MSc.

Tree root biomass production and turnover play key role in improving soil productivity in land use systems involving woody perennials. Also, tree roots provide substantial contribution to the soil carbon pool which however varies with tree species and management regimes. A field study was conducted at Vellanikkara, Kerala, India in a 21-year-old Grevillea robusta stand (460 trees ha-1) to quantify the extent of belowground biomass production, root distribution pattern and carbon sequestration along with soil carbon stocks. A total of 18 trees were excavated using destructive sampling. The study also explored the distribution of C, N, P and K in the various root girth fractions (0-10 cm, 11-20 cm, 21-30 cm, 31-40 cm and tap root) and at various soil depths up to one meter. Mean tree root biomass production based on diameter class ranged from 12.94 to 59.81 kg tree-1 with a mean annual increment (MAI) varying from 0.62 to 2.85 kg tree-1 yr-1 while root biomass production at the stand level was 18.45 Mg ha-1 with a MAI of 0.88 Mg ha-1 yr-1. Based on the diameter class elemental carbon storage in the belowground root biomass of a mean tree varied from 5.58 to 23.58 kg tree-1. Grevillea robusta stand sequestered 8.04 Mg C ha-1 and 77.45 Mg C ha-1 in the root system and soil respectively. On the whole, the net belowground carbon sequestration (soil + roots) was 80.28 Mg C ha-1. Mean nutrients content (kg tree-1) in a mean tree roots were found to be 0.059 kg tree-1 N, 0.003 kg tree-1 P and 0.047 kg tree-1 K. While on the kg ha-1 basis N, P and K in a mean tree were found to be 25.89 kg ha-1, 1.70 kg ha-1 and 21.69 kg ha-1 respectively. Overall, the partitioning in the nutrient content among the root girth classes was found to be in order of tap root > 0-10 cm >11-20 cm > 21-30 cm > 31-40 cm.
Logarithmic spiral trenching was used to study the root distribution pattern using 15 trees. The results revealed a decrease in rooting intensity with increasing lateral distance from tree base. With regards to lateral distance alone as a factor of determining the rooting intensity about 78 % roots were found to be within 1.55 m lateral distance. While using only depth as the sole criteria 74.51 % roots were found confined within top 30 cm depth. This clearly revealed the presence of majority of Grevillea robusta roots in the surface. Another study conducted using 32P study revealed that for 21-year-old Grevillea robusta trees, the active foraging zone lies within top 30 cm depth and 150 cm lateral distance. Overall uniformity in root distribution pattern obtained from excavation, logarithmic spiral trenching and 32P tracer study further demonstrate the surface feeder nature of Grevillea robusta roots.
The higher root biomass and C sequestration in Grevillea robusta suggest its potential to enrich the soil nutrient status and there by offering better understorey productivity in polyculture systems involving silver oak. The root distribution trends though suggest silver oak to be a surface feeder, appropriate density management and planting geometry regulation makes it possible to try intercrops beyond a lateral distance of 2.0 m from the tree base.