Charcoal Volatile Matter Content Influences Plant Growth and Soil Nitrogen Transformations

A series of short‐term greenhouse experiments and laboratory incubations were conducted to evaluate the effect of macadamia ( Macadamia integrifolia Maiden & Betche) nut shell (MNS) charcoal with varying volatile matter (VM) content on soil properties and plant growth in two tropical soils. Lettuce ( Lactuca sativa L.) and corn ( Zea mays L.) were planted in an Andisol amended with four rates of MNS charcoal (0, 5, 10, and 20% w/w) containing relatively high VM content (225 g kg −1 ) with and without N fertilizer. Increasing rates of charcoal without N caused a significant decline in both lettuce and corn growth. Corn growth declined significantly with or without N at the two highest charcoal rates. In a third experiment, corn growth also declined significantly in an Ultisol amended with the MNS charcoal (5% w/w) with and without fertilizers. In a fourth experiment, charcoals with high VM (225 g kg −1 ) showed negative effects on plant growth while the low‐VM (63.0 g kg −1 ) charcoal supplemented with fertilizer showed a significant positive effect on corn growth. Results from the 2‐wk incubation experiments showed that high‐VM charcoal caused a significant decline in soil NH 4 + –N and a significant increase in soil respiration compared with the soil amended with low‐VM charcoal and the soil alone. We propose that phenolic compounds and other products in the high‐VM charcoal stimulated microbial growth and immobilization of plant‐available N. Our results demonstrate that VM content appears to be an important property of charcoal that has short‐term effects on soil N transformations and plant growth. Longer incubation experiments and field trials are needed to further elucidate the role of charcoal VM content on soil processes and plant growth.