Conservation agriculture for cassava and maize in Southwestern Amazon affects soil organic matter molecular-chemical composition and dynamics.

In Brazilian Southwestern Amazon, food security agriculture predominates in smallholder farms where cassava cultivation is usually under conventional tillage (CT). We investigated the impact of conservation agriculture on soil organic matter (SOM) quality and dynamics in an Acrisol (Acre) cultivated with cassava and maize within 13 years. Management systems (P-fertilization, liming, cover crops of legume and legume-grass consortia) under CT and no-tillage (NT) were compared. Samples from experiment and native forest were collected (6 layers within 1 m) in the 10^th and in the 13^th year of the experiment. Besides C and N contents, SOM was investigated by ¹³C isotopic signature, ¹³C NMR spectroscopy and n-alkanes determination. An effect of the conservation system on C concentration was detected only at the second sampling, evidencing the importance of the legume- grass consortia in contributing to C reservoir in tropical soils. However, a mobilization of the endogenous SOM was verified under CT management systems without a change of soil C content. Introduction of agriculture affected SOM composition in the first 5 cm promoting an increase of the O-alkyl and a decrease of alkyl C groups, an increase of microbial-derived n-alkanes and of the degradation of vegetal-derived n-alkanes. Within the timeframe of 10 years, the key factor on the SOM quality in this environment was the land use change rather than the management system that employed only legume as the cover crop. In contrast, the introduction of the legume-grass consortia improved soil C content under conservation management within 4 years.