School of Agriculture, Food and Ecosystem Sciences - Theses

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Author: Handoko × End date: 2019 × Type: PhD thesis × Subject: Wheat ×

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    Analysis and simulation of water-nitrogen interactions of the wheat crop
    Handoko ( 1992)
    This thesis studies the interactions between water and nitrogen in the growth and yield of wheat. The approach used was to investigate those interactions, firstly, by conducting field experiments in two years (1988 and 1989) and, secondly, by developing a crop model which simulates those processes. Data collected from those experiments on a set of 27 large drainage lysimeters (each 6x3x 1.2 m deep), provided data to construct and validate the model. The model was designed for spring wheat crops in semi and environments. Crops were sown on a dry soil but high soil mineral nitrogen content (210 kg ha-1) in 1988. There were four treatments, including the rainfed control, by the use of combination of irrigation and rainout shelters. By contrast, crops were sown on high soil moisture to field capacity in 1989. The level of soil mineral nitrogen remained high (>200 kg ha-1) at sowing because of high mineralization rates during summer period prior to sowing. There were nine treatments comprising two water regimes (rainfed and irrigated during grain filling) and three levels of soil nitrogen, unfertilized, fertilized at 100 kgN ha-1 at 24 days after sowing (DAS) in one, and a further 100 kgN ha-1 at DAS 104. Dunng 1988, soil water deficits were developed before anthesis (under drought treatment) and after anthesis (under rainfed treatment) resulting small crop biomass (<6.8 t ha-1) and nitrogen uptake (<120 kg ha-1) and low yield (<0.8 t ha-1) for both treatments. By contrast, irrigation treatment commencing around terminal spikelet increased crop biomas, nitrogen uptake and grain yield up to 13 t ha-1, 177 kg ha-1 and 3.3 t ha-1, respectively. Irrigation commencing around anthesis also increased those crop components but increases were smaller than earlier irrigation. During 1989, crop biomass at maturity (range from 12.7 to 17.9 t ha-1) and nitrogen uptake (158-316 kg ha-1) were large relative to the 1988 results due to greater available soil water and longer growing season (208 days compared to 177 days in 1988). However, grain yield was relatively small in all treatments ranging from 2.5 to 3.9 t ha-1, because of short duration of grain filling. The narrow ranges of crop biomass, nitrogen uptake and grain yield between treatments in 1989 were due to large amount of initial soil water and mineral nitrogen. In most cases, crop responded to nitrogen treatments only for the first 100 kgN ha-1 application. The common feature of soil nitrogen dynamics observed in both seasons was that nitrate decreased from about floral initiation to anthesis associated with high growth rate. After anthesis, when crop uptake was small, nitrate increased caused by increasing nitrification rates with temperatures. During the time, ammonium nitrogen decreased with time in most cases, related to the high nitrification rates which transforms ammonium into nitrate. Soil moisture was important factor determining mineralization. During 1989, post-anthesis irrigation increased mineralization, particularly the nitrification. By contrast, drought treatment during 1988 resulted in lower mineralization (42 kg ha-1) compared with the other treatments (>120 kg ha-1). In both years, drainage occured if soil moisture was above field capacity. Given low moisture at sowing during 1988, drainage and the associated leaching occured only after the application of irrigation. In 1989, when initial soil moisture was about field capacity, drainage and leaching occured following high rainfall during the first half of the growing season. During the second half, small amount of irrigation (126 mm) did not cause drainage because water loss by evapotranspiration was large (300 mm). A crop model that simulates water and nitrogen water interactions was developed and tested. The model was validated against crop development, above ground biomass and nitrogen, grain yield and nitrogen content, and components of soil water and nitrogen balances that include: soil water content, drainage loss, evapotranspiration, soil nitrate and leaching. The model's prediction showed good agreement with observation except for drainage and the associated leaching, both soil water and nitrate after anthesis, and leaf area index of unfertilized treatments. Variability of measured drainage, caused by small variation of initial soil moisture, and, hence, leaching introduced some problem in this validation. Application of the model may support agronomic decisions including selection of cultivars with distinct developmental behaviour, sowing density, sowing times, timing and rates of nitrogen fertilizer applications, yield forcasting and risk analysis. The model can also be applied to assist the study of crop-environment response.