MAIZE TEMPORAL YIELD VARIABILITY UNDER LONG-TERM MANURE AND FERTILIZER APPLICATION - FRACTAL ANALYSIS

Long-term experiments offer unique possibilities to study the effects of management practices on crops and soils with time. Characterizing t emporal variability of grain yield provides an opportunity to distingu ish between the effects of environment vs. management practices on gra in yield, an important parameter. A long-term study was established in 1912 in western Nebraska as part of a rotation study. In 1953, each p lot was divided into manure (27 Mg ha(-1) annually) and no-manure sect ions to which fertilizer treatments of 0, 45, 90, 135, 180 kg N ha(-1) , and 135 kg N + 80 kg P ha(-1) were applied annually. Grain yield dat a from 1953 to 1993 for these treatments were used for fractal analysi s and determination of fractal dimension (D), which is an indication o f pattern of variation. A D value close to 1 indicates dominance of lo ng-term variation (or trend), while a D value close to 2 indicates dom inance of short-term (year-to-year) variation. Grain yield increased w ith increasing N application rate without manure, but no response to f ertilizer application was observed where manure had been applied. Frac tal dimensions ranged from 1.942 to 1.996, indicating significant domi nation of short-term variation of grain yield in the past 41 yr in all treatments. There was no significant difference between D values for manure or fertilizer treatments. Soil fertility amendments did not all eviate year-to-year variability observed in the corn (Zea mays L.) yie ld. Environmental factors like hail, fall freezing, and temperature va riation during the growing season had a significant effect on the grai n yield over the years. Although management practices can reduce tempo ral grain yield variability in some crops, variations in environmental factors in this study were great enough to dominate the yearly maize grain yield regardless of soil fertility amendments.