长期不同施肥土壤对可溶性有机碳的吸附特征

  【目的】  分析长期不同施肥农田土壤对可溶性有机碳(DOC)浓度与结构的吸附差异特征，以及吸附能力与土壤性质、DOC结构的相关关系，为农田土壤固碳潜力及合理施肥提供理论依据。  【方法】  供试土壤选自长期定位试验中棕壤和红壤两种土壤的不施肥(CK)、单施有机肥(M)、单施化学氮磷钾肥(NPK)和有机肥配施化学氮磷钾肥(NPKM) 4个施肥处理。供试DOC标准溶液由新鲜猪粪提取，提取液DOC浓度为2400 mg/L。以该提取液配置系列DOC浓度溶液，运用Langmuir等温吸附方程拟合土壤对新鲜猪粪来源DOC的吸附浓度变化，采用三维荧光光谱技术分析吸附前后DOC结构及各区域成分相对体积荧光强度变化，分析影响DOC吸附的主要因素。  【结果】  红壤与棕壤对DOC的最大吸附量分别为12.81和10.82 g/kg，两种土壤均表现为NPKM处理的吸附能力高于NPK、CK。新鲜猪粪提取的DOC主要为类酪氨酸蛋白(区域Ⅰ)、类色氨酸蛋白(区域Ⅱ)和溶解性微生物代谢产物(区域Ⅳ)。土壤对这类DOC吸附较多的成分是类酪氨酸和类色氨酸。通过平行因子分析法得出，土壤对新鲜猪粪中的DOC吸附存在2个荧光组分，分子量较高的类蛋白物质(类酪氨酸和类色氨酸)与可溶性微生物代谢产物(C1组分)，分子量较小聚合程度较低的类酪氨酸蛋白物质与可溶性微生物代谢产物(C2组分)。不同施肥处理对C1和C2组分的吸附能力差异显著，对C1组分的吸附能力为M≈NPKM>NPK>CK，对C2组分的吸附能力为NPK>M≈NPKM>CK。土壤对DOC的最大吸附量Q_max与土壤中游离态铁(Fe_d)、络合态铁(Fe_p)、土壤有机质(SOM)呈极显著正相关性，与DOC腐殖化指数HIX和C1组分含量呈极显著正相关性 (P<0.01)，与荧光指数FI呈极显著负相关性 (P<0.01)。  【结论】  从新鲜猪粪提取的DOC结构简单，类酪氨酸蛋白、类色氨酸蛋白和溶解性微生物代谢产物含量较多，腐殖化程度较低。土壤对DOC的吸附过程会受到DOC结构特征和土壤理化性质的影响。土壤有机质(SOM)、Fe_d和Fe_p含量越高，吸附DOC的能力越强。长期施有机肥土壤对DOC中的类腐殖酸等高聚合度的芳香性大分子物质的吸附比例相对较高，而长期单施化肥土壤对分子量较小、聚合程度较低的类酪氨酸蛋白物质吸附量较高。

Adsorption characteristics of dissolved organic carbon by soils under different fertilization models

  【Objective】  We analyzed the adsorption characteristics of dissolved organic carbon (DOC) by soils under long-term fertilization. We aimed to understand the mechanism of long-term manure mineralization in the cultivation of fertile soils.  【Method】  The treatments included four fertilization regimes selected from the long-term experiment on brown soil and red soil. The treatments were no fertilization (CK), organic fertilizer only (M), NPK fertilizer only (NPK), and manure combined with NPK fertilizer (NPKM). The tested DOC was extracted from fresh pig manure with a concentration of 2400 mg/L, and a series of DOC solutions were made by diluting it for the Langmuir isotherm adsorption experiment. Three-dimensional fluorescence spectroscopy was used to analyze the DOC structure and relative volume fluorescence intensity before and after the adsorption by the soils. The main soil factors affecting the adsorption of DOC were analyzed.  【Results】  The maximum adsorption capacity of DOC was 12.81 g/kg in red soil and 10.82 g/kg in brown soil. The adsorption capacity of the two soils treated with NPKM was higher than in NPK. The fresh DOC was composed of tyrosine (region I), tryptophan-like protein (region II), and soluble microbial metabolites (region IV). Parallel factor analysis showed two fluorescent components in the adsorption of DOC by the soil amended with fresh pig manure. These included protein-like substances with higher molecular weight (tyrosine-like and tryptophan-like) and soluble microbial metabolites (C1 component), and tyrosine-like protein substances with a smaller molecular weight with a lower degree of polymerization and soluble microbial metabolites (C2 component). The most adsorbed components of fresh DOC by soil were tyrosine-like and tryptophan-like. The adsorption capacity of the C1 component was M≈NPKM>NPK>CK and NPK>M≈NPKM>CK for the C2 component. The largest adsorption ( Q_max ) was positively correlated with free iron (Fe_d), complex iron (Fe_p), and soil organic matter; positively correlated with humification index (HIX) and C1 component content of DOC ( P < 0.01), and negatively correlated with fluorescence index (FI) (P < 0.01 ) .  【Conclusions】  Fresh DOC contains a relatively high tyrosine-like protein, tryptophan-like protein, and soluble microbial metabolites, with simple structures and a low degree of humification. DOC structures and soil physical and chemical properties affecte the adsorption of DOC by soil. The high content of SOM, Fe_d, and Fe_p in soil leads to intensive adsorption of DOC. Long-term manure application increases the adsorption capacities of DOC components with a high degree of humification. Long-term application of chemical fertilizers increases the adsorption capacities of DOC components with a low degree of humification.