长期不同量秸秆炭化还田下水稻土孔隙结构特征

  【目的】  生物炭被认为是一种能够提高土壤固碳能力、改善土壤结构和减缓全球气候变化的土壤改良剂。土壤孔隙结构直接影响土壤中水、气、热的运动，因此，研究长期施用生物炭对土壤孔隙结构特征的影响，以期为秸秆炭化还田提供理论依据。  【方法】  研究基于2013年建立的水稻秸秆炭化还田长期定位试验，选取在等氮磷钾条件下不施用生物炭 (C0)、施用低量生物炭 (1.5 t/hm2，C1.5)、高量生物炭 (3.0 t/hm2，C3.0)的 3个处理。利用X射线CT扫描和图像处理技术，分析了土壤孔隙结构参数，包括土壤孔隙度、土壤孔隙大小分布、孔隙连通性指数 (欧拉特征值)、各向异性、分形维数、最紧实层孔隙度和最紧实层平均孔隙直径等参数。  【结果】  C1.5和C3.0处理均能显著增加土壤有机碳含量和土壤总孔隙度，降低土壤容重，平均增加或降低比例分别为15.5%、10.5%和7.4%。C1.5与C3.0处理之间的总孔隙度没有显著差异，但孔隙大小分布存在差异。C1.5处理显著增加了大孔隙中当量孔径为100～500 μm和 > 500 μm的孔隙度，增幅分别为81.6%和275.3%，而C3.0处理显著降低了大孔隙中当量孔径100～500 μm的孔隙度，降幅为32.9%。C3.0处理当量孔径 < 25 μm的孔隙度显著大于C0处理和C1.5处理，增幅分别为13.8%和16.3%。C1.5处理的欧拉特征值最低，分形维数、最紧实层孔隙度和平均孔隙直径最大。各处理土壤孔隙的各向异性没有显著差异。  【结论】  长期施用水稻秸秆生物炭能够显著增加稻田土壤有机碳含量和总孔隙度，降低土壤容重。施用适量生物炭会增加土壤大孔隙度和土壤孔隙的连通性，但是过量施用生物炭可能会降低土壤大孔隙度和土壤孔隙的通气导水能力。炭化秸秆还田量与孔隙结构之间的定量关系还需深入研究。

Characterization of soil pore structure of paddy soils under different long-term rice straw biochar incorporation

  【Objectives】  The application of biochar as a soil amendment method for improving carbon sequestration, improving soil structure, and mitigation of global climate change has received considerable attentions over the last decade. Soil pores play a key role in transportation of air, water, and heat. However, knowledge of the long-term effects of biochar on soil pore characteristics under field conditions is limited.   【Methods】  We investigated the effect of successive addition of low-dose rice straw-derived biochar 1.5 t/(hm2?a), C1.5], high-dose rice straw-derived biochar 3.0 t/(hm2 ?a), C3.0], and no biochar addition (control, C0) on soil pore structure in a paddy field over a 6-year (2013?2018) period. Porosity, pore size distribution, connectivity, anisotropy, fractal dimension, porosity, and mean macropore diameter of the limiting layer were measured by X-ray computed tomography (CT) and image processing.  【Results】  We found that biochar amendment significantly increased soil organic carbon content and total porosity by 15.5% and 10.5%, respectively, and decreased soil bulk density by 7.4%. There was no significant difference in soil total porosity under the two biochar amendment treatments (C1.5, C3.0), but the proportion of macropores was significantly different between them. Compared with C0, C1.5 treatment significantly increased porosity of 100?500 μm and >500 μm macropores by 81.6% and 275.3%, respectively, while C3.0 treatment significantly decreased the porosity of 100?500 μm macropores by 32.9%. C3.0 treatment had higher porosity of <25 μm micropores than C0 and C1.5. Meanwhile, connectivity, fractal dimension, porosity, and mean macropore diameter of the compacted layer were highest under C1.5, but anisotropy was similar for the three treatments.  【Conclusions】  Our results indicate that the application of rice straw-derived biochar could increase the soil organic carbon content, total porosity, and decrease the soil bulk density. Proper application rate of biochar could increase the soil macroporosity and connectivity, but excessive application rate of biochar would reduce the soil macroporosity, soil hydraulic conductivity, and gas diffusivity. These results enhance our understanding of the relationship between soil pore structure and biochar application, and provide evidence for decision making process in choosing proper straw management.