Comparison of decision tools to improve the nitrogen management in irrigated maize under Mediterranean conditions in Spain

Recent studies at watershed scale indicate that the excessive use of nitrogen (N) fertilizer in irrigated maize is one of the major contributors to non-point nitrate pollution of waters in some agricultural areas of Spain. There is a need to provide relatively easy and practical decision tools to help farmers to increase N use efficiency in maize fields which simultaneously will increase crop profits and reduce environmental problems from excessive N use. Four field trials with different soil types were conducted in three different locations in Spain during the years 2010 and 2011 to compare three N fertilizer managements. These were (T1) a fixed N rate considering the crop potential of the zone, 200-250 kg N/ha, (T2) a variable rate of N depending on mineral soil N content before plant sowing, (T3) a variable rate of N depending of chlorophyll meter reading (CMR) at V15 stage. A control (T0) treatment without N application and an over fertilized treatment (T4) of 300 kg N/ha were also included. These five treatments were evaluated under 3 different initial soil nitrate concentrations (low, medium, and high), prior to sowing. Total N-fertilizer applied, grain yield and N-fertilizer use efficiency (NUEf) are presented and discussed. Our results showed that the two decision tools evaluated (T2 and T3) allowed for a reduction in total N fertilizer applied in two of the four experimental sites compared to the fixed N rate (T1) without yield penalties. In the other two sites the T3 treatment reduced the N applied but the soil criteria (T2) did not allow to reduce the N applications. The NUEf of the overfertilized plots (T4) was, in general, lower than in the N management treatments T1, T2 and T3. The use of the decision tools did not always significantly increase the NUEf compared with a reduced fixed dose of N fertilizer (T1). The methodology used should be refined to better adjust the N rates to different soil types and environments. However, results indicate that the use of soil tests before planting or chlorophyll meter readings at later vegetative stage can significantly improve the management of N in irrigated maize crops, reducing the negative effects of excessive nitrogen fertilizers use in agricultural areas.