Effects of Combined Application of Biogas Slurry and Chemical Fertilizer on Soil Aggregation and C/N Distribution in an Ultisol

Unreasonable use of chemical fertilizer (CF) on agricultural soil leads to massive losses of soil organic carbon (SOC) and total nitrogen (TN) in tropical and subtropical areas, where soil conditions are unfavorable for aggregate formation. This study evaluated the effects of combined application of biogas slurry (BS) plus CF on soil aggregation and aggregate—associated C/N concentration and storage in an Ultisol. Six treatments included: no fertilizer (T1), CF only (T2), partial (15% (T3), 30% (T4) and 45% (T5)) substitution of TN with BS and BS only (T6). Soil mechanical—stable aggregates (MSAs) formation and stability as well as MSAs—associated C/N concentration and storage were observed in different aggregate sizes (>5, 5–2, 2–1, 1.0–0.5, 0.50–0.25 and <0.25 mm). The proportion of MSAs >5 mm significantly increased with BS substitution (T5), while the proportions of MSAs 1.0–0.5 mm, MSAs 0.50–0.25 mm and MSAs <0.25 mm significantly decreased. Both mean weight diameter and geometric mean diameter were highest in T5, which improved soil aggregation stability as well as resulted in significantly higher SOC and TN concentrations and storage in MSAs >0.5 mm that constituted 72–82% of MSAs. Stepwise regression analysis showed that MSAs >5 mm, SOC in MSAs >5 mm and TN in MSAs >5 mm were the dominant variables affecting aggregate stability. Meanwhile SOC in MSAs <0.25 mm and TN in MSAs 2–1 mm were independent variables affecting SOC and TN concentrations in bulk soils. Therefore, certain rate of combined application of BS plus CF is an effective, eco—friendly way to improve soil quality in an Ultisol.

在热带、亚热带地区，由于土壤条件不利于团聚体形成，农业土壤中化肥（CF）的不合理施用会导致土壤有机碳（SOC）与全氮（TN）大量流失。本研究针对老成土（Ultisol），评估了沼液（BS）配施化肥（CF）对土壤团聚体形成、团聚体关联碳氮浓度及储量的影响。试验共设置6个处理：不施肥组（T1）、单施化肥组（T2）、以沼液分别替代15%（T3）、30%（T4）与45%（T5）全氮的配施组，以及纯沼液组（T6）。针对不同粒径的机械稳定性团聚体（MSAs）（>5、5–2、2–1、1.0–0.5、0.50–0.25及<0.25 mm），测定了其形成与稳定性，以及各粒径MSAs关联的碳氮浓度与储量。结果显示，T5组（45%全氮替代）中>5 mm粒径MSAs的占比显著提升，而1.0–0.5 mm、0.50–0.25 mm及<0.25 mm粒径MSAs的占比则显著降低。T5组的平均重量直径与几何平均直径均为最高，不仅提升了土壤团聚体稳定性，还使占总团聚体72%–82%的>0.5 mm粒径MSAs中的SOC与TN浓度及储量显著升高。逐步回归分析表明，>5 mm粒径MSAs及其内部的SOC、TN含量是影响团聚体稳定性的主导变量；而<0.25 mm粒径MSAs中的SOC与2–1 mm粒径MSAs中的TN含量，则是影响原状土壤整体碳氮浓度的独立变量。综上，沼液与化肥的适宜配施方式，是提升老成土土壤质量的有效且生态友好的途径。