Influence of straw mulching on soil water/salt movement and cotton root distribution under drip irrigation

With wide application of plastic mulching in cotton fields in the arid regions of China, residual plastic films have become a growing concern. Compared with plastic mulching, straw mulching has been noted to safely avoid the residual problem associated with plastic mulching. To explore the feasibility of replacing plastic mulching with straw mulching in a combined mulching and drip irrigation system, the effects of straw mulching on distributions of soil water and salt and cotton root system in drip-irrigated cotton fields were evaluated in this study. In addition, the effect of deep straw mulching (buried a layer of straw at 30 cm soil depth) coupled with drip irrigation was evaluated. Pit test experiments were carried out on two soil types (non-saline and saline-alkali soils) with three straw mulching treatments - surface mulching, deep mulching and non-mulching - coupled with drip irrigation. Samples were taken at cotton batt stage when the distribution of root systems was stable, followed by analyses of water, salt and root distribution characteristics. The results showed that surface mulching was fairly effective in soil moisture conservation and effectively prevented water loss via evaporation and salt accumulation in the plough layer. Deep mulching was generally better in effect regarding water conservation than non-mulching. Deep mulching was only advantageous over surface mulching in soil areas near irrigation lines under the straw layer. While deep mulching significantly improved soil moisture content below the plough layer, it had no apparently effect on salt at the cotton batt stage. Straw mulching significantly influenced the distribution of cotton root systems, especially in deep soil layers, by controlling the movements of water and salt. For non-saline soils, in the 0 28 cm soil layer, the highest root length density, root weight density and root length density proportion occurred under non-mulching. Although surface mulching had the smallest root length density, it also had the largest proportion of root weight density. Moreover, the lowest root weight density was under deep mulching. Deep mulching had the highest root length density in the 28 70 cm soil layer. Surface mulching had the smallest root length density but the largest root length density proportion. Then non-mulching had the lowest root length density proportion. In the 28 56 cm soil layer, deep mulching had both the highest root weight density and root weight density proportion. For saline-alkaline soil, in the 0 28 cm soil layer, both surface and deep mulching treatments had higher root length density and root length density proportion than non-mulching. Then surface mulching had the highest root weight density. The lowest root weight density and root weight density proportion occurred under deep mulching. On the contrary, deep mulching had the highest root weight density proportion and the lowest root weight density in the 28 70 cm soil layer. These findings suggested that surface mulching promoted root development in the plough layers of both saline-alkaline and non-saline soils. Deep mulching inhibited root system development in the upper soil layers but enhanced root system development in soil layers below the 30 cm soil depth. Also straw mulching caused the root system to be thinner and longer under unfavorable salt and alkali conditions. Therefore the combination of straw mulching and drip irrigation was highly promising for efficient cotton production in arid areas.