SIMULATING TRENDS IN SOIL ORGANIC-CARBON IN LONG-TERM EXPERIMENTS USING THE NCSOIL, AND NCSWAP MODELS

NCSOIL simulates C and N transformations in 4 soil organic pools: Pool I labile, Pool I resistant, Pool II, and Pool III, with half-lives of 2, 17, 115 days and about 150 years: respectively. Pool I labile and Pool I resistant represent the microbial biomass. Pool I and Pool II r epresent the potentially mineralizable N, or the biologically active s oil organic matter. The sum of Pools I, Pool II, and Poor III - the so il organic matter - corresponds to the total organic matter minus resi dues. Each residue is described by 2 pools. NCSOIL is a stand-alone mo del. It is also a module of NCSWAP, a larger model which encompasses t he soil-water-air-plant system. A number of systems and treatments, in cluding the Rothamsted nitrate treatment and the Calhoun tracer C data were simulated. The initial level of Pool II and the decay rate const ant of Pool III were calibrated on the basis of measured total soil or ganic matter and above-ground production. Simulated data were sensitiv e to above-ground production as it controlled residues input to soil. Model performance, based on total soil organic matter only, is discuss ed elsewhere in this issue. Most decay rate constants for Pool III ran ged from 1.0E - 5 to 3.0E - 5 d(-1). Rate constants outside this range were associated with peculiarities of the soil or agronomic practices . Levels of biologically active organic matter (Pool I plus Pool II) i n the top soil layers ranged from 4 to 108 mu g N g(-1). They were con sistent with those reported for the potentially mineralizable nitrogen and reflected the agronomic practice and soil fertility level better than did the total soil organic matter. The simulated biologically act ive organic matter presented a 1 year periodic cycle. In the future, a major challenge in modelling studies will be to flee simulations from the calibration process and to devise experimental methods which will provide initial values relevant to the dynamic requirements of the mo del. (C) 1997 Elsevier Science B.V.