The study was conducted at the Yanting Agro-Ecological Station of Purple Soil (31˚16ʹN, 105˚28ʹE, and 420 m altitude Southwest China), which is a research station of the Chinese Ecosystem Research Network (CERN) in Sichuan province. The experimental site has a moderate subtropical monsoon climate, with an annual average air temperature of 17.3˚C and mean precipitation of 824 mm, with rainfall mainly occurring during the summer rainy season [19] . The soil is classified as Eutric Regosols in the FAO Soil Taxonomy and Pup-Orthic-Entisols in the Chinese Soil Taxonomy [20] . It has a clay loam texture, a pH (H₂O: Soil of 2.5:1 w/w) of 8.22, a bulk density of 1.330 kg m⁻³, an organic C content of 8.75 g kg⁻¹ and a total N of 0.62 g kg⁻¹ [18] . The experimental plots were cropped conventionally with winter wheat (Triticum aestivum L.) from late October to May each year and then rotated with summer maize (Zea mays L.) from May through September. Organic fertilizers were applied with farmyard organic manure (fresh pig manure slurry) and wheat and maize residues (straw), while inorganic fertilizers were ammonium bicarbonate (NH₄HCO₃) supplied the required N, the calcium superphosphate (Ca(H₂PO₄)2H₂O) supplied the required P (90 kg P₂O₅ ha⁻¹) and potassium chloride (KCl) required K (36 kg K₂O ha⁻¹) were applied as basal fertilization for both wheat and maize. A yearly amount of N (280 kg N ha⁻¹), split into 130 kg N ha⁻¹ in the wheat season and 150 kg N ha⁻¹ in the maize growing season. A more comprehensive description of the site and field experiment could be found previously in [21] . Soil samples were obtained from experimental plots following a long-term application of organic amendments in field lysimeter plots (Size: 8 × 4 m²) spanning between 2007 and 2022. The soil was used for incubation experiments.

The experiment was set up with plots and laid out in a randomized complete block design with six treatments and three replicates. The six treatments were: (1) No amendment (CK), (2) conventional mineral fertilizers (NPK) (3) organic manure (OM) composed of pig slurry (4) only crop residues at N equivalent to 20% of applied N in treatment of NPK (CR), (5) fresh pig slurry at N equivalent to 40% mineral N plus 60% mineral N at total N rate equivalent within NPK treatment (i.e., OMNPK), and (6) crop residues at 20% equivalent plus 80% mineral N at total N same as in the NPK treatment (i.e., CRNPK).

Three cores of field moist soil samples (0 - 20 cm) were collected from each treatments plots after maize harvesting using a flat-bladed stainless-steel shovel. In each replicated plot, three separate soil samples weighing 300 g and 100 g were taken from every three replicates and homogenized to make a composite sample of 100 g to complete four replicates for each treatment for incubation experiment. Field-moist soil samples were brought to the laboratory, air dried at room temperature for 48 hours, crushed and sieved through a 2 mm mesh to remove large plant materials and stone fragments. Soil samples were kept in plastic bags and stored in a refrigerator at 4˚C until the start of incubation experiments. Soil samples (30.0 g on an oven dried-basis) were placed into 250 mL glass jars for two parallels incubation samples, where the first part was used for CO₂ emissions measurement to monitor C mineralization, and the second part for N mineralization ( $N{H}_4^{+}$-N and $N{O}_3^{-}$-N) analysis. Glass jars bottles were air-tightened and incubated in the dark under controlled conditions (70% water holding capacity and 25 ± 1˚C) [2] . Before measuring C and N mineralization, samples were moistened to 70% water holding capacity and pre-incubated in the dark at room temperature (25˚C) for one week. Pre-incubation was required to minimize any possible effects of sampling time (during late summer), soil temperature and water content lost during field soil sampling and air-drying time [22] and for reducing labile organic matter concentration and activating soil microbial communities [2] .

After pre-incubation, the CO₂ released during the incubation period was used to measure the rate of C mineralization in the soil. Distilled water was added to each glass jars using a micropipette to 70% water holding capacity (7.0 g H₂O/40 g soil). The incubation experiments involved eight treatments with four replicates. Incubation glass jars were weighed weekly, so as to compensate for water losses in the treatments. At the time of the CO₂ gas sampling, the 250 mL glass jars, which were typically covered with plastic wrap, were sealed using an airtight butyl rubber stopper perforated by centered Perspex tubes and flushed with 100% pure dioxygen gas before closure. All glass jars were incubated at 25˚C. Four hours after closure, four samples of 20 ml of the headspace gas were sampled with airtight plastic syringes fitted with three-way stopcocks connected to the glass jars to measure CO₂ concentrations [23] . After sampling, the gas samples were immediately analyzed for CO₂ concentration using a gas chromatograph (Agilent GC-7890B, HP 5890II, Hewlett-Packard, Palo Alto, California, USA) equipped with a flame ionization detector (FID) for CO₂ detection. The CO₂ emissions (FCO₂) and C mineralization (F) and were calculated following Equations (1) - (2), respectively:

$FC{O}_2=\frac{\rho *V*\Delta C*273}{m*\Delta t*\left(273+T\right)}$ (1)

$F=\frac{\rho *V*\Delta C*273}{m*\Delta t*\left(273+T\right)}\ast \frac{12}{44}$ (2)

where, F is the C mineralization (mg kg⁻¹ h⁻¹); FCO₂ is the CO₂ emissions(mg C-CO₂ kg⁻¹ h⁻¹); ρ is the CO₂ gas density (1.965 kg m⁻³); V is the glass jars empty space volume (m⁻³); ∆C is CO₂ concentration (ppm); m is oven-dried basis soil weight (kg); ∆t is incubation time (h); and T is the incubation temperature(˚C).

Conversely, nitrogen mineralization was determined in a parallel soil incubation experiment, where the soil samples (5 ± 0.5 g) were extracted with 25 ml of 0.5 M K₂SO₄ solution and the supernatants were filtered through Whatman filter membranes (0.45 µm) for soil ammonium nitrogen ( $N{H}_4^{+}$-N), and nitrate-nitrogen ( $N{O}_3^{-}$-N) measurements. Thereafter, soil $N{H}_4^{+}$-N, and $N{O}_3^{-}$-N contents in the filtrate were analyzed using a continuous flow autoanalyzer (model AA3, Bran + Luebbe, Norderstedt, Germany). The total carbon mineralized (TCm) and total nitrogen mineralized (TNm) were calculated as the cumulative C and N mineralized during the 13-week incubation period.

Soil samples were collected from different treatment plots and each soil sample (20.0 g) was fumigated with ethanol-free chloroform in a vacuum desiccator for 24 hours at room temperature in the dark. After complete fumigation, 5.0 ± 0.5 g of each soil sample was shaken (1 hr at 120 rpm) with 25 mL of 0.5 M K₂SO₄ solution and then filtered through a Whatman No. 5 filter paper. The filtrates were immediately refrigerated at −18˚C until analysis. At the analysis time, the filtrates were retrieved and left to deep freeze, after which the dissolved organic carbon (DOC), ammonia nitrogen ( $N{H}_4^{+}$-N) and nitrate nitrogen ( $N{O}_3^{-}$-N) in each sample were determined using a continuous flow autoanalyzer (model AA3, Bran + Luebbe, Norderstedt, Germany).

From soil C and N mineralization data, first-order kinetic single exponential model was used to determine the potentially mineralized C and N and the mineralization rate (k) constants following the methods used in numerous previous experiments [2] [24] as described in Equation (3):

$Ym=Yo\left(1-e^{\left(-kt\right)}\right)$ (3)

where, Ym (mg kg⁻¹) is the cumulative concentration of net C or N mineralized after time t; Yo (mg kg⁻¹) is the potentially mineralizable organic C or N pools at (t = o); t (weeks) is the time from the start of incubation experiment; k (week⁻¹) is the rate constant or potential turnover rate of soil labile C and N pools. The inverse of k value (1/k) was calculated and defined as the potential turnover time (week). To identify whether or how organic fertilization affects purplish soil organic C and N quality and availability, we calculated the initial potential rate of C or N mineralization (Yo × k, mg kg⁻¹ week⁻¹) following the methods suggested by Campbell et al. [25] and Saviozzi et al. [26] .

Analysis of variance (ANOVA) was used for statistical analysis of the effects of the six treatments on C and N kinetics parameters in SPSS 16.0 (Chicago, IL, United States). The relationships between the total C mineralized and total organic carbon and total N mineralized and total nitrogen were verified by linear functional regressions. Data are reported in four replicates as mean standard error (±SE). Significant differences in the mean of estimated parameters among treatments were verified by the least significant difference test (LSD), at 95% confidence level (P < 0.05). The data graphics were drawn in Sigma plot software (version 12.5, Systat, Inc, USA).

To understand the carbon mineralization kinetics of upland purplish soil under organic and inorganic treatments, data from incubation experiment were fitted using a single exponential model ( Figure 1 ) and carbon mineralization (Cm) kinetics parameters are shown in Table 1 . The best fit for Cm using the single exponential model, where higher regression coefficients were found (R² = 0.968, OM), (R² = 0.980, CR), (R² = 0.972, NPK), (R² = 0.940, OMNPK) and (R²= 0.946, CRNPK) ( Figure 1 ) and standard error of estimate (SEE) ranging from 583.8 to 1264.6 (data not shown). At the end of 13 weeks, the total C mineralized (TCm) was significantly lower in NPK treatment (10,439 mg kg⁻¹), and TCm was significantly higher in CRNPK treatment with 13787 mg kg^-1. NPK was not significant with CR (11,291 mg kg⁻¹) (P > 0.05) ( Table 1 ). Compared with NPK, TCm for organic amended treatments increased by 8 - 24% throughout the incubation period.

The Co concentration ranging from 15,050 to 17,213 mg kg⁻¹ among all treatments with an increase by 4 - 13% in organic amended treatments compared to inorganic fertilizers (NPK) ( Table 1 ). The Co and TCm showed a similar trend, but no significant difference was observed in Co between the treatments, whereas Co was greater than TCm in both organic amended and inorganic fertilizers treatments after 13 weeks of incubation as shown by Co/TCm ratios bigger than 1 ( Table 1 ), indicating that some of the C fraction was still available for further microbial mineralization.

Organic amended treatments Co/TCm decrease by 3 - 15% compared with the NPK treatment ( Table 1 ). The rate constant (kC) or decay rates for the labile C ranged from 0.0071 to 0.0099 week⁻¹ for the organic amended treatments and NPK treatment ( Table 1 ). Significant differences were observed in kC between NPK and CRNPK treatments (P < 0.05). The organic amended treatments kC increased by 6 - 29% compared with NPK treatment. The estimated potential turnover time (1/kC) ranging from 101.0 to 161.3 for the for the organic amended treatments and NPK treatment, with significant differences between organic amended treatments and NPK treatment (P < 0.05). The organic amended treatments 1/kC decreased by 7 - 36% compared with the NPK ( Table 1 ). The initial potential rate of C mineralization (Co × kC) ranging from 106 to 170 mg kg⁻¹, significantly varied between the organic amended treatments and NPK treatment. The organic amended treatments Co × kC increase by 10 - 37% compared to the NPK treatment ( Table 1 ). The results indicated the TCm, Co, Co × kC and Co/TCm increased while kC showed the most significant decrease in organic amended treatments over the entire 13 weeks period ( Table 1 ).

Mean ± SE: Means in columns followed by different lower cases letter are significantly different (LSD, P < 0.05) and means followed by the same lower cases letter are not significantly different (LSD, P < 0.05). TCm, total C mineralized; Co, potentially mineralizable C; k_C, rate constant for labile C; Co × k_C, initial potential rate of C; 1/k_C, potential C pool turnover. No fertilizer (CK, control), Pig manure (OM), crop residues (CR), inorganic fertilizers (NPK), combined pig manure and inorganic fertilizers (OMNPK), and combined crop residues and inorganic fertilizers (CRNPK).

The N mineralization (Nm) data were fitted using a single exponential model ( Figure 2 ) and the Nm kinetics parameters are shown in Table 2 . The fitted exponential model for N mineralization under all the treatments showed high regression coefficients, which were R² = 0.984, (NPK), R² = 0.981, (OM), R² = 0.986, (CR), R² = 0.983, (OMNPK) and R²= 0.984, (CRNPK) ( Figure 2 ) and low standard error of estimate (SEE) ranging from 24.1 to 31.7 (data not shown). The

total N mineralized (TNm) under all treatments are presented in Table 2 . TNm values ranged from 415 to 618 mg kg⁻¹, with the OM and CRNPK treatments having TMn values significantly than the NPK treatment ( Table 2 ).

The potentially mineralizable N (No) ranged from 753 mg kg⁻¹ to 947 mg kg⁻¹, with the difference between organic and inorganic treatments (NPK) (P < 0.05) ( Table 2 ). Although the No of organic amended treatments increase by 15 - 20% compared to NPK, significant differences were observed between the ratios of No/TNm, which ranged from 1.51 to 2.36 across all treatments (P < 0.05). Interestingly, the No/TNm ratio was greater than 1 ( Table 2 ), showing that TNm released was lower than No for all treatments, which indicated that N fractions was still present for more microbial mineralization.

The rate constant (kN) for the labile N was 0.0033 to 0.0065 week⁻¹ across the five treatments with significant difference between the treatments and higher kN was observed in the treatments OM, CR, and CRNPK treatments (P < 0.05). Moreover, the organic amended treatments kN increased by 3 - 27% compared to NPK treatment ( Table 2 ). The estimated (1/kN) indicated the slow turnover times of the labile N mineralization, varying from 154 to 303 for all treatments.

Significant difference was observed between the treatments in 1/kN (P < 0.05), and the organic amended soil 1/kN decrease by 3 - 35% compared to NPK treatment. More interestingly, a significant difference was observed in the initial potential rate of N mineralization (No × kN), ranging from 3.60 to 6.10 mg kg⁻¹ week⁻¹. The organic amended treatments (No × kN) increase by 18 - 52% compared to NPK ( Table 2 ). Among all the treatments, TNm, kN, and No × kN were greater in the organic amended treatments than in the NPK treatment, whereas the No/TNm and 1/kN showed a contrast trend ( Table 2 ). Furthermore, NPK treatment showed significantly higher TCm/TNm and Co/No ratios compared to the organic amended treatments. Consequently, the organic amended treatments TCm/TNm ratios decrease by 27% ( Table 2 ) and the Co/No decreased by 10 - 14% compared with NPK ( Table 2 ). Moreover, the total C mineralized (TCm) and soil total organic carbon were positively correlated (P < 0.0001) ( Figure 3 ) across all the treatments at the end of the 13-week incubation period. A positive linear relationship was found between total N mineralized and soil total nitrogen (TN) (P < 0.0001) ( Figure 3 ) across all treatments at the end of the incubation of 13 weeks.

Mean ± SE: Means in columns followed by different lower cases letter are significantly different (LSD, P < 0.05) and means followed by the same lower cases letter are not significantly different (LSD, P < 0.05). TNm, total mineralizable organic N; No, potentially mineralizable N; k_N, constant for labile N; 1/k_N, potential N pool turnover. No fertilizer (CK, control), Pig manure (OM), crop residues (CR), inorganic fertilizer (NPK), combined pig manure and inorganic fertilizers (OMNPK), and combined crop residues and inorganic fertilizers (CRNPK).