Abstract
ABSTRACT The replacement of natural vegetation by crop systems directly impacts the soil organic matter fractions. The objective of this study was to evaluate the total organic carbon (TOC) and nitrogen (TN) contents in different fractions of the soil organic matter (SOM) of an Oxisol of the Brazilian semiarid region under different irrigated crops and different soil management systems. Seven treatments were evaluated, which consisted of two soil management systems (no-till and conventional tillage) and three crops (maize, sunflower and sorghum), using as reference the soil under a native forest (NF). The summer crops preceded common bean crops in the autumn-winter. The total organic carbon content, total nitrogen, carbon content in humic substances and their constituents (fulvic acids, humic acids and humin) and labile, non-labile and water-soluble carbon contents were evaluated two years and three months after the experiment implementation to determine the carbon lability (L) lability index (LI), partitioning index (CPI) and management index (CMI). The greatest carbon, nitrogen and organic matter contents in the soil surface layer (0.00-0.05 m) were found in crops under no-till system (NTS), especially maize. The crops under NTS presented greater carbon content in humic substances than the conventional tillage system (CTS) ones in the layer 0.05-0.10 m. The crops under NTS presented greater sustainability in the Brazilian semiarid region compared with those under CTS, as shown by their higher CMI in the soil surface layer.
Highlights
Assessments on changes in the amount of carbon (C) in the fractions of the soil organic matter (SOM) allows to evaluate the possible impacts of soil management in agricultural systems (SILVA et al, 2011)
The reduction in total organic carbon (TOC) and total nitrogen (TN) in the no-till system (NTS) was due to the NTS lower implementation time, since the carbon accumulation process can take 10 to 15 years to be significant (GUARESCHI; PEREIRA; PERIN, 2013)
The amount of plant residues deposited on the soil surface by the sunflower (15.64 Mg ha-1), sorghum (16.39 Mg ha-1) and maize (16.90 Mg ha-1) were greater than the amounts deposited by the native forest (NF) (14.50 Mg ha-1) (SALES et al, 2016)
Summary
Assessments on changes in the amount of carbon (C) in the fractions of the soil organic matter (SOM) allows to evaluate the possible impacts of soil management in agricultural systems (SILVA et al, 2011). The labile SOM fraction is ephemeral, with a high rate of decomposition, releasing nutrients to plants by mineralization, as well as energy and C to soil organisms. This fraction consists mainly of water-soluble carbon (WSC) and labile carbon (LC), which are very sensitive to changes in land use (SCHIAVO et al, 2011). Humic substances are the most assessed soil partition in studies evaluating the impacts of soil management systems on the SOM (ROSSI et al, 2012; CAMPOS et al, 2013; GUARESCHI; PEREIRA; PERIN, 2013)
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