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Soil nutrient cycling, the distribution of soil aggregates, and their stability are directly influenced by soil texture. Different sizes of soil aggregates provide microhabitats for microorganisms and therefore influence soil carbon (C) and nitrogen (N) mineralization. The purpose of the present study was to assess the aggregate stability and dynamics of carbon and nitrogen in macroaggregate size fractions (1-8 mm) with different clay content from meadow soils. Surface soil samples (0-15 cm) were collected from 4- to 5-year-old forage crops. Four macroaggregate size classes were isolated by dry sieving and analyzed for their mass proportions: fine macroaggregates (FM) (less than 1 mm), medium-fine macroaggregates (MFM) (1-2 mm), mediumcoarse macroaggregates (MCM) (2-4 mm), and large-coarse macroaggregates (LCM) (4-8 mm). The dry mean weight diameter (MWD), organic carbon (OC), total nitrogen (TN), carbon and nitrogen of microbial biomass (C-MB, N-MB) were determined. CO2 emission and net nitrogen mineralized (NM) were measured after 14 weeks of incubation. The amounts of FM were significantly lower than those of intermediate macroaggregates (MCM and MFM) and decreased markedly with increasing clay content within soil macroaggregates. In general, the amounts of macroaggregate size fractions were lowest in soils with high clay content. MWD exhibited a significant correlation with particle size distribution, OC, and MB-C. OC, TN, MB-C, and MB-N contents within macroaggregates increased with decreasing macroaggregate size and increasing clay content of macroaggregate fractions. The CO2 emission and NM content increased with increasing macroaggregate size, indicating higher organic C and N mineralization activity in larger macroaggregates. Mineralization of OC was lowest in macroaggregate fractions with the highest clay content. We conclude that clay content can increase the protection of microbial biomass in meadow soils. Small macroaggregates tend to contain more recalcitrant organic matter compared to larger macroaggregates. [ABSTRACT FROM AUTHOR]

Deep soil carbon (>30 cm) is the primary pool of soil organic carbon (SOC). However, deep SOC is often not included in carbon inventories or management strategies, especially in North-Africa. The objective of the study was to determine the distribution and the main factors governing SOC storage along a representative toposequence in North-Eastern Algeria. The soil organic matter (SOM) was characterized by SOC content, carbon stocks, particle size distribution and mineralization potential. The amount and the dynamics of the deep SOC depended on the topographical position. In the mountain soils, most of the SOC stock (100 t C ha −1 ) was in the topsoil (0–30 cm) and associated with the >50 μm fractions (plant materials) whereas in the piedmont and alluvial plain soils, most of the SOC stock (168 t C ha −1 ) was accumulated in subsoil (30–100 cm) and associated with the <20 μm fractions (mineral fractions). The SOC in the subsoil contributed to SOC potential of mineralization and should not be underestimated, especially in the alluvial cultivated plains. This carbon should be considered as an important component in agro-ecological agriculture but it is also a potential source of CO 2 . [ABSTRACT FROM AUTHOR]