Effects Of Nitrogen Reduction Combined With Humic Acid Fertilizer On Apple Saplings And Soil Nutrients Under Water Stress

The Loess Plateau,as the dominant apple growing region in China,moisture is the biggest factor limiting the development of apples,and the unreasonable fertilization methods and dosage of fruit farmers in pursuit of yield also lead to the breeding of many problems in orchards.Previous research on water-fertilizer coupling mostly focused on wheat,rice and corn cash crops,and research on fruit trees mainly focused on mature fruit trees,and the fertilizers used were conventional chemical fertilizers,and the research content was mostly on the effects of high,medium and low fertilizers and water coupling on fruit trees.Few studies have been conducted on the effect of water-fertilizer coupling on the growth of young apple trees.For this reason,in this study,young annual Ruiyang and Ruixue apple trees were used as test materials,and three different fertilization levels were set:conventional chemical fertilizer（F₁）,chemical fertilizer with 20%N reduction and equal amount of humic acid liquid fertilizer（F₂）,chemical fertilizer with 40%N reduction and equal amount of humic acid liquid fertilizer（F₃）,and three irrigation levels:soil water content of 80%（W₁）,60%（W₂）,and 40%（W₃）of the field water holding capacity.Different water gradients and fertilizer application levels were coupled with each other for a total of 9treatments.The water-fertilizer coupling was carried out on young Ruiyang and Ruixue apple trees through pot experiments to investigate the growth condition,physiological characteristics,soil chemical properties,and nutrient uptake and utilization by young 1-year-old apple trees,with a view to providing a relevant basis for the formation of efficient cultivation techniques for new varieties of Ruiyang and Ruixue apples in the Loess Plateau region.The main research results are as follows:（1）The effects of water on plant height growth,basal stem growth,leaf area and new shoot length of Ruiyang and Ruixue at all stages of reproduction were highly significant,while fertilization and water-fertilizer interaction had little effect on them.The maximum values of total plant height,total basal stem growth,leaf area and new shoot length were found in the F₂W₁ treatment,and the F₂W₁ treatment was the most favorable for the growth of young apple trees.（2）Water significantly affected leaf water content,malondialdehyde（MDA）,proline（Pro）content and SPAD value of young apple trees.As the degree of water stress deepened,the relative water content and SPAD values of apple leaves gradually decreased,and malondialdehyde and proline contents significantly increased.The effect of humic acid liquid fertilizer application on leaf MDA,Pro content and SPAD value was not significant,and the response of MDA,Pro content and SPAD value to water was significantly greater than that of fertilizer application.This indicates that water plays a dominant role in the SPAD values of apple leaves.（3）As apple fertility progressed,leaf P_n,T_r and G_s showed a trend of increasing and then decreasing,reaching a maximum at the middle of fertility.The maximum values of leaf P_n,T_r and G_s of young apple trees under coupled water and fertilizer conditions were basically found in F₂W₁ treatment.Under certain fertilizer application conditions,leaf P_n,T_r and G_s increased significantly with the increase of water irrigation,specifically W₁>W₂>W₃,and the effect of water on photosynthetic characteristics of young apple trees was greater than that of fertilizer application.Different water and fertilizer treatments had no specific rule on LWUE of apple saplings.The P_n of young Ruiyang apple trees was closely related to T_r,G_s,LRWC,MDA,Pro,plant height and basal stem growth,and the P_n of young Ruixue apple trees was closely related between G_s and Pro.（4）Water was the main factor influencing the leaf N,P and K contents and soil chemistry of young Ruiyang and Ruixue apple trees.With the same fertilizer application,leaf N,P and K contents increased with the increase of soil water content,specifically W₁>W₂>W₃,which was more favorable for nutrient uptake by young Ruiyang and Ruixue apple trees under adequate water supply.Soil p H and soil effective phosphorus,nitrate and ammonium nitrogen contents decreased significantly with increasing irrigation water,while soil organic matter,total nitrogen and fast-acting potassium contents increased significantly with increasing irrigation water.Fertilizer N reduction with equal N application of humic acid fertilizer had little effect on leaf N,P and K content,but significantly increased soil organic matter and soil fast-acting nutrient content,and significantly decreased soil p H and soil total N content.In conclusion,maintaining the soil water content at 80%of the field water holding capacity,fertilizer with 20%nitrogen reduction and humic acid liquid fertilizer treatment with equal amount of nitrogen could significantly promote the growth of young apple trees and the accumulation of soil nutrients.