Organic fractions: Stratification under soil tillage

Quality and Quantity of Organic Fractions as Affected by Soil Depth in an Argiudoll under Till and No-till Systems

Aims: The aim of this study was to evaluate the long-term effect of tillage systems on the quantity and quality of organic carbon fractions at different soil layers.

Study Design: The experimental design was a split plot with three blocks. The long-term effects (25 years) of conventional- (CT) and no-tillage (NT) systems on a Tipic Argiudoll was sampled at 0-5, 5-10, 10-15 and 15-20 cm soil depth.

Place and Duration of Study: The field experiment was carried out at Tornquist (38° 07’ 06” S - 62°02’ 17” O) and soil sampling was performed during wheat seeding (June 2011).

Methodology: Total soil organic carbon (SOC) content and the following fractions were determined: Coarse particulate (POCc, 105-2000 µm), fine particulate (POCf, 53-105 µm) and mineral-associated (MOC, 0-53 µm) carbon fractions; humic (HA) and fulvic (FA) acids; and total (CHt) and soluble (CHs) carbohydrates. The main physico-chemical properties of HA and FA were analyzed using both FT-IR and fluorescence spectroscopies.

Results: After 25 years, total SOC at the 0-20 cm depth was 9% higher in no-tilled than in tilled soils. The POCf was the SOM fraction that turned out to be the most sensitive to tillage effects. The POCc:POCf:MOC ratio at 0-20 cm was similar for NT (3:14:82) and CT (5:10:84); however, differences were found across soil depths. Tilled soils showed higher aromaticity, starting by CH-degradation, in more superficial soil layers.

Conclusion: The no-tillage system presented a different pattern which can be related to distribution of crop residues and conditions for humification along the soil depth.

Tillage system; soil organic carbon; chemical and physical fractionation.

Galantini, J.A.; M. Duval; J.M. Martinez; V. Mora; R. Baigorri & J.M. García-Mina. 2016. Quality and quantity of organic fractions as affected by soil depth in an argiudoll under till and no-till systems. International Journal of Plant & Soil Science 10 (5) - doi:10.9734/IJPSS/2016/25205

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Soil organic fractions: Soil quality

Soil organic fractions content and quality under wheat production systems in the semiarid Pampas

Soil organic matter (SOM) is the key factor in the production systems, and the knowledge of their fractions dynamic and equilibrium is important to reach high productivity and agriculture sustainability. The effect on organic fractions distribution and quality of the following production systems were studied: continuous wheat (TT), wheat–natural grass each year (TP) and wheat – legume (TL). Labile fractions, physically separated (particulate SOM, MOP) and chemically separated (fulvic acid, AF), showed high sensibility to crop sequence. Continuous wheat affected labile and recalcitrant organic fractions contents. The tillage increase caused SOM decrease, changes in labile to recalcitrant fraction ratios, nutrient losses (N and S) and modified humic acid (AH) structure. The AH from cultivated soils showed higher aromatic groups content and lower aliphatic and N compounds groups than AH from reference soil.

Organic fractions, Production systems, Semiarid region

Galantini J.A. 2001. Contenido y calidad de las fracciones orgánicas del suelo bajo rotaciones con trigo en la región semiárida pampeana. Revista de Investigaciones Agropecuarias (RIA-INTA) 30 (1) 125-146.

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Cover Crops in the Production System

Winter cover crops in soybean monoculture: Effects on soil organic carbon and its fractions

The current agricultural production systems in the Pampas Region have been significantly simplified by cultivating large land areas under no tillage (NT), where soybean is the predominant crop. These systems with long periods of fall-winter fallow and poor annual input of carbon (C) into the soil lead to soil degradation, thereby affecting physical and chemical properties. A 6-year cover crop study was carried out on a Typic Argiudoll under NT in the south of Santa Fe, Argentina. Various winter species were used as cover crops: wheat (W), oat (O), vetch (V), an oat + vetch mixture (O + V) and a control (Ct) treatment without a cover crop. We examined the influence of cover crops on the following soil organic C-fractions: coarse particulate organic carbon (POCc), fine particulate organic carbon (POCf) and mineral-associated organic carbon (MOC) from 2008 to 2011. Aboveground carbon input by the cover crops was related to the June to October rainfalls. In general, the W and O treatments supplied a higher amount of C to the soil; these gramineous species produced 22 and 86% more biomass than O + V and V. The water cost of including cover crops ranged from 13 to 93 mm compared with Ct. However, this water-use did not affect soybean yields. On average, gramineous species (pure stand or mixture) supplied more than 3.0 Mg C ha^-1 year^-1 to the soil, whereas V supplied less than 2.0 Mg C ha^-1 year^-1. Increase in the mean annual C-input by residues into the soil (cover crop + soybean) explained most SOC variation (R² = 0.61; p < 0.05). This relationship was more evident with labile soil organic fractions, both for POCc (R² = 0.91; p < 0.001) and POCc + POCf (R² = 0.81; p < 0.001). The stratification ratios of SOC (SI, 0-5: 10-20 cm) reflected differences among treatments, where >2.0 for W; 1.7 for O, O + V and V, and <1.5 for Ct. Soil physical fractionation by particle size showed that cover crops affected the most dynamic fraction directly associated with residue input (POCc) at 0-5 and 5-10 cm. At 0-5 cm, the effects were observed in the most transformed fractions (MOC and POCf) 4 years after the experiment started, whereas at 0-20 cm, differences in the labile fractions (POCc and POCf) were found at the end of the experiment (6 years). Although C-input by the cover crops fueled decomposition of labile soil organic fractions, concentration of surface SOC and its associated fractions (POCc, POCf and MOC) was modified after 6 years. This effect became noticeable during the third year when the plots under cover crops showed a higher SI than the traditional fallow.

Organic fractions, Carbon balance, carbon stratification index

Duval M., J.A. Galantini, Julia E. Capurro, J.M. Martinez, F.M. López. 2016. Winter cover crops in soybean monoculture: effects on soil organic carbon and its fractions. Soil & Tillage Research 161:95-105 doi:10.1016/j.still.2016.04.006

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