Variation of soil organic matter depends on light-fraction organic matter under long-term monocropping of different crops

Cultivating crops influences soil organic matter (SOM), but the effect of different crops remains unclear, particularly under long-term monocropping. The objective of this study was to identify how different crops influence the content and chemical structures of SOM under long-term monocropping. Here, soils were sampled (0–20 cm) under 27-year soybean and maize monocropping and separated into different physical fractions. The content and chemical structures of SOM in all fractions were determined. SOM contents were higher under soybean than maize in bulk soil and macroaggregates and their light-fractions instead of microaggregates and silt and clay. The difference in SOM chemical structure was observed in aggregates and density fractions rather than bulk soils and supported by the result of principal component analysis. The proportion of O-alkyl C in macro- and microaggregates and all free light fractions and that of aromatic C in mineral-associated fractions were higher, while that of carbonyl C was lower under maize than soybean. These results demonstrated that different crops monocropping influences the content and chemical structures of SOM, and the variations were mainly in the light-fraction SOM and highlight a higher sensitivity of physical fractions than bulk soil to different crops.

Effects of Leafy Biomass of Some Agroforestry Tree Species on Light Fraction Organic Matter Content of Soil

A study on light fraction organic matter was carried out on soil where leafy biomass of Enterolobium cyclocarpum (ENCY), Treculia africana (TRAF), Anogeissus leiocarpus (ANLE), Gliricidia sepium (GLSE), Leuceana leucocephala (LELE) were used as mulch. Soil samples were collected in each plot at two auger points 0-15 cm and 15-30 cm and then homogenized for laboratory analysis. ANLE was observed to have significant higher values (6.37, and 6.52) on soil pH, and at day 28 on soil pH respectively. Light fraction organic carbon (LFC) also experienced significant higher value (3.48 g/kg) at day 28 in ANLE leafy biomass. However, LELE had the significant highest values (0.37 g/kg, and 1.44 g/kg) on light fraction nitrogen (LFN), and at day 42 on LFN respectively, while, microbial population count (MP) was highest in ENCY (183516667 cfu/g), and (726666667 cfu/g) at day 14 respectively. It is therefore concluded that the leafy biomass investigated had improving positive effects on light fraction organic matter. Keywords: Organic matter, Soil content, Light fraction, Leafy biomass, Tree species

Densimetric fractionation of organic matter in an agricultural chronosequence in no-till areas in the Cerrado region, Brazil

Mineralization of organic residues deposited on the soil surface in a no-till system (NT) maintains a continuous flow of different forms of carbon (C), which might interfere with densimetric fractions of soil organic matter (SOM). Currently, there are few studies on variations in these fractions in NT areas with different deployment times in the Cerrado region. Thus, the objective of this study was to evaluate the total C, nitrogen (N), and 13C in the soil, as well as to quantify the mass and contents of C, N, and 13C in the light and heavy fractions of SOM extracted using different solutions (water, sodium iodide [NaI], and sodium polytungstate [SPT]) in areas of Cerrado, pastures, and NT with different deployment times with Distroferric Red Latosol soil. The study areas are located in Montividiu, Goiás (GO), Brazil: (1) pasture of Brachiaria decumbens (PA); (2) NT with three years of deployment with soybean in summer and fallow instead of a second crop (NT3); (3) NT with 15 years of deployment with soybean in the summer and maize/sorghum as the second crop (NT15); and (4) NT with 20 years of deployment with soybean in the summer and maize as the second crop (NT20). These areas were compared to an area of native Cerrado sensu stricto (CE). The NT according to the deployment time showed an increase in the contents of C (0-0.05 m) and N (0-0.2 m) in the soil. The origin of C in the soil of the NT areas is associated with C4 plants; however, in up to 0.2 m of the soil profile, the 13C contents reduced according to NT deployment years. Extraction of light-fraction organic matter (LFOM) with SPT better represents this SOM fraction quantitatively and qualitatively than extraction of LFOM with water and NaI. This pattern is evident because of a greater consistency in the mass, C, N, and 13C contents of the LFOM extracted with SPT among the evaluated areas and the lower C content in residual particulate organic matter among the extractants.