CARDAMOM, MANDARIN AND NITROGEN-FIXING TREES IN AGROFORESTRY SYSTEMS IN INDIAS HIMALAYAN REGION .2. SOIL NUTRIENT DYNAMICS

A study on seasonal soil nutrient dynamics was made in large cardamom (Amonum subulatum) and mandarin (Citrus reticulata) agroforestry syste ms in the Sikkim Himalaya, India. Alnus nepalensis was the N-2-fixing associate in the large cardamom system, and Albizia stipulata in the m andarin agroforestry system. Sites without N-2-fixing species in both agroforestry types comprised native non-symbiotic mixed tree species. Soil was acidic in the cardamom agroforestry and slightly acidic to ne utral in the mandarin agroforestry system. Total-N in soils was the hi ghest in the forest-cardamom stand and the lowest in the mandarin-base d agroforestry systems. Soil ammonium-N and nitrate-N concentrations w ere highly seasonal, and the ratio of seasonal maximum and minimum var ied up to six times. The CIN ratio was higher in cardamom agroforestry indicating lower N availability than in the mandarin agroforestry. Ca rdamom stand with Alnus showed a relatively narrower C/N ratio. N-2-fi xing species help in maintenance of soil organic matter levels with hi gher N-mineralization rate as land use change from natural-forest syst em to agroforestry systems with sparse tree populations. Ratios of ino rganic-P/total-P were lower in cardamom agroforestry than the mandarin agroforestry. Seasonal fluctuation in Ca-PO4, Al-PO4 and Fe-PO4, cont ents regulated the availability of phosphates to some extent for plant uptake.