Characteristics and controlling factors of yellow soil organic carbon mineralization in different fertilization treatments under constantand variable temperature regimes

Objectives Soil organic carbon (SOC) mineralization is an important process of carbon cycle in terrestrial ecosystems, and temperature changes significantly affect the characteristics of SOC mineralization. We investigated the SOC mineralization in response to constant and variable temperatures under different long-term fertilization management, so as to provide a theoretical basis for mitigating the impact of agricultural production on climate change through optimized farmland carbon management.

Methods The soil samples were collected from the yellow soil long-term fertilization experiment at Guizhou Academy of Agricultural Sciences, with the following four treatments: no fertilizer (CK), chemical fertilizer (NPK), organic fertilizer (M), and combined organic and inorganic fertilizer (NPKM). The soil samples were incubated at constant temperature (15℃), and variable temperature (the incubation temperature changed from 10℃→15℃→20℃→15℃→10℃ at interval of 6 hours, and the cycle repeated every 24 hours) for 32 days. The CO₂ effluxes were determined by NaOH absorption method, and soil physicochemical and biological properties were analyzed, in order to elucidate the characteristics of SOC mineralization and its main driving factors under different temperature modes.

Results Under constant temperature, compared with CK treatment (201 mg/kg), the cumulative CO₂ emission significantly increased by 30.6% and 13.3% in M and NPKM treatments (262 and 228 mg/kg, respectively), and decreased by 12.7% in NPK treatment (175 mg/kg). Under variable temperature condition, cumulative CO₂ emission significantly increased by 30% in the M treatment relative to the CK treatment, while there was no significant difference among the CK, NPK, and NPKM treatments. Compared to constant temperature condition, cumulative CO₂ emission increased by 16.2% and 25.6% for the CK and NPK treatments, respectively, under variable temperature condition. However, there was little difference in cumulative CO₂ emission under constant and variable temperature conditions for the M and NPKM treatments. The cumulative CO₂ emissions were significantly and positively correlated with soil pH, SOC, total nitrogen, and microbial biomass carbon (MBC) and nitrogen (MBN) among fertilization treatments under both constant and variable temperature conditions. The changes in CO₂ emission induced by temperature modes were negatively correlated with soil MBC, MBN, microbial respiratory quotient, and enzyme activities related to carbon, nitrogen and phosphorus cycling.

Conclusions Long-term application of organic fertilizer could significantly increase SOC mineralization by increasing substrate availability and microbial biomass, and mitigate the positive effects of variable temperature on SOC mineralization via changing soil microbial respiratory quotient and enzyme activities related to carbon, nitrogen, and phosphorous cycling.