Within plot variability in available soil mineral nitrogen in relation to leaf greenness and yield

Available soil mineral nitrogen (N) varies both temporally and spatially. These variations affect field-scale N-use efficiency. A field study was conducted for three years to investigate spatial variability in available soil mineral N within uniform research plots in relation to leaf greenness or chlorophyll content (plant N sufficiency) and yield. Variations within the plot in available soil mineral N sampled at the 6-ligule stage was related to N fertility: the higher the fertilizer N levels, the higher the variability. The standard deviation for the 200 kg N ha-1 treatment was up to five times higher than the unfertilized control treatment. The nitrate NO3-N accounted for 70 to 80% ofsoil mineral N in fertilized plots compared to 50 to 60% in unfertilized control plots. The variability in grain yield of individual maize (Zea mays L.) plants within a plot was inversely related to soil N fertility: the higher the fertilizer N levels, the lower the yield variability (at 100 or 200 kg N ha-1, yield ranged from 97 to 148 g plant-1, or 10% CV within a year compared to ranges from 0 to 82, or 50% CV in the same year at 0 kg N ha-1). On an individual plant basis, chlorophyll content from the 6-ligule stage through the growing season generally showed much smaller CV's, but had a similar trend to variations in yield. Leaf greenness from 6-ligule stage to silking was significantly correlated with harvest yield (r>0.60, P<0.01), and both also correlated with available soil mineral N, though to a lesser degree (r>0.36). The number of fully expanded leaves prior to silking differentiated N treatments better than did single leaf chlorophyll measurements with higher yields associated with more rapid vegetative development. Our data suggest that multiple core samples are required to estimate available soil mineral N, particularly in fertilized plots that have greater spatial variability. Variability of plant-based measures, such as chlorophyll content, could be used as an indicator of relative plant N sufficiency at early growth stages as spatial variability declined with higher soil N fertility.