Effect of water and fertilizer coupling on root growth, soil water and nitrogen distribution of cabbage with drip irrigation under mulch

Abstract: The water and fertilizer application scheduling is the main factor affecting the efficiency of water and fertilizer use and crop yield as well as the root growth of crops and the distribution of soil water and nitrogen in soil profile. This study focused on the experimental observation and analysis of cabbage root growth, soil water and nitrogen distribution under different water and fertilizer application scheduling with drip irrigation mulch in greenhouse, aiming at putting forward a better water and fertilizer application scheduling for cabbage cultivation in greenhouse in Beijing-Tianjin-Hebei region. Three nitrogen amounts (200, 300 and 400 kg/hm2) were selected from the recommended range of nitrogen application rates in published literatures and three irrigation amounts were determined (irrigation upper/lower limit：90% θf /75%θf, 100%θf /85%θf and 100%θf /75%θf, θf is the field capacity.). Then, three schemes of water and fertilizer application scheduling were established: treatment 1 (low water and high fertilizer), treatment 2 (high water and low fertilizer) and treatment 3 (medium water and medium fertilizer), and field comparison experiments for two seasons were carried out in the greenhouse of the Water Saving Irrigation Demonstration Base of the Ministry of Water Resources. The experimental results showed that the root distribution ratios in 0-20 cm and 0-40 cm soil layers of the three treatments were above 90% and 98% (the proportion of total root weight), respectively. However, the distribution ratio of roots in higher irrigation lower limit (85%θf) was higher than that in lower irrigation lower limits (75%θf) in 0-20 cm soil layers. And a larger amount of water and fertilizer (treatment 3) could increase the total root weight. The water and fertilizer application scheduling of treatment 2 (high water and low fertilizer) could keep higher soil moisture content (average value of soil volume moisture content 28.44%-33.48%) and smaller coefficient of variation (0.08-0.13) in 0-60 cm soil layer during the whole growth period, and the leakage of irrigation water to deep layer (below 60 cm soil layer) was not obvious. The changes of nitrate nitrogen and ammonium nitrogen in the soil during the growth period mainly occurred in the 0-40 cm soil layer, and the change of soil layer below 40 cm was small. For the cabbage with more than 98% of the root system distributed in the soil layer of 0-40 cm, this was beneficial to the absorption and utilization of N by the root system, thus improving the utilization efficiency of nitrogen of cabbage. However, larger amount of irrigation water when fertilizing might lead to the leaching of nitrate nitrogen and ammonium nitrogen to the deep layer (below 60 cm). In addition, after harvesting, the residues of treatment 1 (nitrogen application rate 400 kg/hm2) and treatment 3 (300 kg/hm2) in 0-20 cm soil layer were significantly higher than those of treatment 2 (200 kg/hm2) (P < 0.05), of which treatment 1 was 2.28-2.83 times of treatment 2 and treatment 3 was 1.77-2.45 times of treatment 2. The residual nitrate nitrogen might damage soil environment and cause groundwater pollution. Therefore, comprehensive analysis showed that the suitable nitrogen application rate of cabbage was 200 kg/hm2, the appropriate lower irrigation limit was 85%θf, and the irrigation upper limit was 100%θf. This result could provide technical reference for the water and fertilizer management of cabbage with drip irrigation under mulch and reduction of agricultural non-point source pollution.