Seasonal variation in biomass and nitrogen content of fine roots of bead tree (Melia azedarach) under different nutrient levels in an agroforestry system

The understanding of seasonal fine root distribution in agroforestry systems is important for assessing the root competition between tree and intercrop roots. Growth parameters of bead tree (Melia azedarach Linn.) were evaluated under different levels of N and P in an agroforestry system (pearl millet-wheat rotation under bead tree) in Punjab. Depth-wise (0–15, 15–30, 30–45 and 45–75 cm) distribution of fine root biomass (FRB), concentration and accumulation of N in the fine roots of bead trees were determined during four months (April, July, October and January) representing four seasons in the plantation in its 5th year of age under different nutrient levels. Leaf N concentration was also determined. The diameter at breast height (DBH) and height of 5 year old trees (in January 2010) were the highest (18.14 cm and 14.83 m, respectively) with the application of N and P @ N40P25, N60P37.5, N80P50, N100P62.5 and N120P75 kg ha−1 during 1st, 2nd, 3rd, 4th and 5th year, respectively. Significant variations (P<0.01) were observed for above parameters among soil depths, months and nutrient levels. Fine root biomass and N accumulation were significantly higher in July and October than April and January. These were lowest in unfertilized trees (31.1 g m−2 and 141 mg m−2, respectively) and highest at the highest level of nutrient application (94.2 g m−2 and 1102 mg m−2, respectively) in the soil profile (0–75 cm depth). More than 30% FRB and 37% N accumulation were restricted to 0–15 cm soil depth. Root and leaf N concentration were highest in April (0.96 and 1.10%, respectively) and lowest in January (0.67 and 0.94%, respectively). These also increased significantly (P<0.01) with the application of nutrients. The study signifies that the root competition for moisture and nutrients between tree and intercrop roots would be higher in summer crops than the winter crops.