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In Mediterranean areas, irrigated maize systems are based on winter bare soil periods and synthetic N fertilizer is split in a maximum of three applications leading to a low nitrogen use efficiency (NUE). Spain is one of the largest pig producers worldwide, which leads to an important availability of organic sources. Therefore, application of organic fertilizers is usual in cereals. Moreover, the Spanish biogas sector is under development. Therefore, it is necessary to study organic fertilizer strategies allowing a NUE optimization leading to synchrony between N availability and crop demand. The aim of this work is to assess different strategies to enhance NUE, on irrigated maize (Zea mays L.) and rainfed cereal systems in the Ebro Valley (NE Spain).
Two on-farm field experiments were established in 2022. A rainfed cropping systems (RCS) one in Agramunt (Lleida, NE Spain) and an irrigated one (ICS) in Sucs (Lleida, NE Spain). In RCS the factors evaluated were: (i) use of different fertilizers and (ii) their application date (either pre-sowing or top-dressing). For factor (i) a control, mineral N (50 and 100 kg N·ha-1, M-50 and M-100, respectively), pig slurry (PS) and digested pig slurry (DPS) at 125 kg N·ha-1 (either at sowing, -s, or top-dressing, -t) and composted sewage sludge at pre-sowing (125 kg N·ha-1) top-dressed with (CSS+M-50) or without 50 kg mineral N ha-1 (CSS). For some treatments was not feasible to apply the target rate with the commercial machinery. The effective N rates applied are shown in Figure 1.A. The experimental design is based on randomized blocks with 3 replications. In 2023 triticale (Triticosecale Wittm.) was harvested at the end of flowering for forage. In ICS the factors assessed were: (i) fertigation with liquid fraction of sow slurry (control and fertigation), (ii) the use of cover crops (control, rye (Secale cereale L.), faba bean (Vicia faba L.)) and, (iii) top-dressed urea-ammonium nitrate (0, 50, 100 and 150 kg N·ha-1). The experiment was based on a split-split plot design with 4 replications. The 2023 season was extraordinarily dry and maize could not be planted. Nitrogen efficiency indexes were defined as follows: (i) nitrogen apparent recovery fraction (NRF, %) as the increment of crop aboveground N uptake to applied N, (ii) nitrogen use efficiency (NUE, kg kg−1) as the ratio of forage yield to N supply. Nitrogen supply includes the total N applied as fertilizer and the initial soil nitrate content (SNC). Three for RCS and two for ICS observations of aerial biomass samples for each plot were taken at flowering in RCS and before cover crops termination in ICS, to determine dry matter and N content. In addition, biological nitrogen fixation (BNF) for faba bean was also determined, using the natural abundance method of the 15N isotope (Unkovich et al., 2008). SNC was determined either just before sowing and after crop termination for 0-90 cm in RCS and for 0-60 cm for ICS.
In ICS no differences were found between treatments on forage yield (Figure 1.A), although a trend of higher values was observed when using organic fertilizers compared to M-50 and M-100. CSS applied at around 50 kg N·ha-1 in such a dry year (280 mm, of which 15.5 mm fell from March to May) lead to the highest NRF. The additional application of 50 kg N·ha-1 of mineral fertilizer didn’t improve either efficiency N indices and yield. The application of PS independently of the date, led to the same NRF than the use of mineral N (M-100, C-M-50). In a dry season such as the studied, 50 kg N ha-1 applied with organic fertilizers were enough to reach the highest yields without differences between treatments. In ICS rye achieved the greatest aerial biomass and faba bean the lowest with intermediate values of the control due to spontaneous vegetation (Figure 1.B). N uptake the highest in rye (21 vs 10.5 kg N ha-1). In faba bean BNF accounted for 69% of N acquired (Figure 1.B). Rye showed its ability to mitigate nitrate leaching. However, more research is needed to assess the fate of cover crops nitrogen decomposition.
References:
Unkovich, Murray., & Australian Centre for International Agricultural Research. (2008). Measuring plant-associated nitrogen fixation in agricultural systems. Australian Centre for International Agricultural Research.
| Keywords | cover crop; irrigated maize; nitrogen use efficiency; organic fertilizer; winter cereal |
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