| Large tracts of once productive farmlands in the Indo-Gangetic plains of India have been degraded by intensive cropping and over-irrigation. This has resulted in soil salinization and alkalization. Soil productivity may be restored by afforestation with salt-tolerant tree species. Temporal patterns of biomass accumulation, understorey vegetation development, soil fertility and moisture, tree growth and the nutritional status of plant tissues were studied under greenhouse and field conditions in an age sequence of Prosopis juliflora planted 0-, 5-, 7-, and 30-years ago on similar sodic soils in Haryana, India. The objective was to develop an understanding of plant-soil relationships driving protracted ecological processes involved in the rejuvenation of sodic soils under afforestation, and to address the issues of sustainable management for these forest plantations. The structural (species composition) and functional (nutrient content and biomass) diversity of plantation systems increased with age. The growth of trees altered the microclimate and improved soil moisture status, infiltration capacity, organic C, total N, available P and exchangeable Ca, Mg and K levels. Decreases in pH, electrical conductivity, and exchangeable Na levels were also observed. The ameliorating effects of trees on soil increased with tree occupancy. Soil dynamics reflected a three phase restoration sequence characterized by nominal soil changes during tree establishment phase (0--5 years), marked and rapid changes in a short transition phase that was associated with canopy closure (5--7 years), and gradual stabilization of soil properties in the fallow enrichment phase (7--30 years). The ameliorative mechanisms which contributed to soil remediation were categorized into two parallel sets of major processes: sodicity alleviation associated with salt leaching, and fertility restoration driven by organic matter addition through litter fall and nutrient cycling. The former was typical of the transition phase while the latter characterized the fallow enrichment phase. Effective phytoremediation was confirmed by pot trials assessing the yield and nutritional response of an agricultural crop (wheat) reared on fallow, plantation and farm soils. Contrary to previous studies, results demonstrate that lengthy rotation (>30 years) systems are necessary to sustain future productivity of these plantation ecosystems. |
