Biotic Stress and Yield Loss

As with soil water, Equations 13.10 to 13.16 can be used to simulate indirectcompetition for soil nitrogen. During periods when soil nitrogen is not limiting,plants utilize as much nitrogen as is required using Equation 13.12, which subsequentlyreduces the pool of available soil nitrogen. As the season progresses, if nitrogensupply is insufficient to meet demand, nitrogen uptake is reduced. A subsequentreduction in tissue N content or partitioning of carbon to above-ground growthwill reduce plant growth. Equation 13.11 indicates that when nitrogen supply isless than the overall demand, uptake is limited to the quantity available. However, itprovides no information as to how much is acquired by each species. Therefore,direct competition for nitrogen is not accounted for. Under conditions of limited soilnitrogen supply, should the quantity of available nitrogen be divided among speciesbased on the relative quantity of roots in the soil profile? What if the species differ intheir uptake efficiency? What if the depth of the actual root zone of the two speciesdiffers?Smethurst and Comerford 68 presented a method of simulating direct competitionfor soil nitrate. Their approach required that nitrogen uptake be calculated using conceptsfrom solute transport theory (e.g., Nye and Tinker 69 ). Nitrate uptake (U, molcm 3 d 1 ) of a given species within a rooted layer j can be modeled using the proceduresof Baldwin et al. 70 U j 2r oC lo,jL v,jt [13.17]where r ois mean root radius (cm), is root absorbing power (cm s 1 ,I max/(K m C lo C min), where I max(mol m 2 s 1 ) is the maximum N uptake rate, K m(molcm 3 ) is the solute nitrate concentration at which uptake is 1/2 I max,C lois definedbelow, and C min(mol cm 3 ) is minimum solute concentration required for uptaketo occur, L v,j(cm cm 3 ) is the root length density within the jth layer, t is the time ofintegration (1 d 86400 s), and C lo,j(mol cm 3 ) is nitrate concentration at the rootsurface within layer j:C lo,j vo,j 1 vo,jC l,jo r dz,jeb ,j 1r o r d 2 r D2 1 22 r o o ,jDev br oz,jv o[13.18]where C l,jis average concentration of nitrate in soil solution (mol cm 3 ,C l,j C li,jat t 0), v o,jis water flux at the root surface (cm s 1 , equal to T a,i), r dz,jis the radiusof the depletion zone (cm) around the root, b is buffer power, and D eis the effectivediffusion coefficient (cm 2 s 1 ):D e D l 0.5a,j[13.19]