Objectives To investigate the effects of organic material application on the distribution of soil aggregates and the content of organic carbon and microbial necromass carbonin cotton fields of arid regions.

Methods A six-year field experiment was conducted in Shawan City, a major cotton-producing area in northern Xinjiang. Three treatments were established: sole chemical fertilizer (CF); CF combined with 40 kg/hm² acetic acid (AA); and CF combined with 15000 kg/hm² cattle manure (CM). Cattle manure was applied as a basal dressing, while acetic acid was applied via drip irrigation in six split applications. Soil samples were collected from the 0−20 cm and 20−40 cm layers at the mature stage of cotton. We measured soil organic carbon (SOC) content, the distribution of aggregates of different sizes, and microbial necromass carbon (MNC) content within these aggregates.

Results Compared with the CF treatment, the application of acetic acid and cattle manure significantly increased the proportion of macroaggregates (>2 mm) while decreasing the proportions of microaggregates and silt-and-clay particles. The mass proportion of macroaggregates ranged from 20.92% to 25.95%. Both amendments enhanced SOC and MNC contents. In the 0−20 cm soil layer, SOC increased by 43.81% and 55.65% under AA and CM, respectively, while MNC increased by 15.82% and 29.95%, respectively; the increases with cattle manure were significantly higher than those with acetic acid. In the 20−40 cm soil layer, cattle manure significantly increased SOC and MNC by 43.22% and 36.61%, respectively, whereas acetic acid had no significant effect. Fungal necromass carbon (FNC) constituted 65.95%−79.99% of total MNC, with bacterial necromass carbon (BNC) accounting for the remainder (20.01%−34.05%). The increase in FNC was greater than that in BNC, and cattle manure induced a larger increase in FNC than acetic acid. The enhancement of FNC by both acetic acid and cattle manure was most pronounced in the silt-and-clay fraction, whereas the increase in BNC was greatest in microaggregates. Cattle manure promoted macroaggregate formation and the accumulation of MNC within aggregates; notably, the increases in SOC and MNC in the 0−20 cm layer were 35.94% and 6.35% higher, respectively, than those in the 20−40 cm layer.

Conclusions Combined application of chemical fertilizer with acetic acid or cattle manure significantly increased the contents of soil organic carbon and total microbial-necromass carbonin oasis cotton-field soil, raised the proportion of macro-aggregates, and reduced the proportions of micro-aggregates and silt-clay fractions. The increase magnitude of soil organic carbon in each aggregate fraction followed the order: micro-aggregates > small aggregates > macro-aggregates > silt-clay fractions. The increment of fungal-necromass carbonwas greater in silt-clay fractions, whereas the increment of bacterial-necromass carbonwas higher in micro-aggregates. Compared with acetic acid, cattle manure was more conducive to the accumulation of microbial-residue carbon, especially fungal-residue carbon, and its effects generally extended to the 20−40 cm soil depth.