Effect of manure combined with chemical fertilizers on fruit yield, fruit quality and water and nitrogen use efficiency in watermelon grown in gravel-mulched field

Abstract：【Objective】The use of gravel-sand mulch is a traditional water conservation technique that has been used for centuries in the loess region of northwestern China. A porous layer of gravel and sand about 10 cm thick on the soil surface could reduce the risk of crop failure, which frequently occurs due to a combination of low precipitation and high evaporation that creates severe soil moisture deficits. The gravel-sand mulch could effectively reduce evaporation and runoff, and increase the soil temperature and retain soil moisture. However, because of covering sand layer, it also increases the difficulty of applying farmyard manure. Therefore, the traditional fertilization in this region involves chiefly application of chemical fertilizer, while organic fertilizers are neglected. Long-term surfeit use of excessive chemical fertilizer worsens the physical property of soil, causing soil hardening and leanness. Therefore, drought stress, low fertilizer efficiency and imbalanced fertilization are the limiting factors in gravel-mulched fields. This study examined the effect of the application of manure combined with chemicalfertilizers on fruit yield, quality and water and nitrogen use efficiency of watermelon in gravel-mulched field under the same amount of nutrient application.【Methods】A two-year field experiment was conducted in Gaolan, Gansu Province, China, where watermelon cultivation in gravel-mulched fields has a long history. A late mature watermelon cultivar‘Xi Sha Rui Bao’was used as the material and given 6 fertilization treatments: OF-0%, N fertilizer without organic fertilizer; OF-25%, substitution 25% inorganic-N with organic-N; OF-50%, substitution 50% inorganic-N with organic-N; OF-75% , substitution 75% inorganic-N with organic-N; OF-100%, substitution 100% inorganic-N with organic-N; and CK, no N fertilizer. The treatments were arranged in a randomized manner with three replicates, and the area of test plots was 32 m² . The amounts of nitrogen, phosphorus and potassium applied were 200 kg·hm^-2 , 170 kg · hm^-2 and 260 kg · hm^-2 , respectively. The growth target, yield, quality and the use efficiency of water and N fertilizer of watermelon were determined.【Results】The emergence rate and survival rate of watermelon seedlings increased with the increased application amount of farmyard manure. Compared to chemical fertilizer, 50%-100% substitution of inorganic-N with organic-N improved the survive rate of watermelon by 26.47%-34.16%. Meanwhile, the above-ground dry matter also increased with the addition of farmyard manure in vegetative stage, which was advantageous to the formation of‘source’. The watermelon yield in the treatments with 50%-100% substitution of inorganic-N with organic-N was increased by 63.2%-156.6% compared with N fertilizer without organic fertilizer. Meanwhile, the watermelon quality in treatments with 50%-100% substitution inorganic-N with organic-N was superior to that in treatments with no organic fertilizer. For example, the contents of sugars, effective acids and vitamin C increased by 18.1%-27.1%, 3.4%-5.3% and 18.6%-37.6% respectively, suggesting that organic fertilizer plays an important role in quality formation in watermelon. Total nitrogen in watermelon increased significantly by 58.8%-91.9% at fruit stage, and by 106.2%-119.1% at mature stage, in treatments with 50%-100% substitution of inorganic-N with organic-N. Nitrogen transportation amount was increased significantly by 46.4%-223.8%, and nitrogen fertilizer recovery efficiency by 6.9%-18.5% by applying organic fertilizer. The results showed that more organic fertilizer was propitious to the transportation between the‘source’and the‘sink’. Combined application of chemical fertilizer and organic manure could reduce water consumption and improve water use efficiency in melon grown soil in gravel- mulched fields. Compared to pure chemical fertilizer, soil water use efficiency was increased by 54.9%-176.3% with organic manure treatments.【Conclusion】These results demonstrated that under the same N input level, 50%-100% substitution of inorganic-N with organic-N is a suitable for watermelon in 20 years old sandy land．