Particle-size distribution and associated organic matter under different cropping systems and tillage practices in a semi arid environment.

The aim of this study was to evaluate the long-term impact of different management practice and tillage tools on :1) changes in soil carbon stock with different cropping systems and soil tillage managements; 2) aggregate size distribution and organic carbon concentration of each fraction; 3) organic matter composition and stability Methodology The research was carried out at the Pietranera farm, located in southern part of central Sicily (Italy) (37°32’74” N / 13°31’53” E; elevation 236 m; mean annual precipitation 481 mm; mean air temperature 19 °C) on a soil of a long term experiment. The soil is classified as a fine-clayey, calcareous, mixed, xeric Chromic Pelloxerert with a slope of 4%. The soil were sampled before the start of the experiment and had 471 g kg-1 clay, 225 g kg-1 silt, 304 g kg-1 sand, 18,1 g kg-1 SOC (soil organic carbon), 1,29 g kg-1 N, and a pH of 8,1. The twelve plots (twenty years long term trial) used in this test area follow two crops rotations, wheat-wheat (W) and wheat-bean (WB), with three different soil managements: 1) traditional (CT = conventional tillage) consisting of a ploughing at 30-35 cm depth; 2) dual layers tillage (DL) with a chisel at 40 cm and 15 cm depth; 3) no tillage (NT). Soil sample were taken at 0-20 cm depth, air dried and sieved at 2 mm. Wet aggregate-size fractions, with no chemical dispersion were isolated by mechanic shaking of 50 g air-dry 2 mm sieved sample of soil on a column of 2000, 1000, 500, 250, 75 and 25 μm sieves. After the physical fractionation, we distinguished three main fractions: 75-250 μm (large microggregates), 25-75 μm (small microaggregates), < 25 μm (silt and clay fraction). The relative distribution of nitrogen and organic carbon content as well as the 13C natural abundance were measured for these fractions. Cumulative carbon input of the cropping systems were calculated according to Kong et al. (2005). Results The carbon content in the different experimental plots does not vary greatly, ranging from 18,3 g kg-1 up to 21,2 g kg-1. The highest carbon content values were found under the wheat monocropping system with no-tillage (WNT) (21,2 g kg-1) and conventional tillage management (WCT) (20,8 g kg-1), respectively. Comparing the three different soil management techniques, NT shows a higher influence on organic carbon accumulation (20,4 g kg-1), than the CT and dual layer (DL) (19,7 g kg-1 and 18,3 g kg-1). The estimated cumulative C input, ranged from 6,5 Mg C ha-1 in WBDL to 7,0 Mg C ha-1 in WCT. Annual SOC sequestered is higher for W than WB (0,16 Mg C ha-1 yr-1 vs. 0,05 Mg C ha-1 yr-1, p<0,05). Among the different soil managements techniques the DL seems to be less effective in soil organic carbon protection. Regarding the different soil aggregates, the <25 μm fraction was the most abundant one (mean 688 g kg-1). Conversion from continuous wheat to wheat-bean rotation resulted in no real differences in the large microaggregates percentage (102 g kg-1). Long-term NT systems does not produce a better aggregation state. Comparing the C contribution of different aggregates to the total organic C amount, the following order was discerned: 0-25 μm > 25-75 μm > 75-250 μm > 250-2000 μm. The higher resistance to mineralisation processes of silt-clay associated organic matter is also explained by the higher concentration of N in this fraction (cf. Kleber et al., 2007) than in the bulk soil (average N = 1,30 g kg-1 and 1,17 g kg-1, respectively; p<0,05). Taking, however, the average C concentration in each fraction into the consideration, the 75-250 μm aggregate size show a C enrichment (23,9 g kg-1), while the corresponding values of the 25-75 μm and 0-25 μm fractions do not differ greatly from the bulk soil concentration (19,3 g kg-1 and 19,8 g kg-1 respectively). According to Gerzabek et. al. (2001), we compared C and mass distributions between size fractions and found that large microaggregates (75-250 μm) were enriched in organic C, whereas small microaggregates and silt and clay fraction showed no enrichment or depletion (0,98 and 0,99). The amount of annual sequestered SOC, is a consequence of the cumulative carbon input (fig.1), and, when compared to C the concentration in different soil size fractions, a trend of decreasing annual SOC sequestration with increasing soil size aggregates can be detected (fig.2). The importance of the < 25 μm fraction in characterising the bulk soil features is pointed out by the amount and quality of organic matter stored in small microaggregates. The strong correlation between the bulk SOC concentration and SOC concentration in the <25 μm fraction is only due to wheat residues, since no correlation between the bulk SOC and the SOC concentration in < 25 μm fraction was found under WB rotation (fig 3). Conclusions Fifteen years of conservative tillage and crop rotation did not result in significant differences in C accumulation or aggregate stability (the non-existing differences are not mentioned before in the text). Among the different tillage techniques, NT seems to improve the organic carbon content, mostly due to the contribution of the C stored in small microaggregates and the silt-clay fraction. 75-250 µm fraction show an enrichment of C (C/mass ratio = 1,20) The strong relationship between 13C values of the bulk soil and <25 μm fraction under wheat cropping systems demonstrates the predominant contribution of these crop residues to the composition of organic matter. The higher nitrogen concentration in soils samples under wheat, both in bulk and in < 25 μm fraction, suppose a greater stability of the associated organic matter (by N-bonding mechanisms to mineral surfaces). The comparison of our data with those of other long-term agricultural experiments sites suggests that cropping systems in Sicily have lower efficiency in sequestering C from added C inputs. References Gerzabek, M.H., Haberhauer, G., Kirchmann, H, 2001. Soil organic matter pools and carbon-13 natural abundances in particle-size fractions of a long-term agricultural field experiment receiving organic amendments. Soil Sci. Soc. Am. J. 65: 352-358. Kleber, M., Sollins, P., Sutton, R., 2007. A conceptual model of organo-mineral interactions in soils: Self-assembly of organic molecular fragments into zonal structures on mineral surfaces. Biogeochemistry, 85, 1, 9-24. Kong, A.Y.Y., Six, J., Bryant, D.C., Ford Denison, R., van Kessel, C., 2005. The relationship between carbon input, aggregation, and soil organic carbon stabilization in sustainable cropping systems. Soil Sci. Soc. Am. J. 69:1078–1085.