The impact of wood biochar as a soil amendment in aerobic rice systems of the Brazilian Savannah

<strong>Abstract</strong> <em>Keywords</em>: tropical Savannah, biochar, soil fertility, aerobic rice, grain yield, N2O emission Márcia Thaís de Melo Carvalho (2015). The impact of wood biochar as a soil amendment in aerobic rice systems of the Brazilian Savannah. PhD thesis, Wageningen University, The Netherlands, with summaries in English, Dutch and Portuguese, 160 pp. Rice is a staple food for 3 billion people in the world. In Brazil, rice is a traditional staple food mostly cultivated by smallholder farmers. Rice is better adapted to soil types and climate conditions of the Brazilian tropical Savannah than crops like corn and soybean. However, environmental and socio-economic constraints such as the variable rainfall and the limited access to mineral fertilization is a challenge for sustainable aerobic rice production in Brazil. Yields can vary from 1 to 5 Mg ha-1. In this context, the use of agronomic techniques able to improve soil properties seems a good option to increase quantity and stability of rice production. The use of biochar as a soil amendment represents one such option. Biochar is carbonized biomass, generally a by-product of bioenergy production from biomass. Its use in agricultural soils is inspired by the very fertile <em>Terra Preta</em> soils, which are a result of pre-Columbian human activity in the Amazon region. A key component of the fertility of <em>Terra Preta</em> soils is the high content of C, mostly present in form of pyrogenic C, result of carbonization of organic material. Pyrogenic C is also an important fraction of the soil organic matter present in the weathered soils of the Brazilian Savannah. These soils are mostly acidic, with low soil organic matter content, requiring liming and mineral fertilization if used for agriculture. The biochar tested in the current research is a by-product of charcoal production from eucalyptus wood via slow pyrolysis at 400-500 ○C. It is a porous material with a high C content and K, Ca and Mg availability, which make it a potentially suitable soil amendment for the low fertile soils of the Brazilian Savannah. We applied biochar in a sandy and a clay soil type of the Brazilian Central West region, where over 40% of the Brazilian total crop production is located. We investigated whether biochar amendment improves soil chemical and physical properties and how this in turn affects aerobic rice yields along four cropping seasons after a single biochar application. In both soil types, biochar decreased soil acidity up to 3.5 years after its application. On the clay soil, biochar application decreased the soil water retention capacity but increased the soil organic matter content. The effect of biochar on rice yields on the clay soil were either absent, negative or dependent on the amount of mineral N applied, as well as biochar-induced changes in soil properties, particularly soil water retention and soil organic matter. Most promising results were observed on the sandy soil, where biochar application increased the soil water retention capacity. On the sandy soil, first two seasons were drier than latter two seasons. Accordingly, effects of biochar on rice yields were divergent: the positive effects observed in the first two seasons were absent in subsequent seasons. During this study, weather conditions and rice blast infestations were factors that influenced the observed effects of biochar on rice yields. Further, biochar did not enhance N2O emissions on the clay soil. Based on these results wood biochar could be considered for use in farming systems of the Brazilian Savannah, particularly on sandy soils.