Biochar addition can improve the physical and hydraulic characteristics of sandy soil. This study investigated the effects of biochar on water holding capacity and water movement in sandy soil under drip irrigation. By indoor simulation experiments, the effects of biochar application at five levels (0%, 1%, 2%, 4% and 6%) on the soil water retention curve, infiltration characteristics of drip irrigation and water distribution were tested and analyzed. The results showed that biochar addition rate was positively correlated with water holding capacity of sandy soil and soil available water. Within the same infiltration time, with an increasing amount of added biochar, the diffusion distance of the horizontal wetting front (HWF) tended to decrease, while the infiltration distance of vertical wetting front (VWF) initially declined and then rose. The features of wetted bodies changed from "broad-shallow" to "narrow-deep" type. The relationship between the transport distances of HWF and VWF and the infiltration time was described by a power function. At the same distance from the point source, the larger the amount of added biochar, the higher the soil water content. Biochar had a great influence on the water content of the layer with biochar (0-200 mm) and had some effects at 200-250 mm without biochar; but it had less influence on the soil water content deeper than 250 mm. For the application rate of biochar of 4%, most water was retained within 0-250 mm soil layer. However, when biochar application amount was high (6%), it would be helpful for water infiltration. During the improvement of sandy soil, biochar application rate of 4% in the plow layer had the best effect.

Drip irrigation under plastic mulch is widely used in Xinjiang, Northwest China. It can not only save water, but also reduce nutrient loss and improve fertilizer utilization. However, it is not clear whether the leaching occurs or not, what is the leaching amount? What is the relationship among fertilization, irrigation regimes, loss, cotton absorption, and cotton field under different fertilization and irrigation management under drip irrigation? Studying these issues not only provides reference for the formulation of fertilization and irrigation systems, but also is of great significance for reducing non-point source pollution. A long-term positioning experiment was conducted from 2009 to 2012 in Baotou Lake farm in Korla City, Xinjiang, with drip-irrigated cotton (Gossypium hirsutum L.) under different N fertilizer and irrigation amounts. The treatments were designed comprising Control (CK,0 N, 0 P, and 0 K with an irrigation of 480 mm) and the following three other treatments: (1) Conventional fertilize and irrigation (CON, 357 kg N hm–2, 90 kg P hm–2, 0 kg K hm–2, and irrigation of 480 mm); (2) Conventional fertilization and Optimizing irrigation (OPT, 357 kg N hm–2, 90 kg P hm–2, 62 kg K hm–2, and irrigation of 420 mm); and (3) Optimizing fertilization and irrigation (OPTN, 240 kg N hm–2, 65 kg P hm–2, 62 kg K hm–2, and irrigation of 420 mm). The results found that the leaching would occur in arid area under drip irrigation. The loss of total N, NH4+, P, N and P loss coefficient was higher under conventional fertilize and irrigation treatment while the loss of NO3- was higher under conventional fertilization and optimizing irrigation treatment. The correlations among N, P absorption by cotton, loss of NH4+ and total phosphorus were quadratic function. The total nitrogen loss and cumulative nitrogen application was lineally correlated. The loss of NO3- and cumulative nitrogen application was exponential. The nitrogen and phosphorus absorption by cotton under conventional fertilization and optimizing irrigation treatment was 24.53% and 35.86% higher than that in conventional fertilize and irrigation treatment, respectively. The cotton yield under conventional fertilization and optimizing irrigation treatment obtained higher than that in other three treatments. Therefore, the conventional fertilization and optimizing irrigation treatment was the optimal management of water and fertilizer in our study. These results demonstrate that reasonable water, nitrogen and phosphorus fertilize could not only effectively promote the absorption of nitrogen and phosphorus, but also reduce nitrogen and phosphorus losses under drip fertigation and plastic mulching.

Farmers provided excessive nitrogen fertilizer to obtain high cotton yield in Xinjiang, Northwest China. Although drip irrigation could save water resources and improve nitrogen use efficiency, it is not known if leaching is occurring or not and whether leaching will harm the water environment under different nitrogen application. The purpose of our study was to estimate the effect of different nitrogen (N) application on the N leaching loss in drip irrigated cotton fields in South Xinjiang. A field experiment was conducted with N application rates of 317(N317), 395(N395) and 476(N476) kg hm-2 from 2021 to 2022. The characteristics of N pattern and leaching amount were analyzed. The concentration of NO3- and total nitrogen in the leaching water significantly decreased with the decrease of N application. The proportion of NO3–to total nitrogen reached the highest values of 51.52%-58.16%, 49.53%-56.91%, and 57.52%-62.87% at bud, flower, and boll stages of cotton, respectively. Moreover, the proportion of NO3– to total nitrogen remained at a stable level, which indicated that NO3–was the main pattern of N leaching. The N loss in the treatment N395 and N317 was 79.67% and 67.48% of that in the N476 treatment (1.23 kg hm–2), while the yield was 120.56% and 112.46% of that in the N476 treatment. Compared with conventional fertilizer application (476 kg hm-2), the reasonable N fertilizer application would not only reduce the amount of N leaching, but also increase the cotton yield.