Influences Of Irrigation,Fertilization And Garden Cloth Covering On Distribution And Dynamics Of Root-zone Soil Nitrogen In The Citrus Orchard

Citrus is the number one fruit group in China,with a constant demand for nitrogen at different ages and stages of the tree.Field management activities such as irrigation,fertilizer application and ground cover are important factors in ensuring a good and stable yield of citrus,and these activities can affect soil environmental factors,the spatial and temporal distribution of soil nitrogen and the efficiency of citrus nitrogen fertilizer use in the short term.The different forms of nitrogen in the soil include soil total nitrogen,alkali-hydrolyzable nitrogen,nitrate and ammonium nitrogen,which are important indicators of soil nitrogen availability.In this paper,the spatial and temporal distribution of soil nitrogen in the root zone of citrus under drip irrigation,fertilization and ground cloth covering and its relationship with environmental factors were investigated in a field experiment to provide basic data for the scientific and precise management of drip irrigated citrus orchards in the south.The field trial was conducted from September 2020 to September 2021 in a citrus orchard on the campus of Huazhong Agricultural University on a yellow-brown soil with three treatments:five irrigation levels W1（11 m³/hm²,20 min）,W2（23 m³/hm²,40 min）,W3（33 m³/hm²,60 min）,W4（45 m³/hm²,80 min）and W5（50 m³/hm²,90 min）.min)and W5（50 m³/hm²,90 min）;three N application levels N1（3.75 kg/hm²,5 g/plant）,N2（7.5kg/hm²,10 g/plant）,and N3（11.25 kg/hm²,15 g/plant）;ground cloth cover（C）and no ground cloth cover（NC）.Soil samples were collected at different depths in the root zone before（0 h）and 8,24 and 48 h after drip irrigation,respectively,and soil environmental factors（soil temperature,water content,conductivity and rainfall）were monitored simultaneously in the field to analyze the spatial and temporal distribution of different soil nitrogen forms in the root zone of citrus under the three treatments and their characteristics with environmental factors.The following main results were obtained.（1）The low irrigation amount can both replenish the loss of water and nitrogen in the soil and facilitate the enrichment of soil effective state nitrogen.Before and after drip irrigation,the trends of different forms of soil nitrogen in citrus root zone varied with time under different irrigation amounts.At 48 h after drip irrigation,the soil alkali-hydrolyzable nitrogen and nitrate content of W1 increased the most,by 0.26 and 3.31 times,respectively,while the ammonium nitrogen content only decreased by 3.64%,the soil nitrogen supply capacity of W1 irrigation was the strongest;the trends of alkaline nitrogen and nitrate content of W1,W2 and W3 at low irrigation rates were the same with time,which were"decreasing-rising"and"increasing-decreasing-increasing"type,respectively.The contents of alkali-hydrolyzable nitrogen and nitrate in W1,W2 and W3 showed the same trend with time,which were"decreasing-rising"and"increasing-decreasing-rising"type,respectively,while the ammonium nitrogen content increased gradually with the increase of irrigation amount.The ammonium nitrogen content of W4 and W5 was higher than the other treatments.（2）Low level of nitrogen application helps reduce soil nitrogen losses.Different forms of nitrogen in the soil profile responded differently to different nitrogen application rates.Before and after fertilizer application,soil nitrogen accumulated significantly in the 0-20cm soil layer and decreased with the increase of soil depth,while the change of soil nitrogen content was greatly influenced by the initial soil nitrogen content,and the nitrogen loss in the furrow increased with the increase of nitrogen application.After fertilization,the tendency of different forms of soil nitrogen migration to the furrow was positively correlated with the applied nitrogen.The decreasing trend of soil total nitrogen and alkali-hydrolyzable nitrogen contents in N2 and N3 was obvious,and the increase of nitrate and ammonium nitrogen was smaller than that in N1;while the soil nitrogen content in N1increased,in which the average contents of alkaline nitrogen,nitrate and ammonium nitrogen increased by 1.82,8.36 and 1.08 mg·kg^-1,respectively,and the average content of alkali-hydrolyzable nitrogen,nitrate and ammonium nitrogen increased by 1.82,8.36and 1.08 mg·kg^-1,respectively,and clustered near the citrus root system（20-40 cm soil layer in the monopoly）.（3）The ground cloth cover was conducive to increasing soil temperature,reducing nitrogen leaching,and providing a more stable soil environment for soil nitrogen transformation and migration.The ground cloth cover affected the soil nitrogen content in the root zone,and the content of soil total nitrogen,alkali-hydrolyzable nitrogen and ammonium nitrogen decreased significantly,while the nitrate nitrogen changed in the opposite direction(average content increased by 1.22 mg·kg^-1).In addition,the cover cloth reduced the variation of soil nitrogen with soil depth,increased the average soil temperature by 0.43℃,and decreased the soil volumetric water content and soil electrical conductivity by 0.14 m³/m³ and 0.11 m S/cm,respectively.The variation of soil total nitrogen,alkali-hydrolyzable nitrogen and nitrate with soil environmental factors under the cover cloth was of the"decrease-increase-decrease"type,while the response of ammonium nitrogen to each environmental factor varied.The results showed that the spatial and temporal distribution of soil nitrogen in the root zone of citrus under irrigation,nitrogen application,and covered or uncovered ground cloth differed significantly in the short term.The low irrigation and nitrogen application rates helped to slow down the leaching loss of soil nitrogen to the furrow and deep soil,while the cover cloth reduced the disturbance of the external environment to the shallow soil,maintained the stability of the internal soil environment.The results of this study have practical implications for water and nitrogen application and management in drip irrigated orange orchards,but the dynamic distribution of soil nitrogen in the root zone of citrus during each reproductive period still needs to be further investigated.