Effects of substitution of chemical fertilizer by organic fertilizer on maize growth and field carbon emission in dry farming area of Longzhong, Gansu Province

Objectives Replacing a certain ratio of chemical fertilizers with organic fertilizer effectively reduces the total chemical fertilizer input. Here, we investigated the effects of replacing different ratios of chemical fertilizers with organic sources on maize growth and carbon emission in dry farming areas of central Gansu Province.

Methods A field experiment was conducted in 2019 and 2020 in Lanchou City. Keeping an equal total N input of 200 kg/hm², the five organic fertilizer N substitution rates were 0 (T1), 50.0% (T2), 37.5% (T3), 25.0% (T4), and 12.5% (T5), and there was a no fertilizer control (T6). Leaf area index and dry matter were measured at jointing stage, trumpeting, filling, and maturity, and yield was measured at maturity. Soil pH, soil organic carbon, and total nitrogen content were measured in different soil layers. CO₂ fluxes were monitored every 15 days after sowing until harvest.

Results Throughout the maize reproductive period, leaf area index, growth rate, and grain yield were in the order T1≈T5≈T3 > T4 > T2 > T6. Soil pH was not different (P>0.05) in 0–5, 5–10, and 10–30 cm soil layers between T3 and T1 after harvest. The highest soil organic matter content was recorded at the 0–5 cm layer. The organic matter content of T3 in 0–5 cm soil (P<0.05) was 34.5%, 9.9%, and 38.8% higher than in T1, T4 and T5, respectively. The organic matter of T3 was not different (P>0.05) from T1 in 5‒10 cm soil. In contrast, there was no difference (P>0.05) in the organic matter content among T2, T3, and T4. T3 (P<0.05) increased soil organic matter by 31.4% compared to T1 and 26.1% compared to T5 in 10–30 cm layer. The highest total nitrogen content was recorded in the 5–10 cm soil layer; total N in T3 significantly decreased by 23.6% compared to T1, and the differences among treatments of different proportions of substitution were not significant (P>0.05). Compared to T6, all fertilizer treatments increased total carbon emissions throughout the maize reproductive period. Notably, T1, T3, and T5 had lower total carbon emissions among all fertilizer treatments and were not (P>0.05) different. All fertilizer treatments increased net ecosystem productivity (NEP) and soil carbon balance (SCB) compared to T6, with the average SCB of 2019 and 2020 under T3 having a higher value than other treatments. The maize fields were all sinks for atmospheric CO₂, with T3 showing the highest carbon sink effect. Correlation analysis showed that soil carbon emission was positively correlated with soil organic matter content (r=0.56*) and soil temperature (r=0.93**, P<0.05), and negatively correlated with soil pH (r= –0.77**), SCB was significantly correlated with leaf area index (r=0.75**), dry matter (r=0.75**), seed yield (r=0.93**). The carbon balance was positively correlated with total N (r=0.72**) and temperature (r=0.84**).

Conclusions Applying 200 kg/hm² of pure N in the dry farming area of Longzhong during water abundant years shows that the proportional replacement of chemical ftilizers with organic fertilizer could help improve maize yield. Replacing 37.5% of urea with organic fertilizer maintains a stable yield compared to urea alone, improves carbon sink, soil fertility and pH, but the total soil CO₂ emission is not significantly different. Thus, the appropriate proportional replacement of chemical fertilizer with organic fertilizer in the dry farming area of Longzhong benefits the full-film double-ridge furrow-sown maize.