Effects of water salinity and N application rate on water- and N-use efficiency of cotton under drip irrigation

In arid and semi-arid regions, freshwater scarcity and high water salinity are serious and chronic problems for crop production and sustainable agriculture development. We conducted a field experiment to evaluate the effect of irrigation water salinity and nitrogen（N） application rate on soil salinity and cotton yield under drip irrigation during the 2011 and 2012 growing seasons. The experimental design was a 3×4 factorial with three irrigation water salinity levels（0.35, 4.61 and 8.04 dS/m） and four N application rates（0, 240, 360 and 480 kg N/hm2）. Results showed that soil water content increased as the salinity of the irrigation water increased, but decreased as the N application rate increased. Soil salinity increased as the salinity of the irrigation water increased. Specifically, soil salinity measured in 1：5 soil：water extracts was 218% higher in the 4.61 dS/m treatment and 347% higher in the 8.04 dS/m treatment than in the 0.35 dS/m treatment. Nitrogen fertilizer application had relatively little effect on soil salinity, increasing salinity by only 3%–9% compared with the unfertilized treatment. Cotton biomass, cotton yield and evapotranspiration（ET） decreased significantly in both years as the salinity of irrigation water increased, and increased as the N application rate increased regardless of irrigation water salinity; however, the positive effects of N application were reduced when the salinity of the irrigation water was 8.04 dS/m. Water use efficiency（WUE） was significantly higher by 11% in the 0.35 dS/m treatment than in the 8.04 dS/m treatment. There was no significant difference in WUE between the 0.35 dS/m treatment and the 4.61 dS/m treatment. The WUE was also significantly affected by the N application rate. The WUE was highest in the 480 kg N/hm2 treatment, being 31% higher than that in the 0 kg N/hm2 treatment and 12% higher than that in the 240 kg N/hm2 treatment. There was no significant difference between the 360 and 480 kg N/hm2 treatments. The N use efficiency（NUE） was significantly lower in the 8.04 dS/m treatment than in either the 4.61 dS/m or the 0.35 dS/m treatment. There was no significant difference in NUE between the latter two treatments. These results suggest that irrigation water with salinity 〈4.61 dS/m does not have an obvious negative effect on cotton production, WUE or NUE under the experimental conditions. Application of N fertilizer（0–360 kg N/hm2） could alleviate salt damage, promote cotton growth, and increase both cotton yield and water use efficiency.

Effects of fertigation scheme on N uptake and N use efficiency in cotton

While fertigation can increase fertilizer use efficiency, there is an uncertainly as to whether the fertilizer should be introduced at the beginning of the irrigation or at the end, or introduced during irrigation. Our objective was to determine the effect of different fertigation schemes on nitrogen (N) uptake and N use efficiency (NUE) in cotton plants. A pot experiment was conducted under greenhouse conditions in year 2004 and 2005. According to the application timing of nitrogen (N) fertilizer solution and water (W) involved in an irrigation cycle, four nitrogen fertigation schemes [nitrogen applied at the beginning of the irrigation cycle (N-W), nitrogen applied at the end of the irrigation cycle (W-N), nitrogen applied in the middle of the irrigation cycle (W-N-W) and nitrogen applied throughout the irrigation cycle (N&W)] were employed in a completely randomized design with four replications. Cotton was grown in plastic containers with a volume of 84 l, which were filled with a clay loam soil and fertilized with 6.4 g of N per pot as unlabeled and 15N-labeled urea for 2004 and 2005, respectively. Plant total dry matter (DM) and N content in N-W was significantly higher than in N&W in both seasons, but these were not consistent for W-N and W-N-W treatments. In year 2005, a significantly higher nitrogen derived from fertilizer (NDFF) for the whole plant was found in W-N and N-W than that in W-N-W and N&W. Fertigation scheme had a consistent effect on total NUE: N-W had the highest NUE for the whole plant, but this was not significantly different from W-N. Treatments W-N and W-N-W had similar total NUE, and N&W had the lowest total NUE. After harvesting, the total residual fertilizer N in the soil was highest in W-N, lowest in N-W, but this was not significantly different from N&W and W-N-W treatments. Total residual NO3-N in the soil in N&W and W-N treatments was 20.7 and 21.2% higher than that in N W, respectively. The total 15 N recovery was not statistically significant between the four fertigation schemes. In this study, the fertigation scheme N-W (nitrogen applied at the beginning of an irrigation cycle) increased DM accumulation, N uptake and NUE of cotton. This study indicates that Nitrogen application at the beginning of an irrigation cycle has an advantage on N uptake and NUE of cotton. Therefore, NUE could be enhanced by optimizing fertilization schemes with drip irrigation.

Water adaptive traits of deep-rooted C_3 halophyte(Karelinia caspica(Pall.) Less.) and shallow-rooted C_4 halophyte(Atriplex tatarica L.) in an arid region,Northwest China

This paper focused on the water relations of two halophytes differing in photosynthetic pathway, phe- notype, and life cycle： Karelinia caspica （Pall.） Less. （C3, deep-rooted perennial Asteraceae grass） and Atriplex tatarica L. （C4, shallow-rooted annual Chenopodiaceae grass）. Gas exchange, leaf water potential, and growth characteristics were investigated in two growing seasons in an arid area of Xinjiang to explore the physiological adaptability of the two halophytes. Both K. caspica and A. tatarica showed midday depression of transpiration, in- dicating that they were strong xerophytes and weak midday depression types. The roots of A. tatarica were con- centrated mainly in the 0-60 cm soil layer, and the leaf water potential （~L） increased sharply in the 0-20 cm layer due to high soil water content, suggesting that the upper soil was the main water source. On the other hand, K. caspica had a rooting depth of about 1.5 m and a larger root/shoot ratio, which confirmed that this species uptakes water mainly from deeper soil layer. Although A. tatarica had lower transpiration water consumption, higher water use efficiency （WUE）, and less water demand at the same leaf water potential, it showed larger water stress impact than K. caspica, indicating that the growth of A. tatarica was restricted more than that of K. caspica when there was no rainfall recharge. As a shallow-rooted C4 species, A. tatarica displayed lower stomatal conductance, which could to some extent reduce transpiration water loss and maintain leaf water potential steadily. In contrast, the deep-rooted C3 species K. caspica had a larger root/shoot ratio that was in favor of exploiting groundwater. We concluded that C3 species （K. caspica） tapes water and C4 species （A. tatarica） reduces water loss to survive in the arid and saline conditions. The results provided a case for the phenotype theory of Schwinning and Ehleringer on halophytic plants.