Objectives “ Continuous cropping + cotton stalk returning + high nitrogen fertilizer input” is one of the main technical measures for cotton production in Xinjiang oasis. We studied the effects of reducing chemical fertilizer input on soil organic nitrogen transformation and availability for cotton, and further studied the effect of applying organic fertilizer to substitute the reduced chemical fertilizer input, aiming to provide basis for establishing efficient and cost effective fertilization scheme.

Methods A positioning fertilization experiment was carried out in Erlian Agricultural Test Site, Shihezi City, Xinjiang continuously in 2021 and 2022. The conventional nitrogen application rate (CF) and no nitrogen fertilizer (CK) were used as control, chemical fertilizer reduction treatments included reducing 8% (CF-8%), 16% (CF-16%) and 24% (CF-24%) of the fertilizer input, and organic substitution treatments were the organic fertilizer rate equaling to the reduction rate, including 8%OF, 16%OF, and 24%OF. The soil pH, electrical conductivity (EC), organic matter, and inorganic and active organic N content, the activities of enzymes related to nitrogen conversion were analyzed. Cotton yield and nitrogen use efficiency were investigated.

Results In 2021 and 2022, the amount of nitrogen fertilizer applied was reduced on the basis of CF, and there was no significant change in the content of particulate organic nitrogen (PON) and light fraction organic nitrogen (LFON) in 0−20 and 20−40 cm soil layers. The proportion of dissolved organic nitrogen (DON) and microbial biomass nitrogen (MBN) in total nitrogen (TN) in CF-16% treatment were increased by 14.79% and 19.88%, respectively, and the soil protease activity in 0−20 cm soil layer was also significantly enhanced. The microbial biomass nitrogen (MBN) content, soil nitrate reductase and nitrite reductase activity in 0−20 cm soil layer of CF-24% treatment were decreased by 10.78%, 16.88%, 6.25%, respectively, compared with CF. With the decrease of chemical fertilizer application, the content of NH₄⁺-N and NO₃^－-N accumulated in deep soil decreased, and the yield increase rate of CF-16% treatment was the highest compared with CF in nitrogen reduction treatments, reaching 6.59%. Compared with CF, 16%OF and 24%OF treatments significantly increased soil pH in 20−40 cm soil layer, organic matter content in 0−20 and 20−40 cm soil layer increased by 23.44% and 12.99%, 31.74% and 20.14%, respectively, and soil EC value decreased by 7.74% and 4.72%, 5.64% and 7.02%, respectively, which could improve the soil environment of cotton field. The content of particulate organic nitrogen and soluble organic nitrogen in 0−40 cm soil layer of 16%OF treatment increased by 24.01% and 29.15%, 18.95% and 55.32% on average compared with CF treatment in two years. The proportion of particulate organic nitrogen and soluble organic nitrogen in total nitrogen increased by 16.32% and 32.42% respectively, while the content of light fraction organic nitrogen, LFON/TN, MBN/TN and C/N in 24%OF treatment were the highest in the three organic substitution treatments. The effect of 16%OF treatment on nitrogen agronomic efficiency (NAE), nitrogen partial factor productivity (NPFP), nitrogen contribution rate (NCR), single boll weight and yield was better than that of 8%OF and 24%OF treatments, and the yield increase rate was 17.16%.

Conclusions Appropriate reduction of inorganic nitrogen fertilizer can reduce the accumulation of inorganic nitrogen in soil, improve the nitrogen use efficiency and yield of cotton, and CF-16% treatment performed the best. Replacing part of chemical fertilizer with organic fertilizer can increase the content of active organic nitrogen in soil and the activity of soil nitrogen invertase, increase the potential of soil nitrogen supply, improve the soil microenvironment of cotton field, increase the nitrogen use efficiency of cotton by reducing the content of NO₃⁻-N in 20−60 cm soil layer, and increase the single boll weight and seed cotton yield. 16%OF was the optimal treatment for this experiment, and the cotton yield increased by 17.16%.