Does grazing intensity affects organic matter of a Subtropical Oxisol under integrated crop-livestock system?

Prof. Deborah Dick1, Msc Gabriel Garcia1, Doctor Ibanor Anghinoni1, Doctor Paulo Cesar Carvalho1

1Federal University Of Rio Grande  Do Sul, Porto Alegre, Brazil

The objective of this work was to investigate the influence of  grazing intensity on the SOM composition and C stocks in a Brazilian subtropical soil under Integrated Crop-livestock System (ICLS) after 15 years The experiment was conducted in a typical Red Oxisol in randomized blocks (three replications) with different grazing intensities according to pasture height: 10 (P10), 20 (P20) and 40 cm (P40) cm). Litter and soil samples were collected within 1 m depth from treatments and from a forest area. C and N contents and isotopic signature δ¹³C were determined. Chemical composition was investigated by ¹³C NMR CP/MAS spectroscopy and n-alkane distribution was determined by GC-MS. Grazing intensity did not affect C stocks and greater values were observed in forest soil.  δ¹³C values were around -22 ‰ at 0-5 cm for forest and P40 soils and increased from -17 to -14 ‰ along the profile. We concluded that the contribution of native pasture that consisted mainly of Paspalum notatum (C3) to the SOM composition below 5 cm was not affected by soil use even after 40 years. Residues of secondary forest and cultivated pasture (C4 plants) contributed to SOM composition only in the first 5 cm. Chemical composition of SOM did not differ relevantly among the treatments and, in average, it was made up by 16.7 to 19.8 % alkyl C, 52.3 to 54.7 O-alkyl C, 15.4 to 17 % aromatic C and 11.6 to 12.5 % Carboxylic C. In all analyzed environments, average chain length of n-alkanes decreased with depth indicating an increase of microorganism derived alkane. The ratio Rs/l increased with depth as expected but it was greater under P40 when compared to P10. It seems that lower intensity grazing promotes enrichment of microorganism derived compounds.


Prof. Dr. Deborah Pinheiro Dick graduated in Chemistry at the Federal University of Rio Grande do Sul, Brazil, where she also got a Master degree in Soil Science (1986). In 1993 she obtained her Doctorate in Soil Science at the Technical University of Munich, Germany. In !986, Deborah started as Assistant Professor at the Federal University of Rio Grande do Sul and currently she is Senior Professor at the Physical-Chemistry Department of the same University. Her main research topics are 1) soil organic matter composition and interactions in subtropical and tropical soils under different soil uses (forest, pasture) and management (no-tillage and vegetation burning), 2) Use and obtaining of organic matter and humic fractions from biochar, composts and mineral coal for agronomic uses; 3) Organic matter interactions with pesticides and minerals in tropical and subtropical environments; accelerated degradation of pesticides in adapted soils from subtropical and tropical environments: Dissipation mechanisms of pesticides applied as slow release formulations; 4) Composting of agronomic wastes; fertilizer potential, green-house gases emission and effect of biochar. Since 2012, she is board member of the IHSS.

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