Effects of Combined Application of Biochar and Different Types of Nitrogen Fertilizers on Rapeseed Root Growth and Properties of Purple Soil in Southwest China

To demonstrate the effects of combined application of biochar and different types of nitrogen fertilizers on the growth of plant roots and on purple soil properties such as soil nutrients, soil carbon content and soil respiration, a 206-day greenhouse pot experiment with rapeseed was conducted. Three types of nitrogen fertilizer were used: urea (UR), controlled-release urea (RU), a mixture of 60% urea and 40% controlled-release urea (40% RU), and biochar was added at mass fractions of 0% (C0), 2% (C1) and 4% (C2), with a control treatment (CK) without nitrogen fertilizer and biochar. The results showed that biochar significantly improved soil nutrient status, with the best effect observed when 40%RU was co-applied with biochar. The addition of biochar significantly increased soil total organic carbon (TOC) and particulate organic carbon (POC). Soil respiration increased with increasing biochar application, and the combination of 2% biochar and 40% RU showed a reduction in carbon emissions compared to the UR and RU treatments. The number of rapeseed root tips (NT), forks (NF) and crossings (NC) increased significantly with the addition of biochar, and the combination of biochar and 40% RU was more beneficial for root growth and development than RU and UR. Considering the improvement in soil nutrition, increased soil organic carbon content, reduced carbon emissions, and enhanced rapeseed growth and development, the co-application of 2% biochar and 40% RU is recommended for large-scale application in rapeseed cultivation in the hilly purple soil of southwest China.