The study area is located in the sugarcane planting area of Tianyang District, Baise City, Guangxi Zhuang Autonomous Region (106° 22 ′ 14 ″ E~107° 8 ′ 32 ″, 23° 28 ′ 20 ″~24° 6 ′ 55 ″ N). The study area experiences a subtropical monsoon climate characterized by an annual average temperature of 21.9 °C and an annual precipitation of 1156 mm. Experimental soils classified as yellow brown exhibited the following surface layer (0–20 cm) properties: total nitrogen (0.47 g kg^-1), total phosphorus content (0.25 g kg^-1), total potassium content (6.67 g kg^-1), organic matter content (16.74 g kg^-1), and pH (5.74). The investigated sugarcane cultivar Guitang 46 displays medium-large stem morphology with superior ratooning ability, coupled with high productivity and sucrose accumulation potential. The growth period of sugarcane is from February to December, the planting row spacing is 0.9~1.2 meters, and the theoretical yield is about 90 tons per ha.

Experimental procedures involved selecting the upper stem portions from newly cultivated sugarcane of Guitang 46 in February 2022. These stems were precision-sectioned into uniform double-bud segments using surgical blades, with only intact segments employed for planting. The experimental layout comprised 208 buds in total arranged in 52-bud rows under randomized complete block design. Eight nitrogen-potassium ratio treatments were implemented, each replicated three times. The plot area was 31.2 m² (6.5 m × 4.8 m), and nitrogen fertilizer (urea, containing N 46%), potassium fertilizer (potassium chloride, containing K₂O 60%) and phosphorus fertilizer (calcium superphosphate, containing P₂O₅ 12%) were applied in a ditch at one time. The specific treatment and fertilization statuses are shown in Table 1 .

Soil samples: During the December harvest period, topsoil (0–20 cm depth) was collected from each plot using a five-point sampling method, with five soil cores were homogenized per plot. Following natural air-drying and mechanical grinding, samples were fractionated through a 20-mesh sieve (enzyme activity analysis) and an 80-mesh sieve (physicochemical characterization).

Plant samples: Two representative sugarcane plants per plot were separated into stems and leaves for fresh weight measurement. Samples underwent an initial 105°C desiccation for 30 minutes followed by 65°C drying to constant mass, then ground and sieved through 20-mesh for subsequent analysis.

Soil total nitrogen content exhibited a significant positive correlation with nitrogen fertilizer application rates ( Figure 1a ). The high nitrogen and high potassium treatment (N₃K₂) achieved the maximum nitrogen accumulation (1.07 g/kg), representing a 130% increase compared to the control (P < 0.01), while concurrently reducing soil pH to 4.57 ( Figure 1i ). The content of ammonium nitrogen reached the peak (49.81 mg kg^-1) in the medium nitrogen and medium potassium treatment (N₂K₁), while the content of nitrate nitrogen was the highest (29.65 mg kg^-1) in the low nitrogen and high potassium treatment (N₁K₂), indicating that appropriate nitrogen application promoted nitrate nitrogen accumulation ( Figure 1d ). Phosphorus fractions demonstrated minimal variation across treatments, except for total phosphorus, which significantly increased to 1.38 g kg^-1 under high potassium application (N₂K₂) ( Figures 1b, e ). The contents of total potassium and available potassium exhibited dose-dependent responses, with N₃K₂ treatment achieving the maximum total potassium accumulation (16.44 g kg^-1), representing a 147% increase relative to the control ( Figures 1c, e ). Soil organic matter peaked at 49.99 g kg^-1 in the high nitrogen and medium potassium treatment ( Figure 1h ). Nitrogen activation analysis revealed significant negative correlations between application rates and activation coefficients for both ammonium nitrogen and nitrate nitrogen (P < 0.05). The high nitrogen treatment (N₃K₂) exhibited minimal ammonium nitrogen activation (4.54%) ( Figures 1f, g ), indicating nitrogen transformation inhibition under high nitrogen input. The maximum phosphorus activation (5.52%) occurred in the high nitrogen and high potassium treatment. Table 3 data indicated optimal nitrogen use efficiency (38.47%) under low nitrogen-high potassium conditions, representing a 18.41% increase over conventional fertilization. Excessive nitrogen application (N₃K₁) reduced a nitrogen use efficiency to 22.34%, indicating that high nitrogen input reduced the nutrient use efficiency.

Fertilization regimes significantly influenced soil enzyme activities. The high nitrogen and medium potassium treatment (N₃K₁) yielded the highest catalase activity (0.490 mL g^-1), exceeding the blank (N₀K₀) and conventional (N_rK_r) controls by 64.59% and 56.36%, respectively ( Figure 2a ). Cellulase activity reached peak (0.055 mg g^-1) under the medium nitrogen and medium potassium (N₂K₁) conditions, whereas the high nitrogen and high potassium treatment (N₃K₂) simultaneously enhanced both invertase (3.06 mg g^-1) and urease (0.98 mg g^-1) activities, corresponding to 75.56% and 6.46-fold increases than the control ( Figures 2b, c ). Acid phosphatase activity demonstrated equivalent elevation (2.20 mg g^-1) in the medium nitrogen and medium potassium (N₂K₁), and high nitrogen and high potassium treatments (N₃K₂), which was significantly higher than other treatments (P < 0.05). Correlation analysis showed that urease was significantly positively correlated with total nitrogen (r = 0.89, P < 0.01) and nitrate nitrogen (r = 0.78, P < 0.01) ( Figure 3 ).

The contents of total nitrogen, total phosphorus and total potassium in sugarcane leaves reached the peak (0.99%, 0.59% and 1.89%) in the treatments of medium nitrogen and high potassium (N₂K₂), high nitrogen and high potassium (N₃K₂) and low nitrogen and high potassium (N₁K₂), respectively ( Figure 4a ). The low nitrogen and high potassium treatment (N₁K₂) produced medium stem concentrations of total nitrogen (0.92%) and total potassium (0.81%), whereas the high nitrogen and medium potassium treatment (N₃K₁) achieved optimal total phosphorus content (0.16%) ( Figures 4b, c ). Shoot accumulation of nitrogen and phosphorus peaked under the treatment of high nitrogen and high potassium (N₃K₂) conditions at 288.65kg ha^-1 and 75.03kg ha^-1, respectively ( Figure 4d ). The accumulation of potassium was the largest in the treatment of low nitrogen and high potassium (N₁K₂), ( Figure 4e ) reaching 276.66kg ha^-1. The yield results showed that the high nitrogen and high potassium treatment (N₃K₂) had the highest yield (91.05 t ha^-1), increasing by 55.25% and 32.21% compared with the control and conventional fertilization, respectively ( Figure 4e ). Although the low nitrogen and high potassium treatment (N₁K₂) showed lower yield (71.85 t ha^-1), it demonstrated superior nitrogen use efficiency (38.47%), representing a 21.41% improvement over conventional fertilization ( Table 3 ). Strong positive correlations emerged between yield and both total nitrogen (r = 0.85, P < 0.01) and total potassium (r = 0.89, P < 0.01) ( Figure 3 ).

The total amount of base ions in the low nitrogen and high potassium treatment (N₁K₂) was the highest (0.49 Cmol kg^-1), which was significantly positively correlated with total potassium (r = 0.79, P < 0.01) and available potassium (r = 0.75, P < 0.01) ( Figure 5a ). The treatment of high nitrogen and high potassium (N₃K₂) showed the minimal carbon-to-nitrogen ratio (RCN = 20.91), though still exceeding the national average (11.9), indicating reduced organic matter decomposition rates. Carbon-to-phosphorus ratios (RCP) varied substantially. Control plots displaying the highest values (39.28) and the medium nitrogen and high potassium treatment (N₂K₂) achieving the lowest levels (17.00), which was lower than the national average (61), reflecting enhanced phosphorus utilization efficiency ( Figure 5b ). Nitrogen-to-phosphorus ratios (RNP) peaked in the control treatment (1.89), remaining markedly below the national threshold (10), confirming prevalent nitrogen limitation throughout the study area ( Figure 5c ).

Principal component analysis of nine soil fertility indexes (total nitrogen, nitrate nitrogen, ammonium nitrogen, total phosphorus, available phosphorus, total potassium, available potassium, organic matter, and pH) yielded a single significant component ( Table 3 ), with the Kaiser-Meyer-Olkin measure verifying sampling adequacy (KMO = 0.821). This component accounted for 72.997% of the total variance. According to the results of principal component analysis of soil quality indicators ( Table 4 ), Principal component 1 (72.997%) had the highest loads of total phosphorus (0.907), organic matter (0.902) and available phosphorus (0.969), reflecting nutrient availability. Soil quality assessment revealed significant treatment effects, with the high nitrogen and high potassium treatment (N₃K₂) demonstrating optimal performance (47.72), closely followed by the medium nitrogen and high potassium treatment (N₂K₂, 46.92). The unfertilized control treatment exhibited markedly inferior results (-2.38).

Analysis of soil comprehensive fertility index across fertilization treatments ( Table 5 ) revealed treatment 7 (N₂K₂) achieved the highest index values (0.57), significantly exceeding conventional fertilization (N_rK_r, 0.53) and reaching the medium fertility level. The unfertilized control treatment (N₀K₀) recorded the lowest values (0.34), demonstrating the efficacy of optimized nutrient management in enhancing soil fertility. Significant positive correlations emerged between IFI and both total nitrogen (r = 0.78, P < 0.01) and base ions contents (r = 0.81, P < 0.01) ( Figure 3 ).

Correlation analysis ( Figure 3 ) revealed strong positive associations between crop yield and both urease activity (r = 0.83, P < 0.01) and total potassium content (r = 0.89, P < 0.01) ( Figure 3 ). The total amount of base ions exhibited a significant correlation with total potassium (r = 0.79, P < 0.01) and available potassium (r = 0.75, P < 0.01), confirming its role in promoting nutrient availability. Path analysis identified urease as having the greatest direct influence on yield (1.977), while total potassium exerted substantial indirect effects mediated through urease activity (0.702). The direct effect of nitrate nitrogen on yield was negative (-0.85), but the indirect effect through urease was positive (1.84) ( Table 6 ).

The structural model showed that the direct effect of soil nutrients on yield (0.780) was dominant, and indirectly affected yield through enzyme activity (0.702) ( Figures 6a, b ). The base ions (exchangeable potassium, sodium, calcium, magnesium) were significantly positively correlated with soil nutrient content (r = 0.803), confirming their fertility-enhancing properties. Principal component analysis ( Figure 6c ) demonstrated that the first axis (68.41%) predominantly reflected positive associations between RCN, catalase activity, and pH, while the second axis (23.72%) captured yield-nutrient relationships. Yield was strongly positively correlated with urease, total phosphorus and available phosphorus, contrasting with the negative association observed with pH. Structural equation and redundancy analysis revealed that available phosphorus led to yield formation through direct supply and promotion of base ions activation, while the high nitrogen and medium potassium treatment (N₃K₁) increased nutrient accumulation in a short term, but aggravated nitrate nitrogen accumulation and imbalance of carbon-to-nitrogen ratio.

Grey correlation analysis ( Table 7 ) identified total potassium as exhibiting the strongest yield correlation (0.911), followed sequentially by invertase activity, organic matter content, and exchangeable magnesium, all demonstrating coefficients exceeding 0.9. Nitrate nitrogen showed the weakest association with crop yield among the measured parameters.

This study found that the high nitrogen treatment (N₃K₂) significantly elevated urease activity (0.98 mg g^-1) and nitrate nitrogen content (26.64 mg kg^-1) while inducing soil acidification (pH 4.57), consistent with reported nitrogen-mediated acidification effects (Liu et al., 2024). Contrastingly, the low nitrogen and high potassium treatment (N₁K₂) enhanced nitrogen use efficiency (38.47%) through stimulated urease activity (0.96 mg g^-1) and invertase activity (2.11 mg g^-1), supporting the enzyme-mediated nitrogen transformation mechanism (Holatko et al., 2025). Notedly, phosphorus dynamics revealed differential responses: the medium nitrogen and high potassium treatment (N₂K₂) accumulated total phosphorus (1.38 g kg^-1) but limited available phosphorus increment (3.2%), potentially attributable to iron/aluminum oxides fixation in acidic conditions (Wen et al., 2023).

The high nitrogen and high potassium treatment (N₃K₂) maximized yield (91.05 t ha^-1) through enhanced nitrogen accumulation (288.65 kg ha^-1), yet demonstrated limited nitrogen use efficiency (24.95%), which was consistent with the luxury absorption theory proposed by Bronson (2021). Conversely, the low nitrogen and high potassium treatment (N₁K₂) elevated the nitrogen use efficiency to 38.47% by optimizing the carbon-nitrogen ratio (RCN = 54.23), corroborating potassium-mediated nitrogen assimilation mechanisms reported by Kumari et al. (2023). Path analysis showed that the direct effect of urease on yield (1.977) was significantly higher than other indicators. Grey correlation analysis confirmed the predominant influence of total potassium on yield (r = 0.911), corroborating the potassium-mediated root development-nutrient acquisition regulatory mechanism described by Nabati et al. (2025).

Principal component analysis revealed optimal soil fertility (IFI = 0.57) under the medium nitrogen and high potassium treatment (N₂K₂), concurrent with elevated total amount of base ions (0.48 Cmol/kg) surpassing other treatments. These findings support the established base ion-fertility correlation documented by Ordoñez et al. 2010. The ecological stoichiometric characteristics showed that the soil in the study area is in the states of nitrogen limitation (RNP = 0.42-1.89) and phosphorus sufficiency (RCP = 17.00-22.60), which was lower than the RNP threshold theory proposed by Koerselman and Meuleman (1996). Elevated tropical temperatures and humidity in India likely enhance microbial activity, resulting in accelerated nitrogen mineralization rates (higher RNP values). In this study, observed RNP values remained substantially lower (8.2-12.5) than those reported for Brazilian sugarcane regions, characteristic of nitrogen-limited karst soils (Batjes, 1996). Similarly, reduced RCP values (17.00-22.60 compared to 48–62 in Florida) reflect pronounced phosphorus fixation in karst soils (as mentioned in section 3.1 of the document for acidic soil phosphorus fixation), but long-term excessive phosphorus application by farmers (such as 3.13 kg acre^-1 of superphosphate) improved effectiveness. Structural equation models revealed soil nutrient mediation of yield through enzymatic activity (total effect = 0.702), confirming the elemental coupling hypothesis proposed by Reiners (1986). It was worth noting that the carbon-nitrogen ratio imbalance (RCN = 20.91) caused by high nitrogen treatment was significantly higher than the global average (11.9), suggesting organic matter decomposition inhibition, aligning with Cleveland and Liptzin (2007) regarding RCN-mediated microbial community modulation. These findings offer region-specific implications for nutrient management in tropical/subtropical agricultural ecosystems.

The optimized N:K₂O ≈ 1:1.5 fertilization regime demonstrated dual benefits of sustaining sugarcane yield (82.35 t ha^-1) while preserving soil fertility (IFI = 0.57). This balanced approach maintained ammonium nitrogen activation above 1.5% and RCP below 25, effectively nutrient efficiency with environmental protection. Compared with the conventional recommendations (Delfim et al., 2025), the proposed scheme exhibited 21.41% greater nitrogen use efficiency and superior acidification mitigation (ΔpH = +0.34). However, exceeding 2.3% ammonium nitrogen activation in Zhanjiang, Guangdong, lateritic soils triggered notable nitrate accumulation risks (Xu et al., 2010). The activation coefficient of paddy soil in the Taihu Lake Lake basin > 0.8% is accompanied by nitrogen loss (Yang et al., 2004). In karst sugarcane areas, the 1.5% ammonium nitrogen activation threshold serves as a reliable environmental risk indicator, corresponding to nitrate accumulation below 50 mg kg^-1 (N₂K₂: 45.6 mg kg^-1; Figure 1b ) while maintaining strong yield-ammonium nitrogen correlation (r = 0.9) ( Table 6 ), thereby optimizing nutrient use efficiency. However, long-term high potassium input may lead to soil potassium accumulation (total potassium 16.44 g kg^-1). It was necessary to establish an accurate regulation model based on soil potassium activation coefficient in combination with the dynamic management strategy of potassium fertilizer proposed by Mahmood and Ahmed (2024). The proposed N:K₂O ≈ 1:1.5 in this study fertilization regime enhances both yield and potassium use efficiency during single growing seasons, but requires long-term evaluation due to inherent karst soil characteristics. Potassium fixation in such soils creates disparity between accumulation and effectiveness, while nitrogen nitrification and potassium anion loss pose a risk of exacerbating soil acidification. Implementation of long-term field trials incorporating crop rotation optimization and strategic lime application is recommended to achieve sustainable yield-soil health equilibrium.