Soil organic matter quality, quantity and distribution under different tillage systems

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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Extended abstract 

Soil organic matter (SOM) is made up of compounds with diverse biochemical properties and varying degrees of association to the mineral matrix. These organic compounds are a continuum of materials whose cycling rate ranges from weeks to millennia. This continuum can be conceptually divided into discrete kinetic pools. A large number of SOM fractionation and characterization techniques have been developed to gain an insight into the stabilization and destabilization mechanisms that underlie SOM dynamics in the short and long term (Galantini & Suñer, 2008).

Most studies used to apply only one type of extraction procedure, i.e. either chemical or physical separation. Nevertheless, the effects of cropping practices can be better understood if both methods are combined.

Our hypothesis is that under different tillage systems soil moisture and carbon input change both the quantity and quality of labile soil organic fractions at different depth levels.

The objective of this study was to evaluate the effect of 25 years of tillage (conventional tillage and no-tillage) on the quantity and quality of physically and chemically separated soil organic carbon fractions, and to determine the main physico-chemical changes of humic substances at different soil depths.

Methodology:

The field experiment was carried out at Tornquist (38º 07’ 06” S - 62º 02’ 17” O), Argentina, and soil sampling was performed during wheat seeding (June 2011) at 0-5, 5-10, 10-15 and 15-20 cm depth.

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

Results and conclusions:

After 25 years under two tillage systems, total SOC at the 0-20 cm depth was 9% higher in no-tilled than in tilled soil. The POCf (53 to 105 µm) was the SOM fraction that turned out to be the most sensitive to tillage effects, as others studies (Duval et al., 2013; Galantini et al., 2014; Duval et al., 2016).

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.

Residue cover (NT) and soil tillage mixing (CT) produced contrasting conditions for organic matter transformation and humification. The surface NT soil (0-5 cm) had highest C input with better conditions for its transformation than deeper layer. Under CT, however, the tilled layer (0-10 to 0-15 cm) presented similar residue input and condition for its transformation, which was slow down probably by its driest condition.

The POC_c:POC_f: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.

Soil tillage system produced different trends in the molecular functionality of HA fraction (Galantini & Rosell, 1997; Rosell et al., 2000). The conventional tillage system showed higher molecular humification, starting by CH-degradation, at more superficial soil depths. The no-tillage system, however, presented changes which are more difficult to interpret in the context of a simple humification process where microbial activity, oxygen availability, and/or labile carbon content are governed by soil depth. In fact, some results indicated that, apart from processes linked to SOM aerobic degradation, other relevant factors might be involved, mainly residue cover effect on SOM dynamics and humification across depths.

Distribution of crop residues and soil moisture modified concentration of soil organic fractions as well as the characteristics of humic and, particularly, fulvic acids.