Assessment of nitrogen content, uptake, partitioning, and recovery by cotton crop grown under surface irrigation and drip fertigation by using isotopic technique

Field experiments were carried out through four consecutive years, 1995-1998, to assess the effect of irrigation methods and nitrogen (N) rates on nitrate status of the soil, N uptake, efficiency, partitioning, equivalent nitrogen fertilizer uptake (ENFU) and equivalent total nitrogen uptake (ETNU) by cotton by using the 15N technique. Treatments consisted of two irrigation methods, surface irrigation, and drip fertigation. Drip-fertigated cotton received five different nitrogen rates (0, 60, 120, 180, 240 kg N/ha), while only one rate (180 kg N/ha) was applied to the surface-irrigated cotton. All N fertilizers were applied as urea 46% N. Representative samples of the aboveground portions of cotton plants were harvested from the labeled subplots at physiological maturity and then were separated into leaves, stems, and fruiting forms. Dry matter weight, total N content, N uptake, and 15N excess atom percentage were assessed for each plant fraction, except lint. Soil-water status was monitored by using a neutron probe procedure, and irrigation scheduling was established according to the feedback data obtained. Soil solution nitrate as a function of time and depth was also evaluated by using a nitrate-strips procedure. Results indicated that N content, ETNU, ENFU, N uptake, and partitioning by cotton varied due to N input and irrigation methods. Nitrogen uptake and content in plant's tissues was increased as a function of N input and soil N status. Furthermore, N uptake was very high in a few instances, which might be due to the high residual N in the soil or due to high root activity of the cotton cultivar used in this study (Aleppo 33). Almost 55-63% of the accumulated N was partitioned into the fruiting forms, 27-35% in the leaves, and 9-10% in the stems of the cotton plants grown under drip fertigation. Whereas, under surface irrigation the percentage of accumulated N was partitioned as 57% in the fruiting forms, 34% in the leaves, and 9% in the stems. Fruiting forms and leaves were the major N sinks. Lint yield was highly correlated with N uptake, rates and content in plant tissues. Nitrogen recovery varied with different seasons, N input, and irrigation method. Equivalent total N uptakes, N fertilizer uptake, and the ratio of N uptake, of drip-fertigated relative to surface-irrigated cotton proved to be a good indicator for fertilizer and irrigation management.