Sto­r­a­ge of or­ga­nic car­bon in ag­gre­ga­te and den­si­ty frac­tions of sil­ty soils un­der dif­fe­rent ty­pes of land use


Geoderma 128:63-79 (2005)


The type of land use and soil cultivation are important factors controlling organic carbon storage in soils and they may also change the relative importance of different mechanisms of soil organic matter stabilization. Our objectives were: i) to quantify the soil organic carbon (SOC) and nitrogen (N) storage in silty soils under wheat, maize, grassland, and spruce, ii) to determine the SOC and N storage in water-stable aggregates of different size (< 53 µm, 53-250 µm, 250-1000 µm, 1000-2000 µm, > 2000 µm) and in density fractions (Mineral-associated soil organic matter > 2 g cm-3 (Mineral-SOM), free particulate organic matter < 1.6 g cm-3 (free POM), light occluded particulate organic matter < 1.6 g cm-3 (occluded POM<1.6) and dense occluded particulate organic matter 1.6 to 2.0 g cm-3 (occluded POM1.6-2.0)), and iii) to analyse the stability and turnover of these SOC fractions in the maize soil on the basis of the 13C values. Total SOC stocks down to a depth of 60 cm and including the humus layer were larger at the spruce site (10.3 kg C m-2) as compared with the grassland, wheat and maize (7 to 8 C kg m-2). However, SOC stocks in the mineral soil were smaller in the forest soil than in the agricultural soils. In the arable soils, the aggregate fractions 53-250 µm and 250-1000 µm were the most abundant size fractions, whereas aggregates > 1000 µm were most abundant in the grassland and forest soil. The SOC concentration and the C/N ratio were greater for macroaggregates (> 250 µm) than microaggregates (< 250 µm) in the field and grassland soils. At the maize site the percentage of maize-derived C was smallest in the fraction < 53 µm with 24% and steadily increased with increasing aggregate size to 47% in the fraction > 1000 µm.
The major part (86-91%) of the SOC was associated with the heavy mineral fraction at the grassland, maize and wheat site. In the A horizon of the spruce stand, the particulate organic matter accounted for 52% of the total SOC content. The C/N ratios of density fractions decreased in the order free POM<1.6  > occluded POM > Mineral-SOM for all soils and depths.
The mean age of organic carbon in the water-stable aggregates in the Ap horizon of the maize site increased with decreasing aggregate size from 35 yr (> 1000µm) to 86 yr (< 53 µm). For the density fractions the order was free POM (22 yr) < dense occluded POM1.6-2.0 (49 yr) < Mineral-SOM (63 yr) < light occluded POM<1.6 (83 yr). The results showed that the type of land use affected the distribution pattern of litter carbon to functionally different SOM pools and that increasing SOC concentrations were closely associated with the formation of macroaggregates.

Keywords: Aggregation, land use, nitrogen storage, organic carbon turnover, organic matter fractions, organic matter storage, 13C natural abundance.