CropSyst User Manual - sipeaa

Nitrogen fixationFor crops such as legumes, CropSyst decreases nitrogen demand by supplying additional nitrogenproduced by nitrogen fixing bacteria.whereND pot (kg/ha)is the nitrogen demand .ND act (kg/ha)ND act = ND pot - N fixationis the nitrogen demand accounting for nitrogen fixation for legumes and will replace the normalcrop nitrogen demand (ND).Nfixation (kg/ha)is the fixed nitrogen.The rate of nitrogen fixation follows the development of crop growth. The potential amount of fixednitrogen may be reduced by plant available water, mineral nitrogen content in the root zone and soiltemperature.whereFix r (0-1)N fixation = min [ ND pot · Fix r , 6.0 (kgN/ha)/ day ]Fix r = Fix p · min(Fix w , Fix n , Fix t )is the fraction of daily nitrogen demand supplied by fixation.Fix p (0-1)Fix wis the effect of crop development.Fix p =⎧⎪⎨⎪⎩CGD day⎯⎯⎯⎯⎯⎯⎯⎯CGDflower beginif active growth1.0 if senescence (grain filling)1.0 - [ CGD day - CGDgf begin / CGDfix end - CGDgf begin ] if senescentDuring senescence, fixation is reduced from the start of grain filling to a point halfway betweengrain filling and physiological maturity at which fixation is zero.limits fixation based on plant available water (PAW) in the first 10 to 30 cm of the soil profile.