Objectives Xinjiang is a typical arid area worldwide and cotton crops are mainly watered by drip irrigation under mulch film (DI). Roots are in direct contact with the soil and thus are the first plant organ to be affected by the water and fertilizer application strategy. Many studies have proved that long-term use of drip irrigation technology can have negative impacts on crop root systems. Restriction of root growth under drip irrigation reduces the soil volume that can be explored by the root system and hence the availability and uptake of essential minerals, and finally affects shoot growth and yield. The main objective of the present study is to explore the growth and dynamic changes in the spatial distribution of cotton roots and aboveground growth in response to DI in a field trial.

Methods A field experiment that included two treatments consisting of drip irrigation under mulch film (DI) and flood irrigation under mulch film (FI) was conducted. The monolith method was used to harvest roots at seven time points in the growth periods, and the root length, and shoot and root dry weight were measured.

Results The total root length per plant in the 0-10 cm soil layer was higher under DI, whereas in the 30-60 cm soil layer roots were longer under FI. From 65 to 96 days after sowing (DAS), the rate of increase in root length was lower under DI than FI. Total root length decreased after 125 DAS under DI, and was mainly centered in the 0-40 cm soil layer and at distances of 30-70 cm from drip-lines. The shoot: root ratio at 125 DAS was higher under DI than FI, but at 160 DAS the shoot: root ratio abruptly declined under DI. The decline in root length under DI during advanced growth stages may be attributable to the higher root density in shallow soil layers and the increase in the shoot: root ratio.

Conclusions For the cotton cultivated in the drip irrigation under mulching system, the cotton roots are mainly distributed in 0-40 cm of soil, which is significantly shallower and the total volume of roots is less than those grown under flooding system. After flowering stage, the shoot to root ratio become larger, which will exaggerate the presenility of roots. So it is necessary to study scientific water and fertilizer procedure that is good for building up reasonable root architecture, and regulate the vegetative growth of aboveground parts of cotton.