Impact of Parthenium Hysterophorous L. Invasion on Soil and Plant Species Composition of Grasslands in Central Nepal

Abstract

The present study was undertaken in three grasslands each in Gorkha, Nuwakot and Kathmandu districts where invasion of Parthenium hysterophorus L. (here after referred to as Parthenium) was at initial stage (5-6 years). The research was focused on the change in physico-chemical properties of soil and plant community structure (species richness, density and important percentage) due to Parthenium invasion. The field sampling was done from August to September 2006. In each study site, ten belt transects (size 13 m × 1 m) were defined passing from non invaded part to invaded. In each transect, five quadrats of 1 m × 1 m were sampled at interval of 2 m in such a way that two quadrats lied on invaded, one on transition and two on non-invaded regions. In total 150 quadrats (50 in each study site) were sampled. Soil samples collected from each sampling plot were analyzed for texture, pH, organic matter (OM), total nitrogen (N), available phosphorus (P) and available potassium (K). The soil type was sandy loam in Gorkha, pure loam or sandy loam in Nuwakot and sandy loam or silt loam in Kathmandu. The pH in invaded plot ranged from 5.92-6.73 and in all three sites pH declined from non-invaded plots to invaded plots with slightly acidic or neutral soil. Soil OM and soil N content did not change significantly due to Parthenium invasion. Soil K increased from non-invaded to invaded plots. However, K content increased due to Parthenium invasion at initial stage but it declined with increasing Parthenium density. There were a number of trends in changes in soil properties that might be due to heterogeneous nature of habitat, instead of Parthenium invasion. Plant species richness was high in invaded plots of Gorkha and Kathmandu sites and there was no change in species richness from non-invaded to invaded plots in Nuwakot site. Plant species richness was high in invaded plots probably due to increase in habitat heterogeneity at initial stage of invasion. The non invaded plots in all sites were dominated by Acrachne racemosa which formed continuous mat and suppressed the growth of associated species. Parthenium lacks such type of network but creates shaded condition which favours growth of other plant species under dense canopy at ground level. Dominance shifted from Acrachne racemosa to Parthenium and density of associated species declined with increasing Parthenium density. Many palatable species (Acrachne racemosa (Heyne) Ohwi., Trifolium repens L., Imperata sp., Chrysopogan aciculatus (Retz.) Trin., Sporobolus sp. and Dactyloctenium aegypticum (L.) P. Beauv.) were negatively affected by Parthenium invasion. Thus protection of grazing lands from Parthenium invasion is important to ensure sustainable supply of fodder to livestocks.