# Spectrophotometric Determination of Biopolymers in Alabama Benchmark Soils
## 4 Discussion  
Soil organic matter (SOM) constitutes a major soil health, soil structure, and agriculture sustainability attribute [36]. Under any system of management, soil organic matter content eventually reaches an equilibrium due to biochemical recalcitrance, chemical stabilization, and physical protection from microbial influences [37; 38]. This study provides a spectrophotometric determination of cellulose, AB lignin, and ACE protein content in soils under hay fallow management. The soils used are mostly acidic sandy loam or silt loam and a few alkaline clay soils. It has been shown that spectrophotometric quantifications of biopolymers in soils were faster and more reliable than other analytical chemical procedures [29]. The soil cellulose (0.53-9.79 mg g\({}^{-1}\)) and AB lignin (5.38-21.64 mg g\({}^{-1}\)) measured in this study were comparable with those obtained in previous studies: 6.5 mg lignin g\({}^{-1}\)[39], 32.6 mg lignin g\({}^{-1}\) and 6.5 mg cellulose g\({}^{-1}\)[29], and 11.7 mg lignin g\({}^{-1}\) in top forest soils [25]. Soil lignin content was higher than cellulose and proteins, suggesting the resistance or ease of oxidation of these biopolymers in soils, with lignin being the most resistant and proteins least resistant. The coefficient of variation (CV) of the three biopolymers in the soils used was in the order ACE protein (56.8%) > cellulose (55.2%) > AB lignin (44.7%). This may suggest lignin is a more stable SOM fraction than cellulose and protein. Lignin and cellulose contents in the Houston, Vaiden, and Sumter claye soils were higher than in the other soils, suggesting that lignin oxidation in soils might be retarded by its association with clay minerals [40; 41]. Previous studies [42; 43] indicate cellulose and lignin are closely linked in the soil lignocellulosicoses. Lignin and soil clay particle interactions suggest why the acetyl bromide method for the spectroscopic determination of lignin in soils is better than the CuO oxidation method deemed to underestimate lignin content in soils [25]. Our study indicated low-pH soils had higher protein content than high-pH clay soils. Although soil microbial activity is generally reduced at low pH, fungi thrive in low-pH soils [44; 45]. Previous studies have shown that arbuscular mycorrhizal fungi (AMF) are a significant contributor to soil protein in acidic soils [26]. Unlike cellulose and lignin, which are protected by their association with clay minerals, studies show that the sorption of protein-sized organic N to the soil minerals does not protect the compounds from depolymerization [46]. However, further studies are required to investigate the relative  
Figure 5: Triangle heat maps showing the Pearson correlations between soil biopolymers (cellulose, lignin, and protein) and soil properties. The results for acidic soils are shown in (**A**), while the correlations for all nine soils under study are shown in (**B**). contribution of soil microorganisms and root exudates to the protein content of different soil types. Cellulose and AB lignin showed a strong positive correlation with each other and with SOM (Table 3). The significant relationship between SOM and spectroscopic cellulose and lignin is an indication that the latter are components of SOM and are potential estimators of relatively stable SOM, although lignin and ACE protein did not show a significant relationship with SOC. Soil cellulose, AB lignin, and ACE protein were negatively correlated with soil available P (SAP) (Table 3). Regression analysis showed a significant relationship between SAP and cellulose (r\({}^{2}\) = 0.713, \(p\) = 0.004) and AB lignin (r\({}^{2}\) = 0.53, \(p\) = 0.026). Although cellulose amendments promote P solubilization in soil by microorganisms [47], some studies showed that amending soils with cellulose significantly increased microbial P assimilation and decreased soil P [48].