Pyrogenic conversion of rice straw and wood to biochar increases aromaticity and carbon accumulation in soil

Abstract

Open field burning of rice straw besides causing environmental pollution and human health problems results in loss of organic carbon and essential nutrients. Urgent solution is required for management of rice straw in an eco-friendly and sustainable manner. The aim of the present study was to examine the efficiency of direct return of rice straw into the soil versus biochar generated from rice straw, and compare it with wood derived biochar in influencing soil biological processes and potential to accumulate soil organic C (SOC). We studied the effect of pyrogenic conversion of rice straw and wood chips to biochar on changes in chemical composition of feedstock by Fourier Transform Infrared (FTIR) spectroscopy and C and N mineralization kinetics, microbial metabolism, carbon use efficiency (CUE) and soil enzyme activities in a series of laboratory incubation experiments conducted at 25 °C temperature and field capacity moisture. Pyrogenic conversion of feedstock to biochar resulted in increase in C (39.6% to 69.2%) and N (0.4% to 1.65%) contents and lowering of C/N (99 to 28.4) and H/C (1.94 to 0.60) ratios. Charring resulted in decrease in aliphatic and increase in aromatic components. Decomposition of organic substrates was governed by aromaticity and C/N ratio; in 127 days, only 12% of rice straw biochar embodied C was mineralized as against 76% from rice straw. The addition of biochars resulted in soil N priming (120 ± 24 mg N kg−1) whereas straw application resulted in net N immobilization (−13 mg N kg−1). Compared to unpyrogenic straw, the rice straw biochar led to greater microbial growth, lower metabolic quotient, higher CUE (3.3 times) and decreased microbial respiration, with potential to promote SOC accumulation. It was concluded that addition of rice residue generated biochar into soil is more beneficial for soil biological processes and SOC stabilization to stymie climate change than direct return of plant residue into the soil. The results are significant for devising eco-friendly rice residue management practices in the Indo-Gangetic plains.