Improving soil organic carbon (SOC) levels is fundamental to enhancing cultivated land quality. Straw return has long been practiced as a common soil amendment strategy, but has limited efficiency in SOC accumulation. The conversion of straw into compact rod-shaped granules through crushing and compression enables high-intensity one-time straw input.

However, the effects of the conversion on SOC accumulation remain poorly understood. Besides, high-intensity granulated straw application may induce elemental imbalances (carbon excess with nutrient limitation) due to the high C/nutrient ratio of straw. Whether supplementary nutrient inputs could alleviate microbial nutrient limitation, enhance microbial anabolism, and consequently improve straw carbon accumulation remains an open question.

Recently, researchers from the Institute of Subtropical Agriculture of the Chinese Academy of Sciences systematically investigated the effects of granulated straw application at varying intensities (0, 30, 60, and 90 t/ha) and nutrient supplementation regimes (C/N ratios of 40 and 25) on SOC accumulation and underlying mechanisms. The findings were published in Journal of Integrative Agriculture and Soil Ecology Letters.

Researchers conducted the microcosm experiments in typical newly reclaimed upland soils and infertile paddy soils in subtropical regions. They found distinct carbon conversion patterns between soil types.

After one year of granulated straw application, the efficiency of straw carbon conversion to SOC in upland soils remained relatively stable across increasing application intensities (ranging from 30.8% to 37.5%). In contrast, paddy soils showed a decline from 60.0% to 38.3% with increasing straw input.

The increase in SOC content under high-intensity straw application in both soil types was primarily attributed to plant residue accumulation rather than microbial necromass. This was explained by the physical spatial isolation effect of straw granules, which limits microbial access to and utilization of carbon sources within the granule interior, thereby enhancing straw carbon accumulation in the form of plant residues.

Under 30, 60, and 90 t/ha granulated straw applications, maize yields in upland soils increased by 25.0%, 55.6%, and 83.3%, respectively, compared to controls (none application). In paddy soils, upland rice yields increased by 46.4%, 55.4%, and 64.3%, respectively.

Moreover, researchers found that nutrient supplementation slightly reduced SOC content in upland soils but modestly increased SOC content in paddy soils compared to pure granulated straw application. This was mainly because that nutrient supplementation enhanced microbial necromass carbon and mineral-associated organic carbon content.

Importantly, researchers showed that straw granulation combined with nutrient supplementation boosted crop yields, with maize and upland rice yields increasing by an additional 50.0% and 19.2%, respectively, compared to granulated straw alone.

"The findings show that under high temperature and humidity conditions typical of subtropical regions, granulated straw reduces microbial accessibility to the inside of straw carbon, improving annual straw carbon accumulation efficiency," said Prof. CHEN Xiangbi, the corresponding author of the studies. The study provides a rapid soil improvement way that simultaneously achieves soil carbon sequestration and crop yield enhancement in infertile agricultural soils.