Biotic Stress and Yield Loss

13.4 COMPETITION FOR SOIL NITROGENA relationship between crop productivity and nitrogen (N) application rate has longbeen known to exist and is commonly used for fertilizer management schedules.Because 50 to 80% of the nitrogen in plant leaves is found in photosynthetic proteins,46–48 the observed correlation between light saturated CO 2assimilation (A max,i,mol CO 2m 2 s 1 1.7536 10 5 g CO 2m 2 s 1 ) and leaf nitrogen content (N L,i,g N m 2 ), 49–56 should be no surprise. Dry matter growth increase of species i undernitrogen limited production conditions (dW N,i/dt, g m 2 s 1 ) can be determined byadjusting the light saturated CO 2assimilation rate (A max,i) used to determine A c,iinEquation 13.3 for leaf nitrogen content (N L,i). Therefore, the nitrogen dependent netgrowth function (f i(N L,i)) is:f i(N L,i) A c,i(,A max,i(N L,i))which indicates that CO 2assimilation is dependent upon and A max,i, and the latteris dependent upon leaf nitrogen content. Sinclair and Horie 49 proposed the followingrelationship for quantifying the light saturated CO 2assimilation–N L,irelationship:2(1 exp(a(N L,i b))A max,i [A max,i] 1 [13.10]where [A max,i] represents an absolute maximum CO 2assimilation rate under optimallight, soil water, and leaf nitrogen conditions, a is a shape coefficient, and b is the leafnitrogen content at which CO 2assimilation reaches zero. Estimates of [A max,i], a, andb can be obtained by regressing observed CO 2assimilation (in full sunlight) on leafnitrogen content for each species. 50–57 Since leaf nitrogen content may be critical forplant growth, nitrogen uptake and partitioning within the plant must be accuratelypredicted. When soil nitrogen supply becomes limited, nitrogen uptake will bereduced and nitrogen partitioning to leaves may be modified. Therefore, weedsinduce a stress on the crop through their direct use of stored nitrogen in the root zoneof both species. How can we quantify the demand for nitrogen by both the crop andweed? How does current nitrogen use influence current and future supply of availablesoil nitrogen? What is the effect of limited nitrogen supply on nitrogen uptake? In acompetitive situation, how do we partition the quantity of nitrogen available foruptake between the two species?Several algorithms have been developed to predict soil nitrogen uptake (U i,g Nm 2 d 1 ). 58–61 Typically, nitrogen uptake is predicted as the minimum of (1) thedaily nitrogen demand of the species (N demand,i,g N m 2 d 1 ) and (2) the quantity ofnitrogen available for uptake (R N, g N m 2 ). Thus:U MIN(N demand,R N) [13.11]ten Berge et al. 58 calculated daily nitrogen demand for a given species using:N demand,i MIN(U N,U M,U P) [13.12]