| Poplar is one of the most potentially fast-growing and high-yielding species in forestry cultivation,it is an important plant resource for the construction of China’s timber strategic reserve forest.However,the current productivity of poplar plantations in China is lower than 15 m3 ha-1 yr-1,and far below the world averaged level of 20~30 m3 ha-1 yr-1.The selection of improved varieties and intensive management strategies are essential for achieving its potential production.Populus tomentosa is widely planted on the North China Plain due to its fast growth rate,short rotation period,and white fiber.Compared with other tree species,it has higher requirements for water and nitrogen.Therefore,an appropriate application of water and nitrogen is important to improve the productivity of P.tomentosa plantations.Besides,water and nitrogen application through the drip-irrigated system is beneficial for water-nitrogen use efficiency and environmental protection.The object of the study was 2-to 4-year-old triploid P.tomentosa clone S86((Populus tomentosa×Populus bolleana)×(Populus alba×Populus glandulosa))on sandy loam soil in Gaotang County,Shandong Province,and drip irrigation and nitrogen fertigation(DIF)strategies were applied through the surface drip-irrigated system.Irrigation started when the soil water potential(?s)at 20 cm depth directly beneath the dripper reached-20 k Pa(I20),-33 k Pa(I33)and-45 k Pa(I45),and irrigation ended when the averaged soil water content(SWC)in the wetting zone(i.e.a semi-ellipsoid with the emitter as the center and a long semi-axis of 40 cm and a short semi-axis of 25 cm)reached field capacity.220,260,260 kg N ha-1 yr-1(FH),150,190,190 kg N ha-1 yr-1(FM),80,120,120 kg N ha-1 yr-1(FL)and no nitrogen fertilizer treatment(F0)were applied with water for 2-to 4-year-old P.tomentosa plantations,and a non-irrigated and non-fertigated treatment CK was also set up.There was a total of 13 treatments.By the field measurements and physical and chemical analysis in the lab,we researched the dynamics and spatial distribution of water and nitrogen in root zone in a different-aged stand,the growth,distribution patterns and morphological characteristics of fine root,the characteristics of water and nitrogen absorption and utilization,and the tree growth patterns and organ biomass distribution,aiming to find out the mechanism of water-nitrogen regulation in “root zone-absorptive root-plant” and the related techniques of DIF for P.tomentosa plantation under 2 m×3 m planting density during the short rotation period.The main results and conclusions are as follows:(1)Surface drip irrigation improved SWC in 0~60 cm shallow soil layers effectively.SWC in I20 treatment was significantly higher than that of other treatments by 24.7%.SWC in 80~140 cm deep soil layers increased by the heavy rainfall in the rainy season.Soil mineral nitrogen(Nmin)in root zone was affected by irrigation,fertigation,rainfall and soil depth.Nmin concentrated in the 0~20 cm soil layers(37.7%),and decreased with the increment of soil depth.Less irrigation amount and more fertigation amount could increase Nmin,but there was no interactive effect of irrigation and fertigation on Nmin,and the heavy rainfall had caused nitrogen leaching in the deep soil layer.(2)Fine roots concentrated mostly in 0~30 cm soil layers(43.2%~82.6%,of which I20 treatment accounted for 72.8%),and decreased by the depth of the soil layer.Besides,fine roots distributed close to the trunk(10~30 cm radial range accounted for 67.15% of the 10~150 cm radial range),and it decreased with the increase of the radial distance.I20 treatment significantly promoted fine root biomass density(RBD)in 0~30 cm soil layers in 2-year-old stand and in 30~80 cm soil layers in 3-year-old stand.Specific root length(SRL)was significantly lower in 0~30 cm soil layers in I20 treatment in 2-year-old stand.Fertigation significantly decreased fine root length density(RLD),fine root surface area(RSA),and SRL of 2-year-old stand in 30~80 cm soil layers.Organic matter,ammonium nitrogen and nitrate nitrogen had a significant positive correlation with some fine root traits(RBD,RLD and RSA),and the fine roots showed the preference of nitrate nitrogen absorption.(3)June,July and August were the critical period of stand water consumption under DIF,and stand transpiration(Tr)during June to August accounted for 55.56~59.60% of the total Tr in the growing season;Tr could be promoted by higher irrigation amount and less fertigation amount.Tr was significantly higher in I20 treatment than that of I45 FH and CK from April to June in 3-year-old stand,and I20 FL had the highest Tr.Leaf nitrogen content(>19g kg-1)was high during the whole growing season,which showed a “dilution effect” as leaf area index increased.Organ nitrogen concentration was less affected by DIF,and I20 treatments could significantly increase stand total biomass,so as to increase stand nitrogen uptake significantly.(4)Dirichlet regression model(DRM)was fitted for organ biomass allocation pattern under DIF treatment.The individual size of the tree significantly influenced the aboveground-belowground biomass allocation and leaf-branch-stem biomass allocation,but there was no effect on stump-coarse root-fine root allocation.Besides,I45 FH significantly reduced belowground biomass allocation proportion,highirrigation treatments(I20FH,I20 FM and I20FL)could improve stem biomass allocation proportion,and high-fertigation treatments(I20FH,I33 FH,I45FH)could significantly change the belowground biomass allocation of different organs.There was less effect on fine root biomass allocation among different treatments.(5)Drip irrigation stimulated tree growth in dry seasons and changed seasonal growth patterns in relatively dry years.The maximum DBH increment was one month earlier than other irrigation treatments in May in 2017.There was a significant positive correlation between tree growth and the cumulative irrigation amount and rainfall,and a significant negative correlation between tree growth and Nmin in rainy seasons.DIF significantly stimulated the growth of 2-and 3-year-old stand.There was a significant effect of drip irrigation on stand growth,and there was no significant interactive effect of drip irrigation and fertilization.I20 FH was the optimal treatment for the growth of the short-rotation P.tomentosa S86 plantation,and the averaged woodland productivity was 30.84 m3 ha-1,which was significantly higher than that of CK by 41.94%.While I20F0 was the most beneficial treatment,the economic benefit of 4-year-old stand was 2749 RMB per mu.(6)In summary,the key mechanisms of DIF on the growth of P.tomentosa S86 plantation on sandy loam soil and the related DIF techniques are as follows:(1)April,May and June were the critical water management period for P.tomentosa S86.Drip irrigation could make the supplement of water in root zone,so as to stimulate 2-and 3-year old stand growth.(2)Fine roots concentrated in 0~30 cm shallow soil layers.DIF significantly influenced fine root vertical distribution patterns and some morphological traits.There was a significantly negative correlation between tree growth and Nmin in rainy seasons,which indicated fertigation should be applied with a little amount in multiple times in growing seasons,and fertigation in rainy seasons should be avoided.(3)Fertigation had no significant effect on tree growth and organ nitrogen concentration,but it significantly increased Nmin in root zone.Besides,fertigation could change organ biomass allocation pattern,which indicated the long-term monitor of the effect of nitrogen fertigation on tree growth was necessary.(4)The optimal DIF technique for stand growth was that irrigation initiated when ?s was-20 k Pa with nitrogen application amount was 220,260,260 kg N ha-1yr-1 for 2-to 4-aged stand,and the fertigation frequency was 6 times during the growing season.While the optimal DIF technique for stand economic benefit was that irrigation initiated when ?s was-20 k Pa,and there was no fertigation during the short-rotation. |
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