Abstract

Litterfall in 3-to 6-year-old acacia plantations was quantified using litter traps. Annual litter production in the plantations ranged between 9.3 and 12.0 t ha-1. Monthly litterfall indicated a unimodal pattern with its peak during December-January.The dry months (October to March) accounted for 67.5 to 76.2 of the total litterfall. Litter production differed significantly between locations and months. Annual litter yield in acacia plantations was higher than those reported for other major forest plantation species in India and elsewhere. Decay rate of acacia leaflitter was determined using the mesh bag technique. The dry weight loss of litter (laid on the surface of soil-LSS) at the end of 12 months was 64.4 at Chettikulam (C), 65.6 at Kothermanakkadu (K) and 86 at Kannamkuzhi (Ka). The corresponding values for litter partially buried in the soil (LPB) were 94.5 at site C and 92.6 (after a period of six months) at site Ka. The decay rate was faster in the fertile site (Ka) compared to degraded areas (C and K). In general, the rate of decomposition of acacia leaflitter was lower than the decay rate of leaflitters of major plantation species reported from Kerala. Laboratory studies to quantify the addition of organic carbon (OC) to soil during litter decay indicated that significant amount of OC was added to soil. A survey was conducted in 26 plantations of acacia to assess the status of mycorrhizal associations. It revealed that acacia forms VA mycorrhizal association in Kerala soils. The extent of colonization by VAM fungi was very high (varied between 90 and 100) in majority of the plantations. The VAM colonization was correlated with silt content, clay content, porosity and bulk density of soil; it was not correlated with the soil nutrient status. Glomus spp. were found to be the dominant VAM fungi in acacia plantations. Five VAM fungi were screened for their efficacy in promoting the growth of acacia seedlings. Seedlings inoculated with Glomus fasciculatum had maximum increase in height, shoot and root dry mass, total P content and mycorrhizal colonization. Based on the present study, use of G. fasciculatum as an inoculant VAM fungus for acacia is found to be promising. For assessing the status of root nodulation of acacia in Kerala, soil samples were collected from 26 plantations. The average number of nodules per 100 cm3 of soil, based on sixty samples, was calculated for each plantation. The study indicated that nodule number varied depending upon the soil. Generally, degraded, less fertile soils showed more number of nodules. There was a significant inverse relationship. Nursery experiments, inoculating acacia seeds with Rhizobium cultures isolated from the 26 plantations revealed significant difference between the isolates in their efficiency for nodule formation. There was significant positive correlation. The soil characters of acacia plantations were compared with those of adjacent fallow land to ascertain the effect of acacia growth on the soil properties. Soil analysis revealed that there was no significant difference between acacia plantations and adjacent fallow land in physical and chemical properties of soil as well as soil nutrient contents except in the case of pH which was significantly lower in acacia plantations