsch Road, CEP 98200-000 Ibirubá (RS) Brazil. E-mail: ben-hur.campos@ibiruba.ifrs.edu.br (3) Associate Professor, Soil Department, UFSM. 1000 Roraima Ave., CEP 97105-900 Santa Maria (RS) Brazil. CNPq Scholarship. E-mail: tamado@smail.ufsm.br (4) Associate Professor, Soil Department, Federal University of Rio Grande do Sul, UFRGS. 7712 Bento Gonçalves Ave., CEP 91540-000 Porto Alegre (RS) Brazil. CNPq Scholarship. E-mail: cimelio.bayer@ufrgs.br (5) Researcher, Embrapa Swine and Poultry. Road BR 153 km 110, P.O. BOX 21, CEP 89700-000 Concórdia (SC) Brazil. CAPES/ PNPD Scholarship. E-mail: rodrigo.nicoloso@cnpsa.embrapa.br (6) Researcher, Foundation Center of Experimentation and Research, FUNDACEP. Professor, Cruz Alta University, UNICRUZ. Road RS 342 Km 149, CEP 98100-970 Cruz Alta (RS) Brazil. E-mail: jackson@fundacep.com.br Comissão 2.4 Química do solo 806 Ben-Hur Costa de Campos et al. R. Bras. Ci. Solo, 35:805-817, 2011 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.