Long-term effects of biochar amount on the content and composition of organic matter in soil aggregates under field conditions

PURPOSE: Soil aggregates play an important role in promoting soil fertility, as well as increasing the sink capacity and stability of soil carbon. In this study, we consider the following research questions: 1. Under field conditions, do different dosages of biochar increase the soil aggregation after 3 years of application? 2. How does the application of biochar affect the concentration and distribution of soil total organic carbon (TOC) and total nitrogen (TN) in different sizes of aggregates? 3. Can the application of biochar alter the composition of organic carbon in soil aggregates? MATERIALS AND METHODS: Different amounts of biochar (up to 90 t ha⁻¹) were applied to a calcareous soil in a field experiment in 2009 along with the application of chemical fertilizer annually and the returning of winter wheat and summer maize straws. After 3 years, 0–20-cm soil samples were taken to measure the size distribution of soil water-stable aggregates by wet sieving, the concentrations of TOC and TN in whole aggregates and light or heavy fractions by elemental analysis equipment, and composition of TOC by Fourier transform infrared (FTIR) and pyrolysis-gas chromatography/mass spectrometer (Py–GC/MS). RESULTS AND DISCUSSION: (1) The 3 years of biochar application had no significant effects on degree of soil aggregation but reduced the breakage of large soil aggregates (>1000 μm); (2) biochar significantly increased the contents of TOC and TN in soil macro-aggregates (>250 μm), as well as their ratios to total soil amount. Biochar also significantly increased the contents of TOC and TN in light fractions as well as the C/N ratio, which made the soil organic matter more active. The biochar dosage showed a significant positive correlation with organic carbon, total nitrogen, and C/N ratio in light fraction components of aggregates (>250 μm). Biochar mainly affected the organic matter in the heavy fraction components of macro-aggregates; (3) from the Py–GC/MS results, biochar increased the CO₂ content originated from active organic carbon. CONCLUSIONS: Long-term application of biochar improved the stability of soil aggregates, increased the contents of TOC and TN as well as organic carbon and total nitrogen in macro-aggregates, and usually increased the contents of CO₂ originated from active organic carbon in light fractions. The findings were helpful in evaluating the effects of biochar on soil aggregation and organic matter stability.