| Biomass can be used to produce biofuels,an alternative energy source in the future due to concerns of negative environmental consequences and declining fossil fuel supplies.Energy sorghum(Sorghum bicolor L.Moench)and switchgrass(Panicum virgatum L.)are widely adapted biomass crops with high potential for production of biofuel.The high biomass yields,adaptability to broad geographic range,high water-and nutrient-use efficiency,low nutrient requirements,and potential to grow in marginal environmental conditions make each energy sorghum and switchgrass an ideal feedstock for biofuel production.However,to achieve the production potential,optimal management practices are needed when grown in marginal wastelands.The purposes of this research were to deternine in depth:(i)the biomass yield,nutrient concentration in biomass,crop nutrient uptake,and nitrogen-use efficiency(NUE)of energy sorghum affected by nitrogen(N)fertilizer application rate,(ii)switchgrass plant size,biomass yield,and N recovery responses to N,phosphorus(P),and potassium(K)fertilizer deficiency,and(iii)the yield and quality of switchgrass(each for biomass and forage production)and crop nutrient uptake as affected by N application rate and harvest time.The ultimate goal for biomass production is to maximize the concentration of lignocellulose in the feedstock and minimize the concentration of mineral nutrients and their uptake that would help to reduce fertilization requirements and maintain soil quality.Therefore,we conducted four different studies in energy sorghum and switchgrass for sustainable biomass production on the basis of nutrient demand and supply.In the first study,we investigated the potential biomass yield of energy sorghum,nutrient(N,P,and K)concentration and uptake in harvested biomass,and NUE as influenced by N fertilizer application rate on semiarid sandy wasteland,in order to optimize N fertilizer rate to produce the sustainable biomass yield with least crop nutrient uptake.Five N rates(0,60,120,180 and 240 kg N ha-1)and two sorghum varieties(sweet type GT-8 and biomass type GN-11)were evaluated for two years in 2014 and 2015.Yield increment observed as N level increased,but the stand out treatment appeared to be N rate of 60 kg ha-1 which significantly increased biomass yield vs.controls by 68.8%in 2014 and 64.1%in 2015 for both varieties,with no additional benefits of increased N rates.The crop uptake of N,P,and K increased with N rate increase,and ranged between 32.2-119.1,7.9-19.2,and 22.1-94.0 kg ha-1,respectively,across all N application rates.Furthermore,lowest N rate(60 kg N ha-1)maximized all the components of NUE(i.e.,partial factor productivity,PFP;agronomic NUE,ANUE;apparent N recovery efficiency,ANRE;physiological NUE,PNUE).In the second study,we determined the effects of N,P,or/and K fertilizer deficiency on switchgrass biomass production,nutrient uptake and N recovery efficiency,and soil nitrate-N(NO3--N)accumulation on semiarid sandy wasteland under non-irrigated conditions.Fertilizer treatments i.e.,NPK(complete dose),PK(N omission),NK(P omission),NP(K omission),and a control with no fertilizer input(CK),were set up in 2015 and 2016 in a field of Blackwell switchgrass that was already established in 2008.Biomass yield with the NPK treatment in 2015 was 8.9 Mg ha-1 and in 2016 it was 7.3 Mg ha-1.In 2015,compared with the NPK treatment,a significant yield reduction of 33.7%was found with PK,22.5%with NK,28.1%with NP,and 40.5%with CK;however,in 2016,yield declined significantly only with CK.Plant N content was reduced with N omission treatment;conversely.P and K contents were not affected with P and K omission.Plant N and K uptakes were severely declined by respective nutrient deficiency treatments.Apparent N recovery(ANR)was lower for the treatments NP(21%)and NK(17%),which led to an increase in soil NO3--N accumulation in the top 0-20 cm depth,compared with the NPK treatment(30%).In the third study,we evaluated the effects of five N fertilizer rates(0,60,120,180,and 240 kg N ha-1)and two harvest times(late summer and early fall)in one-cut system on plant size and biomass yield of switchgrass,biomass nutrient(N,P,K)concentration and crop nutrient uptake,NUE,and soil N balance on a sandy wasteland under non-irrigated conditions.Plant height and stem diameter persisted unafected by N rate.Biomass yield did not respond to N rate in 2015;however,application of 60 kg N ha-1 significantly increased AGP biomass yield by 14.3%vs.control(6.3 Mg ha-1)during early fall harvest in 2016.An early fall harvest exhibited 12.5%higher biomass yield than harvest in late summer.AGP nutrient concentration and nutrient uptake increased by increasing N rate,except for concentration of P,which decreased.The N,P,and K uptake ranged between 49-64,8-1 1,and 43-51 kg ha-1,respectively across all N rates.An early fall harvest significantly reduced biomass concentration of N and K by 27.1%and 30%,respectively,and uptake by 19.5%and 22.4%,respectively.Besides,PFP and ANUE were highest with the application rate of 60 kg N ha-1.A soil N deficit in top 0-20 cm depth was observed in control;however.N supply of 60 kg N ha-1,was sufficient for normal plant growth.Meanwhile,N surplus increased as N rate increased which likely boosted NO3--N leaching.In the fourth study,forage and biomass quality of switchgrass,theoretical ethanol yield(TEY)and soil NO3--N content were examined in response to four N fertilizer rates(0,60,120,and 240 kg N ha-1)and two harvest times(late summer and early fall)in one-cut system.Crude protein(CP)content and protein yield of switchgrass,and TEY were significantly increased with an N application rate of 60 kg N ha-1;however,N application did not affect average values of other quality components of forage(acid detergent fiber,ADF;neutral detergent fiber,NDF;dry matter digestibility,DMD;dry matter intake,DMI;total digestible nutrients,TDN;net energy for lactation,NEL;and relative feed value,RFV)and biomass feedstock(soluble sugar,cellulose,hemicellulose,and ash).The soil NO3--N content(top 0-20 cm depth)significantly increased by N application rate at harvest in early fall;although soil NO3--N contents were lower at early fall harvest than green-up stage(after regrowth in mid-May)of switchgrass.Compared with late summer harvest,an early fall harvest significantly decreased average CP content and protein yield,increased average soluble sugar content and TEY,and it did not affect average contents of cellulose,hemicellulose,and ash.Therefore,our findings suggest that an N fertilizer rate exceeding 60 kg ha-1 to an optimal 120 kg N ha-1 would be required for sustainable biomass production of energy sorghum on sandy wasteland to maintain long-term soil productivity.For switchgrass,a balanced N,P,and K fertilizer management approach is essential to sustain biomass yield and stand persistence in long-term on sandy soils.This approach will not merely increase biomass yield,but it will enhance ANR,while avoiding soil NO3--N accumulation which is highly prone to leaching loss particularly on a sandy soil.Furthennore,a lower N fertilizer rate(60 kg N ha-1)is recommended to facilitate sustainable switchgrass biomass production without negatively affecting feedstock quality and soil productivity.Meanwhile,if purpose is to grow switchgrass as livestock enterprise,this low N supply would be beneficial to improve forage quality,while reducing the risk of N losses through NO3--N leaching on sandy soil.Switchgrass forage quality would be higher during late summer with greater CP content,whereas harvest during early fall or further delaying until late fall or after killing frost could provide higher quality biomass for energy purpose,and it would also reduce crop fertilizer requirements. |
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