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. T hese variations affect field-scale N-use efficiency. A field study was cond ucted for three years to investigate spatial variability in available soil mineral N within uniform research plots in relation to leaf greenness or ch lorophyll content (plant N sufficiency) and yield. Variations within the pl ot 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 variabilit y. 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 a ccounted for 70 to 80% of soil mineral N in fertilized plots compared to 50 to 60% in unfertilized control plots. The variability in grain yield of in dividual 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 v ariability (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 gr eenness from 6-ligule stage to silking was significantly correlated with ha rvest 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 rapi d vegetative development. Our data suggest that multiple core samples are r equired to estimate available soil mineral N, particularly in fertilized pl ots that have greater spatial variability. Variability of plant-based measu res, 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.